- Email: [email protected]
Differences in the seismicity preceding the 2007 and 2014 eruptions of Kelud volcano, Indonesia
Journal of Volcanology and Geothermal Research 382 (2019) 50–67
Contents lists available at ScienceDirect
Journal of Volcanology and Geothermal Research journal homepage: www.elsevier.com/locate/jvolgeores
Differences in the seismicity preceding the 2007 and 2014 eruptions of Kelud volcano, Indonesia Sri Hidayati a,⁎, Hetty Triastuty a, Iyan Mulyana a, Sucahyo Adi a, Kazuhiro Ishihara b, Ahmad Basuki a, Heri Kuswandarto a, Budi Priyanto a, Akhmad Solikhin a a b
Center for Volcanology and Geological Hazard Mitigation, Geological Agency, Indonesia Organization of Volcanic Disaster Mitigation, Japan
a r t i c l e
i n f o
Article history: Received 8 August 2016 Received in revised form 9 October 2018 Accepted 15 October 2018 Available online 18 October 2018 Keywords: Kelud volcano VT earthquake Hypocenter Magma migration
a b s t r a c t A significant increase in the number of volcano-tectonic (VT) and VB earthquakes preceded the 2007 and 2014 eruptions of Kelud volcano, Indonesia. The background seismicity rates at the volcano is generally b5 of VTs per month and other types of volcanic earthquakes are rarely recorded. After 17 years of quiescence, the 2007 eruption was preceded by an increase in VTs starting in September 2007, two months prior to the eruption. An intense VT swarm began in October 2007 and led to occurrence of VB, sometimes presenting as repetitive self-similar events. The final precursory seismicity included spasmodic bursts of VTs, repetitive self-similar VBs and low frequency tremor, prior to lava dome formation in the crater lake, on November 3, 2007. After just ~6 years, VTs began to increase again in December 2013. The number significantly intensified in mid January 2014, ~1 month before the eruption. Seismicity continued to intensify including VTs, VBs, and LFs into February 2014, with the final precursory seismicity consisting of a burst of over scale tremor, followed by a short burst of repetitive self-similar events that merged into harmonic, gliding tremor a couple hours prior to the explosive eruption on February 13, 2014. We examine the locations of VTs, the energy release rate, and the precursory seismic pattern prior to both eruptions to understand the differences in the eruptions' style and size. We found that epicentral locations of VTs are similar for both eruptions, but the depths were slightly deeper prior to the 2014 eruption. We also find that the cumulative energy released prior to 2014 is twice that of 2007 and in half the time. The migration of VTs in Kelud volcano may indicate magma intrusion from a deeper storage region in 2014 and at a faster rate, leading to a much more explosive eruption. © 2018 Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
1. Introduction The 1731 m stratovolcano Kelud, located in East Java (Fig. 1) is one of the most active and dangerous volcanoes in Indonesia with eight eruptions of VEI 4 or greater since 1548, five of which have occurred in the last 100 years. Tectonically most of the active volcanoes in Java are located about 100 km above the north-dipping subducting slab of the Sunda Arc (England et al., 2004). According to Hall et al. (2007), Java has a simple structure in which the east-west physiographic zones broadly correspond to structural zones. It can be separated into three
⁎ Corresponding author. E-mail address: [email protected] (S. Hidayati).
structural sectors: West, Central and East Java. The regional stress field of East Java is estimated to be dominantly in the NE-SW direction (Sribudiyani et al., 2003) where Kelud volcano forms an alignment with Kawi-Butak, Arjuno-Welirang, and Penanggungan volcanoes (Smyth et al., 2008) (Fig. 1). Kelud volcano has both explosive and effusive eruptions, and therefore lava flows, lava domes, pyroclastic flows, pyroclastic airfall, and eruption lahar deposits, mostly basaltic andesite in composition, comprise its edifice (Wirakusumah, 1991). The explosive eruptions usually occur in a short time and begin with phreatomagmatic eruptions that are quickly followed by explosive magmatic eruptions that produce pyroclastic flows, ash-fall and lapilli. The effusive eruptions forma lava dome. Strong increases in the number and magnitudes of distal VTs event often precedes volcanic eruptions at closed system volcanoes (White
https://doi.org/10.1016/j.jvolgeores.2018.10.017 0377-0273/© 2018 Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
S. Hidayati et al. / Journal of Volcanology and Geothermal Research 382 (2019) 50–67
51
Fig. 1. Geological Map of East Java (slightly modified from Smyth et al., 2008). Kelud volcano is located within a chain of volcanoes aligned from NE to SW (inset). Rose diagrams (bottom right) show dominant orientations of regional structures for East Java for both for the surface and basement (Sribudiyani et al., 2003).
and McCausland, 2016) in response to the intrusion of new magma. Increases in VT seismicity have preceded most eruptions at Merapi volcano that generated pyroclastic flows due to dome collapse (Hidayati et al., 1998; Ratdomopurbo and Poupinet, 2000; Hidayati et al., 2008b; Santoso et al., 2013). Similar precursory seismicity was also recorded at Unzen volcano, where numbers of VTs increased and hypocenters generally migrated toward the summit in the 6 years prior to the 1991 eruption (Umakoshi et al., 2001). The November 1998 eruption of Volcán de Colima, México was preceded by a 12-months of increased swarms of various types of microearthquakes (Domı́ nguez et al., 2001). Swarms or increases in seismicity may lead to eruption, however their rates, durations and types prior to eruptive activity can vary. White and McCausland 2019 break the precursory period into 4 stages and correlate them to the processes that occur as magma migrates to the surface, which inherently will change slightly from volcano to volcano and eruption to eruption. After the 6 years of distal VT seismicity at Unzen, regular self-similar earthquakes preceded and accompanied dome extrusion (Nakada et al., 1999; Umakoshi et al., 2011). At Usu volcano, only a short swarm of VTs preceded a flank eruption on March 31, 2000 (Zobin et al., 2005). Guntur volcano (Sadikin, 2008; Basuki, 2015) and Gede volcano (Basuki et al., 2013; Hidayat et al., 2013), have experienced several periods of high seismicity of VT with no ensuing eruption, also called ‘failed eruptions’ by Moran et al. (2011). Understanding which seismic unrest will lead to an eruption and the size and timing of that eruption, is a question all agencies responsible for monitoring volcanoes seeks to know. This paper describes the similarities and differences in the observed precursory changes, the temporal spatial distributions
of high frequency volcano tectonic (VT) seismicity, and seismic energy release rates at Kelud volcano preceding the 2007 and 2014 eruptions. In particular, we will discuss the changes in of the precursory seismicity during volcanic unrest and how it relates to the rate of magma supply and explosivity of eruptions at Kelud. 2. Historic and recent eruptions of Kelud volcano The typical eruption styles of Kelud volcano are either a large explosive pyroclastic flow generating eruption or an effusive lava dome forming eruption. In addition, the presence of a crater lake at the summit can lead to destructive syn-eruptive lahars. Dates and sizes of known historical eruptions of Kelud volcano are listed in Table 1 (GVP, 2013; Kusumadinata et al., 1979). The intereruption interval ranges from 1 to 311 years, and the biggest eruptions (≥VEI 4) occurred in 1586, 1641, 1826, 1919, 1951, 1966, 1990 and 2014. All of these major eruptions were following 15–55 years long repose interval, except the eruption in 2014, which occurred only 7 years after the previous effusive eruption in 2007. The explosive eruptions in 1901 and 1919, when the volcano had a crater lake of about 40 Mm3 in volume, resulted in tens of kilometers long lahars causing N5000 casualties (Global Volcanism Program, 2013). To reduce the water volume of the crater, a tunnel has been built and has succeeded in minimizing the distance of lahars in the 1951 and 1990 eruptions. The number of fatalities, caused by both eruptions, were smaller (in 1951, 7 people were killed; in 1990, 34 people were killed, because of roof collapse at an evacuation shelter). However the 1966 eruption was an
52
S. Hidayati et al. / Journal of Volcanology and Geothermal Research 382 (2019) 50–67
Table 1 Eruption year, size and type from the Global Volcanism Program (2013) and CVGHM reports (Kusumadinata et al., 1979). Erupted volumes from Ishihara et al. (2011), Hidayati et al. (2009) and Maeno et al., (2019). Year
VEI Eruption type
1000 1311 1334 1376 1385 1395 1411 1450 1451 1462 1481 1548 1586 1641 1716 1752 1771 1776 1785 1811 1825 1826 1835 1848 1851 1864 1901 1919 1920 1951 1966 1967 1990 2007 2014
3 3 3 3 3 3 3 3 3 3 3 3 5 4 2 2 2 2 2 2 2 4 2 3 2 2 3 4 2 4 4 1 4 2 4
Dome
Explosive Explosive
Explosive
Explosive Explosive Dome Explosive Explosive Explosive Dome Explosive
Inter-eruption interval (years) 311 23 42 9 10 16 39 1 11 19 67 38 55 75 36 19 5 9 26 14 1 9 13 3 13 37 18 1 31 15 1 23 17 7
Volume erupted (106 m3)
Casualties
200 90
? Reported, number unknown Reported, number unknown Reported, number unknown ? ? ? None None ? ? ? 1000. No further explanation ? Reported, number unknown None None ? ? ? None None None None None None Reported, number unknown 5160 None 7 died, 167 injured 210
130 16.3 220
34 None None
190
exception to this reduction in lahar hazard. The crater was excavated to a greater depth by the 1951 eruption, therefore the volume of crater lake water prior to the 1966 eruption had increased to 21.6 Mm 3 and a lahar travelled to 31 km. This eruption killed 210 people. A new tunnel was constructed following the 1966 eruption and the crater lake was reduced to just 1 Mm 3 prior to the 1990 eruption. A strong and explosive eruption (VEI 4, Bourdier et al., 1997) on 10 February 1990 produced a lava plug at the top of the conduit, yet the eruption on 3 November 2007 was almost purely effusive, forming a lava dome in the crater. Unrest began with a swarm of VTs on 10 September 2007, followed by swarms of VB (i.e. shallow VT, hybrid and/or LF earthquakes; Minakami, 1974) and spasmodic tremor prior to lava dome formation on 3 November 2007 (Hidayati et al., 2009). The behavior of the precursory seismicity before this dome extrusion is similar to other domes worldwide such as Soufriere Hills Volcano (Miller et al., 1998; Rowe et al., 2004), Unzen Volcano (Nakada et al., 1999; Umakoshi et al., 2011), Mount St. Helens (Moran et al., 2008) and Sinabung Volcano (McCausland et al., 2019). Phreatomagmatic ash emissions occurred several times as the hot lava dome occupied the lake crater. Six year later, a large and violent phreato-magmatic explosion occurred in the evening of 13 February 2014, at 22:46 local time, destroying the 2007 lava dome. The eruption plume rose to up to 27 km altitude, ejected an estimated 220 Mm3 of volcanic material with heavy ash fall out to distances as great as 200 km (Andreastuti, personal communication, 2016; Maeno et al., 2019). The eruption was preceded by 2 months of increasing in VT and shallow seismicity.
3. Monitoring network Visual observation is conducted from the Kelud volcano observatory at Margomulyo Village, about 7.5 km from the volcano.
Fig. 2. Monitoring network of Kelud volcano. The diamonds and solid squares represent seismic station and tiltmeter, respectively. The triangles represent water temperature sensor (BUOY) at surface, 10 m and 15 m for 2007activity and water temperature measurement at Bladak River outlet (OUT) for 2014 activity.
S. Hidayati et al. / Journal of Volcanology and Geothermal Research 382 (2019) 50–67
Periodic visits are made to assess the visual conditions around the crater. Water level, temperature and color of the lake are monitored for significant changes. Temperature sensors were installed at the surface, and at 10 m and 15 m depth in 2006 (Fig. 2). Continuous water temperature measurements prior to 2014 eruption were made at the outlet to the Bladak river. In addition, periodic measurements of the water chemistry are
53
performed. Deformation at Kelud is measured by tiltmeters, the first of which was installed in early 2007, and a second was installed in North Lirang on 5 February 2014 in response to the change to Alert Level II (Watch). Similarly a CCTV camera was installed at the Lirang seismic station (LRG) and telemetered to the observatory in order to monitor the crater for any visual changes.
Fig. 3. Lake temperature and tilt data for the 2007 and 2014 eruptions. (a) The change of lake temperature prior to lava dome formation in 2007. (b) A sharp deflation was observed from tilt data prior to lava dome formation in 2007. (c) Change in hot spring temperature at outlet (Bladak river) in period of 15 September 2013 to 13 February 2014 (until 20:00 local time). (d) Tilt observation at Lirang Selatan Station during the period of 15 September 2013 to 13 Februarys 2014 (until 22:40 local time). Tilt observation at Lirang Utara Station during 7–14 February 2014 is added at the bottom left. A M4.5 felt earthquake occurred on Feb.11, 2014 at 13:46.
54
S. Hidayati et al. / Journal of Volcanology and Geothermal Research 382 (2019) 50–67
Bladak Outlet
Lirang Utara Lirang Selatan
7 – 14 Feb 2014 Fig. 3 (continued).
Seismic monitoring of Kelud volcano began in 1925 with a single analog seismometer. The network was increased to three stations in 1987, all of which were destroyed in the 1990 eruption. Some stations were replaced after the eruption and since April 2007, five permanent seismic stations (Fig. 2) are used to monitor Kelud volcano. KWH station was located inside the crater, whereas the other three (KLD, SMB, LRG) were on the crater rim, about 600 m from the active vent. Station UMB was added on April 2007 in order to better earthquake locations. It is located about 4.5 km from the crater. All stations are equipped with single component Mark L4-C short period seismometer. Continuous seismic signals are transmitted by radio link to the Kelud volcano observatory (about 7.5 km away from the volcano) and are converted to digital format in real-time at 100 Hz.
2007, the lake temperature gradually rose from background values of 30 °C to 33.2 °C and the water color changed from the typical green to yellowish to bluish. This change in lake color was also accompanied by an increase in the concentration of CO 2 . Lake temperatures rose again on 2 November 2007 and rose even faster starting 24 h later on 3 November (Fig. 3a) (Hidayati et al., 2009). Geochemical analyses (Kunrat, 2009) of the lake water showed two hydrothermal systems: one deep, one shallow. Kunrat attributes the change in lake color and temperature to an increasing contribution to the lake from the deep hydrothermal system. Tiltmeter data from the station located 600 m from the crater on the SW flank (Lirang) showed slow and slight inflation from about 9 October through 2 November at 12:13 WIB (GMT +8), when a sharp change to deflation occurred at the same time there was the sharp increase in lake temperature (Fig. 3b) (Hidayati et al., 2009).
4. Gas, lake and tilt observations prior to eruption 4.1. Prior to the 2007 eruption
4.2. Prior to the 2014 eruption
The first known sign of unrest occurred in July 2007 when intense degassing of the lake floor was observed. Then in August
Water temperature was measured continuously at outlet of Bladak river. Crater lake temperatures began increasing on 10 September
S. Hidayati et al. / Journal of Volcanology and Geothermal Research 382 (2019) 50–67
55
Fig. 4. Time series situation around crater lake on 8 February 2014 (top), 13 February (bottom right) and 14 February 2014 (bottom left).
2013 (Fig. 3c), increasing about 5.5 °C between 10 September 2013 and 22 January 2014. Then, from 23 January until 13 February 2014 a significant increase of 4 °C was observed. No lake color changes were noted. Lirang tiltmeter data showed inflation in both radial and tangential components beginning in early January 2014, and increased sharply just before the 13 February eruption (Fig. 3d). The second tiltmeter installed on 8 February in North Lirang showed no inflation until after a felt M4.5 VT earthquake in the region of Blitar and Kediri (source: BMKG). In response to that earthquake, the tilt inflated drastically, particularly in the radial component and continued to show inflation until the beginning
of the eruption. Both tiltmeters were destroyed in the explosive eruption. Gas emissions coming from the top lava dome had been observed since December 2013. The emission was getting wider along with the appearance of bubbles at the inlet (Fig. 4) in early February 2014. As Kelud volcano started to erupt on 13 February 2014, visual observation could not be done from the observatory. However, a rumbling sound was heard and lightning observed around the crater. The Darwin Volcanic Ash Advisory Center (VAAC) reported that the eruption column reached a height of 17 km.
56
S. Hidayati et al. / Journal of Volcanology and Geothermal Research 382 (2019) 50–67
VT
VB
LF
Fig. 5. Earthquake types at Kelud volcano: VT, VB, LF, tremor, emission and explosion.
S. Hidayati et al. / Journal of Volcanology and Geothermal Research 382 (2019) 50–67
57
Tremor
Emission
Explosion
Fig. 5 (continued).
A gray-black ash emission with a height of 400–600 m was observed on February 14, followed by a 3000 m tall of gray-white ash plume the next day. The eruption decreased to white ash emissions
with a height of about 300 m (Fig. 4). On February 18, lava and secondary eruptions were observed in some rivers such as Ngobo, Mangli (Kediri), Bladak (Blitar), and Konto (Malang) rivers. The
58
S. Hidayati et al. / Journal of Volcanology and Geothermal Research 382 (2019) 50–67
Fig. 6. Daily VT, B-type earthquake and tremor counts at Kelud volcano during January 1989 – March 2014, using KWH seismic station which is located 100 m from the crater. The arrows represent when the eruptions took place.
eruption on February 13, destroyed the 2007 lava dome inside the crater. There was no SO2 measurement prior to the eruption of February 13, 2014. However after the eruption, SO2 measurements were conducted to anticipate possible further eruptions. The SO2 flux on February 17 was 89.3 tons/day and on February 18 ranged from 81.1 to 82.3 tons/day. A decrease of total SO 2 flux indicated that Kelud activity was declining. 5. Earthquake classification and location Seismicity at Kelud is classified into high frequency volcanotectonic earthquakes (VT), shallow VB (Minakami classification scheme), low frequency earthquakes (LF), tremor, emission and explosion signals (Fig. 5). Regional tectonic and teleseismic events are also noted and cataloged, but not used in the assessment of volcanic activity. VTs are broadband and high frequency (3–7 Hz) typically with well-defined P- and S-wave phases. VBs are shallow b1.9 km depth from the summit. The VB can occur as repetitive self-similar swarms (also called multiplets, families or drumbeats) or as non-regularized swarms with no significant similarity among waveforms. LFs are shallow, narrow band and low frequency (1–3 Hz), generally lack clear S-phases. LFs also can occur as drumbeats especially prior to explosive eruption. Tremor can be spasmodic (with rapidly changing amplitudes) or constant amplitude; continuous or occur in bursts; and narrowband, broadband, or
harmonic. Emission events are low frequency (b5 Hz), initial part unclear, and usually followed by ash emitting from the crater. Explosion signals typically start lower frequency and become broadband and have durations that are generally as long as the visible explosion signals. VT seismicity at Kelud volcano usually has sharp P-wave onsets at 4 (sometimes 5) stations and well-defined S-phases. Hypocentral locations were calculated using the GAD program (Nishi, 2005). In order to minimize time-picking errors, locations were determined for events that had at least 4 Pwave and 1S-wave arrival. We assumed an homogeneous isotropic velocity model (Vp = 2.7 km/s and Vp/Vs = 1.73), and only locations with travel time residuals b0.3 s were retained as good locations. With these criteria, we obtain 115 (out of 334) and 78 (out of 974) well-located VT events for 2007 and 2014, respectively.
6. Kelud seismicity The background seismicity of Kelud volcano is quiet, with b5 high frequency volcano-tectonic (VT) earthquakes in a month (Fig. 6). During the period 1989–2014, the seismicity of the volcano was low except in January 1990, September 2007 and January 2014, when the number of VT significantly increased, several weeks prior to eruptions in February 1990 (VEI
S. Hidayati et al. / Journal of Volcanology and Geothermal Research 382 (2019) 50–67
4), November 2007 (VEI 2), and February 2014 (VEI 4), respectively. After 17 years of repose, the effusive lava dome eruption in November 2007 at Kelud volcano was preceded by a 2-month period of high seismic activity that included 3 VT and 2 VB swarms (Hidayati et al., 2009) (Fig. 7a). The first VT swarm occurred on 9 September 2007, causing CVGHM to raise the alert to Level II (Watch). The second VT swarm on the contained larger amplitude and a greater number of earthquakes, but with no significant change in their hypocenters (Fig. 8a), causing CVGHM to raise the alert to Level III (Standby). LFs began occurring on 30 September and continued through 13 October, with almost no LFs after that date (Fig. 7a). The first VB swarm on 16 October 2007 contained the
59
first repetitive self-similar events and led CVGHM to raise the alert level again to Level IV (Warning). Hypocenters for this swarm were in confined to a narrow area, dominantly within 1 km of the crater floor (Fig. 8a). The final seismicity that preceded the dome emergence started on 24 October and was at first dominantly VTs that were larger in number and amplitude than the previous swarm. When VBs returned in significant numbers on 31 October, again they started smaller in amplitude and similar in waveform and became regular in time over 2 days and were larger in amplitude and greater in number than the previous swarm. An extremely regular period of these earthquakes occurred for 70 min at 14:48 WIB on November 2 (Fig. 10a) was preceded by 3 h of slowly increasing (in amplitude) continuous low frequency tremor
Fig. 7. Daily seismicity of Kelud volcano and RSAM prior to eruption a) on November 3, 2007 and b) February 13, 2014. The dots represent starting the alert of each level.
60
S. Hidayati et al. / Journal of Volcanology and Geothermal Research 382 (2019) 50–67
Fig. 7 (continued).
and followed by the same style of tremor. VTs recurred at 19:47 on 2 November increasing in number and size until becoming spasmodic bursts of VTs at 06:31 on 3 November (Fig. 10b). Continuous low frequency tremor began as a rapid series of shallow LFs from 12:32 WIB becoming spasmodic low frequency tremor around 13:11 WIB on November 3, and large amplitude (saturated) by 15:50 (Fig. 10c). It is believed that the dome emerged during this time period, because of the seismicity, the rapid lake temperature change, and the sharp deflation signal on the tiltmeter. The next day, when the weather was clear, a small black dome was spotted in the crater lake. This seismic
progression is similar to other dome extrusions, however the presence of the crater lake changes the last stages of seismicity to tremor instead of continued regular self-similar VBs, likely because of the interactions between the rising magma and its heat with the crater lake. Fig. 8a shows the VT hypocentral distribution during September – November 2007. Most of events are located at depths of 1 to 4 km beneath the summit, and few are deeper, up to 9 km. The distribution during September 10 to October 23, 2007 was wider and deeper, compared with those during
S. Hidayati et al. / Journal of Volcanology and Geothermal Research 382 (2019) 50–67
61
Fig. 8. a. Hypocenter distribution of VT earthquake during September–November 2007. Red solid and open circles represent hypocenter before and after October 23, 2007, respectively. The depth mostly ranges between 1.0 and 4.0 km from the summit. A few are deeper and the deepest one reached 9.0 km. In addition, white squares and house symbol indicates the location of seismic station and the Kelud volcano observatory, respectively. b. Hypocenter distribution of VT earthquake during January–February 2014. Red solid and open circles represent hypocenter before and after February 9, 2014, respectively. The depth mostly ranges between 0.5 and 4.0 km from the summit, and few are deeper and the deepest one located at 11.0 km. In addition, white squares and house symbol indicates the location of seismic station and the Kelud volcano observatory, respectively.
October 24 to November 3. Namely, foci shifted and clustered to shallow depths between 1.0 and 2.5 km beneath the summit approximately 10 days before lava dome extrusion on November 3. Six years of relative seismic quiet were then interrupted by a small increase in VTs in early December 2013. This was then followed by increasing VT and VB seismicity throughout January that included a swarm of VBs 15–17 January 2014; a swarm of VTs from 17 to 21 January; and a larger swarm of VTs on 27 January. The January 27 swarm was followed by a persistent increase in both seismicity types into February (Fig. 7b), including an increase in earthquake amplitudes since the beginning of February. CVGHM raised the alert to Level II (Watch) on 4 February. A sharp increase in both event types and their magnitudes occurred on 6 and 9 February 2014, which can be seen in the Real-time Seismic Amplitude Measurement (RSAM; Endo and Murray, 1991) (Fig. 7b). On 10 February CVGHM raised the alert to Level III (Standby) with an intensification of the cumulative seismic energy release for VTs (Fig. 9). Hypocenters through this time located up to 7 km deep
below the crater and up to 5 km from the crater (Fig. 8b). From 12 February the VT and VB seismicity again increased dramatically, and LFs began to occur (Fig. 7b). Hypocenters of earthquakes in these last days before the eruption were shallower, at 1–4 km depth below the summit (Fig. 8b). On 13 February, the final seismic precursors to the eruption onset began with a 4-min longover-scale-amplitude tremor at 21:11 WIB (Fig. 11). Within 30 min, at 21:51 WIB, repetitive self-similar earthquakes occurred and accelerated in their rate as to merge and become harmonic tremor with gliding frequencies at 22:18 WIB (Fig. 11). The tremor continued for about 1.5 h and culminated in the first eruption (phreatic) at 22:46 (WIB), causing the alert to be raised to Level IV (Warning). This first eruption destroyed seismic station LRG (Lirang). This eruption was followed by the paroxysmal one (VEI 4) at 23:01 WIB, and the remaining four seismic stations were still able to record this eruption. Unfortunately, this eruption could not be seen visually, because of the late hour. At 23:40 all the seismic stations stopped operating except the most distant
62
S. Hidayati et al. / Journal of Volcanology and Geothermal Research 382 (2019) 50–67
Fig. 9. Change in depth, magnitude and cumulative energy release versus time for VT seismicity prior to the 2007 (a) and 2014 (b) eruptions of Kelud. VT depths become shallower and magnitudes become larger before both eruptions of 2007 and 2014. The overall magnitudes are greater for the 2014 eruption (b), with a greater cummulative VT energy (by an order of magnitude) in a shorter time period (1.8 times shorter). No significant change of the depth ranges.
one, UMB. The eruption lasted for 3 h and subsided gradually. Seismicity after the eruption from 14 to 20 February was dominated by continuous tremor, with emission earthquakes beginning to occur within the tremor between 15 and 17 February. A few VTs recorded in the period between 19 and 20 February. The alert level was decreased to Level III on 20 February, and to Level II (Watch) on 28 February. Before the 2014 eruption, the majority of VTs during January until the eruption on February 13 occurred at depths of 0.5 to 4.0 km. The distribution of hypocenters was wider from January until February 9, and then was again more focused just beneath the summit from February 10 through the eruption on 13 February 2014 (Fig. 8b). 7. Discussion The VT activity prior to the 2007 and 2014 eruptions has similarities in the distribution of hypocenters and the migration
of hypocenters prior to eruption, however here we discuss four differences in precursory seismicity in relation to eruption style and size. (1). Prior to the 2007 lava dome eruption, the seismicity was significantly elevated over a 2-month period; whereas prior to the 2014 explosive eruption, it was significantly elevated over a 1-month period with an overall increasing rate of seismicity with time (Fig. 7) to a greater total of seismic energy released (Fig. 9). Fig. 9 shows the change in depth, magnitude, cumulative seismic energy released versus time for VTs. The depth of VTs in 2007 slowly migrated and became shallower from October until before the eruption on 3 November. The depth range was about 1–2 km and became shallower 0–1 km just before eruption. In contrast, the range of depths of VTs in 2014 is greater, about 0–4 km below the summit, from the beginning of January until the eruption on 13
S. Hidayati et al. / Journal of Volcanology and Geothermal Research 382 (2019) 50–67
February. This suggests that the magma may have ascended from deeper and in a shorter period of time possibly retaining more gas or because it contained more gas, thus leading to a more explosive (VEI 4) eruption. (2). Magnitudes of the VTs were larger in 2014 than in 2007, and the cumulative energy release prior to 2007 is half that prior to 2014 and in twice the time, a significant increase in the seismic energy release rate for the more explosive 2014 eruption. The magnitude of VTs in 2014 were mostly larger than 2007 ones, the largest magnitude for 2007 is M1.9 occurred on 1 November at 04:35 and 1.5 km depth from the summit. As for 2014, the largest magnitude is M2.2 occurred on 9 February at 02:15 and 5.5 km depth from the summit. Fig. 9 shows that the cumulative seismic energy released by VTs in 2014 is twice that of 2007, and occurred in half the time: a significant increase in the energy release rate.
The temporal variation of the number of VTs recorded prior to the eruptions in 2007 and 2014 is shown in Fig. 12. Based on the number of events, Zobin (2003) divides swarms of VT into 3 types: 1) one peak 2) multi-peak sequences with the maximum peak at the beginning 3) or multi-peak with the maximum at the end of the sequence. The swarm for 2007 and 2014 eruptions can be classified as type 3, multi-peak sequences with the maximum peak at the end of the sequence. Zobin (2003) suggests that multi-peak temporal variations in the number of seismic events may result from the migration of the seismic cluster.
63
Even though the 2007 and 2014 eruptions have a similar type of VT swarm by Zobin's definition, the details of how the VT seismicity leads into the eruption are significantly different and therefore Zobin's definition does not provide an indication of eruption size. Instead the rate of seismic energy release seems to play an important role. For the 2007 eruption, the VTs gradually increased before the eruption, while for the 2014 eruption, the VTs increased exponentially prior to eruption and over a shorter time interval. This suggests that the magma migration process prior to the 2007 effusive eruption was longer (~2 months), compared to the explosive eruption in 2014 (~1 month). Retrospective InSAR data analysis from the ALOS-PALSAR satellite show unrest began as early as January 2007, 6 months earlier than the surficial and seismic changes (Lubis, 2014). Lubis (2014) found 11 cm of continuous uplift between January and May 2007, and the uplift increased prior to the initial eruption based on tilt data (Fig. 3). Lubis (2014) suggests the displacement was associated with an increased volume of magmatic material in the shallow reservoir and migration of magma to the surface. This difference in the rate of magma ascent is consistent with the very different explosivities of the eruptions: where for a slow ascent the magma can either lose its gas and become less explosive or may initially contain very little gas; or for a rapid ascent, the magma does not have time to lose its gas, or may rise fast because it contains a lot of gas. (3). Well before the 2007 eruption (18 days), VBs regularized in time, waveform and amplitude; whereas only a brief period (minutes) of such events was recorded immediately preceding (hours) the eruption in 2014. (4). Tremor that preceded the 2007 eruption is at times spasmodic in amplitude and low frequency; but the tremor that
Fig. 10. Seismicity prior to the 2007 eruption of Kelud. (a) Repetitive self-similar seismicity on 31 October on station Kawah located about 400 m from the crater lake (Hidayati et al., 2009). (b) Seismicity just prior to and during presumed dome extrusion on 3 November including VTs and spasmodic bursts of VTs. Lake temperatures rose dramatically between 12:00 and 18:00 WIB, tilt rapidly changed to deflation at 12:13 WIB. Inset: spasmodic bursts of VTs on 3 November. (c) Onset of tremor on 3 November at 12:32 WIB and starting as rapid, shallow LF events.
64
S. Hidayati et al. / Journal of Volcanology and Geothermal Research 382 (2019) 50–67
b)
Fig. 10 (continued).
preceded the 2014 was harmonic with gliding frequencies. Thus the pattern of seismicity prior to 2014 eruption showed a clear change. In 2007, the VB earthquakes regularized 18 days prior to the eruption, showing a regular and stationary seismic energy release process, like with many other dome eruptions in the literature (Moran et al., 2008; Umakoshi et al., 2011; Nakada et al., 1999; McCausland et al., 2019; Miller et al., 1998; Rowe et al., 2004); the VBs in 2014 only regularized for a very short time in the final hours before the onset of harmonic, gliding tremor. The tremor that preceded the 2007 eruption was low frequency, whereas the 2014 tremor was harmonic with gliding frequencies. Gliding frequency tremor has been related to
variations of the pressure in the conduit, which modify the gas fraction, the wave velocity and, possibly, the length of the resonator (Lesage et al., 2006). Gliding frequency tremor can be followed by an explosion as what happened in 2009 eruption of Redoubt volcano (Hotovec et al., 2013) and at Arenal volcano (Lesage et al., 2006). This difference in the nature of the tremor may be related to significant differences in style and size of eruption between the effusive eruption in 2007 and the explosive eruption in 2014.
As mentioned previously, the 2014 VEI 4 eruption occurred with a short dormant time interval, 6 years and 3 months after
S. Hidayati et al. / Journal of Volcanology and Geothermal Research 382 (2019) 50–67
65
Fig. 10 (continued).
the 2007 VEI 2 eruption. After the 1990 VEI 4 eruption, Kelud volcano seismic activity became extremely low and only in 1996 did seismicity recur with occasional swarms of VTs and then a consistent increased rate through the onset of unrest in 2007 (see Fig.6). In contrast, seismic activity continued after the 2007 eruption, and turned into increase in 2010. It is interpreted that beneath the volcano the charge and accumulation of magma already started. Furthermore, the amount of material ejected by 2007 eruption is much smaller, ~16.3 × 10 6m 3 (Hidayati et al., 2008a) compared with previous eruptions, in 1919: ~190 × 10 6 m 3 (Kemmerling, 1921);1951: ~200 × 10 6 m 3 (Hadikusumo, 1974); 1966: ~90 × 10 6 m 3 (Kusumadinata et al., 1979) and 1990: ~130 × 10 6 m 3 (Bourdier et al., 1997). Recent average magma production rate of Kelud volcano was calculated to about 5 × 109 kg/y using the erupted volumes and interruptive time (Ishihara et al., 2011). We revise this calculation using the erupted volume from 2014 (220 × 106 m3mean value from Maeno et al, 2019) and find the magma supply rate since 1919 can be estimated as 6.0 × 106 m3/y (Fig. 13). The amount of lava extruded by the 2007 eruption is much smaller than what we would expect from the average production rate. It is suggested that probably the 2007 eruption did not fully eject all of the magma stored under the volcano. This may be why VT activity was resumed soon after the 2007 eruption and why eruption interval between the 2007 and 2014 eruption was much shorter than those of eruptions in 20th century (Table 1). According to Caricchi et al. (2014) the rate of magma supply to upper crust controls the volume of a single eruption. The time interval between magma injections into the subvolcanic reservoir, at a constant magma-supply rate, determine the duration of magmatic activity that precedes eruptions. At Kelud, it seems that the process of magma migration, magma accumulation and energy release occurs in a relatively a short time (1–3 months). The final approach to eruption (1–10 days) is usually characterized by accelerating rates of seismicity (Kilburn, 2003). This is consistent with our seismic observations
where the rate of cumulative seismic energy release is greater in 2014 (reaches 2 times the value of 2007 in half the time), and the resulting explosion was greater (VEI 4 vs. VEI 2). 8. Conclusion Both eruptions in 2007 and 2014 have similar precursory seismicity, initiated by the occurrence of and then swarms of VTs and VBs. Also in both cases there occurred repetitive, selfsimilar VBs and tremor several hours before the eruption. However there are significant differences. The depth range of VT for 2007, 1–2 km and migrated slowly to 0–1 km, is b2014, which is about 0–4 km, indicating the erupted magma was possibly stored at shallower levels. The magnitudes of VTs in 2007 are generally smaller than 2014, and the cumulative energy of 2014 much larger and accumulated over a much shorter time (about half as long). For 2014, once the seismicity begins, it occurs constantly and then sharply increases prior to the VEI 4 eruption; while for 2007 eruption, there is some time when VTs occur and then subsides, and then reoccur and increase to a lesser total value prior to the VEI 2 lava dome eruption. Furthermore repetitive self-similar seismicity occurs as a significant fraction of the precursory seismicity in 2007, but not in 2014. Lastly, both activity recorded occurrence of tremor just before eruption, but for 2014 eruption the tremor is harmonic and has gliding frequencies. We suggest the gliding frequencies relate to variations of the pressure in the conduit. These differences in the seismic progression, event types and energy release rate seem to relate to the significant difference in style and magnitude of eruptions in 2007 (effusive) and in 2014 (explosive). Acknowledgment Authors are grateful to observers of Kelud volcano for their help during the eruption and fieldwork, without
66
S. Hidayati et al. / Journal of Volcanology and Geothermal Research 382 (2019) 50–67
Tremor
Drumbeat
Harmonic Tremor
Figure
Fig. 11. On February 13, tremor recorded for 4 min and drumbeat event occurred at 21:51 (local time) for 27 min and changed to harmonic tremor with gliding frequency at 22:18 led to first eruption at 22:46.
their tireless work and monitoring we would not have the data with which to write this paper. The authors thank Greg Waite and Wendy McCausland for helpful reviews of
the manuscript. This work was supported by Center for Volcanology and Geological Hazard Mitigation, Geological Agency, Indonesia.
Fig. 12. Temporal variation of the number of VT before eruption in 2007 and 2014 as recorded at KWH station.
S. Hidayati et al. / Journal of Volcanology and Geothermal Research 382 (2019) 50–67
Fig. 13. Calculation for magma supply rate.
References Basuki, 2015. Determination of Hypocenter and Velocity Structure in Guntur Volcano Using Recent Data (2010–2014). Master Thesis. Institut Teknologi Bandung. Basuki, A., Mulyana, I., Kriswati, E., 2013. Laporan Penelitian G. Gede, Jawa Barat. Pusat Vulkanologi dan Mitigasi Bencana Geologi. Internal Report. Bourdier, J.L., Pratomo, I., Thouret, J.C., Boudon, G., Vincent, P.M., 1997. Observations, stratigraphy and eruptive processes of the 1990eruption of Kelut volcano, Indonesia. J. Volcanol. Geotherm. Res. 79, 181–203. Caricchi, L., Annen, C., Blundy, J., Simpson, G., Pinel, V., 2014. Frequency and magnitude of volcanic eruptions controlled by magma injection and buoyancy. Nat. Geosci. 7, 126–130. Domı́nguez, T., Zobin, V.M., Reyes-Davila, G.A., 2001. The fracturing in volcanic edifice before an eruption: the June–July 1998 high-frequency earthquake swarm at Volcán de Colima, México. J. Volcanol. Geotherm. Res. 105, 65–75. Endo, E., Murray, T., 1991. Real-time Seismic Amplitude Measurement (RSAM): a volcano monitoring and prediction tool. Bull. Volcanol. 53 (7), 533–545. https://doi.org/ 10.1007/BF00298154. England, P., Engdahl, R., Thatcher, W., 2004. Systematic variation in the depths of slabs beneath arc volcanoes. Geophys. J. Int. 156, 377–408. Global Volcanism Program, 2013. In: Venzke, E. (Ed.), Kelut (263280) in Volcanoes of the World, v. 4.6.7. Smithsonian Institution Downloaded 20 Apr 2018. http://volcano. si.edu/volcano.cfm?vn=263280 https://doi.org/10.5479/si.GVP.VOTW4-2013. Hadikusumo, 1974. The rise and drop of Mt. Kelut crater bottom after paraxysmal eruptions. Tectonophysics 23, 341–347. Hall, R., Clements, B., Smyth, H.R., Cottam, M.A., 2007. A new interpretation of Java's structure. Proceedings, Indonesian Petroleum Association Thirty-First Annual Convention and Exhibition, May 2007 IPA07-G-035. Hidayat, D., Gunawan, H., Hendrasto, M., Basuki, A., Schwandner, F., Marcial, S., Newhall, C., Kunrat, S., 2013. Tectonic or Magma Intusion? New Result From the Analysesofseismicswarm at Gede and SalakVolcano, West Java, Indonesia. oral presentation. 2013. IAVCEI. Hidayati, S., Iguchi, M., Ishihara, K., Purbawinata, M.A., Subandriyo, Sinulingga, I.K., Suharna, 1998. A preliminary result of quantitative evaluation on activity of Merapivolcano. Proc. On Symposium on Japan – Indonesia IDNDR Project, pp. 165–180. Hidayati, S., Basuki, A., Kristianto, Mulyana, I., Budiyanto, A., 2008a. Laporan Penelitian G. Kelud. Pusat Vulkanologi dan Mitigasi Bencana Geologi (Internal report). Hidayati, S., Ishihara, K., Iguchi, M., Ratdomopurbo, 2008b. Focal mechanism of volcanotectonic earthquake at Merapi volcano, Indonesia. Indonesian J. Phys. 19 (2), 75–82. Hidayati, S., Kristianto, Basuki, A., Mulyana, I., 2009. A new lava dome emerged from crater lake of Kelud volcano, East Java, Indonesia. Jurnal Geologi Indonesia 4 (4), 229–238. Hotovec, A.J., Prejean, S.G., Vidale, J.E., Gomberg, J., 2013. Strongly gliding harmonic tremor during the 2009 eruption of Redoubt Volcano. J. Volcanol. Geotherm. Res. 259, 89–99. Ishihara, K., Hendrasto, M., Hidayati, S., 2011. Long-term forecasting of volcanic eruptions in case of Kelud volcano, Indonesia. Ann. Disaster Prev. Res. Inst., Kyoto Univ. 54 (B1), 209–214 (in Japanese, with English abstr.).
67
Kemmerling, G.L.L., 1921. De uitbarsting van den Keloet in dennacht van de 19den op den20sten Mei 1919. Vulkanol. Meded. 2. Kilburn, C.R.J., 2003. Multiscale fracturing as a key to forecasting volcanic eruptions. J. Volcanol. Geotherm. Res. 125, 271–289. Kunrat, S.L., 2009. Geochemical and Thermodynamic Modeling of Volcanic Fluids and Their Interpretation for Volcano Monitoring. Universite Libre de Bruxelles (unpublished paper). Kusumadinata, K., Hadian, R., Hamidi, S., Reksowirogo, L.D., 1979. Data Dasar Gunungapi Indonesia, Bandung: Direktorat Vulkanologi. Lesage, P., Mora, M.M., Alvarado, G.E., Pacheco, J., Métaxian, J.P., 2006. Complex behavior and source model of the tremor at Arenal volcano, Costa Rica. J. Volcanol. Geotherm. Res. 157, 49–59. Lubis, A.M., 2014. Uplift of Kelud volcano prior to the November 2007 Eruption as Observed by L-Band Insar. J. Eng. Technol. Sci. 46 (3), 245–257. Maeno, F., Nakada, S., Yoshimoto, M., Shimano, T., Hokanishi, N., Zaennudin, A., Iguchi, M., 2019. A sequence of a Plinian eruption preceded by dome destruction at Kelud volcano, Indonesia, on February 13, 2014, revealed from tephra fallout and pyroclastic density current deposits. J. Volcanol. Geotherm. Res. 382, 24–41. McCausland, W.A., White, R.A., Indrastuti, N., Gunawan, H., Patria, C., Suparman, Y., Putra, A., Triastuty, H., Hendrasto, M., 2019. Using a process-based model of pre-eruptive seismic patterns to forecast evolving eruptive styles at Sinabung Volcano, Indonesia. J. Volcanol. Geotherm. Res. 382, 253–266. Miller, A.D., Stewart, R.C., White, R.A., Luckett, R., Baptie, B.J., Aspinall, W.P., Latcman, J.L., Lynch, L.L., Voight, B., 1998. Seismicity associated with dome growth and collapse at the Soufriere Hill volcano, Montserrat. Geophys. Res. Lett. 25 (18), 3401–3404. Minakami, T., 1974. Seismology and Volcanoes in Japan. In: Civetta, L., Gasparini, P., Luongo, G., Rapolla, A. (Eds.), Physical Volcanology. Elsevier, pp. 1–27. Moran, S.C., Malone, S.D., Qamar, A.I., Thelen, W.A., W., A.K., Caplan-Auerbach, J., 2008. Seismicity associated with renewed dome building at Mount St. Helens, 2004–2005. US Geol. Surv. Prof. Pap. 1750, 27–60. Moran, Seth, C., Newhall, Chris, Roman, Diana C., 2011. Failed magmatic eruptions: latestage cessation of magma ascent. Bull. Volcanol. 73 (2), 115–122. Nakada, S., Shimizu, H., Ohta, K., 1999. Overview of the 1990–1995 eruption at Unzen Volcano. J. Volcanol. Geotherm. Res. 89 (1999), 22. Nishi, K., 2005. Hypocenter Calculation Software GAD (Geiger's method with Adaptive Damping). JICA Office, Jakarta. Ratdomopurbo, A., Poupinet, G., 2000. An overview of the seismicity of Merapi volcano (Java, Indonesia), 1983–1994. J. Volcanol. Geotherm. Res. 100, 193–214. Rowe, C., Thurber, C., White, R., 2004. Dome growth behavior at Soufriere Hills Volcano, Montserrat, revealed by relocation of volcanic event swarms, 1995–1996. J. Volcanol. Geotherm. Res. 134 (3), 199–221. Sadikin, N., 2008. Study on Volcano-tectonic Earthquakes and Magma Supply System at Guntur Volcano, with Long-term Dormant Period. PhD Thesis. Kyoto University, Japan. Santoso, A.B., Lesage, Philippe, SapariDwiyono, Sri Sumarti, Subandriyo, Jousset, Philippe, Metaxian, Jean-Philippe, Surono, 2013. Analysis of the seismic activity associated with the 2010 eruption ofMerapi Volcano, Java. J. Volcanol. Geotherm. Res. 261, 153–170. Smyth, H.R., Hall, R., Nichols, G.J., 2008. Cenozoic volcanic arc history of East Java, Indonesia: The stratigraphic record of eruptions on an active continental margin. The Geological Society of America, Special Paper 436. Sribudiyani, N.M., Ryacudu, R., Kunto, T., Astono, P., Prasetya, I., Sapiie, B., Asikin, S., Harsolumakso, A.H., Yulianto, I., 2003. The collision of the East Java Microplate and its implication for hydrocarbon occurrences in the East Java Basin. Indonesia Petroleum Association. Proceedings of 29th Annual Convention, pp. 335–346. Umakoshi, K., Shimizu, H., Matsuwo, N., 2001. Volcano-tectonic seismicity at Unzen volcano, Japan, 1985–1999. J. Volcanol. Geotherm. Res. 112, 117–131. Umakoshi, K., Itasaka, N., Shimizu, H., 2011. High-frequency earthquake swarm associated with the May 1991 dome extrusion at Unzen Volcano, Japan. J. Volcanol. Geotherm. Res. 206, 70–79. White, R., McCausland, W., 2016. Volcano-tectonic earthquakes: a new tool for estimating intrusive volumes and forecasting eruptions. J. Volcanol. Geotherm. Res. 309, 139–155. White, R., McCausland, W., 2019. A Process-based Model of Pre-eruption Seismicity Patterns and its use for eruption forecasting at dormant stratovolcanoes. J. Volcanol. Geotherm. Res. 382, 267–297. Wirakusumah, A.D., 1991. Some Studies of Volcanology, Petrology, and Structure of Mt. Kelut, East Java, Indonesia. Victoria University of Wellington PhD thesis. Zobin, V.M., 2003. Introduction to volcanic seismology. Development in Volcanology. 6. Elsevier Science B.V. Zobin, V.M., Nishimura, Y., Miyamura, J., 2005. The Nature of Volcanic Earthquake Swarm preceding the 2000 flank eruption at Usu volcano, Hokkaido, Japan. Geophys. J. Int. 163, 265–275.
Contents lists available at ScienceDirect
Journal of Volcanology and Geothermal Research journal homepage: www.elsevier.com/locate/jvolgeores
Differences in the seismicity preceding the 2007 and 2014 eruptions of Kelud volcano, Indonesia Sri Hidayati a,⁎, Hetty Triastuty a, Iyan Mulyana a, Sucahyo Adi a, Kazuhiro Ishihara b, Ahmad Basuki a, Heri Kuswandarto a, Budi Priyanto a, Akhmad Solikhin a a b
Center for Volcanology and Geological Hazard Mitigation, Geological Agency, Indonesia Organization of Volcanic Disaster Mitigation, Japan
a r t i c l e
i n f o
Article history: Received 8 August 2016 Received in revised form 9 October 2018 Accepted 15 October 2018 Available online 18 October 2018 Keywords: Kelud volcano VT earthquake Hypocenter Magma migration
a b s t r a c t A significant increase in the number of volcano-tectonic (VT) and VB earthquakes preceded the 2007 and 2014 eruptions of Kelud volcano, Indonesia. The background seismicity rates at the volcano is generally b5 of VTs per month and other types of volcanic earthquakes are rarely recorded. After 17 years of quiescence, the 2007 eruption was preceded by an increase in VTs starting in September 2007, two months prior to the eruption. An intense VT swarm began in October 2007 and led to occurrence of VB, sometimes presenting as repetitive self-similar events. The final precursory seismicity included spasmodic bursts of VTs, repetitive self-similar VBs and low frequency tremor, prior to lava dome formation in the crater lake, on November 3, 2007. After just ~6 years, VTs began to increase again in December 2013. The number significantly intensified in mid January 2014, ~1 month before the eruption. Seismicity continued to intensify including VTs, VBs, and LFs into February 2014, with the final precursory seismicity consisting of a burst of over scale tremor, followed by a short burst of repetitive self-similar events that merged into harmonic, gliding tremor a couple hours prior to the explosive eruption on February 13, 2014. We examine the locations of VTs, the energy release rate, and the precursory seismic pattern prior to both eruptions to understand the differences in the eruptions' style and size. We found that epicentral locations of VTs are similar for both eruptions, but the depths were slightly deeper prior to the 2014 eruption. We also find that the cumulative energy released prior to 2014 is twice that of 2007 and in half the time. The migration of VTs in Kelud volcano may indicate magma intrusion from a deeper storage region in 2014 and at a faster rate, leading to a much more explosive eruption. © 2018 Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
1. Introduction The 1731 m stratovolcano Kelud, located in East Java (Fig. 1) is one of the most active and dangerous volcanoes in Indonesia with eight eruptions of VEI 4 or greater since 1548, five of which have occurred in the last 100 years. Tectonically most of the active volcanoes in Java are located about 100 km above the north-dipping subducting slab of the Sunda Arc (England et al., 2004). According to Hall et al. (2007), Java has a simple structure in which the east-west physiographic zones broadly correspond to structural zones. It can be separated into three
⁎ Corresponding author. E-mail address: [email protected] (S. Hidayati).
structural sectors: West, Central and East Java. The regional stress field of East Java is estimated to be dominantly in the NE-SW direction (Sribudiyani et al., 2003) where Kelud volcano forms an alignment with Kawi-Butak, Arjuno-Welirang, and Penanggungan volcanoes (Smyth et al., 2008) (Fig. 1). Kelud volcano has both explosive and effusive eruptions, and therefore lava flows, lava domes, pyroclastic flows, pyroclastic airfall, and eruption lahar deposits, mostly basaltic andesite in composition, comprise its edifice (Wirakusumah, 1991). The explosive eruptions usually occur in a short time and begin with phreatomagmatic eruptions that are quickly followed by explosive magmatic eruptions that produce pyroclastic flows, ash-fall and lapilli. The effusive eruptions forma lava dome. Strong increases in the number and magnitudes of distal VTs event often precedes volcanic eruptions at closed system volcanoes (White
https://doi.org/10.1016/j.jvolgeores.2018.10.017 0377-0273/© 2018 Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
S. Hidayati et al. / Journal of Volcanology and Geothermal Research 382 (2019) 50–67
51
Fig. 1. Geological Map of East Java (slightly modified from Smyth et al., 2008). Kelud volcano is located within a chain of volcanoes aligned from NE to SW (inset). Rose diagrams (bottom right) show dominant orientations of regional structures for East Java for both for the surface and basement (Sribudiyani et al., 2003).
and McCausland, 2016) in response to the intrusion of new magma. Increases in VT seismicity have preceded most eruptions at Merapi volcano that generated pyroclastic flows due to dome collapse (Hidayati et al., 1998; Ratdomopurbo and Poupinet, 2000; Hidayati et al., 2008b; Santoso et al., 2013). Similar precursory seismicity was also recorded at Unzen volcano, where numbers of VTs increased and hypocenters generally migrated toward the summit in the 6 years prior to the 1991 eruption (Umakoshi et al., 2001). The November 1998 eruption of Volcán de Colima, México was preceded by a 12-months of increased swarms of various types of microearthquakes (Domı́ nguez et al., 2001). Swarms or increases in seismicity may lead to eruption, however their rates, durations and types prior to eruptive activity can vary. White and McCausland 2019 break the precursory period into 4 stages and correlate them to the processes that occur as magma migrates to the surface, which inherently will change slightly from volcano to volcano and eruption to eruption. After the 6 years of distal VT seismicity at Unzen, regular self-similar earthquakes preceded and accompanied dome extrusion (Nakada et al., 1999; Umakoshi et al., 2011). At Usu volcano, only a short swarm of VTs preceded a flank eruption on March 31, 2000 (Zobin et al., 2005). Guntur volcano (Sadikin, 2008; Basuki, 2015) and Gede volcano (Basuki et al., 2013; Hidayat et al., 2013), have experienced several periods of high seismicity of VT with no ensuing eruption, also called ‘failed eruptions’ by Moran et al. (2011). Understanding which seismic unrest will lead to an eruption and the size and timing of that eruption, is a question all agencies responsible for monitoring volcanoes seeks to know. This paper describes the similarities and differences in the observed precursory changes, the temporal spatial distributions
of high frequency volcano tectonic (VT) seismicity, and seismic energy release rates at Kelud volcano preceding the 2007 and 2014 eruptions. In particular, we will discuss the changes in of the precursory seismicity during volcanic unrest and how it relates to the rate of magma supply and explosivity of eruptions at Kelud. 2. Historic and recent eruptions of Kelud volcano The typical eruption styles of Kelud volcano are either a large explosive pyroclastic flow generating eruption or an effusive lava dome forming eruption. In addition, the presence of a crater lake at the summit can lead to destructive syn-eruptive lahars. Dates and sizes of known historical eruptions of Kelud volcano are listed in Table 1 (GVP, 2013; Kusumadinata et al., 1979). The intereruption interval ranges from 1 to 311 years, and the biggest eruptions (≥VEI 4) occurred in 1586, 1641, 1826, 1919, 1951, 1966, 1990 and 2014. All of these major eruptions were following 15–55 years long repose interval, except the eruption in 2014, which occurred only 7 years after the previous effusive eruption in 2007. The explosive eruptions in 1901 and 1919, when the volcano had a crater lake of about 40 Mm3 in volume, resulted in tens of kilometers long lahars causing N5000 casualties (Global Volcanism Program, 2013). To reduce the water volume of the crater, a tunnel has been built and has succeeded in minimizing the distance of lahars in the 1951 and 1990 eruptions. The number of fatalities, caused by both eruptions, were smaller (in 1951, 7 people were killed; in 1990, 34 people were killed, because of roof collapse at an evacuation shelter). However the 1966 eruption was an
52
S. Hidayati et al. / Journal of Volcanology and Geothermal Research 382 (2019) 50–67
Table 1 Eruption year, size and type from the Global Volcanism Program (2013) and CVGHM reports (Kusumadinata et al., 1979). Erupted volumes from Ishihara et al. (2011), Hidayati et al. (2009) and Maeno et al., (2019). Year
VEI Eruption type
1000 1311 1334 1376 1385 1395 1411 1450 1451 1462 1481 1548 1586 1641 1716 1752 1771 1776 1785 1811 1825 1826 1835 1848 1851 1864 1901 1919 1920 1951 1966 1967 1990 2007 2014
3 3 3 3 3 3 3 3 3 3 3 3 5 4 2 2 2 2 2 2 2 4 2 3 2 2 3 4 2 4 4 1 4 2 4
Dome
Explosive Explosive
Explosive
Explosive Explosive Dome Explosive Explosive Explosive Dome Explosive
Inter-eruption interval (years) 311 23 42 9 10 16 39 1 11 19 67 38 55 75 36 19 5 9 26 14 1 9 13 3 13 37 18 1 31 15 1 23 17 7
Volume erupted (106 m3)
Casualties
200 90
? Reported, number unknown Reported, number unknown Reported, number unknown ? ? ? None None ? ? ? 1000. No further explanation ? Reported, number unknown None None ? ? ? None None None None None None Reported, number unknown 5160 None 7 died, 167 injured 210
130 16.3 220
34 None None
190
exception to this reduction in lahar hazard. The crater was excavated to a greater depth by the 1951 eruption, therefore the volume of crater lake water prior to the 1966 eruption had increased to 21.6 Mm 3 and a lahar travelled to 31 km. This eruption killed 210 people. A new tunnel was constructed following the 1966 eruption and the crater lake was reduced to just 1 Mm 3 prior to the 1990 eruption. A strong and explosive eruption (VEI 4, Bourdier et al., 1997) on 10 February 1990 produced a lava plug at the top of the conduit, yet the eruption on 3 November 2007 was almost purely effusive, forming a lava dome in the crater. Unrest began with a swarm of VTs on 10 September 2007, followed by swarms of VB (i.e. shallow VT, hybrid and/or LF earthquakes; Minakami, 1974) and spasmodic tremor prior to lava dome formation on 3 November 2007 (Hidayati et al., 2009). The behavior of the precursory seismicity before this dome extrusion is similar to other domes worldwide such as Soufriere Hills Volcano (Miller et al., 1998; Rowe et al., 2004), Unzen Volcano (Nakada et al., 1999; Umakoshi et al., 2011), Mount St. Helens (Moran et al., 2008) and Sinabung Volcano (McCausland et al., 2019). Phreatomagmatic ash emissions occurred several times as the hot lava dome occupied the lake crater. Six year later, a large and violent phreato-magmatic explosion occurred in the evening of 13 February 2014, at 22:46 local time, destroying the 2007 lava dome. The eruption plume rose to up to 27 km altitude, ejected an estimated 220 Mm3 of volcanic material with heavy ash fall out to distances as great as 200 km (Andreastuti, personal communication, 2016; Maeno et al., 2019). The eruption was preceded by 2 months of increasing in VT and shallow seismicity.
3. Monitoring network Visual observation is conducted from the Kelud volcano observatory at Margomulyo Village, about 7.5 km from the volcano.
Fig. 2. Monitoring network of Kelud volcano. The diamonds and solid squares represent seismic station and tiltmeter, respectively. The triangles represent water temperature sensor (BUOY) at surface, 10 m and 15 m for 2007activity and water temperature measurement at Bladak River outlet (OUT) for 2014 activity.
S. Hidayati et al. / Journal of Volcanology and Geothermal Research 382 (2019) 50–67
Periodic visits are made to assess the visual conditions around the crater. Water level, temperature and color of the lake are monitored for significant changes. Temperature sensors were installed at the surface, and at 10 m and 15 m depth in 2006 (Fig. 2). Continuous water temperature measurements prior to 2014 eruption were made at the outlet to the Bladak river. In addition, periodic measurements of the water chemistry are
53
performed. Deformation at Kelud is measured by tiltmeters, the first of which was installed in early 2007, and a second was installed in North Lirang on 5 February 2014 in response to the change to Alert Level II (Watch). Similarly a CCTV camera was installed at the Lirang seismic station (LRG) and telemetered to the observatory in order to monitor the crater for any visual changes.
Fig. 3. Lake temperature and tilt data for the 2007 and 2014 eruptions. (a) The change of lake temperature prior to lava dome formation in 2007. (b) A sharp deflation was observed from tilt data prior to lava dome formation in 2007. (c) Change in hot spring temperature at outlet (Bladak river) in period of 15 September 2013 to 13 February 2014 (until 20:00 local time). (d) Tilt observation at Lirang Selatan Station during the period of 15 September 2013 to 13 Februarys 2014 (until 22:40 local time). Tilt observation at Lirang Utara Station during 7–14 February 2014 is added at the bottom left. A M4.5 felt earthquake occurred on Feb.11, 2014 at 13:46.
54
S. Hidayati et al. / Journal of Volcanology and Geothermal Research 382 (2019) 50–67
Bladak Outlet
Lirang Utara Lirang Selatan
7 – 14 Feb 2014 Fig. 3 (continued).
Seismic monitoring of Kelud volcano began in 1925 with a single analog seismometer. The network was increased to three stations in 1987, all of which were destroyed in the 1990 eruption. Some stations were replaced after the eruption and since April 2007, five permanent seismic stations (Fig. 2) are used to monitor Kelud volcano. KWH station was located inside the crater, whereas the other three (KLD, SMB, LRG) were on the crater rim, about 600 m from the active vent. Station UMB was added on April 2007 in order to better earthquake locations. It is located about 4.5 km from the crater. All stations are equipped with single component Mark L4-C short period seismometer. Continuous seismic signals are transmitted by radio link to the Kelud volcano observatory (about 7.5 km away from the volcano) and are converted to digital format in real-time at 100 Hz.
2007, the lake temperature gradually rose from background values of 30 °C to 33.2 °C and the water color changed from the typical green to yellowish to bluish. This change in lake color was also accompanied by an increase in the concentration of CO 2 . Lake temperatures rose again on 2 November 2007 and rose even faster starting 24 h later on 3 November (Fig. 3a) (Hidayati et al., 2009). Geochemical analyses (Kunrat, 2009) of the lake water showed two hydrothermal systems: one deep, one shallow. Kunrat attributes the change in lake color and temperature to an increasing contribution to the lake from the deep hydrothermal system. Tiltmeter data from the station located 600 m from the crater on the SW flank (Lirang) showed slow and slight inflation from about 9 October through 2 November at 12:13 WIB (GMT +8), when a sharp change to deflation occurred at the same time there was the sharp increase in lake temperature (Fig. 3b) (Hidayati et al., 2009).
4. Gas, lake and tilt observations prior to eruption 4.1. Prior to the 2007 eruption
4.2. Prior to the 2014 eruption
The first known sign of unrest occurred in July 2007 when intense degassing of the lake floor was observed. Then in August
Water temperature was measured continuously at outlet of Bladak river. Crater lake temperatures began increasing on 10 September
S. Hidayati et al. / Journal of Volcanology and Geothermal Research 382 (2019) 50–67
55
Fig. 4. Time series situation around crater lake on 8 February 2014 (top), 13 February (bottom right) and 14 February 2014 (bottom left).
2013 (Fig. 3c), increasing about 5.5 °C between 10 September 2013 and 22 January 2014. Then, from 23 January until 13 February 2014 a significant increase of 4 °C was observed. No lake color changes were noted. Lirang tiltmeter data showed inflation in both radial and tangential components beginning in early January 2014, and increased sharply just before the 13 February eruption (Fig. 3d). The second tiltmeter installed on 8 February in North Lirang showed no inflation until after a felt M4.5 VT earthquake in the region of Blitar and Kediri (source: BMKG). In response to that earthquake, the tilt inflated drastically, particularly in the radial component and continued to show inflation until the beginning
of the eruption. Both tiltmeters were destroyed in the explosive eruption. Gas emissions coming from the top lava dome had been observed since December 2013. The emission was getting wider along with the appearance of bubbles at the inlet (Fig. 4) in early February 2014. As Kelud volcano started to erupt on 13 February 2014, visual observation could not be done from the observatory. However, a rumbling sound was heard and lightning observed around the crater. The Darwin Volcanic Ash Advisory Center (VAAC) reported that the eruption column reached a height of 17 km.
56
S. Hidayati et al. / Journal of Volcanology and Geothermal Research 382 (2019) 50–67
VT
VB
LF
Fig. 5. Earthquake types at Kelud volcano: VT, VB, LF, tremor, emission and explosion.
S. Hidayati et al. / Journal of Volcanology and Geothermal Research 382 (2019) 50–67
57
Tremor
Emission
Explosion
Fig. 5 (continued).
A gray-black ash emission with a height of 400–600 m was observed on February 14, followed by a 3000 m tall of gray-white ash plume the next day. The eruption decreased to white ash emissions
with a height of about 300 m (Fig. 4). On February 18, lava and secondary eruptions were observed in some rivers such as Ngobo, Mangli (Kediri), Bladak (Blitar), and Konto (Malang) rivers. The
58
S. Hidayati et al. / Journal of Volcanology and Geothermal Research 382 (2019) 50–67
Fig. 6. Daily VT, B-type earthquake and tremor counts at Kelud volcano during January 1989 – March 2014, using KWH seismic station which is located 100 m from the crater. The arrows represent when the eruptions took place.
eruption on February 13, destroyed the 2007 lava dome inside the crater. There was no SO2 measurement prior to the eruption of February 13, 2014. However after the eruption, SO2 measurements were conducted to anticipate possible further eruptions. The SO2 flux on February 17 was 89.3 tons/day and on February 18 ranged from 81.1 to 82.3 tons/day. A decrease of total SO 2 flux indicated that Kelud activity was declining. 5. Earthquake classification and location Seismicity at Kelud is classified into high frequency volcanotectonic earthquakes (VT), shallow VB (Minakami classification scheme), low frequency earthquakes (LF), tremor, emission and explosion signals (Fig. 5). Regional tectonic and teleseismic events are also noted and cataloged, but not used in the assessment of volcanic activity. VTs are broadband and high frequency (3–7 Hz) typically with well-defined P- and S-wave phases. VBs are shallow b1.9 km depth from the summit. The VB can occur as repetitive self-similar swarms (also called multiplets, families or drumbeats) or as non-regularized swarms with no significant similarity among waveforms. LFs are shallow, narrow band and low frequency (1–3 Hz), generally lack clear S-phases. LFs also can occur as drumbeats especially prior to explosive eruption. Tremor can be spasmodic (with rapidly changing amplitudes) or constant amplitude; continuous or occur in bursts; and narrowband, broadband, or
harmonic. Emission events are low frequency (b5 Hz), initial part unclear, and usually followed by ash emitting from the crater. Explosion signals typically start lower frequency and become broadband and have durations that are generally as long as the visible explosion signals. VT seismicity at Kelud volcano usually has sharp P-wave onsets at 4 (sometimes 5) stations and well-defined S-phases. Hypocentral locations were calculated using the GAD program (Nishi, 2005). In order to minimize time-picking errors, locations were determined for events that had at least 4 Pwave and 1S-wave arrival. We assumed an homogeneous isotropic velocity model (Vp = 2.7 km/s and Vp/Vs = 1.73), and only locations with travel time residuals b0.3 s were retained as good locations. With these criteria, we obtain 115 (out of 334) and 78 (out of 974) well-located VT events for 2007 and 2014, respectively.
6. Kelud seismicity The background seismicity of Kelud volcano is quiet, with b5 high frequency volcano-tectonic (VT) earthquakes in a month (Fig. 6). During the period 1989–2014, the seismicity of the volcano was low except in January 1990, September 2007 and January 2014, when the number of VT significantly increased, several weeks prior to eruptions in February 1990 (VEI
S. Hidayati et al. / Journal of Volcanology and Geothermal Research 382 (2019) 50–67
4), November 2007 (VEI 2), and February 2014 (VEI 4), respectively. After 17 years of repose, the effusive lava dome eruption in November 2007 at Kelud volcano was preceded by a 2-month period of high seismic activity that included 3 VT and 2 VB swarms (Hidayati et al., 2009) (Fig. 7a). The first VT swarm occurred on 9 September 2007, causing CVGHM to raise the alert to Level II (Watch). The second VT swarm on the contained larger amplitude and a greater number of earthquakes, but with no significant change in their hypocenters (Fig. 8a), causing CVGHM to raise the alert to Level III (Standby). LFs began occurring on 30 September and continued through 13 October, with almost no LFs after that date (Fig. 7a). The first VB swarm on 16 October 2007 contained the
59
first repetitive self-similar events and led CVGHM to raise the alert level again to Level IV (Warning). Hypocenters for this swarm were in confined to a narrow area, dominantly within 1 km of the crater floor (Fig. 8a). The final seismicity that preceded the dome emergence started on 24 October and was at first dominantly VTs that were larger in number and amplitude than the previous swarm. When VBs returned in significant numbers on 31 October, again they started smaller in amplitude and similar in waveform and became regular in time over 2 days and were larger in amplitude and greater in number than the previous swarm. An extremely regular period of these earthquakes occurred for 70 min at 14:48 WIB on November 2 (Fig. 10a) was preceded by 3 h of slowly increasing (in amplitude) continuous low frequency tremor
Fig. 7. Daily seismicity of Kelud volcano and RSAM prior to eruption a) on November 3, 2007 and b) February 13, 2014. The dots represent starting the alert of each level.
60
S. Hidayati et al. / Journal of Volcanology and Geothermal Research 382 (2019) 50–67
Fig. 7 (continued).
and followed by the same style of tremor. VTs recurred at 19:47 on 2 November increasing in number and size until becoming spasmodic bursts of VTs at 06:31 on 3 November (Fig. 10b). Continuous low frequency tremor began as a rapid series of shallow LFs from 12:32 WIB becoming spasmodic low frequency tremor around 13:11 WIB on November 3, and large amplitude (saturated) by 15:50 (Fig. 10c). It is believed that the dome emerged during this time period, because of the seismicity, the rapid lake temperature change, and the sharp deflation signal on the tiltmeter. The next day, when the weather was clear, a small black dome was spotted in the crater lake. This seismic
progression is similar to other dome extrusions, however the presence of the crater lake changes the last stages of seismicity to tremor instead of continued regular self-similar VBs, likely because of the interactions between the rising magma and its heat with the crater lake. Fig. 8a shows the VT hypocentral distribution during September – November 2007. Most of events are located at depths of 1 to 4 km beneath the summit, and few are deeper, up to 9 km. The distribution during September 10 to October 23, 2007 was wider and deeper, compared with those during
S. Hidayati et al. / Journal of Volcanology and Geothermal Research 382 (2019) 50–67
61
Fig. 8. a. Hypocenter distribution of VT earthquake during September–November 2007. Red solid and open circles represent hypocenter before and after October 23, 2007, respectively. The depth mostly ranges between 1.0 and 4.0 km from the summit. A few are deeper and the deepest one reached 9.0 km. In addition, white squares and house symbol indicates the location of seismic station and the Kelud volcano observatory, respectively. b. Hypocenter distribution of VT earthquake during January–February 2014. Red solid and open circles represent hypocenter before and after February 9, 2014, respectively. The depth mostly ranges between 0.5 and 4.0 km from the summit, and few are deeper and the deepest one located at 11.0 km. In addition, white squares and house symbol indicates the location of seismic station and the Kelud volcano observatory, respectively.
October 24 to November 3. Namely, foci shifted and clustered to shallow depths between 1.0 and 2.5 km beneath the summit approximately 10 days before lava dome extrusion on November 3. Six years of relative seismic quiet were then interrupted by a small increase in VTs in early December 2013. This was then followed by increasing VT and VB seismicity throughout January that included a swarm of VBs 15–17 January 2014; a swarm of VTs from 17 to 21 January; and a larger swarm of VTs on 27 January. The January 27 swarm was followed by a persistent increase in both seismicity types into February (Fig. 7b), including an increase in earthquake amplitudes since the beginning of February. CVGHM raised the alert to Level II (Watch) on 4 February. A sharp increase in both event types and their magnitudes occurred on 6 and 9 February 2014, which can be seen in the Real-time Seismic Amplitude Measurement (RSAM; Endo and Murray, 1991) (Fig. 7b). On 10 February CVGHM raised the alert to Level III (Standby) with an intensification of the cumulative seismic energy release for VTs (Fig. 9). Hypocenters through this time located up to 7 km deep
below the crater and up to 5 km from the crater (Fig. 8b). From 12 February the VT and VB seismicity again increased dramatically, and LFs began to occur (Fig. 7b). Hypocenters of earthquakes in these last days before the eruption were shallower, at 1–4 km depth below the summit (Fig. 8b). On 13 February, the final seismic precursors to the eruption onset began with a 4-min longover-scale-amplitude tremor at 21:11 WIB (Fig. 11). Within 30 min, at 21:51 WIB, repetitive self-similar earthquakes occurred and accelerated in their rate as to merge and become harmonic tremor with gliding frequencies at 22:18 WIB (Fig. 11). The tremor continued for about 1.5 h and culminated in the first eruption (phreatic) at 22:46 (WIB), causing the alert to be raised to Level IV (Warning). This first eruption destroyed seismic station LRG (Lirang). This eruption was followed by the paroxysmal one (VEI 4) at 23:01 WIB, and the remaining four seismic stations were still able to record this eruption. Unfortunately, this eruption could not be seen visually, because of the late hour. At 23:40 all the seismic stations stopped operating except the most distant
62
S. Hidayati et al. / Journal of Volcanology and Geothermal Research 382 (2019) 50–67
Fig. 9. Change in depth, magnitude and cumulative energy release versus time for VT seismicity prior to the 2007 (a) and 2014 (b) eruptions of Kelud. VT depths become shallower and magnitudes become larger before both eruptions of 2007 and 2014. The overall magnitudes are greater for the 2014 eruption (b), with a greater cummulative VT energy (by an order of magnitude) in a shorter time period (1.8 times shorter). No significant change of the depth ranges.
one, UMB. The eruption lasted for 3 h and subsided gradually. Seismicity after the eruption from 14 to 20 February was dominated by continuous tremor, with emission earthquakes beginning to occur within the tremor between 15 and 17 February. A few VTs recorded in the period between 19 and 20 February. The alert level was decreased to Level III on 20 February, and to Level II (Watch) on 28 February. Before the 2014 eruption, the majority of VTs during January until the eruption on February 13 occurred at depths of 0.5 to 4.0 km. The distribution of hypocenters was wider from January until February 9, and then was again more focused just beneath the summit from February 10 through the eruption on 13 February 2014 (Fig. 8b). 7. Discussion The VT activity prior to the 2007 and 2014 eruptions has similarities in the distribution of hypocenters and the migration
of hypocenters prior to eruption, however here we discuss four differences in precursory seismicity in relation to eruption style and size. (1). Prior to the 2007 lava dome eruption, the seismicity was significantly elevated over a 2-month period; whereas prior to the 2014 explosive eruption, it was significantly elevated over a 1-month period with an overall increasing rate of seismicity with time (Fig. 7) to a greater total of seismic energy released (Fig. 9). Fig. 9 shows the change in depth, magnitude, cumulative seismic energy released versus time for VTs. The depth of VTs in 2007 slowly migrated and became shallower from October until before the eruption on 3 November. The depth range was about 1–2 km and became shallower 0–1 km just before eruption. In contrast, the range of depths of VTs in 2014 is greater, about 0–4 km below the summit, from the beginning of January until the eruption on 13
S. Hidayati et al. / Journal of Volcanology and Geothermal Research 382 (2019) 50–67
February. This suggests that the magma may have ascended from deeper and in a shorter period of time possibly retaining more gas or because it contained more gas, thus leading to a more explosive (VEI 4) eruption. (2). Magnitudes of the VTs were larger in 2014 than in 2007, and the cumulative energy release prior to 2007 is half that prior to 2014 and in twice the time, a significant increase in the seismic energy release rate for the more explosive 2014 eruption. The magnitude of VTs in 2014 were mostly larger than 2007 ones, the largest magnitude for 2007 is M1.9 occurred on 1 November at 04:35 and 1.5 km depth from the summit. As for 2014, the largest magnitude is M2.2 occurred on 9 February at 02:15 and 5.5 km depth from the summit. Fig. 9 shows that the cumulative seismic energy released by VTs in 2014 is twice that of 2007, and occurred in half the time: a significant increase in the energy release rate.
The temporal variation of the number of VTs recorded prior to the eruptions in 2007 and 2014 is shown in Fig. 12. Based on the number of events, Zobin (2003) divides swarms of VT into 3 types: 1) one peak 2) multi-peak sequences with the maximum peak at the beginning 3) or multi-peak with the maximum at the end of the sequence. The swarm for 2007 and 2014 eruptions can be classified as type 3, multi-peak sequences with the maximum peak at the end of the sequence. Zobin (2003) suggests that multi-peak temporal variations in the number of seismic events may result from the migration of the seismic cluster.
63
Even though the 2007 and 2014 eruptions have a similar type of VT swarm by Zobin's definition, the details of how the VT seismicity leads into the eruption are significantly different and therefore Zobin's definition does not provide an indication of eruption size. Instead the rate of seismic energy release seems to play an important role. For the 2007 eruption, the VTs gradually increased before the eruption, while for the 2014 eruption, the VTs increased exponentially prior to eruption and over a shorter time interval. This suggests that the magma migration process prior to the 2007 effusive eruption was longer (~2 months), compared to the explosive eruption in 2014 (~1 month). Retrospective InSAR data analysis from the ALOS-PALSAR satellite show unrest began as early as January 2007, 6 months earlier than the surficial and seismic changes (Lubis, 2014). Lubis (2014) found 11 cm of continuous uplift between January and May 2007, and the uplift increased prior to the initial eruption based on tilt data (Fig. 3). Lubis (2014) suggests the displacement was associated with an increased volume of magmatic material in the shallow reservoir and migration of magma to the surface. This difference in the rate of magma ascent is consistent with the very different explosivities of the eruptions: where for a slow ascent the magma can either lose its gas and become less explosive or may initially contain very little gas; or for a rapid ascent, the magma does not have time to lose its gas, or may rise fast because it contains a lot of gas. (3). Well before the 2007 eruption (18 days), VBs regularized in time, waveform and amplitude; whereas only a brief period (minutes) of such events was recorded immediately preceding (hours) the eruption in 2014. (4). Tremor that preceded the 2007 eruption is at times spasmodic in amplitude and low frequency; but the tremor that
Fig. 10. Seismicity prior to the 2007 eruption of Kelud. (a) Repetitive self-similar seismicity on 31 October on station Kawah located about 400 m from the crater lake (Hidayati et al., 2009). (b) Seismicity just prior to and during presumed dome extrusion on 3 November including VTs and spasmodic bursts of VTs. Lake temperatures rose dramatically between 12:00 and 18:00 WIB, tilt rapidly changed to deflation at 12:13 WIB. Inset: spasmodic bursts of VTs on 3 November. (c) Onset of tremor on 3 November at 12:32 WIB and starting as rapid, shallow LF events.
64
S. Hidayati et al. / Journal of Volcanology and Geothermal Research 382 (2019) 50–67
b)
Fig. 10 (continued).
preceded the 2014 was harmonic with gliding frequencies. Thus the pattern of seismicity prior to 2014 eruption showed a clear change. In 2007, the VB earthquakes regularized 18 days prior to the eruption, showing a regular and stationary seismic energy release process, like with many other dome eruptions in the literature (Moran et al., 2008; Umakoshi et al., 2011; Nakada et al., 1999; McCausland et al., 2019; Miller et al., 1998; Rowe et al., 2004); the VBs in 2014 only regularized for a very short time in the final hours before the onset of harmonic, gliding tremor. The tremor that preceded the 2007 eruption was low frequency, whereas the 2014 tremor was harmonic with gliding frequencies. Gliding frequency tremor has been related to
variations of the pressure in the conduit, which modify the gas fraction, the wave velocity and, possibly, the length of the resonator (Lesage et al., 2006). Gliding frequency tremor can be followed by an explosion as what happened in 2009 eruption of Redoubt volcano (Hotovec et al., 2013) and at Arenal volcano (Lesage et al., 2006). This difference in the nature of the tremor may be related to significant differences in style and size of eruption between the effusive eruption in 2007 and the explosive eruption in 2014.
As mentioned previously, the 2014 VEI 4 eruption occurred with a short dormant time interval, 6 years and 3 months after
S. Hidayati et al. / Journal of Volcanology and Geothermal Research 382 (2019) 50–67
65
Fig. 10 (continued).
the 2007 VEI 2 eruption. After the 1990 VEI 4 eruption, Kelud volcano seismic activity became extremely low and only in 1996 did seismicity recur with occasional swarms of VTs and then a consistent increased rate through the onset of unrest in 2007 (see Fig.6). In contrast, seismic activity continued after the 2007 eruption, and turned into increase in 2010. It is interpreted that beneath the volcano the charge and accumulation of magma already started. Furthermore, the amount of material ejected by 2007 eruption is much smaller, ~16.3 × 10 6m 3 (Hidayati et al., 2008a) compared with previous eruptions, in 1919: ~190 × 10 6 m 3 (Kemmerling, 1921);1951: ~200 × 10 6 m 3 (Hadikusumo, 1974); 1966: ~90 × 10 6 m 3 (Kusumadinata et al., 1979) and 1990: ~130 × 10 6 m 3 (Bourdier et al., 1997). Recent average magma production rate of Kelud volcano was calculated to about 5 × 109 kg/y using the erupted volumes and interruptive time (Ishihara et al., 2011). We revise this calculation using the erupted volume from 2014 (220 × 106 m3mean value from Maeno et al, 2019) and find the magma supply rate since 1919 can be estimated as 6.0 × 106 m3/y (Fig. 13). The amount of lava extruded by the 2007 eruption is much smaller than what we would expect from the average production rate. It is suggested that probably the 2007 eruption did not fully eject all of the magma stored under the volcano. This may be why VT activity was resumed soon after the 2007 eruption and why eruption interval between the 2007 and 2014 eruption was much shorter than those of eruptions in 20th century (Table 1). According to Caricchi et al. (2014) the rate of magma supply to upper crust controls the volume of a single eruption. The time interval between magma injections into the subvolcanic reservoir, at a constant magma-supply rate, determine the duration of magmatic activity that precedes eruptions. At Kelud, it seems that the process of magma migration, magma accumulation and energy release occurs in a relatively a short time (1–3 months). The final approach to eruption (1–10 days) is usually characterized by accelerating rates of seismicity (Kilburn, 2003). This is consistent with our seismic observations
where the rate of cumulative seismic energy release is greater in 2014 (reaches 2 times the value of 2007 in half the time), and the resulting explosion was greater (VEI 4 vs. VEI 2). 8. Conclusion Both eruptions in 2007 and 2014 have similar precursory seismicity, initiated by the occurrence of and then swarms of VTs and VBs. Also in both cases there occurred repetitive, selfsimilar VBs and tremor several hours before the eruption. However there are significant differences. The depth range of VT for 2007, 1–2 km and migrated slowly to 0–1 km, is b2014, which is about 0–4 km, indicating the erupted magma was possibly stored at shallower levels. The magnitudes of VTs in 2007 are generally smaller than 2014, and the cumulative energy of 2014 much larger and accumulated over a much shorter time (about half as long). For 2014, once the seismicity begins, it occurs constantly and then sharply increases prior to the VEI 4 eruption; while for 2007 eruption, there is some time when VTs occur and then subsides, and then reoccur and increase to a lesser total value prior to the VEI 2 lava dome eruption. Furthermore repetitive self-similar seismicity occurs as a significant fraction of the precursory seismicity in 2007, but not in 2014. Lastly, both activity recorded occurrence of tremor just before eruption, but for 2014 eruption the tremor is harmonic and has gliding frequencies. We suggest the gliding frequencies relate to variations of the pressure in the conduit. These differences in the seismic progression, event types and energy release rate seem to relate to the significant difference in style and magnitude of eruptions in 2007 (effusive) and in 2014 (explosive). Acknowledgment Authors are grateful to observers of Kelud volcano for their help during the eruption and fieldwork, without
66
S. Hidayati et al. / Journal of Volcanology and Geothermal Research 382 (2019) 50–67
Tremor
Drumbeat
Harmonic Tremor
Figure
Fig. 11. On February 13, tremor recorded for 4 min and drumbeat event occurred at 21:51 (local time) for 27 min and changed to harmonic tremor with gliding frequency at 22:18 led to first eruption at 22:46.
their tireless work and monitoring we would not have the data with which to write this paper. The authors thank Greg Waite and Wendy McCausland for helpful reviews of
the manuscript. This work was supported by Center for Volcanology and Geological Hazard Mitigation, Geological Agency, Indonesia.
Fig. 12. Temporal variation of the number of VT before eruption in 2007 and 2014 as recorded at KWH station.
S. Hidayati et al. / Journal of Volcanology and Geothermal Research 382 (2019) 50–67
Fig. 13. Calculation for magma supply rate.
References Basuki, 2015. Determination of Hypocenter and Velocity Structure in Guntur Volcano Using Recent Data (2010–2014). Master Thesis. Institut Teknologi Bandung. Basuki, A., Mulyana, I., Kriswati, E., 2013. Laporan Penelitian G. Gede, Jawa Barat. Pusat Vulkanologi dan Mitigasi Bencana Geologi. Internal Report. Bourdier, J.L., Pratomo, I., Thouret, J.C., Boudon, G., Vincent, P.M., 1997. Observations, stratigraphy and eruptive processes of the 1990eruption of Kelut volcano, Indonesia. J. Volcanol. Geotherm. Res. 79, 181–203. Caricchi, L., Annen, C., Blundy, J., Simpson, G., Pinel, V., 2014. Frequency and magnitude of volcanic eruptions controlled by magma injection and buoyancy. Nat. Geosci. 7, 126–130. Domı́nguez, T., Zobin, V.M., Reyes-Davila, G.A., 2001. The fracturing in volcanic edifice before an eruption: the June–July 1998 high-frequency earthquake swarm at Volcán de Colima, México. J. Volcanol. Geotherm. Res. 105, 65–75. Endo, E., Murray, T., 1991. Real-time Seismic Amplitude Measurement (RSAM): a volcano monitoring and prediction tool. Bull. Volcanol. 53 (7), 533–545. https://doi.org/ 10.1007/BF00298154. England, P., Engdahl, R., Thatcher, W., 2004. Systematic variation in the depths of slabs beneath arc volcanoes. Geophys. J. Int. 156, 377–408. Global Volcanism Program, 2013. In: Venzke, E. (Ed.), Kelut (263280) in Volcanoes of the World, v. 4.6.7. Smithsonian Institution Downloaded 20 Apr 2018. http://volcano. si.edu/volcano.cfm?vn=263280 https://doi.org/10.5479/si.GVP.VOTW4-2013. Hadikusumo, 1974. The rise and drop of Mt. Kelut crater bottom after paraxysmal eruptions. Tectonophysics 23, 341–347. Hall, R., Clements, B., Smyth, H.R., Cottam, M.A., 2007. A new interpretation of Java's structure. Proceedings, Indonesian Petroleum Association Thirty-First Annual Convention and Exhibition, May 2007 IPA07-G-035. Hidayat, D., Gunawan, H., Hendrasto, M., Basuki, A., Schwandner, F., Marcial, S., Newhall, C., Kunrat, S., 2013. Tectonic or Magma Intusion? New Result From the Analysesofseismicswarm at Gede and SalakVolcano, West Java, Indonesia. oral presentation. 2013. IAVCEI. Hidayati, S., Iguchi, M., Ishihara, K., Purbawinata, M.A., Subandriyo, Sinulingga, I.K., Suharna, 1998. A preliminary result of quantitative evaluation on activity of Merapivolcano. Proc. On Symposium on Japan – Indonesia IDNDR Project, pp. 165–180. Hidayati, S., Basuki, A., Kristianto, Mulyana, I., Budiyanto, A., 2008a. Laporan Penelitian G. Kelud. Pusat Vulkanologi dan Mitigasi Bencana Geologi (Internal report). Hidayati, S., Ishihara, K., Iguchi, M., Ratdomopurbo, 2008b. Focal mechanism of volcanotectonic earthquake at Merapi volcano, Indonesia. Indonesian J. Phys. 19 (2), 75–82. Hidayati, S., Kristianto, Basuki, A., Mulyana, I., 2009. A new lava dome emerged from crater lake of Kelud volcano, East Java, Indonesia. Jurnal Geologi Indonesia 4 (4), 229–238. Hotovec, A.J., Prejean, S.G., Vidale, J.E., Gomberg, J., 2013. Strongly gliding harmonic tremor during the 2009 eruption of Redoubt Volcano. J. Volcanol. Geotherm. Res. 259, 89–99. Ishihara, K., Hendrasto, M., Hidayati, S., 2011. Long-term forecasting of volcanic eruptions in case of Kelud volcano, Indonesia. Ann. Disaster Prev. Res. Inst., Kyoto Univ. 54 (B1), 209–214 (in Japanese, with English abstr.).
67
Kemmerling, G.L.L., 1921. De uitbarsting van den Keloet in dennacht van de 19den op den20sten Mei 1919. Vulkanol. Meded. 2. Kilburn, C.R.J., 2003. Multiscale fracturing as a key to forecasting volcanic eruptions. J. Volcanol. Geotherm. Res. 125, 271–289. Kunrat, S.L., 2009. Geochemical and Thermodynamic Modeling of Volcanic Fluids and Their Interpretation for Volcano Monitoring. Universite Libre de Bruxelles (unpublished paper). Kusumadinata, K., Hadian, R., Hamidi, S., Reksowirogo, L.D., 1979. Data Dasar Gunungapi Indonesia, Bandung: Direktorat Vulkanologi. Lesage, P., Mora, M.M., Alvarado, G.E., Pacheco, J., Métaxian, J.P., 2006. Complex behavior and source model of the tremor at Arenal volcano, Costa Rica. J. Volcanol. Geotherm. Res. 157, 49–59. Lubis, A.M., 2014. Uplift of Kelud volcano prior to the November 2007 Eruption as Observed by L-Band Insar. J. Eng. Technol. Sci. 46 (3), 245–257. Maeno, F., Nakada, S., Yoshimoto, M., Shimano, T., Hokanishi, N., Zaennudin, A., Iguchi, M., 2019. A sequence of a Plinian eruption preceded by dome destruction at Kelud volcano, Indonesia, on February 13, 2014, revealed from tephra fallout and pyroclastic density current deposits. J. Volcanol. Geotherm. Res. 382, 24–41. McCausland, W.A., White, R.A., Indrastuti, N., Gunawan, H., Patria, C., Suparman, Y., Putra, A., Triastuty, H., Hendrasto, M., 2019. Using a process-based model of pre-eruptive seismic patterns to forecast evolving eruptive styles at Sinabung Volcano, Indonesia. J. Volcanol. Geotherm. Res. 382, 253–266. Miller, A.D., Stewart, R.C., White, R.A., Luckett, R., Baptie, B.J., Aspinall, W.P., Latcman, J.L., Lynch, L.L., Voight, B., 1998. Seismicity associated with dome growth and collapse at the Soufriere Hill volcano, Montserrat. Geophys. Res. Lett. 25 (18), 3401–3404. Minakami, T., 1974. Seismology and Volcanoes in Japan. In: Civetta, L., Gasparini, P., Luongo, G., Rapolla, A. (Eds.), Physical Volcanology. Elsevier, pp. 1–27. Moran, S.C., Malone, S.D., Qamar, A.I., Thelen, W.A., W., A.K., Caplan-Auerbach, J., 2008. Seismicity associated with renewed dome building at Mount St. Helens, 2004–2005. US Geol. Surv. Prof. Pap. 1750, 27–60. Moran, Seth, C., Newhall, Chris, Roman, Diana C., 2011. Failed magmatic eruptions: latestage cessation of magma ascent. Bull. Volcanol. 73 (2), 115–122. Nakada, S., Shimizu, H., Ohta, K., 1999. Overview of the 1990–1995 eruption at Unzen Volcano. J. Volcanol. Geotherm. Res. 89 (1999), 22. Nishi, K., 2005. Hypocenter Calculation Software GAD (Geiger's method with Adaptive Damping). JICA Office, Jakarta. Ratdomopurbo, A., Poupinet, G., 2000. An overview of the seismicity of Merapi volcano (Java, Indonesia), 1983–1994. J. Volcanol. Geotherm. Res. 100, 193–214. Rowe, C., Thurber, C., White, R., 2004. Dome growth behavior at Soufriere Hills Volcano, Montserrat, revealed by relocation of volcanic event swarms, 1995–1996. J. Volcanol. Geotherm. Res. 134 (3), 199–221. Sadikin, N., 2008. Study on Volcano-tectonic Earthquakes and Magma Supply System at Guntur Volcano, with Long-term Dormant Period. PhD Thesis. Kyoto University, Japan. Santoso, A.B., Lesage, Philippe, SapariDwiyono, Sri Sumarti, Subandriyo, Jousset, Philippe, Metaxian, Jean-Philippe, Surono, 2013. Analysis of the seismic activity associated with the 2010 eruption ofMerapi Volcano, Java. J. Volcanol. Geotherm. Res. 261, 153–170. Smyth, H.R., Hall, R., Nichols, G.J., 2008. Cenozoic volcanic arc history of East Java, Indonesia: The stratigraphic record of eruptions on an active continental margin. The Geological Society of America, Special Paper 436. Sribudiyani, N.M., Ryacudu, R., Kunto, T., Astono, P., Prasetya, I., Sapiie, B., Asikin, S., Harsolumakso, A.H., Yulianto, I., 2003. The collision of the East Java Microplate and its implication for hydrocarbon occurrences in the East Java Basin. Indonesia Petroleum Association. Proceedings of 29th Annual Convention, pp. 335–346. Umakoshi, K., Shimizu, H., Matsuwo, N., 2001. Volcano-tectonic seismicity at Unzen volcano, Japan, 1985–1999. J. Volcanol. Geotherm. Res. 112, 117–131. Umakoshi, K., Itasaka, N., Shimizu, H., 2011. High-frequency earthquake swarm associated with the May 1991 dome extrusion at Unzen Volcano, Japan. J. Volcanol. Geotherm. Res. 206, 70–79. White, R., McCausland, W., 2016. Volcano-tectonic earthquakes: a new tool for estimating intrusive volumes and forecasting eruptions. J. Volcanol. Geotherm. Res. 309, 139–155. White, R., McCausland, W., 2019. A Process-based Model of Pre-eruption Seismicity Patterns and its use for eruption forecasting at dormant stratovolcanoes. J. Volcanol. Geotherm. Res. 382, 267–297. Wirakusumah, A.D., 1991. Some Studies of Volcanology, Petrology, and Structure of Mt. Kelut, East Java, Indonesia. Victoria University of Wellington PhD thesis. Zobin, V.M., 2003. Introduction to volcanic seismology. Development in Volcanology. 6. Elsevier Science B.V. Zobin, V.M., Nishimura, Y., Miyamura, J., 2005. The Nature of Volcanic Earthquake Swarm preceding the 2000 flank eruption at Usu volcano, Hokkaido, Japan. Geophys. J. Int. 163, 265–275.