Abstract
A set of short aperture seismic arrays was operated in the Garhwal Himalaya close to the Main Central Thrust and recorded a large number of small local earthquakes. This study pertains to the inversion of the body wave arrival time data of these earthquakes to construct a seismic velocity model for the region. The analysis indicates a systematic variation in the P-wave velocities of the upper crustal rocks. We estimate (i) signi®cantly lower seismic velocities within the Middle-Lesser Garhwal Himalaya, and (ii) higher seismic velocities in the interface zone between the Middle-Lesser and Higher Garhwal Himalaya. Seismic activity is mostly con®ned to a relatively narrow north-east dipping zone in the upper 4 km of the crust characterized by a relatively higher P-wave velocity. This active seismicity represents reverse thrusting along steep north-easterly dipping parallel slip surfaces within this zone forming a ramp in the crystalline formations of Higher Himalaya. The lower velocity zone exhibits a low level of seismicity but appears to be associated with an increased landslide hazard.q2001 Elsevier
Science Ltd. All rights reserved.
1. Introduction
From seismological considerations, the Garhwal segment of the Himalayan mountain chain is distinctive. Located within a seismic gap (Khattri and Tyagi, 1983; Khattri, 1987), small and moderate magnitude earthquakes and severe landslides have frequently occurred here in recent times. In contrast, a total absence of large earthquake occur-rence for several decades makes the region a possible zone of high seismic risk. For that reason implementation and construction of hydroelectric power projects of the Garhwal Himalaya is beset with controversy.
In an effort to shed light on some of these issues, by quantitatively assessing the seismicity and possible seismic hazards, a systematic investigation of microseismicity was undertaken in phases between December 1979 and June 1990 along the entire Garhwal Himalayan mountain chain, in the vicinity of the Main Central Thrust (MCT). The details of this investigation and the ensuing results have already been published in the literature (Gaur et al., 1985; Khattri et al., 1989; Sarkar et al., 1993). The following uncertainties about the data and interpretations presented in those studies may be recalled. (1) In the absence of an appropriate regional travel time table or seismic velocity
model, a homogeneous semi-in®nite earth medium has been assumed for all hypocentral parameter estimations. (2) For operational reasons, the time scale on analog seismic records was compressed so that the records could be chan-ged less frequently. As a result for all earthquakes occurring within the recording array, while the errors of reading of the arrival times of the direct P-phases were at the most 0.2 s those for the direct S-phases were often more than 1.0 s (3) Even for earthquakes whose epicentral distances exceeded the crossover distance for the ®rst crustal layer in the region, the ®rst P-phases had to be considered as direct due to the constraints of the assumed earth model. These limitations resulted in errors in estimates of epicentral locations and focal depths, greater than 1.5 and 6.0 km respectively, for at least 25% of all considered earthquakes.
For the present study, we winnowed the entire data set and selected data for only those earthquakes which had (1) occurred within or close to the recording arrays (2) been simultaneously recorded by at least four recording stations and (3) from previous estimations, focal depths in 0±20 km range and estimated errors of hypocentral locations within 1.0 km. Only 166 earthquakes could meet these constraints. The locations of these earthquake epicentres and the record-ing stations, which provided the related seismic wave arrival time data, are shown in Figs. 1 and 2.
Our study is in two parts: (1) inversion of selected P and S
1367-9120/01/$ - see front matterq2001 Elsevier Science Ltd. All rights reserved. PII: S 1 3 6 7 - 9 1 2 0 ( 0 0 ) 0 0 0 2 3 - 7
arrival times for simultaneous estimation of hypocentral parameters of the earthquakes and the local P and S wave velocity values and (2) re-interpretation of ®rst P-motion data.
2. Arrival time data
2.1. Method of analysis
We used Thurber's (1983) program in three successive iterations, in a manner explained below, to invert the arrival time data for simultaneous and improved estimations of hypocentral location and earth model parameters. The input data of the ®rst iteration comprised of (1) an assumed one dimensional two layered earth model with constant P
and S wave velocity values in each layer as estimated from previous studies (Chander et al., 1986; Kumar et al., 1989; Salam, 1988; (Fig. 3) and (2) the hypocentral location para-meters and arrival time data pertaining to the selected 166 earthquakes (Sarkar, 1983). The earthquake epicenters delineate a generally NW±SE oriented belt, well de®ned in its central part over approximately (60£36)2 area but diffuse at its two ends (Fig. 2).
During the ®rst iteration we considered a (60£36£20)
km3 volume of the earth subsurface directly below the
epicentral zone and divided it into 588 rectangular elements,
each having (10£6£2) km3 volume and P and S wave
velocity values at its eight nodes as speci®ed by the assumed earth model. The inversion results from this ®rst iteration were selectively used in the following manner as input for the second iteration. Of the 166 earthquakes, revised
epicenters of 138 were located within the 60£36 km2area. These 138 epicenters delineated a revised NW±SE directed
seismic belt whose central part, covering (42£30) km2
area, was most well de®ned. A reduced (42£30£20)
km3volume of earth subsurface directly below this epicen-tral zone, subdivided into 588 rectangular elements, each
with volume (7£5£2) km3 and P and S wave velocity
values at its eight nodes assigned from results of the ®rst iteration and the arrival times of these 138 earthquakes formed the input data set for the second iteration. Similarly during the third iteration, only those 122 of the previous 138 earthquakes whose revised epicenters were located within the (42£30) km2area were selected. The most well de®ned part of the revised epicentral belt now covered an area of 30£18 km2. A volume of (30£18£20) km3of the earth directly below this epicentral zone was divided into 588
rectangular elements, each with volume (5£3£2) km3
and P and S wave velocities at its eight nodes assigned from the second iteration results and the arrival times of these selected 122 earthquakes formed the input data set for this last iteration.
Thus we had three sets of inversion results using rela-tively coarse, medium and ®ne scale grids.
3. Results of analysis
3.1. The reliability of the estimated P and S wave velocity values
Since more than 90% of the earthquake hypocentres in the data set are estimated to have depths of less than 6.0 km, it has been possible to reliably resolve variations in the
seismic velocity in the earth only within 0±6 km depth. Also the quantity and distribution of the arrival time data of the S-phases is far less than that of the P-phases. Conse-quently there is poorer resolution of the S-wave velocity variations. Only the estimated variations of the P-wave velocity values in this depth range warrant interpretation.
Table 1 gives an overview of the comparative standard error of the results obtained by the three successive runs of the inversion scheme.
The standard error of the estimated P-wave velocity values from all three iterations is small in magnitude but those from the latter two are larger. A larger standard error implies ®ner scale variations in estimated P-wave velocity values. However, since the results of the second iteration re¯ect a larger reduction in data variance when compared with the one dimensional case, we prefer to interpret the P-wave velocity values using that iteration.
3.2. Resolving power of the results
For an overview of the resolving power of the inversion results we conducted two separate synthetic tests. First we considered a checkerboard resolution test (Zhao and Hase-gawa, 1993; Kayal and Zhao, 1998). The checkerboard was made by randomly assigning positive and negative pertur-bations of the order of 5% to P-wave velocity values at all three dimensional grid nodes of the model space as had been obtained from the inversion results. P-wave travel times from the 138 earthquake hypocentres to the recording stations were calculated using the velocity values estimated
Fig. 2. The locations of the seismograph stations and earthquake epicentres are shown. The four sections along which the estimated seismic velocities have been projected for analysis of the results of this study are also identi-®ed here.
Fig. 3. One dimensional two-layered Earth model which was the initial input model for the simultaneous inversion scheme.
Table 1
Iteration Description of grid size
Decrease in data variance from the one-dimensional case (%)
Standard error of estimated P velocity values (km/s)
1 Coarse grid 21.6 0.0276
2 Medium grid 32.4 0.0428
at the grid nodes. Normally distributed random errors with 0.1 s standard deviation, were added to these travel times. This error prone data was inverted while keeping the hypo-central parameters ®xed and using the checkerboard as the initially assumed model. The image of the synthetic inver-sion of the checkerboard identi®ed the regions of good and poor resolution. The errors incurred in velocity values at nodes at depths of 0, 2 and 4 km for 7 km grid spacing showed that the resolution was generally good over the entire area of study but was consistently superior in its central part. The power of resolution was highest at 0± 2 km depth but gradually diminished beyond 4 km. However, a similar test with 10 km grid spacing indicated that the checkerboard pattern was reconstructed with poorer resolution. This implies that the ®ner details of the
subsur-face structure are lost in the latter case. We thus conclude that the tomographic image obtained from our study has a spatial resolution of at least 7 km in the upper 4 km of the subsurface.
Next we performed a restoring resolution test (e.g. Zhao et al., 1992). Here we used the tomographic image available from the simultaneous inversion of our actual data as the synthetic model, calculated travel times for this model and introduced normally distributed random errors having stan-dard deviation 0.1 s to these. This error prone data set was inverted only for location parameters of the test hypocen-tres. The results showed that the error of locations never exceeded 3.0 km and was less than 1.0 km for 89% cases. The standard deviation of the distribution of the location errors was less than 0.75 km.
The above numerical analysis provides suf®cient con®-dence in the location parameters and velocity values simul-taneously estimated from the actual data set.
3.3. Comments on estimates of hypocentral locations and travel time residuals
For most of the 138 earthquakes, the estimated locations are close to those estimated earlier (Sarkar, 1983) with shifts at most of the order 0.5 and 1.0 km for the epicenters and focal depths respectively. The larger shifts are always for earthquakes located outside the array.
The travel time residuals (recorded±calculated) for all earthquakes are positive and less than 0.20 s for 75% of the cases. The largest residuals of the order of 0.9 s, are contributed by earthquakes located outside the array, at shallow depths and at smallest epicentral distances from Dunda and Mahidanda, the southern stations of the array (Fig. 1). This may imply that the near surface material close to these stations has P-wave velocity values lower than has been predicted by the computational procedure.
3.4. Variations of estimated P-wave velocity values
The estimated P-wave velocity values at different grid nodes of the three dimensional model exhibit systematic variations, both in depth and laterally. The depth variations are shown in Figs. 4±6 in which velocity values have been projected along the sections A, B, C and D (see Fig. 2).
Similarly the lateral variations at speci®c depths viz. near the surface, 2 and 4 km below mean sea level are shown in Figs. 7±9.
The projections along the three E±W oriented sections shown in Figs. 4a, 5 and 6 reveal the following:
1. At any speci®c depth within 0±2 km range, the estimated P-wave velocity values at all grid nodes generally show negligible lateral variation. Also the rate of depth-wise variation of these values is generally smooth. However, compared to the velocity values at grid nodes near the projected position of Sayanachatti, those near Dunda are lower by several times the standard error of estimation. This pattern of variation is manifest in the shapes of the constant velocity contours at these two localized regions (see Figs. 5 and 6).
2. In the 2±4 km depth range, the rate of depth-wise varia-tion of P-wave velocity values along grid nodes in the small localized zone below and around Dunda is reduced
considerably. Also the grid nodes around the projected
position of Dunda encompassing approximately
(10£10) km2 area of the model space, have distinctly
low P-wave velocity values as compared to the surround-ing nodes. This implies that a zone of signi®cantly low P-wave velocity, at least 4 km thick, may exist in the subsurface below and around Dunda.
The low velocity zone is evident in Fig. 4b, the N±S oriented projection also. Further this ®gure indicates that the zone is localized in the southern part of the array. We are of the opinion that this lateral limit is real and is not a computational manifestation of poor seismic ray coverage. For more than 75% of the 138 earthquake hypocentres with (i) revised locations well within the array, between 0 and 6 km depth and (ii) travel time residuals less than 0.2 s, are situated outside and north of the low velocity zone (Fig. 4b). However, our results cannot provide any reliable lateral limit for the southern end of this zone. Another interesting feature evident in Fig. 4b is that earthquakes in the 0±2 km depth range are generally located a little (less than 3 km) south of those in the 2±4 km depth range indicating a north dipping zone of earthquake distribution.
The isovelocity contours in Figs. 7±9 also provide insight into the pattern of lateral variations in P-wave velocity at shallow depths. Fig. 7 shows that near the ground surface the estimated P-wave values at grid nodes close to the projected position of Dunda are comparatively lower than those at the surroundings nodes; in contrast, nodes near the projected position of Sayanachatti have values comparably higher than the surrounding nodes. The pattern of change of the estimated velocity values along adjacent contours reaf®rms the patterns in the constant velocity layers around Dunda and Sayanachatti as was evident in the projections along the E±W oriented sections (Figs. 4a, 5 and 6). The difference between the velocity values in these two regions is more than the standard errors of estimation. Fig. 8 exhibits similar variation patterns at 2 km depth although
Fig. 7. Isovelocity contours near the surface of the Earth. The numbers along each contour indicate the corresponding estimated P-wave velocity value there. The dots denote the locations of earthquake epicentres with focal depths less than 2.0 km. The seismograph station locations (Sayana-chatti Ð SAY, Barkot Ð BAR, Dunda Ð Dun, Mahidanda Ð MAH, and Bhatwari Ð BHA) and the surface geology are also shown.
the differences between the velocity values surrounding Dunda and Sayanachatti are reduced and appear less signif-icant when compared with the standard errors of estimation. At 4 km depth (Fig. 9) the contours are diffuse and suggest further minor variation of P-wave velocity around Dunda and Sayanachatti. In regions beyond 4 km depth, data reso-lution is poor. However Figs. 7, 8 and 9 provide evidence that at least down to 4 km depth, the region around Saya-nachatti has signi®cantly higher velocity as compared to that around Dunda.
The areas of lower and higher velocity at the shallowest level (Fig. 7) display a NW±SE trend with Sayanachatti and Bhatwari forming two end points of the high velocity zone and Dunda forming a single center of low velocity. The epicenters are generally located in the transition zone between these two zones. In the next deeper level of 2 km (Fig. 7) the lowest velocity value center shifts northward from Dunda, while the higher velocity zone now shows a single high point between Sayanachatti and Bhatwari. The transition zone shows a much more rapid rate of change of velocity compared with the shallower level. The epicentres occur slightly to the north of the zone of transition at this level. At the 4 km depth level, the lower velocity zone becomes narrower and better de®ned while the high velocity zone also becomes a little bit tighter.
3.5. Interpretations of the variations in velocity
The six stations of the seismic array, whose recorded data is considered here, straddles an area where rock units are distinct and bounded by tectonic features identi®ed from surface geology (Fig. 1). Dunda, Mahidanda and Lambgaon lie in the inner Lesser Himalaya where Proterozoic quart-zite, slate and limestone are identi®ed; Barkot lies in the middle Lesser Himalaya amidst predominantly argillo-calcareous metasediments. The south dipping North Almora Thrust has been geologically identi®ed to separate these two belts of Lesser Himalaya. Sayanachatti and Bhatwari are situated in the Higher Himalaya where essentially schist,
1. At shallow depths generally the rocks are horizontally strati®ed in thin sub-parallel layers.
2. The upper crustal rocks of inner Lesser Himalaya near Dunda and Mahidanda have signi®cantly lower P-wave velocity. In contrast, the upper crustal rocks of Higher Himalaya near Sayanachatti have comparatively higher P-wave velocity.
3. The higher velocity zone coincides with the surface exposure of crystalline formations of the Higher Hima-laya, which at its southern margin are demarcated by the MCT (Fig. 4b). Estimated P-wave velocities in such
rocks are generally (5.3^0.3) km/s (see, for example,
Birch, 1942). In the exposed Proterozoic formations between Dunda and MCT and the under-thrusted argillo-calcareous metasediments of the Middle-Lesser Himalaya, exposed SW of Dunda, estimated P-wave velocities are generally (4.3^0.4) km/s (see, for exam-ple, Birch, 1942). It is suggested that the lower velocity zone is located within these formations.
4. The majority of the small earthquakes occurred in the higher velocity material and very few occurred in the lower velocity material at these depths. Thus the small earthquake activity in the region is mostly con®ned to the higher velocity material, near Sayanachatti.
5. Because the surface trace of the MCT is identi®ed to be very near Sayanachatti we consider this small earthquake activity to be indicative of an active MCT zone.
4. First motion data
We winnowed the total P-wave polarity data from the 138 earthquakes considered for the arrival time inversion program and selected only those readings, which were most reliable and de®nitely unambiguous. Ninety-eight polarity readings from 37 earthquakes were thus available for this study. Of these hypocentres, 32 had revised loca-tions within the higher P-wave velocity rock material iden-ti®ed above and contributed 89 of the polarity readings.
4.1. Fault plane solutions
We projected all polarity data in a composite plot using an equal area upper hemisphere projection (Fig. 10). The
projected data are distributed non-uniformly over the projected focal sphere with more than 40% occurring near the periphery. This is because the earthquake hypocentres occur in a comparatively small volume of rock and are mostly located directly below some recording station. The nodal planes are poorly constrained and yield two equally possible solutions. These are shown in Table 2.
Our preference is for the reverse thrust solution (solution 2) with the nodal plane NP1 as the fault plane. We give the following reasons for our selection. Two composite fault plane solutions for small upper crustal earthquakes, locally recorded during December 1979 to June 1990 and located in two speci®c regions between the Tons-Yamuna and Bhagir-athi-Alkananda river valleys are available in literature (Khattri et al., 1989; Sarkar et al., 1993). The two regions ¯ank the hypocentral zone of the present 37 earthquakes from either side and are located close to it. Both composite solutions are well-constrained and exhibit thrust mechan-isms. It has been argued elsewhere (Sarkar et al., 1993) that upper crustal earthquakes in the Garhwal Himalaya generally occur due to thrusting. A strike slip fault model for earthquakes occurring in the intervening area between
The data relate to small earthquakes whose hypocentres were generally well distributed within a 170£30£20 km3 volume of upper crustal Garhwal Himalayan rocks, extend-ing from west of the Tons river valley to the Alaknanda river valley and to epicentres distributed on either sides of the MCT (Gaur et al., 1985; Khattri et al., 1989; Sarkar et al., 1993). It may thus appear surprising that the data we used for this study, the highest quality subset of the original data set, relate only to those earthquakes which occurred in and around the Yamuna-Bhagirathi river valley. This is possibly because (i) in the entire eleven year period of ®eld record-ings, the portable arrays operating in and around the Yamuna-Bhagirathi river valleys were densest and most well distributed in azimuth and (ii) in terms of epicentral distances, these arrays were ideally suited for more reliable P-wave arrival time recordings.
Our analysis has discovered the presence of a shallow zone of low P-wave velocity and a low level of small earth-quake seismicity, latitudinally con®ned in the Lesser Hima-laya, to the Yamuna-Bhagirathi river valley region. Our data do not reveal its longitudinal boundaries. However, we are of the opinion that since the analysis pertains to earthquakes which belong to the major continuous small earthquake cluster (Gaur et al., 1985; Khattri et al., 1989; Sarkar et al., 1993), it is most probable that this low velocity zone actually extends in lateral directions throughout the entire Lesser Garhwal Himalaya, from the Tons river valley to the Alaknanda river valley. Further, due to constraints on the resolution of data because of uneven distribution of the focal depths, we are unable to provide estimates for the maximum depth of this zone. Results of two systematic magnetic surveys in the Ganga-Yamuna river valley have been inter-preted to identify a narrow highly conductive zone in Garh-wal Lesser Himalaya, in the same localized region as this low P velocity zone but at 15±20 km depth (Arora and
Fig. 10. Composite fault plane solution using equal area upper hemisphere projection. The open circles denote dilatations while the closed circles denote compressions. The solid curves identify the nodal planes of the preferred reverse thrust solution (see text for details).
Table 2
NP1 NP2
Solution Strike Dip Slip Strike Dip Slip
1 N 1728 808to the east 88 N 2648 888to the south 728
small earthquakes indicate thrust motion on high angle slip surfaces. The occurrence of these earthquakes, a part of the major small earthquake cluster (Gaur et al., 1985; Khattri et al., 1989; Sarkar et al., 1993), is not con®ned to the area but rather a part of the ongoing crustal deformation in the entire Garhwal Higher Himalaya. We suggest that a crustal shear zone with a card deck mechanism of simple shear exist in the interface zone between Higher and Lesser Garhwal Himalaya. Within this zone there are numerous parallel slip surfaces, dipping steeply northeastwards, across which reverse dip slip motion occurs causing small earthquakes. It has already been established that the Higher Himalaya and Lesser Himalaya are both rising but the Higher Himalaya rises more relative to the Lesser Himalaya. This uplift of the Higher Himalaya has been linked to the convergence of the Indian and Eurasian plates (e.g. Seeber and Gornitz, 1983). We suggest that this uplift is associated with the small earth-quake activity within this upper crustal ramp zone.
The observed anomalous low velocity structure may be caused by a variety of geological situations. For example such a variation can result due to (i) a lateral change of facies, from a higher carbonate content facies to a higher arenaceous content facies, from south to north, (ii) a temperature elevation caused by a deeper seated magma intrusion or (iii) thrusting in a ramp thrust environment. Although presently we have no de®nite evidence, because of the general tectonic style of the region this later model is considered to be quite plausible. However, V. Raiverman (Oil and Natural Gas Commission, India) is of the opinion that the low velocity is possibly related to elevated tempera-tures at depth in the region (personal communication).
The presence of lower velocity rocks in the Garhwal Lesser Himalaya may have direct bearing on the possible natural hazards of the region. For higher incidences of land-slide hazards are generally to be expected within such rock materials. Landslide susceptibility zoning in different river catchment areas of Garhwal Himalaya, using geological and geomorphological data, have indicated a high probability of major mass movement activity in Garhwal Lesser Himalaya (Pachauri et al., 1998). It is pertinent to mention here a systematic ground and satellite imagery survey conducted immediately after the occurrence of the Chamoli earthquake (mb6.3) on March 29, 1999 in the Garhwal Higher
Hima-laya. The survey showed that although damage to buildings
generally be associated with the zones of landslides in the region.
2. A crustal shear zone in the interface zone between the Higher and Lesser Garhwal Himalaya with considerably high P-wave velocity and pronounced seismic activity is identi®ed. In this zone, there are numerous parallel slip surfaces, steeply dipping to the northeast, along which reverse thrust motion occur to cause these small earth-quakes. We propose that this activity is associated with the uplift of the Higher Himalaya relative to the Lesser Himalaya and is a consequence of the general plate convergence process of the region.
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