Seismicity in the Kingdom of Bhutan ( ): Evidence for crustal transcurrent deformation

Transcription

1 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 111,, doi: /2004jb003087, 2006 Seismicity in the Kingdom of Bhutan ( ): Evidence for crustal transcurrent deformation Dowchu Drukpa, 1,2 Aaron A. Velasco, 1 and Diane I. Doser 1 Received 14 March 2004; revised 24 September 2005; accepted 12 January 2006; published 10 June [1] We investigate moderate-sized historic to recent earthquakes in the Bhutan Himalaya spanning the years from 1937 to We find that few moderate-sized earthquakes occurred in the region during this time period. In order to better characterize the seismicity, we relocate all earthquakes and estimate focal mechanisms for events for which we have adequate data. We use first motion data for older events and waveform model digital seismograms for three earthquakes that occurred in 1980 (M w = 6.4), 1995 (M w = 5.4), and 2003 (M w = 5.4). For the modern events, we utilize three techniques for focal mechanism and depth determination: regional full waveform time domain modeling of the two moderate (M w = 5.4) events, and surface wave spectral method and body wave depth modeling approach for the M w = 6.4 event. We find that the first motion and digital data focal mechanisms are mostly strike-slip, with midcrustal to deep crustal depths. Although no recent great event has occurred along the main Himalayan thrust, the Indian plate is undergoing significant, midcrustal to deep crustal transcurrent deformation, likely due to oblique convergence of the Indian-Asian collision in this region. Unlike oceanic-continent oblique subduction, strike-slip and thrust partitioning occurs throughout the crust, not in distinct zones. The geological and geophysical nature of the Bhutan Himalaya appears significantly different than other Himalayan regions due to the strikeslip nature of many of the events, an out of sequence thrust fault, and the implications of deformation in the Shillong Plateau. Citation: Drukpa, D., A. A. Velasco, and D. I. Doser (2006), Seismicity in the Kingdom of Bhutan ( ): Evidence for crustal transcurrent deformation, J. Geophys. Res., 111,, doi: /2004jb Introduction [2] The high mountains of Himalaya result from the Indian tectonic plate colliding into Asia, a process that began in the late Eocene [Molnar and Tapponnier, 1975; Hodges, 2000]. Deformation resulting from the collision stretches approximately 2500 km between the eastern and western syntaxes, and also affects the region behind the arc with the Tibetan Plateau, the highest plateau in the world. The Himalayan region is one of the most seismically active zones in the world (Figure 1) and is susceptible to great earthquakes. Recent work has highlighted the seismic hazards and the seismic gaps that exist in the region [e.g., Bilham et al., 2001]. Seismicity shows that the region is active with both large and moderate-sized events (Figure 1). Because of a relatively short instrumental record of earthquakes, the distribution of seismicity throughout the Himalayan region appears nonuniform [Ni and Barazangi, 1984]. Figure 1 shows the shallow (<35 km) and deeper (35 km) earthquakes from 1977 to 2002 from the National Earthquake Information Center (NEIC) and the Harvard Centroid 1 Geological Sciences, University of Texas at El Paso, El Paso, Texas, USA. 2 Now at the Geological Survey of Bhutan, Thimphu, Bhutan. Copyright 2006 by the American Geophysical Union /06/2004JB003087$09.00 Moment Tensor (HCMT) catalog. The Tibetan Plateau has shallow and diffuse seismicity, with both strike-slip and normal faulting. Along the syntaxes of the Himalayan front, the seismicity is deeper and shows both thrusting and strikeslip earthquakes. Along the Himalayan front, including Nepal and areas to the west, the seismicity appears both shallow and deep, and tends to be clustered. The events are thrust, which should be expected given the convergent nature of this collision zone. [3] The Kingdom of Bhutan is a small country situated in the eastern portion of the Himalayas between Tibet (China) to the north and India to the south (Figure 1). In contrast to the western and central portion of the Himalayas, the seismotectonics of the Bhutan Himalayas are virtually unknown. Unlike other portions of the Himalayas, the Bhutan region appears to have been seismically inactive at the magnitude >6.5 level for at least the past 60 years. However recent studies of monastery records within Tibet and Bhutan suggest that a large (M w 7.6) earthquake occurred in the region in 1713 [Bilham et al., 2001] and GPS/geodesy studies indicate that a slip potential of 4 m (corresponding to a M w > 8 earthquake) exists along this portion of the Himalayan seismic belt [Bilham et al., 2001]. [4] Our study focuses on the relocation of moderate-sized earthquakes (M > 4.5) occurring in the Bhutan region between 1937 and 1998, determination of earthquake focal mechanisms when sufficient first motion data were avail- 1of14

2 Figure 1. NEIC ( ) seismicity of the Himalaya region for shallow (<35 km, open circles) and deeper (>35 km, solid circles) earthquakes. Also shown are the Harvard centroid moment tensor (CMT) solutions from 1977 to Bhutan region is indicated by rectangle. able, and waveform modeling of digital seismograms for three earthquakes of M w > 5.0 occurring in 1980, 1995, and Our results suggest that although no recent event has occurred along the main Himalayan thrust, the Indian plate is undergoing significant, midcrustal to deep crustal transcurrent deformation due to the Indian-Asian collision. This study provides a seismotectonic framework in which to interpret seismic data that was collected by a temporary network that was operating in Bhutan from January 2002 to March 2003 [Velasco et al., 2003]. 2. Tectonic Setting and Seismicity [5] The Himalayan arc system is formed as result of continent-continent collision between the Indian plate and Eurasian plate that began in the late Eocene time [Molnar and Tapponnier, 1975; Hodges, 2000]. From south to north, the Himalayan region consists of five major shear zones: the Main Frontal Thrust (MFT), the Main Boundary Thrust (MBT), the Main Central Thrust (MCT), the South Tibetan Detachment System (STDS), and the Indus-Tsangpo Suture Zone (ITSZ), each of which span the entire 2500 km long Himalayan arc. The Bhutan Himalaya contains all of these features but is the least studied section of the Himalaya (Figure 2). Limited geological and tectonic information exists from the work of the Geological Survey of Bhutan, the Geological Survey of India, Gansser [1983], and the recent work of Grujic et al. [1996] and Grujic et al. [2002]. The presence of an out of sequence thrust, the Kakhtang thrust (KT) [Grujic et al., 1996; Grujic et al., 2002] located between the MCT and the STDS (Figure 2), and a klippen of low-grade sedimentary rocks in the midst of the Greater Himalayan Sequence (GHS) are two of the prominent differences between the Bhutan Himalaya and western section of the Himalayas. The Kakhtang thrust in Bhutan could be a structure on which late shortening has led to the uplift of the Khula-Kangri and Kanga Punsum area [Edwards et al., 1996]. [6] The MCT forms a large embayment in eastern Bhutan called the Shumar-Kuru Chu spur (Figure 2). This spur is believed to be equivalent to the large Darjeeling spur in the Sikkim Himalaya [Gansser, 1983] located just west of Bhutan. Both spurs show intense internal structural complications and plunge northward, perhaps reflecting an old but reactivated structural alignment of the Indian shield [Gansser, 1983]. The MBT and MCT define the edges of the Lesser Himalaya, a zone of complex folds and thrusts formed by the pushing of the orogenic core out over the stable foreland. The Lesser Himalaya in Bhutan is narrower and more discontinuous than in other parts of the Himalayan arc [Grujic et al., 2002]. [7] Seismicity in relationship to the major faults in the region has been examined by numerous studies. Most of the Himalayan earthquake epicenters lie along a relatively narrow belt, approximately 50 km wide, between the MCT and the MBT (Figure 1) and are concentrated just south of the MCT [Ni and Barazangi, 1984]. Seeber and Armbruster [1981] suggested that two prominent seismic belts in the Himalayan region are associated with two distinct seismogenic faults, the detachment faults and the steeper dipping faults of the MCT and the MBT. A study conducted in the Nepal Himalaya and vicinity show that most of the seismic activity is associated with the MBT [Gupta and Singh, 1980]. [8] Past studies, as well as Harvard CMT solutions, indicate that most earthquakes along the Himalayan region have fault plane solutions that represent predominantly thrust faulting (Figure 1) [Fitch, 1970; Baranowski et al., 2of14

3 Figure 2. Seismicity of the Bhutan region from the NEIC for the years from 1923 to KT, Kakhtang Thrust; MCT, Main Central Thrust; MBT, Main Boundary Thrust; STDS, South Tibetan Detachment System. 1984; Searle, 1996; Chen and Kao, 1996]. Baranowski et al. [1984] found that slip vectors are locally perpendicular to the mountain ranges with steeper plunges (25 ) in the western Himalaya, in contrast to shallow plunges of 10 in the eastern Himalaya. This may reflect deformation of the overriding thrust plate or steepening of the main underthrusting zone beneath the Greater Himalaya in the west and underthrusting of the Indian plate beneath, at least, the Lesser Himalaya in the eastern section [Baranowski et al., 1984]. [9] The presence of strike-slip and normal faulting has also been reported in the Himalayan region [Chen and Kao, 1996]. Strike-slip faulting becomes more frequent near the eastern and western Himalayan syntaxes (Figure 1). Gupta and Singh [1980] determined that the presence of normal and strike-slip faulting in the region could be a consequence of faults and folds trending normal and oblique to the present-day direction of deformation. A more recent study in the Sikkim province west of Bhutan identifies pervasive crustal seismicity and strike-slip events in the midcrust along the Goalpara lineament [De and Kayal, 2003], a linear topographic feature extending from southwest Bhutan to the Sikkim province. A general plot of earthquake data from obtained from the NEIC catalog for the Bhutan region shows more concentrated seismicity on the southeastern and southwestern borders of Bhutan, primarily within the Lesser Himalaya (Figure 2). Hypocentral depths of almost all events have been reported in the range of km. However, three Harvard centroid moment tensor (CMT) solutions for events in the vicinity of Bhutan show strike-slip faulting at depths of km (Figure 2), thus highlighting the amount of strike-slip faulting occurring in the region. [10] GPS measurements [Bilham et al., 2001] have determined 20 ± 3 mm/yr of convergence between India and southern Tibet. A region about 50 km wide within southern Tibet absorbs 80% of the convergence, while the remaining 20% is accommodated within the surrounding region (essentially a slip potential of 2 m per century) [Bilham et al., 2001]. The convergence directions in Bhutan appear to be directed northeast-southwest determined from GPS studies measured at Lhasa, China, north of the Bhutan border [Larson et al., 1999]. Thus a small amount of oblique convergence should be expected. [11] Space and time patterns of seismicity have led many authors to propose the existence of seismic gaps [e.g., Seeber and Armbruster, 1981; Satyabala and Gupta, 1996; Bilham et al., 1998, 2001] along the Himalayan collision zone. An important factor in determining the present seismic slip potential within the Bhutan Himalaya is the location of the 1897 Shillong earthquake rupture zone. Gahalaut and Chander [1992] and Bilham and England [2001] suggest that the 1897 event did not occur along the Himalayan detachment plane, called the Himalayan Sole Thrust (HST). In fact, Bilham and England [2001] suggest the event produced 18 m of slip along a reverse fault dipping steeply away from the Himalayas, forming part of a pop-up structure. Furthermore, the seismic moment release along the HST over the past 100+ years is 25% lower [Bilham and Gaur, 2000] than previously estimated. One region identified as having a minimum potential slip of at least 4 m [Bilham et al., 2001] is the Shillong plateau located just south of Bhutan. However, the impact of the uplift of the Shillong plateau on the slip potential in Bhutan is poorly understood. 3. Analysis of Seismicity ( ) [12] We have undertaken a comprehensive study of the seismicity of Bhutan from the late 1930s to 2003 using regional and teleseismic arrival times, first motion and waveform data to determine earthquake locations and source parameters. This study represents the first comprehensive analysis of earthquakes for the Bhutan region and 3of14

4 Table 1. Earthquakes in the Study Area Before and After Relocation With Corresponding Depth, Magnitude, Ellipse Angle, and Major and Minor Axis After ISC Location Relocation Event Date Time, UT Mag m b Depth, km Lat Lon Lat Lon Angle Major Axis Minor Axis 1 31 Mar : Feb : Jan : Jan : Jan : Jan : Feb : Feb : Aug : Aug : Feb : Jul : Feb : Sep : Nov : Nov : Jan : Jul : Nov : Feb : Apr : Aug : Jan : Dec : Oct : Jul : Dec : Apr : Jul : Aug : will provide a framework for ongoing analysis of data collected by a temporary network operating from January 2002 to March 2003 [Velasco et al., 2003]. The following sections describe the analytical techniques used in this study Relocations [13] Earthquake catalogs from both the National Earthquake Information Center (NEIC) and International Seismological Centre (ISC) do not show any major earthquakes within the Bhutan Himalaya during the last century (Figure 2). The largest earthquake occurred on January 21, 1941 with a magnitude 6.75 (Figure 2 and Table 1). Aside from this event, most other events have magnitudes ranging between 4 and 6.5 as shown in Table 1. From Figure 2, we see events concentrated within the Sub-Himalayan and Lesser Himalayan in the southern part of the country. Since the earthquakes are not relocated, the locations reported to the ISC and NEIC may not be reliably correlated to major faults. The hypocentral depth of almost all the earthquakes reported to the ISC and NEIC range between 20 and 35 km. Harvard centroid moment tensor (HCMT) solutions for some of the recent earthquakes in the Kingdom of Bhutan and vicinity show strike-slip faulting and deeper hypocentral depths (35 44 km). In order to better determine epicentral locations and depths, we relocate historic events. [14] We use a bootstrap relocation technique [Petroy and Wiens, 1989] for relocating historic earthquakes in Bhutan. This technique is designed for relocating historic events or events in regions with poor station coverage. Because most of the events in the Bhutan region have poor station coverage, especially the events that occurred prior to the establishment of the World Wide Standardized Seismographic Network (WWSSN), the bootstrap technique is a suitably robust method for the relocations. The bootstrap method estimates the traveltime error by using an original set of traveltimes and iteratively generates new locations by resampling/replacement of the original traveltime data. The resulting cloud of relocations is used to generate a 90% confidence error ellipse. While this technique has a wide range of applications, it has limitations. The bootstrap technique fails when the sample size is too small (<10) and when distributions have infinite moments. This procedure is also computationally intensive. [15] We selected earthquakes of M > 6 occurring between 1937 and 1963 and of M > 4.0 occurring since 1964 for our relocation studies. This range of magnitudes was selected in an effort to insure sufficient phase data for relocations. P and S wave traveltimes were then collected from the International Seismological Summary (ISS) (pre-1964) and International Seismological Centre (ISC) (post-1963). Depths for the pre-1964 events in Table 1 are assumed for carrying out the relocation process. Most of the events that occurred prior to the WWSSN (pre-1964) have poor station coverage, and in some cases where less than 10 stations recorded the event, the particular event could not be relocated. Overall a total of 30 events have been relocated (Table 1). Each event was relocated 100 times and the resulting cloud of earthquakes was used to determine a 90% confidence error ellipse that defines the uncertainty in the 4of14

5 Figure 3. Error ellipses (90% confidence) for relocated epicenters (see Table 1). Major tectonic features of the Bhutan Himalaya [see Grujic et al., 2002] and relocated earthquakes. KT, Kakhtang Thrust; MCT, Main Central Thrust; MBT, Main Boundary Thrust; STDS, South Tibetan Detachment System. relational procedure (Table 1 and Figure 3). Only 10 earthquakes shifted more than 50 km from their original location (Table 1). This implies that the ISC location is generally well constrained. [16] Events with smaller 90% error ellipses indicate more confidence in the location and are attributable to better station distributions whereas relocated events that have bigger error ellipses indicate that the epicenter locations are poorly constrained. Events that occurred prior to 1964 have larger major and minor axes, and therefore larger error ellipses (Table 1). In the case of events 1, 2, 3, 6, and 7 the error ellipse is too large to be shown in Figure 3. Most error ellipses have their major axes aligned in the northeastsouthwest direction implying greater uncertainty in that direction. This northeast-southwest orientation of the error ellipse is most likely a factor of the greater number of stations in Europe. Also, some of the error ellipses have east-west orientation and this could be attributable to stations from India and the former Soviet Union. [17] The relocated epicenters (Figure 3) concentrate within the same three regions (eastern border, Shumar-Kuru Chu spur region, western border) as events from the NEIC catalog (Figure 2), suggesting these regions have remained seismically active throughout the past 60+ years. A coherent relationship cannot be established between the seismicity and the major fault systems in the study area due to the limited amount of data, and the lack of depth control for the event relocation (Figure 3). However, there is a concentration of earthquake epicenters in the east central and the southeastern part of the country, where the largest magnitude earthquake in the Bhutan region (the 1941 event) occurred Focal Mechanisms and Depth [18] In order to define the nature of deformation in the Bhutan Himalaya, we determine the focal mechanisms of both historic and recent events. Since we have a limited amount of high-quality waveform data for events prior to 1980, we use first motion data and a grid search technique for the older events. We then use surface wave spectral inversion and time domain waveform modeling to determine source parameters for more recent events Analysis Using First Motion Data [19] Focal mechanisms of twelve earthquakes (Table 2) are determined by means of first motion polarities using the ISC listings and the grid search technique of Whitcomb et al. [1973]. The mechanisms for the rest of the events could not be evaluated due to emergent nature of the first motion. Focal mechanism results of focal mechanisms from the first polarity studies are shown in Figure 4. Nearly all mechanisms are considered to be strike-slip (B axis plunge >50 ) following the classification scheme of Frohlich [1992]. The presence of predominantly strike-slip motion in the current study area is not consistent with the geologically mapped major thrust faults in the region and is in contrast to thrust faulting seen to the west. This could reflect a change in the stress field near the eastern syntaxis. Alternatively, this could be the result of localized changes due to the Sikkim and Shumar-Kuru Chu spurs. Ni and Barazangi [1984] suggested that a strike-slip mechanism in the region where one of the nodal planes is trending transverse to the Himalayan structural grain and has similar trend to the Yadong-Gulu rift could represent a possible relationship between transverse structural features in the underthrusting Indian plate and the upper Himalayan blocks and Tibet. 5of14

6 Table 2. Results of Focal Mechanism Solutions From First Motion Polarities Nodal Plane 1 Nodal Plane 2 T Axis P Axis Event Strike Dip Rake Strike Dip Rake Strike Plunge Strike Plunge Figure 4. Lower hemisphere projections of first motion data and focal mechanisms for Bhutan earthquakes. Numbers in brackets denote event numbers. Impulsive first motions are indicated by dots (solid dots are compressions), emergent first motions are indicated by squares. Solid triangles denote best P axis orientations. Contour lines indicate uncertainty limits on P axis orientations assuming one good quality, and two lesser quality readings are in error as determined by the grid search computer program FOCPLT [Whitcomb, 1973]. 6of14

7 Figure 5. Focal mechanisms from first motion data (small focal spheres) and waveform modeling (larger spheres). Event numbers are keyed to Tables 1 and 2. Small arrows indicate strike of P axes. Bold arrow at Lhasa indicates direction of plate convergence from GSP studies [Larson et al., 1999]. [20] The strikes of the P axes show variability across region (Figure 5). In the western border region, P axes trend east-west (events 16, 19, and 21) perpendicular to the Darjeeling spur. P axes for event 15 in central Bhutan are directed northwest southeast. In the Shumar-Kuru spur region, there appears to be a transition from NW (events 13 and 30) to NE (event 29) orientation. In the eastern border region (events 14, 18, and 23), P axes strike northsouth to northeast-southwest. The NW-SE strike direction is comparable to the direction of GPS studies measured at Lhasa, China [Larson et al., 1999] Spectral and Time Domain Waveform Modeling [21] We estimate depth and moment tensors for three moderate sized events that have occurred in Bhutan since We obtained long-period digital data from the Incorporated Research Institutions for Seismology (IRIS) Data Management Center (DMC) for Global Seismic Stations (GSN) and Chinese Digital Station Network (CDSN) stations. The data include events that occurred on 19 November 1980 (M w = 6.4), 17 February 1995 (M w = 5.4), and a recent event on 25 March 2003 (M w = 5.4). Although CMT solutions exist for these events, we wish to apply more accurate velocity models for the region in order to obtain more precise depth estimates and focal mechanisms. We apply two techniques that are based on the size and station coverage of each event. We rely on time domain modeling for regionally recorded events (1995 and 2003), and a surface wave spectral technique for the larger teleseismically recorded event (1980). [22] For regional time domain waveform modeling, we calculate reflectivity Green s functions [e.g., Kennett, 1983; Randall et al., 1995]. We investigated several velocity models, but settled on a velocity model that was developed from surface waves for the Tibetan Plateau [Romanowicz, 1982]. Synthetic seismograms were generated based on the three fundamental faults: vertical strike slip, vertical dip slip, and 45 dip slip [Langston, 1981]. We then invert for the five moment tensors necessary for describing the deviatoric and compensated linear vector dipole (CLVD) of a fault system [e.g., Langston, 1981]. We invert for a suite hypocentral depths, and select the optimum hypocentral depth with the minimum RMS. [23] For the surface wave spectral technique, we analyze long-period ( s) Rayleigh and Love waves. We invert the spectra using a moment tensor inversion developed by Kanamori and Given [1981], modified to a two- 7of14

8 Figure 6. Normalized error (r) versus depth for (left) Rayleigh waves and (right) a joint Rayleigh and Love wave inversion for the 1980 event. Note that Love waves are not sensitive to depth. step method. In the first step, we estimate the rupture duration, and in the second step, we estimate the moment tensor and depth [Romanowicz and Guillemant, 1984; Zhang and Kanamori, 1988a, 1988b]. Given the poor azimuthal coverage for the 1980 event, we do not search for an optimal centroid location. We consider a suite of Earth models and assumed source locations to estimate the confidence bounds of the source parameters. For attenuation models, we use the models of Masters and Gilbert [1983] (MG), Dziewonski and Steim [1982] (DS), and Dziewonski and Anderson [1981] (PREM). For the source velocity structure, we considered two standardized earth models (PREM and a model by Regan and Anderson [1984] (RA)). We also computed the excitation functions for a model obtained by Li and Mooney [1998] for the Tibetan Plateau. This model is a simple one-layer average crustal model that is placed on top of the PREM mantle (referred to as CHINA-AVG). The range of results found for different models used is combined with formal inversion uncertainties to approximate our uncertainties in source parameters Teleseismic Body Wave Depth Modeling [24] Since depth uncertainties are large for the above techniques, we model teleseismic body waves for depth phases. Since the body waves are complex due to nearsource crust structure complexities such as dipping structures, we forward model teleseismic body waves by computing simple teleseismic body waves using the a ray theory approximation approach [e.g., Langston, 1984] for a suite of depths. We use a velocity structure with a 60 km thick crust and a P wave velocity structure of 6.5 km/s over a mantle of 8.1 km/s. The synthetics include depth phases pp and sp, so that comparison to the teleseismic data can be used to distinguish depth phases. We then directly compare the results to data and compute a misfit. From this, we can distinguish the depth phases and thus confidently determine depth The 1980 (M w = 6.4) Event [25] For time domain modeling, only one station (KAAO) lies at a regional distance, and thus we choose to use the teleseismic surface wave inversion. In the first step of the surface wave inversion, we invert the phase for duration assuming a simple trapezoid-shaped source function, which can be parameterized in terms of its duration (t). Given the relatively small size of this event, we find a very short duration, between 1 5 s. Since this essentially is a point source solution, we do not consider source finiteness for this event. We estimate the depth and moment tensor in the second step of the inversion from the long-period ( s) surface waves. We investigate a suite of excitation and attenuation models and plot the weighted RMS error (r) [Zhang and Lay, 1989] versus depth for Rayleigh waves and the joint Rayleigh and Love wave inversion (Figure 6). The joint inversion produces a flatter curve since Love waves have little depth resolution. Using Rayleigh waves alone, we found a deep source depth of 55 ± 10 km for the combinations of earth excitation and attenuation models appropriate for this region. For the joint inversion the source depth estimate is approximately 65 ± 20 km (Figure 5). We obtain a good fit to the spectra at 200 s (Figure 7) but find considerable scatter at longer periods due to the relatively small size of the event for this technique. Thus, in our final inversion, we down weight the longer-period information. 8of14

9 Figure 7. (a) Final fit for the joint Rayleigh and Love wave inversion at 200 s. (b) Best double-couple solution from the Harvard CMT solution and our inversion results. The best double-couple focal mechanism has a strike of 217 ±10 ; dip of 52 ±10 ; and rake of 211 ±10 with a 20% nondouble-couple component. This is an oblique strike-slip solution. The moment estimate is dyn cm (M w = 6.4), which is comparable to the Harvard CMT value of dyn cm (M w = 6.4). Our solution is deeper than the CMT depth of 45 km, but both have similar focal mechanisms. [26] We confirmed our depth result with the timing of pp and sp, of which pp occurs about s after the direct P wave (Figure 8). We forward model short-period teleseismic stations ANTO (D = 47.7 ; Az = 300 ), BCAO (D = 70.7 ; Az = 265 ), CTAO (D = 72.8 ; Az = 124 ), GRFO (D = 62.0 ; Az = 314 ), KONO (D = 61.5 ; Az = 325 ), and MAJO (D = 42.3 ; Az = 65 ) using the focal mechanism and moment determined above. Note the complexity of the data waveforms, which likely result from the complex source region structure. Although the waveforms are complex, we see that a solution between 50 and 60 km matches most of the waveforms. We calculate a total misfit from differencing the observed and synthetic, and find that the 50 and 60 km misfit are virtually identical. Thus we conclude that this event occurred within the Indian plate The 1995 (M w = 5.2) Event [27] The 1995 event has adequate azimuthal coverage with six regional stations (CHTO, KMI, WMQ, NIL, LZH, HYB). We obtain an optimum hypocentral depth of 34 km with a minimum misfit error of 0.33, and a focal mechanism of strike of 245, dip of 87, and rake of 27. This is a strike-slip solution and a good fit to the data (Figure 9). The CMT solution gives a hypocentral depth of 35 km for this particular event. The scalar moment magnitude is dyn cm (M w = 5.2) with a CLVD component of 4%. This moment is slightly lower than the Harvard CMT scalar moment of dyn cm (M w = 5.4). [28] With this moderate-sized event, teleseismic waveforms tend to exhibit low signal to noise for the P wave, especially for a strike-slip event where the takeoff angles of the P waves are near the nodes of the focal sphere. We tested our depth result with only one teleseismic waveform 9of14

10 Figure 8. Depth forward modeling for the 1980 event. We use the focal mechanism and moment determined from the surface wave spectral modeling. Note the complexity of the data waveforms, demonstrating the impact of the complex source structure. We forward model short-period teleseismic stations ANTO (D = 47.7 ; Az = 300 ), BCAO (D = 70.7 ; Az = 265 ), CTAO (D = 72.8 ; Az = 124 ), GRFO (D = 62.0 ; Az = 314 ), KONO (D = 61.5 ; Az = 325 ), and MAJO (D = 42.3 ; Az=65 ). Synthetic waveforms for depths km have the minimum misfit. that had good (>3.0) signal to noise for the P wave arrival. We forward modeled broadband station ANTO (D = 50.2 ; Az = 300 ) using the focal mechanism and moment determined above, and find that a depth of approximately 40 km matches the waveform in timing and amplitude (Figure 10). Again, the data waveform exhibits complexities beyond our simple synthetic waveform, but we can see a strong pp arrival within this complexity. Thus we confirm that this event occurred in the midcrust The 2003 (M w = 5.4) Event [29] The 2003 event has excellent azimuthal coverage with six regional stations (LSA, KMI, WMQ, ENH, XAN, NIL), although some of the stations used for the 1995 event were unavailable or not running. We obtain an optimum hypocentral depth of 35 km with a minimum misfit error of 0.12, and a focal mechanism of strike of 135, dip of 69, and rake of 183 (Figure 11). Like the 1995 event, this event is strike-slip and occurs a similar depth. The CMT solution gives a hypocentral depth of 55 km for this particular event, which is deeper than what we obtain. The scalar moment magnitude is dyn cm (M w = 5.2) with a CLVD component of 6%. This moment is slightly lower than the Harvard CMT scalar moment of dyn cm (M w = 5.4). 10 of 14

11 Figure 9. Waveform modeling results for the 1995 earthquake. (left) Depth and focal mechanism estimates plus the Harvard CMT mechanism. (right) Final fit for both vertical and transverse components for the time domain inversion. [30] As with the 1995, the moderate-sized event creates low signal-to-noise rations for the P waves. We confirmed our depth result with one teleseismic waveform that had good (>2.0) signal to noise for the P wave arrival. We forward modeled broadband station HIA (D = 32.8 ; Az = 38.8 ) using the focal mechanism and moment determined above, and find that a depth of approximately 30 km matches the waveform in timing and amplitude (Figure 12). We can identify a strong pp arrival, and thus we confirm that this event occurred in the midcrust. 4. Discussion [31] Our results demonstrate that there is significant midcrustal and deep crustal strike-slip deformation occurring within the Indian crust, a unique result for continentalcontinental collision in the Bhutan Himalayan orogen. Figure 13 shows a schematic diagram of the location of the three modeled events in relation to the geologic structure of Bhutan. The generalized crustal structure is based on the gravity profile work of Cattin et al. [2001] and the geology profiles obtained by Grujic et al. [2002]. The 1980 event occurred at the base of the Indian crust. The 1995 and 2003 events have similar size, focal mechanisms, and midcrustal depths. Since their epicenters are over 200 km apart, they are not necessarily related to juxtaposed faults. However, their similarity in depth, size, and mechanism suggests that Figure 10. Data and synthetic vertical waveforms for station ANTO for the 1995 event. The synthetics were calculated using the focal mechanism and moment determined from the regional waveform modeling. A depth of 40 km best matches this waveform, confirming our regional waveform modeling result that this event occurred at midcrustal depths. 11 of 14

12 Figure 11. Waveform modeling results for the 2003 earthquake. (left) Depth and focal mechanism estimates plus the Harvard CMT mechanism. (right) Final fit for both vertical and transverse components for the time domain inversion. the process of strike-slip faulting is an important component of deformation in this region. Furthermore, our first motion mechanism results also show the extent and dominance of this strike-slip process in the region. [32] The presence of predominantly strike-slip motions in the study area is not consistent with the geologically mapped major thrust faults of the region and is in contrast to thrust faulting seen to the west. However, deep strike-slip faulting has been previously documented by Chen and Kao [1996] in the Himalaya west of this region. Ni and Barazangi [1984] suggested that a strike-slip mechanism in the Himalaya could represent a possible relationship between transverse structural features in the underthrusting Indian plate and the upper Himalayan blocks and Tibet. Interestingly, GPS measurements made at Lhasa to the north of Bhutan show a deflection from pure convergence [Larson et al., 1999], indicating that the convergence in Bhutan is oblique. The idea of slip partitioning has been suggested for many years [e.g., Fitch, 1970], and these moderate-sized earthquakes accommodate the strike-slip component of the oblique convergence. Furthermore, unlike oceanic-continent oblique subduction [e.g., Fitch, 1970] where strike-slip faults occur behind a subduction zone, we are documenting midcrustal to deep crustal strike-slip partitioning that must be occurring without mapped significant strike-slip faults. The thrust component of convergence is accommodated for Figure 12. Data and synthetic vertical waveforms for station HIA for the 2003 event. The synthetics were calculated using the focal mechanism and moment determined from the regional waveform modeling. A depth of 30 km best matches this waveform, confirming our regional waveform modeling result that this event occurred at midcrustal depths. 12 of 14

13 Figure 13. Schematic illustrating the location of the 1980, 1995, 2003 earthquakes in relation to the Indian and Asia plates based on gravity profiles [Cattin et al., 2001] and geology [Grujic et al., 2002]. The location of the 1980 event appears to be on the bottom of the Indian plate, while the 1995 and 2003 events occurred in the mid crust. KT, Kakhtang Thrust; MCT, Main Central Thrust; MBT, Main Boundary Thrust; STDS, South Tibetan Detachment System; HST, Himalayan Sole Thrust. during large great earthquakes in the region, discussed below. [33] Large earthquakes are not uncommon in the Himalayan collision zone, and many studies have concluded the existence of current seismic gaps along the Himalayan collision zone [e.g., Seeber and Armbruster, 1981; Khattri, 1987; Khattri and Tyagi, 1983; Bilham et al., 1998, 2001]. The only great earthquake thought to have occurred in the Bhutan region was in However, Bilham and England [2001] concluded that 1897 Shillong earthquake occurred on a reverse fault dipping steeply away from the Himalaya, not one of the major faults such as the MFT, MBT or MCT. The location of the 1897 event also influences the average seismic moment calculation for the Himalayan region [Bilham and Gaur, 2000]. Thus Bilham and England [2001] reported a possible lower seismic risk in the Bhutan Himalaya that other regions in the Himalaya. Furthermore, they suggested that the absence of slip beneath the Bhutan Himalaya during the 1897 event indicates a 400 km seismic gap between the 1934 and 1950 great earthquakes. Slip rate calculations show that the faults bounding the Shillong plateau could absorb one third of the convergence rate of the Himalaya and this consequently increases the recurrence period between great earthquakes in the Bhutan Himalaya [Bilham and England, 2001]. Similarly, the significant transcurrent deformation we report may also be relieving some stress on the main Himalayan thrust, thus elongating any recurrence interval. [34] Two distinct models exist for the ongoing deformation of the Himalayas. Hauk et al. [1998] utilize observations from the INDEPTH profile and attribute the high mountains to accommodation of the Tibetan plate as it pushes over a crustal ramp in the HST. This model essentially calls for brittle behavior of the entire crust under Bhutan. Grujic et al. [2002] utilize geologic data from Bhutan and attribute the high relief to the southward extrusion of high temperature ductile rock within a channel from midcrustal depths. This model predicts ductile deformation and thus little seismicity within the ductile, midcrustal layer. Jackson [2002] reported that focal depths of earthquakes and gravity anomalies on the continents suggest that seismic activity is prevalent only within the seismogenic upper and lower crust. In this study, however, we identify strike-slip events at midcrustal and deep crustal depths. Thus our results are inclusive regarding the models that exist for the high relief of the Bhutan Himalaya. In the Sikkim province adjacent to Bhutan, De and Kayal [2003] also identify seismicity that occurs throughout the crust and strike-slip events in the midcrust along the Goalpara lineament. Thus significant transcurrent deformation is occurring in Bhutan that is not currently accounted for in any of the previous orogenic models. [35] The seemingly low seismic activity in the Bhutan region, as compared to other parts of the Himalayas, could be due to a lack of local seismic stations. Alternatively, the Shillong plateau, which can accommodate convergence between the India and Eurasia continental plates may play a role, as suggested by Bilham and England [2001]. Another possibility is the Bhutan region s comparatively lower elevation imparts a smaller gravitational load than the topography in the western and central Himalayas. Further, analysis of data from a temporary seismic network in Bhutan operating from January 2002 to March 2003 will help in determining the current seismicity and style of deformation [Velasco et al., 2003] and assist in testing among these hypothesis. 5. Conclusion [36] We studied moderate-sized earthquakes occurring in the Bhutan region between 1937 and We determined earthquake focal mechanisms using first motion data where available, and waveform modeling of digital seismograms for three earthquakes of M w > 5.2 occurring in 1980, 1995, and Our results suggest that although no recent event has occurred along the main Himalayan thrust, the Indian plate is undergoing significant, transcurrent deformation throughout its depth extent, likely due to oblique convergence of the Indian-Asian collision in this region. 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(velasco@geo.utep.edu) D. Drukpa, Geological Survey of Bhutan, P.O. Box 173, Thimphu, Bhutan. 14 of 14