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Seismotectonics and seismogenesis of Mw7.8 Gorkha earthquake and its aftershocks
Accepted Manuscript Seismotectonics and Seismogenesis of Mw7.8 Gorkha Earthquake and its Aftershocks B.R. Arora, B.K. Bansal, Sanjay K. Prajapati, Anup K. Sutar, Shailesh Nayak PII: DOI: Reference:
S1367-9120(16)30231-0 http://dx.doi.org/10.1016/j.jseaes.2016.07.018 JAES 2763
To appear in:
Journal of Asian Earth Sciences
Received Date: Revised Date: Accepted Date:
5 November 2015 18 July 2016 20 July 2016
Please cite this article as: Arora, B.R., Bansal, B.K., Prajapati, S.K., Sutar, A.K., Nayak, S., Seismotectonics and Seismogenesis of Mw7.8 Gorkha Earthquake and its Aftershocks, Journal of Asian Earth Sciences (2016), doi: http://dx.doi.org/10.1016/j.jseaes.2016.07.018
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Seismotectonics and Seismogenesis of Mw7.8 Gorkha Earthquake and its Aftershocks
B. R. Arora1,*, B. K. Bansal1, Sanjay K. Prajapati2, Anup K. Sutar1 and Shailesh Nayak1 1
Ministry of Earth Sciences, Prithvi Bhavan, Lodhi Road, New Delhi-110003 2
National Center for Seismology, Ministry of Earth Sciences, Mausam Bhavan, New Delhi-110003
*Corresponding author: Email: [email protected] Abstract: The April 25, 2015, Mw7.8 Gorkha Earthquake in central Nepal was followed by intense aftershock activity, including Mw6.7 shock on April 26, 2015 and Mw 7.3 shock on May 12, 2015. Synthesis of the focal mechanisms, space-time distribution of seismic activity in relation to previously imaged crustal velocity and resistivity structures reveals focusing of the Mw7.8 Gorkha earthquake near the upper surface of the thin fluid-filled low velocity and high conducting layer immediately above the plane of the detachment. On the geophysical sections, the detachment is identified as a sharp positive velocity interface. Modulation of frictional coupling and mechanical weakening by high-pore pressure fluids counteract the arc-normal stresses creating conditions for failure and nucleation of the Gorkha earthquakes on a plane subparallel with the detachment. Spatio-temporal patterns in aftershock activity indicate rapid alteration of main shock-induced stress fields, triggering a strong aftershock of Mw 6.7. Large stress drop and increased energy released by the Mw6.7 event facilitates upward injection of high pore-pressure fluid fluxes into the hidden out-of-sequence thrust. It is suggested that
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decrease in shear strength along the hidden thrust plane due to the diffusion of high pore pressure fluids created conditions favourable to trigger Mw7.3 aftershock. Key words: Gorkha earthquake, Himalayan seismicity, Detachment, Fluid dynamics, Transient stress transfer, Triggered aftershocks 1.
Introduction The Gorkha earthquake of Mw 7.8 struck the central Nepal on April 25, 2015 (Fig. 1),
accounting for more than 8500 deaths and leaving behind over 3.5 million homeless. The epicenter of the earthquake is placed 80 km WNW of the Kathmandu (Avouac et al. 2015), the capital city of the Nepal (Fig. 1). Since a great/major earthquake has been anticipated in the central Nepal for a long time (c.f. Bilham, 2015; Ader et al., 2012), a modern monitoring network comprising seismometers (Pandey et al., 1999) as well as high time resolution GPS receivers have been in operation (Bettinell et al., 2006), making the present earthquake to be the best recorded event in Himalaya. The high quality data has facilitated prompt characterization of the earthquake source and rupture dynamics (Fan and Shearer, 2015, Galetzka et al., 2015; Grandin et al., 2015; Denolle et al., 2015). The GCMT solution estimated moment magnitude (Mw) of 7.8 and focal depth of 12-15 km. The fault plane solution of the main shock suggests thrust faulting as the source mechanism, the nodal plane striking 293o with gentle northeast dip of 7o symbolizing the source fault (Table 1). All these parameters led Avouac et al. (2015) to suggest that the Gorkha earthquake of April 25, 2015 resulted from unzipping the lower edge of the locked portion of the detachment (MHT). The Mw7.8 Gorkha earthquake was followed by intense aftershock activity, including Mw6.7 shock on April 26, 2015. Largest aftershock of Mw 7.3 occurred on May 12, 2015, 17 days after the main shock. Consistent with the stress conditions due to the on-going collision, fault plane solutions (FPS) favour dominance of thrust 2
mechanism in generating these strong aftershocks. However, their linkages with the detachment or other tectonic features as well as the process of nucleation are poorly understood. In the past decades, large scale seismic active (INDEPTH) and passive (HIMNIT, HiCLIMB) arrays in the Nepal and elsewhere along the Himalayan arc have shown that intracrustal sections scanning detachment depth are dominated by a pair of sub-parallel negative and positive velocity interfaces (Schulte-Pelkum et al., 2005; Nábelek et al., 2009; Singh et al. 2010; Acton et al., 2011, Caldwell et al., 2013). The coincident magnetotelluric (MT) measurements along select profiles help to view the thin low velocity layer immediately overlying the positive interface (MHT) as a fluid-filled horizon (Rawat et al., 2014). The presence of fluids influence the rheological and mechanical properties of crustal rocks and thus geophysical images sensitive to fluids are becoming effective tool to search how the spatial distribution of fluids control the process of earthquake nucleation and occurrences in varied tectonic settings (Ogawa and Honkura, 2004; Umeda et al., 2006; Jiracek et al. 2007; Wannamaker et al, 2009; Heise et al., 2012). Taking advantage of the existing geophysical images in the central Nepal, in the present communication, we integrate the existing knowledge on the subsurface structures with the most coherent features of the current sequel of seismicity, e.g. focal mechanism, focal depth variation, slip distribution, time evolution of aftershock activity etc. to constrain the seismotectonics and process of seismogenesis.
2.
Geological and Tectonic Setting Gorkha earthquake of April 25, 2015 is located in beneath the Lesser Himalaya (LH).
The LH domain to the north is bounded by intra-continental Main Central Thrust (MCT) across which Higher Himalayan (HH) crystalline override southward (Yin, 2006). The LH on south is
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bounded successively by the Main Boundary Thrust (MBT) and the Himalayan Frontal Thrust (HFT), enclosing in between the narrow Sub-Himalaya (SH). The HFT defines the southern limit of the deformation in the Himalayan system and stands out as a first order topographic break against the flat Indo-Gangetic Plains (IGP). This sequence of northeast dipping parallel thrusts sole down to the Main Himalayan Thrust (MHT), later separating the top of the down-going Indian Plate from the overriding Himalayan wedge. This thrust fault system with their roots in the MHT provides the basic geometry for the foreland propagating thrust sheets.
Seismic activity along the Himalayan arc is the manifestation of the release of inter-plate strains resulting from continued convergence between India-Eurasia Plates. Convergence rate estimated at 18-21±2.5 mm/yr across central Nepal is considered to be absorbed by the underthrusting of Indian Plate beneath the overriding wedge of the Himalaya (Bettinelli, et al., 2006; Ader et al., 2012; Stevens and Avouac, 2015). Approximately 100 km wide segment of the MHT beneath the SH and LH remains locked during the inter-seismic period (Banerjee and Burgmann, 2002; Stevens and Avouac, 2015). This locked segment of the MHT slips episodically to generate major and great earthquakes along the Himalayan arc (Seeber et al. 1981; Ni and Barazangi, 1984). The MHT beneath the High and Tethys Himalaya slips aseismically (Avouac, 2003, Herman et al., 2010). However, it still is contentious whether the entire convergence is consumed by slips on the locked segment of the MHT (Cattin and Avouac, 2000) or displacements at other structures, especially out-of-sequence account for partial convergence (Herman et al., 2010). The transition zone from the locked to creeping segments of the MHT define the steep mid-crustal ramp, which at surface correspond to narrow belt of small and moderate magnitude earthquakes (Pandey et al., 1995). This narrow belt, named as
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Himalayan seismic belt (HSB), is regular feature of the entire Himalayan arc (Arora et al., 2012). At surface, the crustal ramp in the MHT also connects to the physiographic transition. In the central Nepal Himalaya, Wobus et al. (2005) inferred an unmapped hidden out-of sequence thrust running 20-30 km south of the surface trace of the MCT. The out-of-sequence thrust transfers slip upward from the basal detachment (MHT), transporting the overlying HH crystalline southward over long distances as Higher Himalayan klippen (Celerier et al., 2009; Srivastava and Mitra, 1994). The Kathmandu klippen in the LH (Fig. 2) is an example of the southward migration of the sheet of HH crystalline enclosing a small oval shaped Tethyan marine sediments (Yin, 2006)
3.
Spatial and Temporal Characteristics of Seismicity
The Mw7.8 Gorkha earthquake of April 25, 2015 was followed by intense aftershock activity confined primarily to ESE of the main shock (Fig.2). Reviewed catalogue of USGS (http://earthquake.usgs.gov/earthquakes/search/) lists 161 aftershocks with M≥4 within first 17 days of the main shock on April 25, 2015 (Fig. 2). The aftershocks included two events with Mw>6 and at least 23 with Mw>5. Slip distribution inverted using long period body waves (Fig. 3) in agreement with the inversion of co-seismic GPS/InSAR deformations indicated unilateral rupture for the main shock, propagating ESE to a distance of 140 km with over all width of 50 km (USGS, http://earthquake.usgs.gov/learn/topics/Nepal_Slides.pdf; Galetzka et al., 2015; Lindsey et al., 2015). Spatio-temporal evolution of aftershocks during the first 17 days of the main event spread all through the length and width of the rupture zone (Figs. 2 and 3). Calculations have shown that static stress changes due to main shock of April 25, 2015 accounted for the wide spread aftershock activity, including clustering of events immediately on 5
south-eastern extremity (Galetzka et al., 2015). The time evolution of aftershock activity during this period, which is complete for events with minimum magnitude (Mc) detection threshold of 4.4 (Fig. 4a), closely follows the Omori’s modified law with a clear exponential decay in seismic activity with the passage of time (Fig. 5a). In this overall decreasing trend of aftershock activity around the main rupture, largest aftershock of Mw7.3 occurred near town Kodari on May 12 2015 (S. No 7 in Table 1). The Mw7.3 Kodari event produced independent rupture of about 40x30 km (USGS Nepal_slides; http://earthquake.usgs.gov/learn/topics/Nepal_Slides.pdf) and generated its own sequence of aftershocks including the strongest aftershock of Mw 6.2 (S. No 8 in Table 1) (Figs. 2 and 3). Most aftershocks in this cloud exhibit the characteristics of secondary aftershocks in the sense that they have tendency to cluster both in time and close to the location of the major aftershocks (Mw7.3 and Mw6.7) which triggered them. Such behavior indicates that the static stress field established by main shock is rapidly altered by the aftershock-induced stresses (Felzer et al., 2003). Since occurrence of secondary aftershocks creates sharp peak in the aftershock cumulative decay curve (Fig. 5c) and hence the trend predicted by Omori’s equation established by the events recorded before the occurrence of this Mw7.3 earthquake does not fit the observed trend for full period (Fig. 5c), as noted independently by Adhikari et al., (2015). Instead, cluster of aftershock of Mw7.3 event define its own magnitude-frequency relation (Fig. 5b). The decay of aftershock sequences of the Kodari Mw7.3 earthquake is represented by an independent Omori’s equation with primary parameters (‘p’ and ‘k’) different from those obtained with data recorded immediately after the main shock of April 25, 2015 (Figs. 2 & 3). Relatively higher ‘p’ value in the rupture zone of the Mw 7.8 Gorkha earthquake correspond to large slip rates (peak >6 m; Galetzka et al., 2015) in this zone as compared to the low ‘p’ and small slip rates (~4 m, USGS Nepal slides, ibid) registered with in the rupture zone of Mw 7.3
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Kodari earthquake. In agreement with similar correlations seen in numbers of active fault zones, it can be surmised that frictional heat generated by deformations on the source fault modulate the viscoelastic properties of the medium to cause faster decay of aftershock in the region of high ‘p’ value (Kisslinger and Jones, 1991; Wiemer and Katsumata, 1999). The estimated b-values also show statistically higher value in the rupture zone of the Mw 7.3 Kodari earthquake than in the Mw 7.8 Gorkha earthquake (Fig. 4a.b). Relatively higher value of ‘b’ and marginally lower value of ‘p’ in the rupture zone of the Mw 7.3 Kodari earthquake are sensitive pointers of the presence of fluids in highly fractured rock matrix (Wiemer and Katsumata, 1999; Bayrak and Öztürk, 2004; Mishra and Zhao, 2003; Singh et al., 2011). These evidences are later used to indicate the role of fluids in triggering Mw7.3 Kodari earthquake.
4.
Seismicity-Tectonic Linkage
4.1
Seismogenic crustal structures Passive receiver function (RF) technique as applied to large scale Hi-CLIMB (Nábelek et
al., 2009) and HIMNIT broadband seismic arrays (Schulte-Pelkum et al., 2005) has given new insight on the geometry and nature of the MHT beneath the central Nepal. Fig. 6 shows the section of RF along the two profiles in the central Nepal (See Fig. 2 for the locations of the profiles). The RF profile AA’ is positioned some 50 km east of rupture zone of Mw7.3 earthquake on May 12, 2015 and profile BB’ just cuts the rupture zone of the main shock (Fig. 2). The intra-crustal section, in the depth range of 8-20 km, is dominated by a pair of sub-parallel negative and positive velocity converters (Fig. 6a,b). This pair of negative-positive converters is a general attribute of the entire Himalayan arc but has been variedly related with the basal
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detachment (Schulte-Pelkum et al., 2005; Nábelek et al., 2009; Singh et al. 2010; Acton et al., 2011, Caldwell et al., 2013). Nábelek et al. (2009) and Acton et al. (2011) related the detachment with a coherent negative converter, which further north beneath the Tethys Himalaya connects up with the strong Low Velocity Layer (LVL) imaged on the INDEPTH profile (Nelson et al., 1996). Given that in the collision domain, part of the down going plate is scraped and recycled in to the over-riding wedge (Bollinger et al., 2004), Schulte-Pelkum et al. (2005) identified the positive velocity interface as the MHT, i.e. the top of the descending competent Indian plate. The overlying negative seismic converter marks the top of a thin LVL sandwiched between the MHT (detachment) and the overriding Himalayan wedge (Schulte-Pelkum et al., 2005; Nábelek et al., 2009). The relation of the LVL with the seismic active detachment becomes more apparent in the Garhwal Himalaya, where both MT and the RF measurements were made on a common profile (Fig. 7; Caldwell et al., 2013, Rawat et al. 2014). On the imaged resistivity section, a low-angle northeast dipping high conductivity layer (HCL) is the most dominant feature in the depth range of 8-13 km all through the SH and LH (Fig. 7b). The imaged HCL, in terms of depth, thickness and lateral extent completely overlaps the LVL mapped by RF images (Fig. 7; Caldwell et al., 2013). The MT measurements in line with the RF profile in Nepal (BB’ in Fig. 2) also indicate that area of LVL immediately overlying the MHT is demarcated by a HCL (Lemonnier et al., 1999). This LVL/HCL embedded between the MHT and the overriding wedge of the Himalaya is attributed to the impoundment of upward propagating metamorphic fluids trapped by tectonically induced neutral buoyancy (Connolly and Podlachikov, 2004; Rawat et al., 2014). Thick pile of sediments in the IGP underthrusts beneath the SH and LH along with the down going Indian cratonic plate. The water driven off these
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sediments contribute to the trapped fluid flux immediately above the plane of the detachment (Hyndman, 1988).
4.2
Seismotectonics and role of fluids in seismogenesis
Identification of causative fault of the earthquake on the imaged crustal structure relies largely upon the accuracy with which focal depths are estimated. In the reviewed USGS catalogue used here, the estimates of focal depth are poorly constraint; as in most cases inverted values echo the default value of 10 km adopted in the travel time inversion. The most robust estimates of focal depths are provided by Denolle et al. (2015) by the inclusion of depth phases. Of the 5 M>6 large earthquakes processed by them, reliable depth estimates could be extracted only for 3 earthquakes of M7.8 (15 km, S. No 1), M6.7 (18 km, S. No 5) and M7.3 (15 km S. No 7 in Table 1), the records of two other large aftershocks (S. No 2 and 8) were obscured by the surface waves of previous larger events. The focal depth and low-angle nodal plane of the main shock align quite closely with the depth and geometry of the negative velocity interface beneath the LH (Fig. 6a, b). The focal depths as well as dip of nodal plane for some other select events of M>5 for which GCMT solutions are available (Table 1), show scattered picture but a tendency for hypocenters to cluster around upper surface of the fluid filled LVL/HCL (bounded between dashed blue and red lines in Fig. 6a, b) is discernible. The presence of over-pressured fluids in the thin layer immediately above the detachment modify the friction processes and, thus, control the inter plate coupling (Heise et al., 2013). In addition, mechanical weakening effects arising due the fluid-filled porosity counteract the fault-normal stresses (Byerlee, 1990; Harris, 1998), creating conditions favorable for thrust-type episodic displacement in the compression
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environment. The Gorkha earthquake with its hypocenter rooted in hydrated detachment appears to be a good example of the fluid induced seismicity. The tendency for large earthquakes to align with the plane of negative velocity convertor, just a few kilometres above the top of down going Indian plate, was emphasised by Rawat et al. (2014) in respect of M>6 1991-Uttarkashi and 1999-Chamoli earthquakes (Fig. 7).
The M7.8 Gorkha earthquake occurred on the near sub-horizontal detachment beneath the LH, whereas the Mw6.7 event of April 26, 2015 and Mw 7.3 earthquake of May 12, 2015 occurred further down-dip where the detachment with overlying LVL begins to dip to form the steep crustal ramp (Figs. 6a,b). The high conductance and low velocity of the ramp block signifies high degree of fluid interconnectivity, causing substantial reduction in the shear strength (Cox, 2005; Jiracek et al., 2007). Such a low shear strength block embedded between the locked segment of the detachment to the south and the creeping segment to the north tends to be store house of strains. The release of these continuously accumulating strains under deviatoric stresses, arising from an elevated topography, generate small but more frequent earthquakes, confined to narrow HSB (Bollinger et al., 2004; Rawat et al., 2014; Arora et al., 2012). Both Mw6.7 and Mw 7.3 events are embedded in this background clustered seismicity.
This M6.7 aftershock was anomalous in a sense that it registered nearly double stress drop (40±1.5 MPa) as compared to the M7.8 main shock of April 25, 2015 (22.9±1.9 MPa) and subsequent event of M7.3 on May 12, 2015 (20.2±1.3 MPa) (Denolle et al., 2015). This Mw6.7 aftershock was also anomalous as it radiated as much energy as the subsequent larger Mw7.3 aftershock (Denolle et al., 2015). The relatively higher value of stress drop recorded in
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association with the Mw6.7 events of April 26, 2015 is consistent with the findings of Yamada et al. (2010), who found that aftershocks located close to edge of large slip patches of the main shock have larger stress drops. Their analysis also showed that shear stress is accentuated at the edge of the fault plane, where the successive seismic activity is sometimes triggered. It is likely that static stress field transfer in conjunction with stress accentuation near the terminal end of main shock rupture raise the strain level in the already pre-stressed ramp to trigger a strong aftershock of M6.7.
Travel time inversions for a group of aftershocks, clustered around the Mw7.3 aftershock, yield estimates of focal depths different from the default value of 10 km, as noted in earlier study (Bai et al., 2016). Hypocenters of large numbers of aftershocks in this cluster are located at shallow depths with dip angles steeper than the MHT (blue dots in Fig. 6a). The hypocenters clearly have tendency to align on a dipping plane which following the slope of ramp on the MHT extend into the Himalayan wedge (Figs. 6a). Structurally, these seismicity planes can be viewed as the hidden out of sequence thrust that has been inferred to rise from the detachment level (Wobus et al., 2005; Sapkota et al., 2013). The high radiated energy and stress drop further modify the in-situ stress conditions to facilitate the injection of fluids fluxes into the overlying strata either through stress transfer or via fault-valve mechanism (Connolly and Podlachikov, 2004; Jiracek et al., 2007; Sibson, 1992). Analogous to the reservoir triggered seismicity and active experiments injecting fluid in to deep boreholes (Talwani, 2007), fluid diffusion due to elevated pore-pressure causes reduction in shear strength of fault planes (Yamada et al. 2015) facilitating occurrence of strong aftershock of May 12, 2015 with relatively low stress drop.
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5.
Discussion Integration of observed signatures of seismicity-tectonic linkages permits us to
conceptualize a simplified seismotectonic model for the Mw7.8 Gorkha earthquake and accompanying strong aftershocks (Fig. 8). Compatible with of the cratonic character of the underthrusted Indian plate, MHT is demarcated as a low-angle northeast dipping positive velocity interface at depth of 15-18 km (Fig. 8, red line; Schulte-Pelkum et al., 2005; Rawat et al., 2014). The seismic velocity and resistivity depth sections unambiguously reveal a thin fluidfilled layer immediately overlying the MHT. The Gorkha earthquake with its nodal plane and hypocenter rooted in hydrated detachment appears to be a good case wherein the fluid induced mechanical weakening and friction modulation appears to have contributed significantly to the processes of seismogenesis. In addition, the fluids would effectively reduce the friction coefficient at the MHT, lower friction has been shown to be the requisite to account the on-going convergence rate entirely by slip on the MHT (Cattin and Avouac, 2000; Herman et al., 2010). Accentuation of stresses (slip) at the terminal end of the elongated rupture in conjunction with clustering pattern in aftershock activity favours that static stress field due to the main shock load the pre-stressed lower edge of the MHT to trigger a strong M6.7 aftershock. Another strong earthquake of magnitude M7.3 occurred in close proximity to the fluid enriched ramp. Hypocenters of many aftershocks of this event of May 12, 2015 are located at shallow depth with dips of nodal plane steeper than the detachment (e.g. earthquakes numbered 2, 6, 8 and 9). This seismic plane in Fig 8 (green line) is seen as extension of the hidden out of sequence thrust that following the slope of the ramp has been inferred to rise from the detachment level (Wobus et al., 2005; Sapkota et al., 2013). At the seismogenic depth of about 10 km, pore-pressure is high
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enough to infiltrate the fluid pulses upward by several km, especially in over pressurized hidden thrust and shear zones (Becken and Ritter, 2012; Connolly and Podlachikov, 2004; Wannamaker et al., 2002). The loading of out-of-sequence thrust by fluid pulses reduce the shear strength of fault planes (Yamada et al. 2015) creating conditions favorable to trigger the large Mw7.3 aftershock at the junction of the fluid-filled ramp and hidden out-of-the sequence thrust. This supports the suggestion of Denolle et al. (2015) that event of M7.3 is not rooted on the plate interface but rather on a sub-parallel fault which has been proposed based on structural/geomorphic/erosion/deformation data (Sapkota et al., 2012;Mugnier et al., 2013; Parameswaran et al., 2015; Bai et al., 2016).
6.
Conclusions
In sum up, the seismotectonic model suggested here to explain the seismic activity in the central Nepal during April-May, 2015 has two principal kinematic components; First, strain resulting from on-going collision between India-Eurasian plates cause large slip on the locked segment of the detachment (MHT) to generate the Mw7.8 Gorkha earthquake. The second component involves the role of hidden out-of-sequence that aligned with crustal ramp structure extends into the shallow depth section of the overriding wedge. Potential of kinematic models incorporating thrusting on MHT and out-of sequence in accounting the crustal shorting across Himalaya remain a contentious issue (Herman et al., 2010). Compatibility of GPS derived rate of convergence with the long term slip rate determined from deformed Holocene terraces around the HFT in Nepal Himalaya support the hypothesis where crustal shortening is entirely taken up by slip along the MHT (Cattin and Avouac, 2000). Nucleation of great and major earthquakes on
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the locked segment of the MHT is consistent with the above hypothesis. Kinematic model developed to test whether out-of-sequence thrusting can reproduce GPS derived convergence rates required unrealistic large thrusting rate on the MCT (Herman et al., 2010). Therefore, consistent with the observations of Cattin and Avouac (2000), simple model involving thrusting on the MHT is preferred although occasional out-of-sequence thrusting as a source of large earthquake cannot be excluded (Herman et al., 2010). Static stress transfer, proximity of the fluid-rich ramp, injection of high pore pressure fluids may be favourable factors contributing to occurrence of earthquake on the out-of-Sequence thrust, like Mw7.3 Kodari earthquake. Further modelling similar to that initiated by Denolle et al. (2015) and Lindsey et al. (2015) simulating the above hydrological and geometrical factors should be a good future exercise with fresh upcoming geodetic and seismological data sets.
Acknowledgements: We acknowledge with thanks many informal discussions with our colleagues, Vineet Gahalaut, Sumer Chopra, G. Suresh from the National Center of Seismology that provided clarity on many aspects on seismic activity in the Himalaya. The research presented here is funded by the Ministry of Earth Sciences, New Delhi. We appreciate thoughtful comments and suggestions from Roland Burgmann, Guest Editor and an anonymous reviewer, which helped us to improve the original manuscript.
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25
Figure Captions
Figure 1:
Topographic and tectonic map of central Himalaya showing epicenters (yellow stars) of the Mw7.8 Gorkha (April 25, 2015) and Mw7.3 Kodari (May 12, 2015) earthquakes. The Epicenter of the 1934 Nepal-Bihar (red star) and historical (peach stars) 1505 and 1833 earthquakes of M~8 are also marked (adopted from Ambraseys and Douglas, 2004). Red circles: events with 7> M≥ 6; Yellow circles: select 7> M≥ 6 earthquakes; Blue dots: events with 6> M ≥ 4.5 for the period of 1964-2010 (ISC catalogue). Distribution of blue dots represents the level of background seismicity confined to narrow Himalayan Seismic Belt (HSB). The square box marks the area of the central Nepal shown in Figs. 2 and 3.
Figure 2: The distribution of the aftershocks of the Mw7.8 Gorkha earthquake of 25th April 2015 (adopted from USGS Reviewed catalogue; http://earthquake.usgs.gov/earthquakes/search/). Spatialtemporal clustering patterns are highlighted using different colour codes for events before (magenta symbols) and after (green symbols) the strongest Mw7.3 aftershock of May 12, 2015. Lines AA’ and BB’ mark the receiver function profiles, shown in Figs 6a, b respectively. Location of select earthquakes projected on lines AA’ and CC’ are superimposed respectively on RF section in Figs. 6a,b with respect to their distance from the Main Boundary Thrust (MBT). Beach balls in red colour represent Fault Plane Solutions (FPS) for the Mw7.8 Gorkha earthquake and two M>6 aftershocks (Listed at S. Nos. 1, 2, 5 in Table 1). FPS for Mw7.3 Kodari and one M>6 aftershocks listed at S. Nos. 7, 8 in Table 1) are shown as blue beach balls. 26
Red and blue boxes mark the spatial window considered for estimating ‘b’ value and Omori decay curves in Figs. 4 and 5.
Figure 3:
Distribution
of
aftershocks
in
relation
to
slip
distribution
(USGS
Nepal_slides;
http://earthquake.usgs.gov/learn/topics/Nepal_Slides.pdf) demarcating independent ruptures zones of Mw7.8 (April 25, 2015) and Mw7.3 Kodari (May 12, 2015) earthquakes. Spatialtemporal clustering patterns are highlighted using different colour codes for events before and after the strongest Mw7.3 aftershock of May 12, 2015. Clustering pattern in aftershocks immediately following the Mw6.7 (April 26, 2015) and Mw7.3 (May 12, 2015) aftershocks are highlighted independently.
Figure 4:
Gutenberg-Richter frequency-magnitude fit {log10(N) = a-bM} to (a) aftershock sequence following the Mw7.8 Gorkha earthquake, 25th April 2015 and (b) for the Mw7.3 Kodari earthquake of 12th May 2015. In both cases aftershocks falling in the spatial window marked by red and blue boxes in Fig. 2 are used. The total numbers of aftershocks, number of events with varying magnitude range are also listed along side in respective fits. The b-value is estimated using maximum likelihood method of Aki (1965).
Figure 5: p
The modified Omori fit {n (t) = k/(c+t) } to the cumulative numbers of aftershock sequences recorded during (a) the first 17 days after the occurrence of April 25, 2015 Mw7.8 Gorkha
27
earthquake and (b) first 50 days after the occurrence of May 12, 2015 earthquake. In both cases aftershocks falling in the spatial window marked by red and blue boxes in Fig 2 are used. (c) Time plot of cumulative numbers of aftershocks since the occurrence of Mw7.8 Gorkha earthquake April 25, 2015; steep rise corresponds with the occurrence of May 12, 2015 earthquake. Prediction of Omori fit (green curve) obtained based on the data of first 17 days does not explain the sequence after the occurrence of Mw7.3 aftershock on May 12, 2015. The constant k, c and p, which vary between earthquake sequences, are estimated using formulation outlined in Utsu (1961).
Figure 6:
Stack of Receiver Function (RF) images along (a) Profile AA’ and (b) Profile BB’ (modified respectively from Schulte-Pelkum et al., 2005 and Nábelek et al., 2009). See Fig. 2 for profile locations. Horizontal distances are referenced to the Main Boundary Thrust (MBT). The pair of positive (red line) and negative (broken blue in ‘a’ and black in ‘b’) delineate respectively the detachment (MHT) and top of the overlying Low Velocity Layer (LVL). Circles in section (a) indicate hypocenters of select events determined using depth phase (magenta), GCMT solutions (green) and travel time inversion (blue). Short lines on hypocenter location in vertical section (b) depict dip of shallow nodal plane. Numbers refers to S.No. of earthquakes as listed in Table 1. (c) MT inverted electrical resistivity section (modified from Lemonnier et al., 1999). Red curve shows the detachment as mapped by background seismicity where as black line indicates negative velocity interface imaged in (b) and major thrust boundaries of the Himalayan collision belt.
Figure 7: 28
(a) Schematic structural cross-section across the Garhwal Himalaya, (b) MT inverted resistivity section (c) RF images (modified from Rawat et al., 2014). Imaged High Conductivity Layer (HCL) and pair of negative (blue broken line) and positive (red broken) help delineate the detachment and the overlying the Low Velocity Layer (LVL)/HCL. The hypocenters of large magnitude earthquakes including 1991 Uttarkashi (white star) and 1999 Chamoli earthquake (red star) are also superimposed.
Figure 8:
Simplified crustal cross-section across the Nepal Himalaya showing interpreted seismotectonic model for M7.8 Gorkha earthquake and following aftershocks. The blue line represents top of the low velocity/ high conductivity layer (LVL/HCL) indicating a thin fluid-filled layer (blue hatched) immediately above the Main Himalayan Thrust (detachment) identified as positive velocity converter (red line). Hypocenters of main shock and large aftershocks determined using depth phase (magenta), GCMT solutions (green) and travel time inversion (blue and orange) are superposed together with bars representing dip of nodal planes of select events (numbered 1 to 9). Modulation of frictional coupling and mechanical weakening by high-pore pressure fluids facilitate slips to cause main shock (No 1) of April 25, 2015 on the locked segment of the detachment beneath Lesser Himalaya. Static stress change due to the main shock trigger a strong M6.7 aftershock (No 5) where both NE dipping detachment and overlying fluid-filled layer show sharp ramp structure. Large stress drop and increased energy released by the Mw6.7 event facilitate upward propagation of fluids along the hidden out-of-sequence thrust to trigger Mw7.3 earthquake of May 12, 2015 (No. 7). Relatively higher dip of nodal plane (Nos. 2, 6, 9) and
29
alignment of hypocenters (blue circles) of aftershocks of May 12, 2015 favour the extension of hidden thrust rising from the ramp into the shallow overriding wedge (dash red lines).
30
Table
Table 1: Location parameters and GCMT Fault Plane Solution for the April 25, 2015 Gorkha earthquake and a few large aftershocks in central Nepal. S. No
Date
Time
Latitude (°)
Longitude (°)
Magnitude
1
2015-04-25
06:11:58
28.2305
84.7314
7.8
Focal Depth (km) 15.0*
Centroid Depth (km) 12
Dip (°)
Strike (°)
Rake (°)
7
293
96
2
2015-04-25
06:45:29
28.2244
84.8216
6.7
10.0
21
23
308
131
3
2015-04-25
17:42:53
28.2380
85.8290
5.3
10.0
21
40
339
-105
4
2015-04-25
23:16:18
27.7993
84.8715
5.1
13.6
15
40
201
-20
5
2015-04-26
07:09:20
27.7711
86.0173
6.7
18.0*
21
14
289
98
6
2015-04-26
16:26:09
27.8297
85.8650
5.2
14.0
20
26
305
115
7
2015-05-12
07:05:28
27.8087
86.0655
7.2
15.0*
12
11
307
117
8
2015-05-12
07:36:59
27.6250
86.1617
6.2
15.0
20
28
299
116
9
2015-05-16
11:34:14
27.5603
86.0734
5.4
7.0
12
34
324
138
Time, location parameters and magnitude of the listed events are extracted from http://earthquake.usgs.gov/earthquakes/search/ whereas Focal Plane solution including centroid depth are adopted from http://www.globalcmt.org/CMTsearch.html. * Depth adopted from Denolle et al. (2015)
Highlights-Arora-Nepal-JAES
•
Interrelation between electrical resistivity, fluid &
•
Geophysical images of Main Himalayan Thrust and their linkages with Mw7.8 Nepal earthquakes
•
Role of fluids dynamic in seismogenesis.
•
Role of transient stress transfer in triggering earthquakes
31
seismicity in collision zone
S1367-9120(16)30231-0 http://dx.doi.org/10.1016/j.jseaes.2016.07.018 JAES 2763
To appear in:
Journal of Asian Earth Sciences
Received Date: Revised Date: Accepted Date:
5 November 2015 18 July 2016 20 July 2016
Please cite this article as: Arora, B.R., Bansal, B.K., Prajapati, S.K., Sutar, A.K., Nayak, S., Seismotectonics and Seismogenesis of Mw7.8 Gorkha Earthquake and its Aftershocks, Journal of Asian Earth Sciences (2016), doi: http://dx.doi.org/10.1016/j.jseaes.2016.07.018
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Seismotectonics and Seismogenesis of Mw7.8 Gorkha Earthquake and its Aftershocks
B. R. Arora1,*, B. K. Bansal1, Sanjay K. Prajapati2, Anup K. Sutar1 and Shailesh Nayak1 1
Ministry of Earth Sciences, Prithvi Bhavan, Lodhi Road, New Delhi-110003 2
National Center for Seismology, Ministry of Earth Sciences, Mausam Bhavan, New Delhi-110003
*Corresponding author: Email: [email protected] Abstract: The April 25, 2015, Mw7.8 Gorkha Earthquake in central Nepal was followed by intense aftershock activity, including Mw6.7 shock on April 26, 2015 and Mw 7.3 shock on May 12, 2015. Synthesis of the focal mechanisms, space-time distribution of seismic activity in relation to previously imaged crustal velocity and resistivity structures reveals focusing of the Mw7.8 Gorkha earthquake near the upper surface of the thin fluid-filled low velocity and high conducting layer immediately above the plane of the detachment. On the geophysical sections, the detachment is identified as a sharp positive velocity interface. Modulation of frictional coupling and mechanical weakening by high-pore pressure fluids counteract the arc-normal stresses creating conditions for failure and nucleation of the Gorkha earthquakes on a plane subparallel with the detachment. Spatio-temporal patterns in aftershock activity indicate rapid alteration of main shock-induced stress fields, triggering a strong aftershock of Mw 6.7. Large stress drop and increased energy released by the Mw6.7 event facilitates upward injection of high pore-pressure fluid fluxes into the hidden out-of-sequence thrust. It is suggested that
1
decrease in shear strength along the hidden thrust plane due to the diffusion of high pore pressure fluids created conditions favourable to trigger Mw7.3 aftershock. Key words: Gorkha earthquake, Himalayan seismicity, Detachment, Fluid dynamics, Transient stress transfer, Triggered aftershocks 1.
Introduction The Gorkha earthquake of Mw 7.8 struck the central Nepal on April 25, 2015 (Fig. 1),
accounting for more than 8500 deaths and leaving behind over 3.5 million homeless. The epicenter of the earthquake is placed 80 km WNW of the Kathmandu (Avouac et al. 2015), the capital city of the Nepal (Fig. 1). Since a great/major earthquake has been anticipated in the central Nepal for a long time (c.f. Bilham, 2015; Ader et al., 2012), a modern monitoring network comprising seismometers (Pandey et al., 1999) as well as high time resolution GPS receivers have been in operation (Bettinell et al., 2006), making the present earthquake to be the best recorded event in Himalaya. The high quality data has facilitated prompt characterization of the earthquake source and rupture dynamics (Fan and Shearer, 2015, Galetzka et al., 2015; Grandin et al., 2015; Denolle et al., 2015). The GCMT solution estimated moment magnitude (Mw) of 7.8 and focal depth of 12-15 km. The fault plane solution of the main shock suggests thrust faulting as the source mechanism, the nodal plane striking 293o with gentle northeast dip of 7o symbolizing the source fault (Table 1). All these parameters led Avouac et al. (2015) to suggest that the Gorkha earthquake of April 25, 2015 resulted from unzipping the lower edge of the locked portion of the detachment (MHT). The Mw7.8 Gorkha earthquake was followed by intense aftershock activity, including Mw6.7 shock on April 26, 2015. Largest aftershock of Mw 7.3 occurred on May 12, 2015, 17 days after the main shock. Consistent with the stress conditions due to the on-going collision, fault plane solutions (FPS) favour dominance of thrust 2
mechanism in generating these strong aftershocks. However, their linkages with the detachment or other tectonic features as well as the process of nucleation are poorly understood. In the past decades, large scale seismic active (INDEPTH) and passive (HIMNIT, HiCLIMB) arrays in the Nepal and elsewhere along the Himalayan arc have shown that intracrustal sections scanning detachment depth are dominated by a pair of sub-parallel negative and positive velocity interfaces (Schulte-Pelkum et al., 2005; Nábelek et al., 2009; Singh et al. 2010; Acton et al., 2011, Caldwell et al., 2013). The coincident magnetotelluric (MT) measurements along select profiles help to view the thin low velocity layer immediately overlying the positive interface (MHT) as a fluid-filled horizon (Rawat et al., 2014). The presence of fluids influence the rheological and mechanical properties of crustal rocks and thus geophysical images sensitive to fluids are becoming effective tool to search how the spatial distribution of fluids control the process of earthquake nucleation and occurrences in varied tectonic settings (Ogawa and Honkura, 2004; Umeda et al., 2006; Jiracek et al. 2007; Wannamaker et al, 2009; Heise et al., 2012). Taking advantage of the existing geophysical images in the central Nepal, in the present communication, we integrate the existing knowledge on the subsurface structures with the most coherent features of the current sequel of seismicity, e.g. focal mechanism, focal depth variation, slip distribution, time evolution of aftershock activity etc. to constrain the seismotectonics and process of seismogenesis.
2.
Geological and Tectonic Setting Gorkha earthquake of April 25, 2015 is located in beneath the Lesser Himalaya (LH).
The LH domain to the north is bounded by intra-continental Main Central Thrust (MCT) across which Higher Himalayan (HH) crystalline override southward (Yin, 2006). The LH on south is
3
bounded successively by the Main Boundary Thrust (MBT) and the Himalayan Frontal Thrust (HFT), enclosing in between the narrow Sub-Himalaya (SH). The HFT defines the southern limit of the deformation in the Himalayan system and stands out as a first order topographic break against the flat Indo-Gangetic Plains (IGP). This sequence of northeast dipping parallel thrusts sole down to the Main Himalayan Thrust (MHT), later separating the top of the down-going Indian Plate from the overriding Himalayan wedge. This thrust fault system with their roots in the MHT provides the basic geometry for the foreland propagating thrust sheets.
Seismic activity along the Himalayan arc is the manifestation of the release of inter-plate strains resulting from continued convergence between India-Eurasia Plates. Convergence rate estimated at 18-21±2.5 mm/yr across central Nepal is considered to be absorbed by the underthrusting of Indian Plate beneath the overriding wedge of the Himalaya (Bettinelli, et al., 2006; Ader et al., 2012; Stevens and Avouac, 2015). Approximately 100 km wide segment of the MHT beneath the SH and LH remains locked during the inter-seismic period (Banerjee and Burgmann, 2002; Stevens and Avouac, 2015). This locked segment of the MHT slips episodically to generate major and great earthquakes along the Himalayan arc (Seeber et al. 1981; Ni and Barazangi, 1984). The MHT beneath the High and Tethys Himalaya slips aseismically (Avouac, 2003, Herman et al., 2010). However, it still is contentious whether the entire convergence is consumed by slips on the locked segment of the MHT (Cattin and Avouac, 2000) or displacements at other structures, especially out-of-sequence account for partial convergence (Herman et al., 2010). The transition zone from the locked to creeping segments of the MHT define the steep mid-crustal ramp, which at surface correspond to narrow belt of small and moderate magnitude earthquakes (Pandey et al., 1995). This narrow belt, named as
4
Himalayan seismic belt (HSB), is regular feature of the entire Himalayan arc (Arora et al., 2012). At surface, the crustal ramp in the MHT also connects to the physiographic transition. In the central Nepal Himalaya, Wobus et al. (2005) inferred an unmapped hidden out-of sequence thrust running 20-30 km south of the surface trace of the MCT. The out-of-sequence thrust transfers slip upward from the basal detachment (MHT), transporting the overlying HH crystalline southward over long distances as Higher Himalayan klippen (Celerier et al., 2009; Srivastava and Mitra, 1994). The Kathmandu klippen in the LH (Fig. 2) is an example of the southward migration of the sheet of HH crystalline enclosing a small oval shaped Tethyan marine sediments (Yin, 2006)
3.
Spatial and Temporal Characteristics of Seismicity
The Mw7.8 Gorkha earthquake of April 25, 2015 was followed by intense aftershock activity confined primarily to ESE of the main shock (Fig.2). Reviewed catalogue of USGS (http://earthquake.usgs.gov/earthquakes/search/) lists 161 aftershocks with M≥4 within first 17 days of the main shock on April 25, 2015 (Fig. 2). The aftershocks included two events with Mw>6 and at least 23 with Mw>5. Slip distribution inverted using long period body waves (Fig. 3) in agreement with the inversion of co-seismic GPS/InSAR deformations indicated unilateral rupture for the main shock, propagating ESE to a distance of 140 km with over all width of 50 km (USGS, http://earthquake.usgs.gov/learn/topics/Nepal_Slides.pdf; Galetzka et al., 2015; Lindsey et al., 2015). Spatio-temporal evolution of aftershocks during the first 17 days of the main event spread all through the length and width of the rupture zone (Figs. 2 and 3). Calculations have shown that static stress changes due to main shock of April 25, 2015 accounted for the wide spread aftershock activity, including clustering of events immediately on 5
south-eastern extremity (Galetzka et al., 2015). The time evolution of aftershock activity during this period, which is complete for events with minimum magnitude (Mc) detection threshold of 4.4 (Fig. 4a), closely follows the Omori’s modified law with a clear exponential decay in seismic activity with the passage of time (Fig. 5a). In this overall decreasing trend of aftershock activity around the main rupture, largest aftershock of Mw7.3 occurred near town Kodari on May 12 2015 (S. No 7 in Table 1). The Mw7.3 Kodari event produced independent rupture of about 40x30 km (USGS Nepal_slides; http://earthquake.usgs.gov/learn/topics/Nepal_Slides.pdf) and generated its own sequence of aftershocks including the strongest aftershock of Mw 6.2 (S. No 8 in Table 1) (Figs. 2 and 3). Most aftershocks in this cloud exhibit the characteristics of secondary aftershocks in the sense that they have tendency to cluster both in time and close to the location of the major aftershocks (Mw7.3 and Mw6.7) which triggered them. Such behavior indicates that the static stress field established by main shock is rapidly altered by the aftershock-induced stresses (Felzer et al., 2003). Since occurrence of secondary aftershocks creates sharp peak in the aftershock cumulative decay curve (Fig. 5c) and hence the trend predicted by Omori’s equation established by the events recorded before the occurrence of this Mw7.3 earthquake does not fit the observed trend for full period (Fig. 5c), as noted independently by Adhikari et al., (2015). Instead, cluster of aftershock of Mw7.3 event define its own magnitude-frequency relation (Fig. 5b). The decay of aftershock sequences of the Kodari Mw7.3 earthquake is represented by an independent Omori’s equation with primary parameters (‘p’ and ‘k’) different from those obtained with data recorded immediately after the main shock of April 25, 2015 (Figs. 2 & 3). Relatively higher ‘p’ value in the rupture zone of the Mw 7.8 Gorkha earthquake correspond to large slip rates (peak >6 m; Galetzka et al., 2015) in this zone as compared to the low ‘p’ and small slip rates (~4 m, USGS Nepal slides, ibid) registered with in the rupture zone of Mw 7.3
6
Kodari earthquake. In agreement with similar correlations seen in numbers of active fault zones, it can be surmised that frictional heat generated by deformations on the source fault modulate the viscoelastic properties of the medium to cause faster decay of aftershock in the region of high ‘p’ value (Kisslinger and Jones, 1991; Wiemer and Katsumata, 1999). The estimated b-values also show statistically higher value in the rupture zone of the Mw 7.3 Kodari earthquake than in the Mw 7.8 Gorkha earthquake (Fig. 4a.b). Relatively higher value of ‘b’ and marginally lower value of ‘p’ in the rupture zone of the Mw 7.3 Kodari earthquake are sensitive pointers of the presence of fluids in highly fractured rock matrix (Wiemer and Katsumata, 1999; Bayrak and Öztürk, 2004; Mishra and Zhao, 2003; Singh et al., 2011). These evidences are later used to indicate the role of fluids in triggering Mw7.3 Kodari earthquake.
4.
Seismicity-Tectonic Linkage
4.1
Seismogenic crustal structures Passive receiver function (RF) technique as applied to large scale Hi-CLIMB (Nábelek et
al., 2009) and HIMNIT broadband seismic arrays (Schulte-Pelkum et al., 2005) has given new insight on the geometry and nature of the MHT beneath the central Nepal. Fig. 6 shows the section of RF along the two profiles in the central Nepal (See Fig. 2 for the locations of the profiles). The RF profile AA’ is positioned some 50 km east of rupture zone of Mw7.3 earthquake on May 12, 2015 and profile BB’ just cuts the rupture zone of the main shock (Fig. 2). The intra-crustal section, in the depth range of 8-20 km, is dominated by a pair of sub-parallel negative and positive velocity converters (Fig. 6a,b). This pair of negative-positive converters is a general attribute of the entire Himalayan arc but has been variedly related with the basal
7
detachment (Schulte-Pelkum et al., 2005; Nábelek et al., 2009; Singh et al. 2010; Acton et al., 2011, Caldwell et al., 2013). Nábelek et al. (2009) and Acton et al. (2011) related the detachment with a coherent negative converter, which further north beneath the Tethys Himalaya connects up with the strong Low Velocity Layer (LVL) imaged on the INDEPTH profile (Nelson et al., 1996). Given that in the collision domain, part of the down going plate is scraped and recycled in to the over-riding wedge (Bollinger et al., 2004), Schulte-Pelkum et al. (2005) identified the positive velocity interface as the MHT, i.e. the top of the descending competent Indian plate. The overlying negative seismic converter marks the top of a thin LVL sandwiched between the MHT (detachment) and the overriding Himalayan wedge (Schulte-Pelkum et al., 2005; Nábelek et al., 2009). The relation of the LVL with the seismic active detachment becomes more apparent in the Garhwal Himalaya, where both MT and the RF measurements were made on a common profile (Fig. 7; Caldwell et al., 2013, Rawat et al. 2014). On the imaged resistivity section, a low-angle northeast dipping high conductivity layer (HCL) is the most dominant feature in the depth range of 8-13 km all through the SH and LH (Fig. 7b). The imaged HCL, in terms of depth, thickness and lateral extent completely overlaps the LVL mapped by RF images (Fig. 7; Caldwell et al., 2013). The MT measurements in line with the RF profile in Nepal (BB’ in Fig. 2) also indicate that area of LVL immediately overlying the MHT is demarcated by a HCL (Lemonnier et al., 1999). This LVL/HCL embedded between the MHT and the overriding wedge of the Himalaya is attributed to the impoundment of upward propagating metamorphic fluids trapped by tectonically induced neutral buoyancy (Connolly and Podlachikov, 2004; Rawat et al., 2014). Thick pile of sediments in the IGP underthrusts beneath the SH and LH along with the down going Indian cratonic plate. The water driven off these
8
sediments contribute to the trapped fluid flux immediately above the plane of the detachment (Hyndman, 1988).
4.2
Seismotectonics and role of fluids in seismogenesis
Identification of causative fault of the earthquake on the imaged crustal structure relies largely upon the accuracy with which focal depths are estimated. In the reviewed USGS catalogue used here, the estimates of focal depth are poorly constraint; as in most cases inverted values echo the default value of 10 km adopted in the travel time inversion. The most robust estimates of focal depths are provided by Denolle et al. (2015) by the inclusion of depth phases. Of the 5 M>6 large earthquakes processed by them, reliable depth estimates could be extracted only for 3 earthquakes of M7.8 (15 km, S. No 1), M6.7 (18 km, S. No 5) and M7.3 (15 km S. No 7 in Table 1), the records of two other large aftershocks (S. No 2 and 8) were obscured by the surface waves of previous larger events. The focal depth and low-angle nodal plane of the main shock align quite closely with the depth and geometry of the negative velocity interface beneath the LH (Fig. 6a, b). The focal depths as well as dip of nodal plane for some other select events of M>5 for which GCMT solutions are available (Table 1), show scattered picture but a tendency for hypocenters to cluster around upper surface of the fluid filled LVL/HCL (bounded between dashed blue and red lines in Fig. 6a, b) is discernible. The presence of over-pressured fluids in the thin layer immediately above the detachment modify the friction processes and, thus, control the inter plate coupling (Heise et al., 2013). In addition, mechanical weakening effects arising due the fluid-filled porosity counteract the fault-normal stresses (Byerlee, 1990; Harris, 1998), creating conditions favorable for thrust-type episodic displacement in the compression
9
environment. The Gorkha earthquake with its hypocenter rooted in hydrated detachment appears to be a good example of the fluid induced seismicity. The tendency for large earthquakes to align with the plane of negative velocity convertor, just a few kilometres above the top of down going Indian plate, was emphasised by Rawat et al. (2014) in respect of M>6 1991-Uttarkashi and 1999-Chamoli earthquakes (Fig. 7).
The M7.8 Gorkha earthquake occurred on the near sub-horizontal detachment beneath the LH, whereas the Mw6.7 event of April 26, 2015 and Mw 7.3 earthquake of May 12, 2015 occurred further down-dip where the detachment with overlying LVL begins to dip to form the steep crustal ramp (Figs. 6a,b). The high conductance and low velocity of the ramp block signifies high degree of fluid interconnectivity, causing substantial reduction in the shear strength (Cox, 2005; Jiracek et al., 2007). Such a low shear strength block embedded between the locked segment of the detachment to the south and the creeping segment to the north tends to be store house of strains. The release of these continuously accumulating strains under deviatoric stresses, arising from an elevated topography, generate small but more frequent earthquakes, confined to narrow HSB (Bollinger et al., 2004; Rawat et al., 2014; Arora et al., 2012). Both Mw6.7 and Mw 7.3 events are embedded in this background clustered seismicity.
This M6.7 aftershock was anomalous in a sense that it registered nearly double stress drop (40±1.5 MPa) as compared to the M7.8 main shock of April 25, 2015 (22.9±1.9 MPa) and subsequent event of M7.3 on May 12, 2015 (20.2±1.3 MPa) (Denolle et al., 2015). This Mw6.7 aftershock was also anomalous as it radiated as much energy as the subsequent larger Mw7.3 aftershock (Denolle et al., 2015). The relatively higher value of stress drop recorded in
10
association with the Mw6.7 events of April 26, 2015 is consistent with the findings of Yamada et al. (2010), who found that aftershocks located close to edge of large slip patches of the main shock have larger stress drops. Their analysis also showed that shear stress is accentuated at the edge of the fault plane, where the successive seismic activity is sometimes triggered. It is likely that static stress field transfer in conjunction with stress accentuation near the terminal end of main shock rupture raise the strain level in the already pre-stressed ramp to trigger a strong aftershock of M6.7.
Travel time inversions for a group of aftershocks, clustered around the Mw7.3 aftershock, yield estimates of focal depths different from the default value of 10 km, as noted in earlier study (Bai et al., 2016). Hypocenters of large numbers of aftershocks in this cluster are located at shallow depths with dip angles steeper than the MHT (blue dots in Fig. 6a). The hypocenters clearly have tendency to align on a dipping plane which following the slope of ramp on the MHT extend into the Himalayan wedge (Figs. 6a). Structurally, these seismicity planes can be viewed as the hidden out of sequence thrust that has been inferred to rise from the detachment level (Wobus et al., 2005; Sapkota et al., 2013). The high radiated energy and stress drop further modify the in-situ stress conditions to facilitate the injection of fluids fluxes into the overlying strata either through stress transfer or via fault-valve mechanism (Connolly and Podlachikov, 2004; Jiracek et al., 2007; Sibson, 1992). Analogous to the reservoir triggered seismicity and active experiments injecting fluid in to deep boreholes (Talwani, 2007), fluid diffusion due to elevated pore-pressure causes reduction in shear strength of fault planes (Yamada et al. 2015) facilitating occurrence of strong aftershock of May 12, 2015 with relatively low stress drop.
11
5.
Discussion Integration of observed signatures of seismicity-tectonic linkages permits us to
conceptualize a simplified seismotectonic model for the Mw7.8 Gorkha earthquake and accompanying strong aftershocks (Fig. 8). Compatible with of the cratonic character of the underthrusted Indian plate, MHT is demarcated as a low-angle northeast dipping positive velocity interface at depth of 15-18 km (Fig. 8, red line; Schulte-Pelkum et al., 2005; Rawat et al., 2014). The seismic velocity and resistivity depth sections unambiguously reveal a thin fluidfilled layer immediately overlying the MHT. The Gorkha earthquake with its nodal plane and hypocenter rooted in hydrated detachment appears to be a good case wherein the fluid induced mechanical weakening and friction modulation appears to have contributed significantly to the processes of seismogenesis. In addition, the fluids would effectively reduce the friction coefficient at the MHT, lower friction has been shown to be the requisite to account the on-going convergence rate entirely by slip on the MHT (Cattin and Avouac, 2000; Herman et al., 2010). Accentuation of stresses (slip) at the terminal end of the elongated rupture in conjunction with clustering pattern in aftershock activity favours that static stress field due to the main shock load the pre-stressed lower edge of the MHT to trigger a strong M6.7 aftershock. Another strong earthquake of magnitude M7.3 occurred in close proximity to the fluid enriched ramp. Hypocenters of many aftershocks of this event of May 12, 2015 are located at shallow depth with dips of nodal plane steeper than the detachment (e.g. earthquakes numbered 2, 6, 8 and 9). This seismic plane in Fig 8 (green line) is seen as extension of the hidden out of sequence thrust that following the slope of the ramp has been inferred to rise from the detachment level (Wobus et al., 2005; Sapkota et al., 2013). At the seismogenic depth of about 10 km, pore-pressure is high
12
enough to infiltrate the fluid pulses upward by several km, especially in over pressurized hidden thrust and shear zones (Becken and Ritter, 2012; Connolly and Podlachikov, 2004; Wannamaker et al., 2002). The loading of out-of-sequence thrust by fluid pulses reduce the shear strength of fault planes (Yamada et al. 2015) creating conditions favorable to trigger the large Mw7.3 aftershock at the junction of the fluid-filled ramp and hidden out-of-the sequence thrust. This supports the suggestion of Denolle et al. (2015) that event of M7.3 is not rooted on the plate interface but rather on a sub-parallel fault which has been proposed based on structural/geomorphic/erosion/deformation data (Sapkota et al., 2012;Mugnier et al., 2013; Parameswaran et al., 2015; Bai et al., 2016).
6.
Conclusions
In sum up, the seismotectonic model suggested here to explain the seismic activity in the central Nepal during April-May, 2015 has two principal kinematic components; First, strain resulting from on-going collision between India-Eurasian plates cause large slip on the locked segment of the detachment (MHT) to generate the Mw7.8 Gorkha earthquake. The second component involves the role of hidden out-of-sequence that aligned with crustal ramp structure extends into the shallow depth section of the overriding wedge. Potential of kinematic models incorporating thrusting on MHT and out-of sequence in accounting the crustal shorting across Himalaya remain a contentious issue (Herman et al., 2010). Compatibility of GPS derived rate of convergence with the long term slip rate determined from deformed Holocene terraces around the HFT in Nepal Himalaya support the hypothesis where crustal shortening is entirely taken up by slip along the MHT (Cattin and Avouac, 2000). Nucleation of great and major earthquakes on
13
the locked segment of the MHT is consistent with the above hypothesis. Kinematic model developed to test whether out-of-sequence thrusting can reproduce GPS derived convergence rates required unrealistic large thrusting rate on the MCT (Herman et al., 2010). Therefore, consistent with the observations of Cattin and Avouac (2000), simple model involving thrusting on the MHT is preferred although occasional out-of-sequence thrusting as a source of large earthquake cannot be excluded (Herman et al., 2010). Static stress transfer, proximity of the fluid-rich ramp, injection of high pore pressure fluids may be favourable factors contributing to occurrence of earthquake on the out-of-Sequence thrust, like Mw7.3 Kodari earthquake. Further modelling similar to that initiated by Denolle et al. (2015) and Lindsey et al. (2015) simulating the above hydrological and geometrical factors should be a good future exercise with fresh upcoming geodetic and seismological data sets.
Acknowledgements: We acknowledge with thanks many informal discussions with our colleagues, Vineet Gahalaut, Sumer Chopra, G. Suresh from the National Center of Seismology that provided clarity on many aspects on seismic activity in the Himalaya. The research presented here is funded by the Ministry of Earth Sciences, New Delhi. We appreciate thoughtful comments and suggestions from Roland Burgmann, Guest Editor and an anonymous reviewer, which helped us to improve the original manuscript.
14
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Figure Captions
Figure 1:
Topographic and tectonic map of central Himalaya showing epicenters (yellow stars) of the Mw7.8 Gorkha (April 25, 2015) and Mw7.3 Kodari (May 12, 2015) earthquakes. The Epicenter of the 1934 Nepal-Bihar (red star) and historical (peach stars) 1505 and 1833 earthquakes of M~8 are also marked (adopted from Ambraseys and Douglas, 2004). Red circles: events with 7> M≥ 6; Yellow circles: select 7> M≥ 6 earthquakes; Blue dots: events with 6> M ≥ 4.5 for the period of 1964-2010 (ISC catalogue). Distribution of blue dots represents the level of background seismicity confined to narrow Himalayan Seismic Belt (HSB). The square box marks the area of the central Nepal shown in Figs. 2 and 3.
Figure 2: The distribution of the aftershocks of the Mw7.8 Gorkha earthquake of 25th April 2015 (adopted from USGS Reviewed catalogue; http://earthquake.usgs.gov/earthquakes/search/). Spatialtemporal clustering patterns are highlighted using different colour codes for events before (magenta symbols) and after (green symbols) the strongest Mw7.3 aftershock of May 12, 2015. Lines AA’ and BB’ mark the receiver function profiles, shown in Figs 6a, b respectively. Location of select earthquakes projected on lines AA’ and CC’ are superimposed respectively on RF section in Figs. 6a,b with respect to their distance from the Main Boundary Thrust (MBT). Beach balls in red colour represent Fault Plane Solutions (FPS) for the Mw7.8 Gorkha earthquake and two M>6 aftershocks (Listed at S. Nos. 1, 2, 5 in Table 1). FPS for Mw7.3 Kodari and one M>6 aftershocks listed at S. Nos. 7, 8 in Table 1) are shown as blue beach balls. 26
Red and blue boxes mark the spatial window considered for estimating ‘b’ value and Omori decay curves in Figs. 4 and 5.
Figure 3:
Distribution
of
aftershocks
in
relation
to
slip
distribution
(USGS
Nepal_slides;
http://earthquake.usgs.gov/learn/topics/Nepal_Slides.pdf) demarcating independent ruptures zones of Mw7.8 (April 25, 2015) and Mw7.3 Kodari (May 12, 2015) earthquakes. Spatialtemporal clustering patterns are highlighted using different colour codes for events before and after the strongest Mw7.3 aftershock of May 12, 2015. Clustering pattern in aftershocks immediately following the Mw6.7 (April 26, 2015) and Mw7.3 (May 12, 2015) aftershocks are highlighted independently.
Figure 4:
Gutenberg-Richter frequency-magnitude fit {log10(N) = a-bM} to (a) aftershock sequence following the Mw7.8 Gorkha earthquake, 25th April 2015 and (b) for the Mw7.3 Kodari earthquake of 12th May 2015. In both cases aftershocks falling in the spatial window marked by red and blue boxes in Fig. 2 are used. The total numbers of aftershocks, number of events with varying magnitude range are also listed along side in respective fits. The b-value is estimated using maximum likelihood method of Aki (1965).
Figure 5: p
The modified Omori fit {n (t) = k/(c+t) } to the cumulative numbers of aftershock sequences recorded during (a) the first 17 days after the occurrence of April 25, 2015 Mw7.8 Gorkha
27
earthquake and (b) first 50 days after the occurrence of May 12, 2015 earthquake. In both cases aftershocks falling in the spatial window marked by red and blue boxes in Fig 2 are used. (c) Time plot of cumulative numbers of aftershocks since the occurrence of Mw7.8 Gorkha earthquake April 25, 2015; steep rise corresponds with the occurrence of May 12, 2015 earthquake. Prediction of Omori fit (green curve) obtained based on the data of first 17 days does not explain the sequence after the occurrence of Mw7.3 aftershock on May 12, 2015. The constant k, c and p, which vary between earthquake sequences, are estimated using formulation outlined in Utsu (1961).
Figure 6:
Stack of Receiver Function (RF) images along (a) Profile AA’ and (b) Profile BB’ (modified respectively from Schulte-Pelkum et al., 2005 and Nábelek et al., 2009). See Fig. 2 for profile locations. Horizontal distances are referenced to the Main Boundary Thrust (MBT). The pair of positive (red line) and negative (broken blue in ‘a’ and black in ‘b’) delineate respectively the detachment (MHT) and top of the overlying Low Velocity Layer (LVL). Circles in section (a) indicate hypocenters of select events determined using depth phase (magenta), GCMT solutions (green) and travel time inversion (blue). Short lines on hypocenter location in vertical section (b) depict dip of shallow nodal plane. Numbers refers to S.No. of earthquakes as listed in Table 1. (c) MT inverted electrical resistivity section (modified from Lemonnier et al., 1999). Red curve shows the detachment as mapped by background seismicity where as black line indicates negative velocity interface imaged in (b) and major thrust boundaries of the Himalayan collision belt.
Figure 7: 28
(a) Schematic structural cross-section across the Garhwal Himalaya, (b) MT inverted resistivity section (c) RF images (modified from Rawat et al., 2014). Imaged High Conductivity Layer (HCL) and pair of negative (blue broken line) and positive (red broken) help delineate the detachment and the overlying the Low Velocity Layer (LVL)/HCL. The hypocenters of large magnitude earthquakes including 1991 Uttarkashi (white star) and 1999 Chamoli earthquake (red star) are also superimposed.
Figure 8:
Simplified crustal cross-section across the Nepal Himalaya showing interpreted seismotectonic model for M7.8 Gorkha earthquake and following aftershocks. The blue line represents top of the low velocity/ high conductivity layer (LVL/HCL) indicating a thin fluid-filled layer (blue hatched) immediately above the Main Himalayan Thrust (detachment) identified as positive velocity converter (red line). Hypocenters of main shock and large aftershocks determined using depth phase (magenta), GCMT solutions (green) and travel time inversion (blue and orange) are superposed together with bars representing dip of nodal planes of select events (numbered 1 to 9). Modulation of frictional coupling and mechanical weakening by high-pore pressure fluids facilitate slips to cause main shock (No 1) of April 25, 2015 on the locked segment of the detachment beneath Lesser Himalaya. Static stress change due to the main shock trigger a strong M6.7 aftershock (No 5) where both NE dipping detachment and overlying fluid-filled layer show sharp ramp structure. Large stress drop and increased energy released by the Mw6.7 event facilitate upward propagation of fluids along the hidden out-of-sequence thrust to trigger Mw7.3 earthquake of May 12, 2015 (No. 7). Relatively higher dip of nodal plane (Nos. 2, 6, 9) and
29
alignment of hypocenters (blue circles) of aftershocks of May 12, 2015 favour the extension of hidden thrust rising from the ramp into the shallow overriding wedge (dash red lines).
30
Table
Table 1: Location parameters and GCMT Fault Plane Solution for the April 25, 2015 Gorkha earthquake and a few large aftershocks in central Nepal. S. No
Date
Time
Latitude (°)
Longitude (°)
Magnitude
1
2015-04-25
06:11:58
28.2305
84.7314
7.8
Focal Depth (km) 15.0*
Centroid Depth (km) 12
Dip (°)
Strike (°)
Rake (°)
7
293
96
2
2015-04-25
06:45:29
28.2244
84.8216
6.7
10.0
21
23
308
131
3
2015-04-25
17:42:53
28.2380
85.8290
5.3
10.0
21
40
339
-105
4
2015-04-25
23:16:18
27.7993
84.8715
5.1
13.6
15
40
201
-20
5
2015-04-26
07:09:20
27.7711
86.0173
6.7
18.0*
21
14
289
98
6
2015-04-26
16:26:09
27.8297
85.8650
5.2
14.0
20
26
305
115
7
2015-05-12
07:05:28
27.8087
86.0655
7.2
15.0*
12
11
307
117
8
2015-05-12
07:36:59
27.6250
86.1617
6.2
15.0
20
28
299
116
9
2015-05-16
11:34:14
27.5603
86.0734
5.4
7.0
12
34
324
138
Time, location parameters and magnitude of the listed events are extracted from http://earthquake.usgs.gov/earthquakes/search/ whereas Focal Plane solution including centroid depth are adopted from http://www.globalcmt.org/CMTsearch.html. * Depth adopted from Denolle et al. (2015)
Highlights-Arora-Nepal-JAES
•
Interrelation between electrical resistivity, fluid &
•
Geophysical images of Main Himalayan Thrust and their linkages with Mw7.8 Nepal earthquakes
•
Role of fluids dynamic in seismogenesis.
•
Role of transient stress transfer in triggering earthquakes
31
seismicity in collision zone