Seismic evidence for significant crustal thickening beneath Jabalpur earthquake, 21 May 1997, source region in Narmada Son lineament, central India

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1 GEOPHYSICAL RESEARCH LETTERS, VOL. 32, L22306, doi: /2005gl023580, 2005 Seismic evidence for significant crustal thickening beneath Jabalpur earthquake, 21 May 1997, source region in Narmada Son lineament, central India S. S. Rai National Geophysical Research Institute, Hyderabad, India T. Vijay Kumar School of Earth Sciences, SRTM University, Nanded, India S. Jagadeesh National Geophysical Research Institute, Hyderabad, India Received 20 May 2005; revised 19 September 2005; accepted 17 October 2005; published 19 November [1] We investigate crustal properties beneath the deep crustal (35 km) Jabalpur earthquake of 21 May 1997, Narmada-Son Lineament (NSL), central India, in search of a possible cause of stress accumulation in the region. Teleseismic receiver functions computed at nine digital seismographs along a 250 km long profile suggests Moho downwarp to 52 km across the width of the lineament, in contrast with an average 40 km depth elsewhere. In addition, the crust beneath the NSL has higher V p /V s of 1.84 compared to 1.73 in the surrounding, suggestive of a high-density mafic mass at depth that compensates the crustal root, also supported by small topographic variation (200 m) across the lineament. Presence of such an anomalous mass in the deep crust may lead to gravitationally induced stresses in the lower crust that contribute to the failure of rock along the pre-existing Narmada-Son fault leading to the earthquake in deep crust. Citation: Rai, S. S., T. Vijay Kumar, and S. Jagadeesh (2005), Seismic evidence for significant crustal thickening beneath Jabalpur earthquake, 21 May 1997, source region in Narmada Son lineament, central India, Geophys. Res. Lett., 32, L22306, doi: /2005gl Introduction [2] The Narmada-Son Lineament (NSL) is a major tectonic feature through central India (Figure 1). Widely considered a paleo-rift, it evolved during mid-late Archaean and has reactivated several times in geological past [Choubey, 1971]. Narmada South Fault (NSF) bounds the lineament to the south and Narmada North Fault (NNF) to the north. The region between these two fault systems is covered with alluvial deposits and late Archean greenstone belts. To the north, the NSL lies in contact with Precambrian terrains like Vindhyan basin and Bundelkhand craton while in the south Deccan volcanics covers the region. The lineament has been seismically active, testified by occurrence of at least six earthquakes of magnitude >5.0 in the last hundred years three of them in the study region (Figure 1). The Jabalpur earthquake (Mw 5.8) of 21 May Copyright 2005 by the American Geophysical Union /05/2005GL [Singh et al., 1999] is unique in terms of (i) its large focal depth (35 38 km) and (ii) high stress drop of 300 bars [Singh et al., 2003] in contrast with 100 bars generally observed for intraplate earthquakes [Kanamori and Anderson, 1975]. The moment tensor inversion of this event suggests focal mechanism with strike 61, dip64 and rake 74. The Jabalpur earthquake has been associated with the Narmada South Fault characterized as a thrust type fault with left lateral strike slip component. [3] There is no unanimity on the cause for intraplate earthquakes, yet it has been linked to the zones of weakness [Hinze et al., 1988] and localized stress concentration [Campbell, 1978]. In typical ancient rift zones like New Madrid and Amazon, the earthquake occurrences are attributed to localized stress generation due to density heterogeneity in the deep crust [Zoback and Richardson, 1996]. In this context, we briefly present the current state of knowledge about the crust beneath NSL. [4] The crustal structure beneath NSL and adjoining terrains has been investigated along a profile using wideangle reflection [Kaila et al., 1989; Murthy et al., 1998] and gravity field [Verma and Banerjee, 1992] measurements. Reflection seismic studies suggest a three-layer crust with P- wave velocities 6.6, 6.4 and 6.8 km/s respectively. The surface layer with 6.6 km/s has variable thickness along the profile, 2 km within NSL. The upper crust shows a horst like feature between fault zones, NNF and NSF and Moho is modeled at a depth of km using PmP travel time data. Seismic data does not reveal any deep structural heterogeneity beneath the lineament. This inference is however, not convincing because nearly horizontal ray paths are difficult to calculate with conventional ray tracing techniques [Wilson et al., 2004] and shot receiver offsets are not sufficient to sample many of the rays. In addition, the crustal thickness inferred using refraction data is essentially averaged value over hundreds of km. Bouguer gravity field show 40 mgal variation across the profile and has been modeled due to mafic intrusion of laterally varying geometry in the shallower crust [Verma and Banerjee, 1992]. NSL is characterized by moderate heat flow regime of mw/m 2 inferred from silica content in the rocks and very shallow borehole measurements [Gupta, 1995]. L of5

2 Figure 1. Geological map of the Study area. The Narmada-Son Lineament (NSL) with Narmada North Fault (NNF) and Narmada South Fault (NSF) are indicated along with important geological features in the neighborhood. Jabalpur earthquake of 21 May 1997 is shown with a star mark. Filled circles indicate the location of seismic stations along with their names. JBP is broadband station; all others are short period (2s). The reflection seismic profile is also marked. [5] Although the mentioned geophysical results do not bring out any unusual structural feature in the deep crust, several hypothesis like serpentinized/mafic lower crust and bending of NSL have been proposed as the cause of stress build up in the deep crust [Kayal, 2000; Rao et al., 2002]. To investigate the role of any possible structural control or presence of compositionally distinct rock on the earthquake generation, we study the variation in crustal property of the NSL through the receiver function analysis of teleseismic waveforms recorded over nine locations across the lineament. 2. Receiver Functions Analysis [6] We analyzed waveforms from nine tri-axial digital seismographs (Figure 1) operated during Feb Sept The location of our stations coincides with earlier reflection seismic and gravity profiles. Station HRP is on the late Archaean granite (Bundelkhand craton); BTG, DMH and MJK are sited in Proterozoic Vindhyan Basin and KTG, JBP and TRN are in the NSL region. The two southern stations TKR and CTR are on the Deccan Volcanics. The station configuration includes SM3KV Russian make 2 seconds period seismometer and REFTEK 72A/07 data logger with time tagged through GPS. Figure S1 (auxiliary material 1 ) shows the global distribution of events with magnitude >5.0 recorded by the network and analyzed. In addition, we used data from JBP broadband station (with CMG 40T sensor and REFTEK data logger) operated by the Geological Survey of India during The 1 Auxiliary material is available at ftp://ftp.agu.org/apend/gl/ 2005GL receiver function approach enhances structural response near the receiver to the incident P-wave by isolating P to S conversions produced at the crust and mantle interfaces. Considering the fact that majority of the seismometers are short period (2s) we checked individual seismogram and those with strong P-arrivals on the vertical component were source normalized using time domain iterative deconvolution method [Ligoria and Ammon, 1999] to generate receiver functions (RF). These RFs have been stacked in ±5 distance range to generate a smoother RF while minimizing the effect of azimuthally varying local structure beneath the station. The stacked RFs for delta range is presented in Figure 2 for all the stations and the P to S converted phase (Pms) from Moho depth (m) is marked. The plot shows shift in the Pms time indicating possible variation in Moho depth around station JBP and TRN. Examination of azimuthal variation in RF at JBP (Figure 3) clearly reveals a systematic delay of up to 1.6 s in Pms time for E-SE events. In the azimuth range this phase is preceded by a strong arrival at 2 s and therefore, it is necessary to examine if the phase at 7s interpreted as Pms is really a Moho conversion or multiple of the phase at 2s. To resolve this issue, we plotted the RF in this azimuth sector as a function of distance (Figure S2, auxiliary material) examining travel time gradient for these two phases. For 2s phase, travel time increases with distance while for the phase at 7s it decreases with distance. This clearly indicates that phase at 7s is a converted phase from Moho and 2s phase is a multiple corresponding to a converted phase at 0.6s which is most likely merged with the P- phase in the RF. To verify this we computed the RF in higher frequency range (Gaussian width 7) that shows the positive conversion at 0.55s. This conversion is possibly from a high velocity layer (Vp = 6.7 km/s) at a depth of 2 km beneath the station JBP inferred through the modeling of reflection seismic data [Murthy et al., 1998]. [7] To bring out quantitative estimate of the crustal parameters (Moho depth H and average V p /V s ) we modeled Figure 2. Stacked radial receiver function for individual station are presented with Moho converted (Pms) marked on them. The RFs are azimuthally averaged for the distance range Number of events (N) used for stacking is shown on the right side. Pms time is delayed at JBP and TRN. 2of5

3 shows a more mafic composition (V p /V s 1.84) in contrast with intermediate composition (V p /V s 1.73) elsewhere. 3. Travel Time Residuals [9] Our estimation of crustal thickness relies principally on the time of P to S converted phase identified in the RFs. To provide a rough crosscheck we also computed the P wave travel time residual relative to event average for the array for two opposite azimuths, SE and NW (Figure Figure 3. Azimuthal variation of radial RF at JBP. The Pms time are delayed at 7.1 s for events from E- SE direction compared to 5.3 s for those from other directions. radial RFs following a robust grid-search stacking [Zhu and Kanamori, 2000] of converted (Pms), and reverberated (PpPms and PpSms) phases with weight of 0.7, 0.25 and 0.05 respectively. Moho depth estimate is dependent on both V p and V p /V s. A recently available P velocity model across the profile [Murthy et al., 1998] using deep seismic sounding data suggests that the average crustal P- velocity varies in a narrow range of km/s for all the stations except beneath JBP where it is 6.5 km/s. These velocities were used in estimating the crustal thickness. The H, V p /V s estimation graph for selected stations is presented in Figure 4. To ascertain that Moho signal and the multiples are identified correctly in the RF, we have marked their arrival time for selected receiver functions at these stations. These times have been computed for the crustal parameters estimated from grid-search approach and considering the event distance. Wherever multiples (in particular, PpPms) were not very clear, we used the V p /V s value from the neighboring station to compute H. We also examined the error in the crustal parameter estimate due to inaccuracy in velocity determination, recomputing H and Vp/Vs for the average P velocity varying from 6.2 to 6.6 km/s. We conclude that Moho depth and Vp/Vs changes by less than 2 km and 0.02 respectively. [8] Figure 5 shows Moho depth and V p /V s variation across the NSL along with the surface topography. Azimuthally averaged Moho depth (H) values for individual stations were plotted below all the stations except for JBP where Moho depths computed for NW and SE azimuths events were projected with horizontal offset. Two significant observations are made from this result. Firstly, while the average Moho depth is 40 km, below the Narmada South Fault (NSF) and its neighborhood it is depressed to 52 km. Secondly, the average crustal composition is found to vary significantly. The region with depressed Moho Figure 4. Grid search stacking of RFs to compute Moho depth (H) and V p /V s. Each H and V p /V s is obtained by weighted sum of amplitudes from P to S conversion (Pms) and its multiples (PpPms and PpSms) in the receiver functions at predicted times for the crustal parameters. To the right of each H-Vp/Vs plot, we have marked the predicted arrival time of Moho converted phase and their multiples for selected individual events. Distance and azimuth are on the left side of the RF. 3of5

4 Figure 5. Moho depth and V p /V s deduced from search stacking are plotted along the profile beneath the stations except at JBP, where events from NW and SE are plotted with offset in respective direction. The sharp Moho offset 12 km is clear in the NSL region. On top of the figure, surface topography is also shown. S3, auxiliary material). For both these directions the residual pattern is almost similar, like s delay for stations within NSL while other stations record early arrival up to 0.2 s. Usually, shallower anomalies produce azimuth independent residual, while deep-seated features have azimuthally varying signature. Crustal thickening produces time delay of 0.03 s/km, which in the present case would be 0.4 s for 12 km change in Moho depth. Considering the inter-station spacing of 25 km and the azimuthally invariant anomaly, we expect that the delayed residual observed beneath the NSL could be explained largely due to the crustal thickening model inferred from receiver function. 4. Discussion [10] Our study suggests that the average Moho depth in the region is 40 km except beneath the NSL where it is the locally depressed, over a width of about 40 km, beneath the Narmada South Fault zone (NSF) to 52 km. The region of depressed Moho is characterized by high V p /Vs ratio (1.84) compared to 1.73 mapped elsewhere along with no topographic expression and almost a featureless isostatic anomaly [National Geophysical Research Institute (NGRI), 1975]. The high V p /V s ratio suggests mafic to ultra mafic lower crustal composition and a smaller density contrast, perhaps as little as 300 kg/m 3 with the surrounding mantle. This accounts for negative correlation between elevation and crustal thickness and explains the absence of any topographic variation across the lineament. Presence of deep Moho beneath the rift flank provides constraint on the mechanism likely to balance rift shoulder uplift. Both upward Moho flexure and lower crustal ductile flow are likely to compensate flank uplift. In case of flexure uplift, the Moho is shallower, while lower crust ductile flow is associated with crustal thickening [Tiberi et al., 2003]. Hence, our model favors lower crustal ductile flow as the possible mechanism to support the NSL rift shoulder uplift. [11] Presence of such high-density crustal root has serious implication in generating localized gravitationally induced elastic stress field in the deep crust [Jeffreys, 1976; Goodacre and Hasegawa, 1980], which may contribute to the failure of rock along pre-existing zone of weakness. An approximate estimate of the maximum stress difference caused due to undulation at the crustmantle boundary is given by s max = b(r m r c )g dh, where dh is Moho undulation and b is a function of the degree of regional support of stress and (r m r c ) refer to density difference between mantle and the crust. If the region is in local isostatic equilibrium b = 1. Considering (r m r c ) = 300 Kg/m 3 and dh = 10 km, s max 300 bars. This represent lower limit to the maximum stress difference in the crust and mantle. Presence of such significant stress field possibly explains the unusually large stress drop (300 bars) associated with Jabalpur earthquake. In addition, the inferred mafic-ultramafic material beneath the deep crust of NSL would locally modify the intraplate stress and would enhance the likelihood of reactivation of pre-existing fault zone leading to deep crustal earthquake beneath the NSL. Present data is, however, inadequate to answer if depressed Moho is a regional feature beneath the Narmada South fault or localized one and await result from another experiment across NSL currently in progress. [12] Acknowledgments. The Deep Continental Studies program of Dept. of Science and Technology, New Delhi supported this study. We are thankful to the Geological Survey of India for providing data of the JBP station. We appreciate the comments from reviewers that helped improving the manuscript. References Campbell, D. L. (1978), Investigation of the stress concentration mechanism for intraplate earthquakes, Geophys. Res. Lett., 5, Choubey, V. D. (1971), Narmada-Son Lineament, India, Nature, 232, Goodacre, A. K., and H. S. Hasegawa (1980), Gravitationally induced stress at structural boundaries, Can. J. Earth Sci., 17, Gupta, M. L. (1995), Thermal regime of the Indian Shield, in Terrestrial Heat Flow and Geothermal Energy in Asia, edited by M. L. Gupta and M. 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