Seismicity associated with the 1995–1996 eruptions of Ruapehu volcano, New Zealand: narrative and insights into physical processes

Journal of Volcanology and Geothermal Research 90 Ž1999. 1–18

Seismicity associated with the 1995–1996 eruptions of Ruapehu volcano, New Zealand: narrative and insights into physical processes C.J. Bryan ) , S. Sherburn Institute of Geological and Nuclear Sciences, Wairakei Research Centre, PriÕate Bag 2000, State Highway 1, Wairakei, Taupo, New Zealand Received 2 July 1997; accepted 7 January 1999

Abstract Seismicity associated with the 1995–1996 eruptions of Ruapehu reflects processes associated with the intrusion of magma to shallow depth in the volcanic edifice, its subsequent eruption, and changes in the volcanic plumbing system resulting from the eruptions. The sequence consisted of two distinct periods of eruptive activity, the first lasting from September 17, 1995 until late-October or early-November 1995 and the second from June 16, 1996 until late-July 1996. Immediately prior to and during the early stages of the 1995 eruptions, seismicity was similar to that recorded at Ruapehu during the previous 25 years, with the exception of the occurrence between September 17 and 25, 1995 of sub-1 Hz tremor, which is thought to represent magmatic intrusion. In early-October, a new pattern of seismicity was established with the disappearance of the 2 Hz tremor resonator source and a change to wideband Ž2–10 Hz. tremor and wideband volcanic earthquakes. This change coincided with the ejection of the last vestiges of Crater Lake and the change in eruptive style from phreatomagmatic to magmatic. Seismicity associated with the 1996 eruptions was very similar to that which accompanied the later part of the 1995 activity, implying the same eruption processes and that no significant changes had occurred in the volcanic plumbing system during the intervening period. Geochemical, geodetic, and seismic precursors to the 1995 eruptions were both minor and inconsistent, highlighting the difficulty in forecasting these eruptions. Furthermore, deep volcano-tectonic earthquakes were extremely rare throughout the 1995 and 1996 eruptive sequences, suggesting that stresses associated with magmatic intrusion were minor. This most likely resulted from the existence of either an open or a ductile pathway from the deep magma source to the surface prior to the eruptions and because the volume of magma intruded and subsequently erupted was relatively small Ž- 0.05 km3 .. q 1999 Elsevier Science B.V. All rights reserved. Keywords: seismicity; Ruapehu; volcanic earthquakes; volcanic tremor

1. Introduction This paper contains a description and an interpretation of the seismicity that accompanied the 1995– )

Corresponding author. Fax: q64-7-374-8211

1996 eruptions of Ruapehu volcano, New Zealand. This seismicity differed in many respects from that which had been typical of Ruapehu since 1971 and reflects changes in the processes generating the seismicity. We focus on changes in the seismicity as the eruptive sequence progressed, and how these changes

0377-0273r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 7 - 0 2 7 3 Ž 9 9 . 0 0 0 1 6 - 5

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reflect ongoing volcanic processes and modifications to the volcanic plumbing system. Mt. Ruapehu Ž2797 m, Fig. 1. is a 250,000-yearold, active, predominantly andesitic composite volcano, situated in the centre of the North Island of New Zealand ŽHoughton et al., 1987.. All historic activity has occurred from a summit crater ŽGregg, 1960; Houghton et al., 1987. which, except for a period following the 1945 eruption, has been occupied by a crater lake ŽGregg, 1960.. Following isolated eruptions in June and July 1995, significant volcanic activity ŽVEI 3r4. commenced on September 17, 1995 with eruptions generating many ash plumes which rose to 10–12 km elevation and numerous lahars which ejected most of the ; 9 = 10 6 m3 of water from Crater Lake ŽRuapehu Surveillance Group, 1996.. Following a relative lull in activity, three large eruptions in October generated ash plumes to 8–11 km elevation and dispersed ash and scoria downwind. These emptied Crater Lake of all remaining water, and eruptions changed from phreatomagmatic to magmatic in style. Activity slowly waned, and by mid-November, Crater Lake had started to reform. Eruptive activity resumed in June 1996, quickly ejecting the small volume of lake water. Several distinct eruptive episodes over the next 6 weeks dispersed ash to about 200 km from the volcano, and bombs to approximately 1.5 km.

tered by standard analog UHF telemetry techniques ŽLesage et al., 1995. to the unmanned Chateau Observatory at the base of the volcano. Data were then sent by telephone line to the Wairakei office of the Institute of Geological and Nuclear Sciences, 100 km to the north, where they were recorded in analog and digital form, and were available for volcano monitoring and hazard assessment. In addition to the seismographs, on September 22, 1995, an acoustic microphone was installed at the Chateau Observatory ŽFig. 1.. Acoustic data were recorded in analog form during the 1995 eruptive activity. Visual observations of Ruapehu, particularly the timing and style of eruptions and their effects, were not uniform during the 1995–1996 eruptions as there were no trained observers stationed at Ruapehu and poor weather sometimes made observations impossible. During 1995, observations were made during overflights, by staff at Ruapehu studying lahar or airfall deposits, or by members of the media. During 1996, financial constraints limited the number of overflights and, consequently, observations were less frequent than in 1995. Thus, the quality of the observational data does not match that of the seismic data, and we are often unable to quantify simple parameters such as the exact time of the start of an eruption, the maximum height of an eruption plume, or the eruption style. 2.1. Seismic data processing

2. Seismic, acoustic, and observational data The permanent seismic network monitoring Ruapehu consists of five seismographs ŽFig. 1.. A summit seismograph, DRZ, located approximately 700 m north of the active crater provided near-source data. Two seismographs on the lower flanks of the volcano, TUV, and CNZ, 8 and 9 km from the crater, respectively, and one on Ngauruhoe volcano, NGZ, 12 km from the crater, were the primary sources of seismic data during the frequent periods when DRZ was overloaded. The fifth seismograph, MGZ, is located 30 km to the north and is low-gain. Consequently, it recorded very little of the eruptionrelated seismicity. All seismic stations consist of identical, verticalcomponent, 1-Hz seismometers with data teleme-

The seismic data analysed in this paper come primarily from two sources, continuous waveforms recorded on analog chart recorders at a speed of 60 mmrmin and 3-min average frequency spectra calculated by a tremor monitor system ŽHurst, 1985.. The tremor monitor system provides data similar to that produced by the more well-known RSAM ŽEndo and Murray, 1991. and SSAM ŽRogers and Stephens, 1995. software, but has a resolution of 0.1 Hz at frequencies of 0.05 to 4.05 Hz, 0.2 Hz at frequencies of 4.05 to 8.05 Hz, and 0.4 Hz at frequencies of 8.05 to 16.05 Hz. It was particularly useful when strong or persistent seismicity caused adjacent analog traces to overlap and the style or intensity of seismicity to become indistinguishable. This system remains on scale for all tremor recorded at Ruapehu, but over-

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Fig. 1. Map showing the locations of the permanent seismographs on and around Mt. Ruapehu. The acoustic microphone was located at the Chateau Observatory which is nearly coincident with site CNZ. Crater lake Žshaded region., elevation contours at 500-m intervals, roads surrounding Ruapehu, and the Whangaehu River are shown for reference. The inset shows the locations of Mt. Ruapehu in the centre of the North Island of New Zealand and of the Wairakei Research Centre where data analyses were performed.

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Fig. 2. Seismic parameters for July 1994–December 1996. From top to bottom, plots show the daily maximum spectral amplitude of the 2 Hz Ždashed line. and 7 Hz Žsolid line. tremor measured at the DRZ site with the 10 eruptive phases described in the text labelled, the dates of occurrence of the different types of tremor, the number of volcanic earthquakes Žve. per day, the dates of occurrence of the two types of volcanic earthquakes, the number of volcano-tectonic earthquakes Žvt. per day, and the dates of occurrence of shallow and deep volcano-tectonic earthquakes. The triangles above the plot represent discrete eruptions; the bars, periods during which eruptive activity was more-or-less continuous, and the ???, the tailing off of the eruptive activity and our lack of information about the exact dates on which eruptive activity terminated. The two dashed boxes in the uppermost plot identify the times shown in Fig. 3. Two and seven hertz tremor were nearly continuous during the entire period. In contrast, sub-1 Hz and wideband tremor were associated only with intrusive and eruptive activity, respectively. Volcanic earthquakes changed from 2 Hz to wideband at about the same time as the last water was emptied from Crater Lake. Few deep volcano-tectonic earthquakes occurred either prior to or during the eruptive activity, suggesting that an open or ductile vent structure existed at Ruapehu prior to the commencement of the eruptions.

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loads for earthquakes larger than M L f 2.8 when recorded at DRZ and M L f 3 when recorded at all other sites. However, a comparison of spectra from clipped and non-clipped earthquakes shows that even when the system is overloaded, the spectra remain a good representation of the frequency content of these events. Amplitude magnitudes of volcanic and volcanotectonic earthquakes recorded during the eruptions have been determined using a table specifically developed for Ruapehu ŽLatter, J.H., personal communication, 1996.. Comparison of these magnitudes with those determined by the New Zealand National Seismograph Network suggest that this table may

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overestimate the magnitude of earthquakes by as much as 0.5. For this reason, magnitudes reported in this paper are considered provisional and are given for internal comparative purposes only.

3. Seismicity narrative The 1995–1996 Ruapehu eruptions are divided into 10 phases based on changes in seismicity and, to a lesser extent, visual observations of the volcano ŽFigs. 2 and 3; Table 1.. While the narrative below gives an overview of the seismic and volcanic activity during each of these phases, a more detailed

Fig. 3. Maximum spectral amplitude at DRZ during the most active periods of the 1995–1996 eruptions. Spikes rising above the general trend of the amplitude data represent earthquakes local to Ruapehu. The eruptive phases described in the text are labelled. The bars above the plot mark the periods of most intense eruptive activity. E denotes a discrete eruption; L, a lahar; and T, precursory tremor.

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Table 1 Summary of important volcanic and seismic events at Ruapehu, 1995–1996 Pre-eruption To Sept. 17, 1995 Two and 7 Hz tremor independent; changes in Crater Lake water temperature do not correlate with seismicity changes. Phase 1 (Sept. 17–30, 1995) Sept. 17 Changes in the amplitude and frequency content of tremor precede a moderate lahar-producing eruption through Crater Lake, accompanied by a M L 3.6 volcanic earthquake. Sept. 17–25 Sub-1 Hz tremor common, and sometimes triggered by volcano-tectonic earthquakes. Sept. 19 M L 3.4 volcanic earthquake accompanies a lahar-producing eruption through Crater Lake. Sept. 23 M L 3.2 volcanic earthquake accompanies an eruption through Crater Lake; three lahars generated. Sustained eruption begins. Sept. 24–25 Tremor and numerous volcanic earthquakes accompany a 10 km high eruption column; sustained lahars. Phase 2 (Sept. 30–Oct. 7, 1995) Sept. 30–Oct. 7 Tremor and volcanic earthquakes accompany moderate but intermittent eruptions. Phase 3 (Oct. 7–14, 1995) Oct. 7 M L ) 3.6 volcanic earthquake accompanies an ash eruption to 7.5 km. Oct. 11 An 8-h episode of wideband tremor accompanies an eruption of about 0.02 km3 of ash; plume to 8–10 km. Crater Lake emptied. Oct. 14 A 5-h episode of wideband tremor accompanies an eruption of about 0.01 km3 of ash; plume to 11 km. Phase 4 (Oct. 14–NoÕ. 9, 1995) Oct. 14–31 Small volcanic earthquakes accompany eruptions of ash to a few hundred meters above the crater. Intra-eruption (NoÕ. 9, 1995–June 10, 1996) Nov. 1995– Seven hertz tremor sustained at ; 3 mmrs; about 20 shallow volcano-tectonic earthquakes per day. Feb. 1996 Feb.–June 1996 ; 50 shallow volcano-tectonic earthquakes per day; a small lava extrusion seen in Crater Lake. Phase 5 (June 10–16, 1996) June 10 Minor, intermittent tremor. June 15 Strong wideband tremor. Six small Ž M L - 2. volcano-tectonic earthquakes. Phase 6 (June 16–July 1, 1996) June 16 Wideband volcanic earthquakes and tremor accompany an ash eruption to several kilometers above the crater; a small lahar empties Crater Lake. June 17–18 Numerous volcanic earthquakes accompanied by ground-coupled airwaves and a change to strombolian style eruptions; molten material erupted to several hundred meters above the crater and bombs to 1.5 km. Phase 7 (July 1–15, 1996) July 1–6 Many small wideband volcanic earthquakes and intermittent tremor. July 7 Continuous tremor and volcanic earthquakes with ground-coupled airwaves accompany the eruption of ash to 5 km elevation. July 8–10 Minor tremor and volcanic earthquakes accompany intermittent ash eruptions. Phase 8 (July 15–19, 1996) July 15–16 Strong tremor. Ash eruption to an elevation of 7 km. Numerous discrete volcanic earthquakes with ground-coupled airwaves. Phase 9 (July 19–24, 1996) July 19–24 Tremor and volcanic earthquakes accompany ash eruptions. Phase 10 (July 24–Aug. 11, 1996) July 24–Aug. 11 Intermittent, low-amplitude tremor and minor, low-elevation eruptions.

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Table 1 Žcontinued. New background and post-eruption From Aug. 11 Seven hertz tremor stabilises at about 2 mmrs; shallow volcano-tectonic earthquakes become common. Sept. 9 Last observed ash eruption. All dates are in Universal Time, 12 h behind New Zealand Standard Time.

description can be found in Bryan and Sherburn Ž1999.. Eruptive periods, values of seismic parameters, and time lines showing the occurrence of the different types of seismicity from July 1994 to December 1996 are shown in Fig. 2. The types of seismicity referred to in the narrative are described in more detail in Sherburn et al. Ž1999..

which had a dominant frequency noticeably lower than the typical 2 Hz tremor. In contrast, the latter eruption was accompanied by a M L 2.4 volcanic earthquake, but was not accompanied by any change in tremor. 3.2. Phase 1: initial eruptiÕe phase (September 17 to 30, 1995)

3.1. Pre-eruption (to September 17, 1995) During 1994 and much of 1995, seismicity at Ruapehu was characterised by the occurrence of volcanic tremor with spectral peaks of 2 and 7 Hz and shallow volcano-tectonic earthquakes at a rate of about 5 per day ŽFig. 2; Table 1.. Throughout the period, 7 Hz tremor occurred at a near-constant level of 1 mmrs at DRZ. Intervals of strong Ž) 5 mmrs at DRZ. 2 Hz tremor ŽFig. 4a. occurred in May 1994, November 1994, April 1995, May 1995, June 1995, and September 1995 ŽFig. 2.. Fewer than 20 volcanic earthquakes were recorded during this period. Although Crater Lake water temperature fluctuated between 10 and 608C throughout this period, changes in the lake water temperature did not correlate with changes in the seismicity ŽBryan and Sherburn, 1999.. Neither were small phreatic eruptions through the crater lake in January 1995 accompanied by any change in the background seismicity. Although increases in the concentrations of Mg 2q and Cly in the lake water between April 19 and May 4, indicators of injection of magma into the lake andror vent beneath the lake ŽRuapehu Surveillance Group, 1996., coincided with enhanced tremor, the relationship between these two processes is not known. The only significant events during this period to be indisputably accompanied by seismic activity were small eruptions through Crater Lake on June 28, and July 2, 1995 ŽFig. 2.. The former eruption was accompanied by a M L 3.2, 2 Hz volcanic earthquake, and preceded by 20 min of volcanic tremor, some of

Seismicity during this period changed with the level and style of volcanic activity. Between September 17 and 23, seismicity was characterised by the occurrence of 2 Hz volcanic earthquakes and of tremor with dominant frequencies of sub-1 Hz, 2 Hz, and 7 Hz, whereas from September 24 until September 30, seismicity was characterised by the occurrence of 2 Hz volcanic earthquakes and the gradual appearance of wideband tremor ŽFig. 2.. Discrete eruptions on September 17, 19, and 23 ŽFigs. 2 and 3. were accompanied by volcanic earthquakes of M L 3.6, 3.4, and 3.2, respectively, and generated lahars which flowed into the Whangaehu River ŽFig. 1. and as many as two additional catchments ŽTable 1.. The first recorded instance at Ruapehu of tremor with a frequency less than 1 Hz ŽFig. 4b. began approximately 1 h prior to the September 17 eruption. Further intervals of this low-frequency tremor then occurred during the next 1–1.5 weeks. Two periods of sub-1 Hz tremor on September 19 and one on September 23 appeared to have been triggered by volcano-tectonic earthquakes ŽTable 1.. Enhanced 7 Hz tremor occurred about 10 h prior to the September 17 eruption ŽFig. 2.. It recurred on September 18 between 0100 and 0630 UT, and from September 24 until November 1995, varied in tandem with the 2 Hz tremor. Two hertz tremor continued throughout this period ŽFig. 2.. The tremor amplitude ratio DRZrCNZ at 2 Hz was unchanged from that characteristic of 1994 and early-1995, suggesting that the source of 2 Hz tremor remained that which was active at

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Ruapehu prior to the start of the eruptions. Discrete 2 Hz volcanic earthquakes also occurred during this period, with the larger events accompanying eruptions. A few volcano-tectonic events, the largest of M L 2.8, occurred during this period, with some, but not all triggering intervals of sub-1 Hz tremor. These were the only deep volcano-tectonic earthquakes during 1995. An eruption at 0457 UT on September 23 ŽFigs. 2 and 3; Table 1. generated a plume to an elevation of 10–12 km, minor ashfall to the northeast, and lahars which flowed into the Whangaehu and two other catchments and down the recently vacated upper part of the skifield on the northwestern side of the mountain. It signalled a change from discrete volcanic and seismic events to more continuous activity. During the next 3 days, periods of relative volcanic and seismic quiescence were interspersed with periods of eruptive activity which generated ash plumes to as high as 10 km elevation and ejected more than half of the 9 = 10 6 m3 of water from Crater Lake. Two hertz volcanic earthquakes were common and wideband tremor began to appear, whereas the sub-1 Hz tremor disappeared ŽFig. 2.. Numerous airwaves were recorded on the acoustic microphone at the base of the volcano. From September 26 until September 30, moderate volcanic tremor and some 2 Hz volcanic earthquakes were recorded. A notable decline in the tremor amplitude on September 30 ŽFig. 3. marked the close of the initial phase of the eruption. 3.3. Phase 2: relatiÕe lull in actiÕity (September 30 to October 7, 1995) From September 30 until October 6, volcanic tremor and volcanic earthquakes continued ŽFigs. 2 and 3; Table 1.. Eruptive activity appeared to have declined significantly from the level of the previous week; however, on most days during this interval, the summit was obscured due to poor weather, resulting in little observational data. On October 6 moderate amplitude tremor recommenced. At DRZ, this tremor was almost wideband, with equal amplitudes at frequencies between 1.5 and 3.5 Hz and with only a slightly lower amplitude at 7 Hz, while at lowerelevation sites ŽCNZ, TUV, NGZ., a dominant spectral peak of 1. 5 Hz was observed ŽFig. 4c..

3.4. Phase 3: second eruptiÕe phase (October 7 to 14, 1995) An eruption commencing at 0203 UT on October 7 ŽFig. 3. was accompanied by the largest volcanic earthquake Ž M L ) 3.6. of the 1995–1996 eruptive episode ŽTable 1.. This multiple-pulse earthquake overloaded the low-elevation analog seismographs for 15 min, and was dominated by frequencies of about 2 Hz. The effects of the eruption were similar to those of eruptions in late September, with an ash column to 8 km elevation, the ejection of blocks to 1 km from the vent, and a lahar in the Whangaehu catchment. The accompanying airwaves were the last acoustic waves recorded during the 1995 eruptions. Intervals of tremor on October 11 between 0800 and 1600 UT and on October 14 between 0400 and 0900 UT accompanied the last two major eruptions of the 1995 activity ŽFigs. 2 and 3; Table 1.. These eruptions emitted about 0.02 and 0.01 km3 of ash, and generated ash plumes to 8–10 and 11 km, respectively, and emptied the remaining water from Crater Lake. At DRZ, the frequency spectra of both periods of tremor were nearly flat between 2 and 10 Hz, while at lower-elevation sites there was a broad high, peaked at 1–3 Hz ŽFig. 4d.. 3.5. Phase 4: initial decline (October 14 to NoÕember 9, 1995) Following the October 14 eruption, there was a marked reduction in the overall level of seismicity and a change in its style ŽFigs. 2 and 3.. Two types of seismicity became predominant ŽFig. 5.: wideband volcanic earthquakes that had a waveform envelope similar to that of typical 2-Hz volcanic earthquakes, and short-duration, shallow, volcano-tectonic events which were often followed by volcanic earthquakes or wideband volcanic tremor. During this period, the larger volcanic earthquakes were usually accompanied by the eruption of ash, typically to a few hundred meters above the crater ŽTable 1.. 3.6. Intra-eruption (NoÕember 9, 1995 to June 10, 1996) Seven hertz tremor recommenced at DRZ on November 9, with an amplitude sustained at about

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3 mmrs until January 1996, at which time it decreased to about 2 mmrs ŽFig. 2; Table 1.. Shortduration, shallow, volcano-tectonic earthquakes continued to be recorded at a rate of about 20 per day, and at lower-elevation sites were characterised by a low-amplitude, high-frequency onset followed by a low-frequency, higher-amplitude wave ŽFig. 6., simi-

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lar to hybrid events recorded at Galeras, Redoubt, and Pinatubo ŽLahr et al., 1994; Chouet, 1996.. The number of shallow volcano-tectonic earthquakes abruptly increased to more than 50 per day in lateFebruary ŽFig. 2; Table 1., and in late-March, the top of a small lava extrusion was seen within Crater Lake ŽOtway, P., pers. commun. 1996., suggesting that these events may have signalled a small dome building event such as occurred at Redoubt, Alaska in 1990 ŽPower et al., 1994.. During this period, melting of snow and ice around the crater and condensation of fumarolic vapours contributed to the re-formation of Crater Lake, although to only approximately 1% of its pre-eruption volume. 3.7. Phase 5: 1996 pre-eruption (June 10 to 16, 1996) On June 10, 1996, weak, intermittent volcanic tremor was observed at low-elevation stations, the first tremor noticed at these sites since October 1995 ŽTable 1.. On June 15, two further periods of tremor, this time much stronger, were recorded ŽFigs. 2 and 3; Table 1.. The first period had frequency characteristics similar to that recorded during late-October 1995: wideband at DRZ, but peaked at around 1 Hz at lower-elevation sites ŽFig. 7a., whereas tremor during the second period was strongly peaked at 1.5–4 Hz at DRZ, and at 0.5–4 Hz at lower-elevation sites ŽFig. 7b.. Six small Ž M L - 2. volcanoFig. 4. Three-minute averaged spectral amplitude at the DRZ, CNZ, and NGZ seismic sites. Ža. April 12, 1995. The typical background tremor at Ruapehu is dominated by a broad spectral peak at 1.8–2.3 Hz recorded at all sites. A 6–7 Hz peak is also frequently recorded at DRZ, but is never recorded at lower-elevation sites. Žb. September 19, 1995. The discrete sub-1 Hz spectral peak recorded at all sites is not seen in the spectra of typical background Ruapehu tremor. As this tremor was recorded by short-period seismometers, the true amplitude was slightly higher than that recorded due to the instrument roll-off. This tremor is inferred to represent magma intrusion. Žc. October 6, 1995. Tremor recorded at the summit is becoming wideband with nearly equal amplitudes at frequencies of 1.5–3.5 Hz, and only a slightly lower amplitude at 5–10 Hz. At the lower-elevation sites, the tremor spectra are peaked at about 1.5 Hz. Žd. October 11, 1995. The tremor becomes wideband as the remaining water in Crater Lake is expelled. At DRZ, the tremor spectrum is nearly white between 2 and 10 Hz, while at lower-elevation sites, spectra show a broad peak between 1 and 3 Hz.

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Fig. 5. The two types of earthquakes recorded after the October 14, 1995 eruption: those with a waveform envelope similar to that of typical 2 Hz volcanic earthquakes, but with a wideband spectrum; and short-duration, volcano-tectonic earthquakes. The volcanic earthquakes generally accompanied ash emission.

tectonic earthquakes occurred during the first hour of this tremor ŽTable 1.. When large enough to be recorded at CNZ, shallow Ž- 500 m depth. volcanotectonic earthquakes are emergent with a cigar-shaped waveform; however, these six events had impulsive first arrivals, clearly showed the arrival of later phases, and had the largest amplitudes near the beginning of the signal. Hence, we infer that these earthquakes occurred deeper within the volcano than the intra-eruption volcano-tectonic earthquakes. No volcanic activity is believed to have occurred during either period of tremor on June 15; however, the mountain was obscured by cloud and visual observation was impossible. Following the end of the second period of tremor, volcanic seismicity returned to the background style of the previous few months. 3.8. Phase 6: 1996 eruption stage 1 (June 16 to July 1, 1996) Wideband volcanic tremor and wideband volcanic earthquakes preceded and then accompanied the be-

ginning of the 1996 eruptions ŽFig. 2; Table 1.. This eruptive activity was characterised by the generation of ash columns to several kilometers above the top of the volcano, the ejection of bombs to 1.5 km from the crater, and the formation of a small lahar which quickly emptied Crater Lake. During the 1996 activity, ground-coupled airwaves were frequently recorded as far as 30 km from Ruapehu, and loud bangs were often heard at the base of the mountain, and in two cases, at distances of 100–200 km. 3.9. Phase 7: 1996 eruption stage 2 (July 1 to 15, 1996) On July 1, 1996, an increase in tremor amplitude and the occurrence of numerous, small, multiplepulse, wideband volcanic earthquakes marked the beginning of this phase ŽFigs. 2 and 3; Table 1.. Significant eruptive activity resumed on July 7 with the eruption of ash to 5 km elevation. The seismicity accompanying this and later periods of strong erup-

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Fig. 6. Waveforms and spectrograms from a volcano-tectonic earthquake which occurred at 0523 UT on October 25, 1995. Spectrograms are calculated using 1 s waveform sections overlapped by 0.5 s. Dark shades indicate high energy and light shades, low energy. Waveforms from these events are characterised by a low-amplitude, high-frequency onset followed by a dominant low-frequency, high amplitude arrival. Waveforms are ordered by increasing epicentral distance with epicentral distance given beneath each trace. These events are similar to the hybrid events recorded at other many other volcanoes ŽLahr et al., 1994; Chouet, 1996..

tive activity during 1996 were characterised by tremor which was preceded, accompanied, or followed by volcanic earthquakes. 3.10. Phase 8: 1996 eruption stage 3 (July 15 to 19, 1996) Strong wideband volcanic tremor abruptly recommenced at 0230 UT on July 15 ŽFigs. 2 and 3. and was accompanied by an ash column which rose to an elevation of 7 km ŽTable 1.. Lower-amplitude tremor and numerous discrete volcanic earthquakes displaying clear ground-coupled airwaves characterised the seismicity during the remainder of this eruptive phase ŽFigs. 2 and 3..

ŽFig. 2; Table 1.. Ash-poor plumes reached heights of as much as 10 km elevation and booms were heard at the base of the volcano and as far as 150 km to the northeast. 3.12. Phase 10: decline in actiÕity (July 24 to August 11, 1996) Minor eruptive activity continued to be accompanied by intermittent, low-amplitude tremor until early August, with the more-sustained periods of low-amplitude tremor lasting for a few hours at a time ŽFig. 2; Table 1.. By mid-August, the eruptive activity had essentially ended.

3.11. Phase 9: 1996 eruption stage 4 (July 19 to 24, 1996)

3.13. Post-eruption (from August 11, 1996)

Wideband volcanic tremor and volcanic earthquakes accompanied this phase of eruptive activity

Shallow volcano-tectonic earthquakes reappeared on the DRZ seismograms in mid-August at a rate of

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cated in distinct zones within this model structure. Analysis of tremor data suggests that 2 Hz tremor is sourced within the single-phase vapour region ŽHurst and Sherburn, 1993. and 7 Hz tremor, at the base of Crater Lake ŽSherburn et al., 1999.. As noted by Christenson and Wood Ž1993., the single-phase vapour region changes size in response to variations in the heat input from the deeper magmatic source, thus leading to small variations in the dominant frequencies and locations of the source regions of the tremor. Volcanic earthquakes contain the same frequency content as the 2 Hz tremor, and, consequently, are thought to be sourced in the same region as the 2 Hz tremor. Volcano-tectonic earthquakes, on the other hand, are sourced outside of the vent system, but within the surrounding volcanic edifice ŽHurst, 1998.. 4.2. Eruption model

Fig. 7. Frequency spectra for the two intervals of tremor on June 15, 1996. Ža. During the first interval, spectra are similar to those recorded during the eruptive activity on October 11 and 14, 1995. Žb. During the second interval, spectra at all sites are peaked at frequencies of less than 4 Hz.

about 5 per day ŽFig. 2.. Varying numbers of this type of earthquake and 7 Hz tremor generally of - 2 mmrs amplitude characterised Ruapehu seismicity until the end of 1996 ŽFig. 2; Table 1..

4. Model for temporal changes in seismicity 4.1. Model of the Õent prior to the eruptions Sherburn et al. Ž1999. present a model for the shallow Ruapehu vent system prior to the 1995 eruptions based on the long-term analysis of seismic data and detailed studies of the vent system ŽHurst et al., 1991; Christenson and Wood, 1993; Hurst and Sherburn, 1993; Hurst, 1998.. Within this model, there are four regions: degassing magma, a singlephase vapour, a two-phase vapour-liquid, and a single-phase liquid ŽFig. 8a.. The postulated source regions of the different types of seismicity are lo-

We suggest that the changes in the seismicity observed during the 1995–1996 eruptions resulted from modifications to both the Ruapehu vent structure and to the surrounding volcanic structure and from changes in the eruptive processes as the eruptions progressed. We note, in particular, the scarcity of volcano-tectonic earthquakes prior to and during the 1995 eruptions but many between the 1995 and 1996 eruptions, the occurrence of tremor enhanced in low frequencies only early in the eruptive sequence, and the cessation of 2 Hz volcanic seismicity and the subsequent commencement of wideband volcanic seismicity ŽFig. 2.. The changes in the seismicity occurred as the volcano moved from one state to the next. Our interpretation of the seismicity is limited by our reliance on poor and often incomplete visual observations and on data from vertical-component, short-period, low dynamic range seismometers. However, we infer that an open or ductile vent structure allowing the passage of magma without rock fracture and preventing the buildup of stress existed at Ruapehu prior to the 1995 eruptions. This structure was modified during the initial phases of the eruption. Initial intrusion of magma modified the dimensions of the source regions of the various types of tremor and altered the stress field in the vent and

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Fig. 8. Ža. Model for the volcanic plumbing system of Ruapehu prior to the 1995 eruptive activity. The model consists of four regions: degassing magma, single-phase vapour, two-phase vapour-liquid, and single-phase liquid. ŽThe single-phase liquid surrounds the two-phase zone, but is not shown on the diagram.. Two hertz tremor and volcanic earthquakes are sourced within the single-phase vapour, 7 Hz tremor at the base of Crater Lake, and volcano-tectonic earthquakes outside of the conduit. Žb. Sub-1 Hz tremor and deep volcano-tectonic earthquakes occur as magma is intruded into the volcano. Žc. Wideband tremor replaces the sub-1, 2, and 7 Hz tremor as the vent system is disrupted and eruptions change from phreatomagmatic to magmatic. Žd. Continuation of seismicity with the same frequency characteristics as that accompanying the eruptions of late 1995, suggesting no change in either the vent system or the eruptive style during the intra-eruptive period. The prevalence of airwaves during the 1996 activity reflects the existence of magma exposed within the vent and the occurrence of dry eruptions which were more intense than the dry eruptions of October and November 1995.

surrounding region, resulting in the disappearance of some of the tremor sources and the excitation of new

sources ŽFig. 8b.. During the first month of eruptive activity, eruption of ; 0.05 km3 of magmatic mate-

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rial and the ejection of Crater Lake resulted in a change from seismic activity in discrete frequency bands to wideband seismicity and in a drying out of the vent, with a consequent transition from discrete phreatomagmatic eruptions to more continual, dry, ash-rich eruptions ŽFig. 8c.. No further significant changes to the Ruapehu structure occurred between mid-October 1995 and June 1996, although a lava spine is inferred to have been emplaced in Crater Lake during the intra-eruptive period, coincident with the occurrence of enhanced volcano-tectonic activity. 4.3. Pre-eruption open Õent The lack of deep volcano-tectonic earthquakes beneath Ruapehu immediately prior to the start of the eruptions on September 17, 1995 and the paucity of them during ensuing activity suggest that the Ruapehu vent system was open or ductile prior to the 1995 eruptions. Volcano-tectonic earthquakes reflect brittle failure of the country rock in response to the effect of stresses that may be caused by the movement and degassing of magma, and their occurrence is a common precursor to volcanic activity ŽMcNutt, 1996.. The almost complete absence of this type of earthquake at Ruapehu is therefore very unusual, although lack of volcano-tectonic precursory activity has been noted at other volcanoes such as Pavlof ŽNeal, 1996.. Prior to the 1995 eruptions, an average heat flux through Crater Lake of about 200 MW ŽBibby, H., personal communication, 1997. and high vent temperatures ŽChristenson and Wood, 1993. would have been sufficient to keep the magmatic conduit open. Therefore, the passage of magmatic fluids to the surface during the early phases of the 1995 eruptions could proceed with little or no impediment. Furthermore, the small magmatic volume of the eruptive products Ž; 0.05 km3 ; Ruapehu Surveillance Group, 1996; Nakagawa et al., in press. is consistent with the low-level of volcano-tectonic earthquake activity. 4.4. Effects of magma intrusion (pre-eruption, phase 1) We infer that sub-1 Hz tremor observed between September 17 and September 25, 1995 ŽTable 1. resulted from magmatic intrusion ŽFig. 8b.. This

tremor is unlikely to have resulted from resonances of either the crater lake or of the volcanic edifice as then it would have formed part of the stable background tremor and would have been recorded on other occasions. In addition, on several occasions during September 1995, the sub-1 Hz tremor was apparently triggered by the rarely observed deep volcano-tectonic earthquakes, suggesting proximity between the sources of these types of seismicity. The shape of the waveforms, the frequency content, and the impulsive onsets of these volcano-tectonic earthquakes suggest a source region a few kilometers beneath Crater Lake, significantly deeper than the depth of less than 1000 m beneath Crater Lake observed for the 2 Hz tremor source region ŽHurst, 1998.. Furthermore, the relative amplitude of the sub-1 Hz tremor at CNZ as compared to that at DRZ is about 10 times greater than that for the 2 Hz tremor, suggesting a source deeper than the 2 Hz source. We speculate that magma degassing and intrusion may have been the cause of this tremor. Petrologic modelling of the 1995–1996 eruption products led Nakagawa et al. Žin press. to propose that magma injection occurred as a series of pulses between April and September 1995 with the last significant magma injection occurring at least several days prior to the October 11, 1995 eruption, timing in broad agreement with that of the occurrence of the sub-1 Hz tremor. 4.5. Disruption to the magmatic system and ejection of Crater Lake (phases 1–3) The change in tremor at DRZ from that consisting of discrete frequency bands of sub-1 Hz, 2 Hz, and 7 Hz to wideband Ž2–10 Hz. between the start of the eruptions and early October ŽFig. 2; Table 1. requires accompanying changes in the conduit and vent regions and in the eruptive processes. We suggest that two factors may have played an important role in causing these changes in tremor frequency: structural modifications within the conduit and the loss of Crater Lake. If, as Christenson and Wood Ž1993. suggest, the single-phase vapour region changes size in response to variations in the heat input from the deeper magmatic source, an increase in the heat flow prior to and early in the eruption would result in changes in the location and size of

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the seismic sources, altering the types of seismicity which could occur. At the start of the 1995 eruptions, Crater Lake contained approximately 9 = 10 6 m3 of water with a maximum depth of approximately 130 m. By September 27, more than half of this water had been ejected, with the final vestiges of the lake disappearing during the October 11, 1995 eruption ŽTable 1.. As the volume of Crater Lake was reduced and the eruptions became drier, the tremor became wideband. We suggest that the Crater Lake water may have acted as a cap above the vent and restricted the resonant frequencies of the tremor sources. The decrease in the confining pressure of over 10 bars resulting from the ejection of the lake water and the accompanying venting of magmatic gases directly to the atmosphere rather than to the confined lake would have altered the stress field within the vent system, and, consequently, may have gradually allowed the tremor sources to vibrate in more resonant modes ŽFig. 8c.. Furthermore, the ejection of Crater Lake and the consequent change from wet to dry eruptions must have changed the physics of the eruptive process. Further periods of activity in October, including the large eruptions on October 11 and 14, were accompanied by seismicity with the same characteristics as that which accompanied the October 7 eruption after the initiating earthquake. This implies that no further changes were occurring in the magmatic plumbing system or the eruptive style. 4.6. Recommencement of eruptiÕe actiÕity (phases 5–6) Frequency characteristics of tremor and volcanic earthquakes during the 1996 activity were similar to that of the post-October 7, 1995 activity ŽTable 1.. Thus, we suggest that no significant changes had occurred in the vent plumbing system during the intra-eruptive period and that the eruptive style in 1996 was similar or identical to that of late 1995. However, we note the prevalence of ground-coupled airwaves in 1996, and suggest that they result from the exposure of magma within the vent and the occurrence of dry eruptions which were more intense than the dry eruptions of the declining stages of the 1995 eruptions.

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5. Discussion 5.1. Precursory actiÕity Geochemical, geodetic, and volcanic precursors to the 1995 eruptions were inconsistent. The concentrations of Mg 2q and Cly ions in Crater Lake water began to show marked increases between mid-April and early-May 1995, indicating the intrusion of fresh magma into Crater Lake ŽRuapehu Surveillance Group, 1996.. However, successive geodetic measurements did not reveal any increase in the length of a survey line across Crater Lake until the August 15–September 20 survey interval ŽRuapehu Surveillance Group, 1996.. As an eruption had occurred on September 17, it is possible that the length change may have been a result of that eruption rather than a precursor to future activity. Even the discrete eruption on September 17, 1995 was not an indication of the activity to come. During the 1970s and 1980s Ruapehu had a history of discrete eruptions; thus, based on the precedent of the previous 25 years, there was no reason to expect larger eruptions to follow the September 17 eruption. Similar discrete eruptions in late-June and early-July 1995 had not been followed by an increasing trend of eruptive activity; in fact, eruptive activity had ceased and seismicity had returned to background levels. Short-term precursory seismic activity to the 1995 eruptions was also minor and ambiguous. The amplitude of the 2 Hz tremor at DRZ varied from - 2 mmrs to over 5 mmrs during 1995 ŽFig. 2.; however, it is common for changes in the amplitude of the 2 Hz tremor to occur without any eruptive activity ŽSherburn et al., 1999, companion paper.. The initial appearance of the sub-1 Hz tremor ŽFig. 2. prior to the September 17, 1995 eruption was recognised only in hindsight as it was obscured on the analog seismograms by the typical 2 Hz tremor. Further occurrences of this tremor on September 18 and 19 were recognised as anomalous at Ruapehu, and, therefore, an indication of a change in subsurface processes; however, as tremor of this frequency had never before been recognised at Ruapehu, its implication was not fully understood at that time. Precursory seismic activity prior to the 1996 eruptions was also minor ŽFig. 2; Table 1.. The recom-

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mencement of wideband tremor on June 10, 1996 went unnoticed as it was barely above the background amplitude and because it occurred during a period of strong wind which partially obscured the analog seismograms. In contrast, the occurrence of significantly stronger wideband tremor Ž65 mmrs at DRZ; Fig. 3. on June 15, 1996 which was similar to that which was recorded in September and October 1995 was interpreted as an intrusive episode; Civil Defence was notified of the change in seismic activity and warned that an eruption could occur at any time. The eruption began on June 16, approximately 40 h after the start of this tremor. 5.2. Eruption forecasting The minor nature of the precursory activity prior to both the 1995 and 1996 eruptions highlights the difficulty in forecasting eruptions at some volcanoes. It has become accepted that seismicity often changes noticeably during the months prior to eruptive activity such as occurred at Mount St. Helens ŽEndo et al., 1981., Nevado Del Ruiz ŽNieto et al., 1990., and Pinatubo ŽPinatubo Volcano Observatory Team, 1991.. However, notable activity can also occur over a much shorter time scale such as at Hekla ŽGronvold et al., 1983; Gudmundsson et al., 1992. and Redoubt ŽPower et al., 1994., or not at all such as at Pavlof ŽNeal, 1996., Arenal ŽMunoz et al., 1993., and Ruapehu. It is these latter cases which confound the eruption forecasting issue. The lack of precursors to the 1995 Ruapehu eruptions is different from the lack of precursory activity preceding late phases of a sequence of eruptions, such as occurred at Mt. Spurr in August 1992 ŽAlaska Volcano Observatory, 1993.. At Spurr, previous recent eruptions should have suggested the likelihood of further eruptions with little or no warning. Lack of precursory activity has been interpreted to represent either an open or ductile vent condition preventing the buildup of stress or the occurrence of an eruption involving only a small amount of magma ŽMcNutt, 1996.. At Ruapehu, the average high heat flux through Crater Lake and the scarcity of volcano-tectonic earthquakes prior to the 1995 and 1996 eruptions argue that the former condition is satisfied, while the total erupted volume of ; 0.05 km3 satisfies the latter condition. This combination

of the existence of an open vent and the occurrence of relatively small eruptions at Ruapehu ŽNakagawa et al., in press. further underscores the difficulty of forecasting eruptions at this volcano.

6. Conclusions Analog and digital data have been examined to provide a descriptive seismic history of the 1995– 1996 eruptions of Mt. Ruapehu, New Zealand. Analysis of analog seismograms and digital tremor data provided insight into both intrusive and extrusive processes occurring at Ruapehu throughout the eruptive sequence. Precursory activity was minor prior to both the initial eruptions in September 1995 and the recommencement of activity in June 1996, reflecting both the existence of an open conduit prior to the commencement of the eruptions and the small scale of the eruptions. However, we were able to forecast the 1996 eruptions based on pre-eruptive changes in the frequency content and amplitude of the tremor and knowledge gained during 1995. Changes in the seismicity throughout the eruptive sequence reflected ongoing changes to the shallow vent system and in the eruptive style. Deep-seated volcano-tectonic earthquakes which might be expected to accompany movement of magma from depth within the volcano to a shallow magma reservoir were only rarely observed. The sub-1 Hz tremor observed in mid-late September 1995 may have reflected this magma movement, but, taken in concert with the paucity of volcano-tectonic earthquakes, suggest that the existing conduit was sufficiently open or ductile to allow this movement without significant rock fracture. Changes in the frequencies of the volcanic earthquakes and tremor during September and October 1995 reflected disruption to the pre-eruptive vent system and a change from wet to dry eruptions. The similarities between the seismicity in June 1996 and that of late-October 1995 suggest that there were no major changes in the conduit plumbing system or in the eruptive processes during the intervening period. Furthermore, the similarity in seismicity accompanying the late-October 1995 and July 1996 eruptions suggests that the late 1995 eruptions were of such a nature that they had little lasting effect on the conduit system.

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The data collected during the eruptive sequence, although limited in dynamic range and frequency content and of only a single component, are of a better quality than any previously collected during an eruption of Ruapehu. Analysis of these data have not only yielded insights into previously unrecognised sub-surface processes, but have also yielded information which will be of help in assessing the implications of future seismicity at Ruapehu. Acknowledgements This work would not have been possible without the eruption observations provided by other staff of the IGNS Wairakei Research Centre. Jan Harris and Brian Ferris of the Seismological Observatory provided invaluable assistance in the day-to-day operation of the Wairakei Seismic Laboratory. Assistance with the installation and operation of the portable seismographs was provided by IGNS staff at the Seismological Observatory and at Technical Services. The manuscript has greatly benefited from reviews by H. Bibby, E. Endo, A. Hurst, F. Klein, and an anonymous reviewer. This work was funded by the NZ Foundation for Research Science and Technology contracts C05402 and C05529. References Alaska Volcano Observatory, 1993. Mt. Spurr’s 1992 eruptions. EOS, Trans. Am. Geophys. Union 74 Ž217., 221–222. Bryan, C.J., Sherburn, S., 1999. Volcano-seismic activity at Ruapehu, 1995. In: Scott, B.J., Sherburn, S. Žcompilers., Volcano and Geothermal Observations 1995, New Zealand Volcanological Record, 24, pp. 37–48. Chouet, B.A., 1996. Long-period volcano seismicity: its source and use in eruption forecasting. Nature 380, 309–316. Christenson, B.W., Wood, C.P., 1993. Evolution of a vent-hosted hydrothermal system beneath Ruapehu Crater Lake, New Zealand. Bull. Volcanol. 55, 547–565. Endo, E.T., Murray, T., 1991. Real-time seismic amplitude measurement ŽRSAM.: a volcano monitoring and prediction tool. Bull. Volcanol. 53, 533–545. Endo, E.T., Malone, S.D., Nosen, L.L., Weaver, C.S., 1981. Locations, magnitudes, and statistics of the March 20–May 18 earthquake sequence. In: Lipman, P.W., Mullinaux, D.R., ŽEds.., The 1980 Eruptions of Mount St. Helens, Washington, US Geol. Surv. Prof. Pap., 1250, pp. 93–107. Gregg, D.R., 1960. The geology of Tongariro subdivision. N.Z. Geol. Surv. Bull. 40, 152.

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