The role of crystallized magma and crustal fluids in intraplate seismic activity in Talala region (Saurashtra), Western India: An insight from local earthquake tomography

TECTO-127112; No of Pages 11 Tectonophysics xxx (2016) xxx–xxx

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The role of crystallized magma and crustal fluids in intraplate seismic activity in Talala region (Saurashtra), Western India: An insight from local earthquake tomography P. Mahesh a,⁎, Sandeep Gupta b a b

Institute of Seismological Research (ISR), Raisan, Gandhinagar 382009, India CSIR-National Geophysical Research Institute, Hyderabad 500007, India

a r t i c l e

i n f o

Article history: Received 30 October 2015 Received in revised form 4 May 2016 Accepted 12 May 2016 Available online xxxx

a b s t r a c t The Talala region in Saurashtra, Western India is one of the seismically active intraplate regions on the Earth. In recent past, this region has been site of moderate magnitude earthquakes as well as swarm-type earthquake activities. To understand the processes of earthquake generation in this intraplate setting, we constrained the earthquake distribution pattern along with the crustal seismic P-wave velocity (Vp) and Vp/Vs variations, using local earthquakes data. We inverted 2470 P- and 2230 S-wave arrival times from 550 earthquakes which were recorded over 11 seismic stations during 2007 to 2012. The earthquakes distribution shows that the seismicity is following ~NNE–SSW trend, extending for a distance of ~25 km and up to 15 km in depth. The seismic tomographic images show that the swarm-type earthquake activities at shallower depths are mostly in the zone of lower Vp and lower Vp/Vs. Whereas, the moderate magnitude earthquakes are occurring in a ~ NW trending zone of higher Vp and higher Vp/Vs, possibly indicating a zone of crystallized mafic magma, which was transported from deeper Earth. This zone represents a pronounced heterogeneity and provides locale for stress accumulation in this region. After 2001 Bhuj earthquake (Mw 7.7), due to stress perturbation the ~NNE–SSW trending fault got activated and caused bigger earthquakes in this region. Moreover, the crystallized mafic magma is possibly feeding fluids at shallower depths for causing the swarm-type earthquake activities in this region. © 2016 Elsevier B.V. All rights reserved.

1. Introduction The Talala region in the Junagadh district of southern Saurashtra (Fig. 1a), about 200 km south to the devastating intraplate 2001 Bhuj earthquake (Mw 7.7), is one of the seismically active regions of Gujarat state in western India. Based on historical and instrumentally recorded seismicity of the Saurashtra peninsula; this region is critically stressed and seismic hazard in this region is appreciable. The Talala region falls under two seismic zones IV and III of the seismic zoning map of India (BIS, 2002) with likely earthquakes of magnitudes 7 and 6, respectively. In recent past, this region has been site of various moderate magnitude earthquakes e.g., November 6, 2007 (Mw 5.0), November 6, 2007 (Mw 4.8) and October 20, 2011 (Mw 5.1) (stars in Fig. 1b). These earthquakes were felt in much of the Saurashtra peninsula. Apart from bigger earthquakes, the swarm-type earthquake activity lasting for 2–3 months have also been observed in this region in the years 2001, 2004, 2007 and 2011 (Rastogi et al., 2013). These swarm-type earthquake activity ⁎ Corresponding author. E-mail addresses: [email protected] (P. Mahesh), [email protected] (S. Gupta).

were observed soon after the monsoon (heavy rains), which raised the water table by 30–70 m, and were suggested to be triggered by percolated rain waters through extensional fractures (Rastogi et al., 2013). Using the observed reservoir and rainfall data from this region; the observed episodic swarm-type earthquake activities in the region were related to a poroelastic response of the seismogenic crust to surface water flux, leading the pore pressure changes at depth (Hainzl et al., 2015). Yadav et al. (2011) studied the spatio-temporal properties as well as Coulomb stress transfer assessments to understand the seismic characteristics of the 2007 Talala earthquake sequence. The authors observed that the spatial aftershock distribution was along a NE–SW striking fault. They also found more fractures and high surface heat flux beneath the 2007 mainshock hypocenter, and the static Coulomb stress changes due to the co-seismic slip of the main shock and enhanced off fault aftershock occurrence. Rastogi et al. (2013) reported that the 2011 Talala earthquake and aftershocks defined a shallow ENE 40-km-long trend; and the increased seismicity in this region was caused by stress perturbation due to the 2001 Bhuj earthquake. Rastogi et al. (2013) also reported a low b-value (0.67) associated with the 2011 Talala sequence and suggested for increased heterogeneity and low stress within the crustal mass indicating the presence of more fractures. The seismic

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Please cite this article as: Mahesh, P., Gupta, S., The role of crystallized magma and crustal fluids in intraplate seismic activity in Talala region (Saurashtra), Western India: An ..., Tectonophysics (2016), http://dx.doi.org/10.1016/j.tecto.2016.05.025

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Fig. 1. (a) The simplified regional geotectonic map of the Saurashtra peninsula, western India (modified after Biswas, 1987), which is shown by the box in the map of India (insert map, in the left-bottom side). The study region, the Talala region is marked by the rectangular box. The small red line shows the ~N-S trending fault as reported by GSI (2000). The ~W–E trending, dash line shows the deep seismic profile (Rao and Tiwari, 2005). The pink diamond shows the location of a volcanic plug at Junagadh. (b) The enlarged map of study region showing bigger earthquakes (M N 4.5 pink stars; 3 b M ≤ 4.5 blue circles), seismic stations (gray triangles), volcanic plug at Junagadh (pink diamond). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

tomography study (Singh et al., 2013), using 1135 P- and 1125 S- from 236 aftershocks of the 2011 earthquake, reported the source region of the 2011 Talala earthquake was associated with low Poisson's ratio anomalies guarded by prominent high Poisson's ratio anomalies along the active fault zone having the strike-slip motion. Using the 2011 Talala earthquake aftershocks, a 3-D micro-structure study (Singh and Mishra, 2015) reported that the strong variation in crack attributes and interconnection of secondary pores from surface to the deeper depths might have facilitated the processes of generating monsoon-induced micro-to-moderate earthquake sequence in this region. The hypocenter distribution of bigger earthquakes in this region shows an upward propagation of earthquakes with time. Also, in absence of proper crustal structure and detailed surface conditions, Hainzl et al. (2015) classified their estimations of time-dependent

pore-pressure changes at depth as a first-order approximation. Thus, it is possible that rather than monsoon-induced process, there may be some other process(es) causing bigger earthquakes and the swarmtype earthquake activity in this region. Therefore, to re-look the monsoon-induced seismicity model for earthquakes occurrence in this intraplate setting, we studied the distribution of the recorded earthquakes from this region during 2007–2012, and generated 3-D seismic tomographic images by inverting the arrival times of these local earthquakes. 2. Overview of geotetonics The Saurashtra peninsula constitutes a crucial component of the geodynamical system of the western continental margin of India. The

Please cite this article as: Mahesh, P., Gupta, S., The role of crystallized magma and crustal fluids in intraplate seismic activity in Talala region (Saurashtra), Western India: An ..., Tectonophysics (2016), http://dx.doi.org/10.1016/j.tecto.2016.05.025

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HYP:2011/11/01/18/35/58.7 70.493°E 21.116°N 6.0 M2.9

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Time (s) Fig. 2. The example of analyzed 3-component waveforms of an earthquake (1 November, 2011, 18:35:58.70 UTC, 70.493°E, 21.116°N, 6 km depth and magnitude 2.9) recorded at two seismic stations VRD (epicentral distance = 10 km) and UNA (epicentral distance = 47 km). The P- and S-phases picked on these waveforms are also shown.

Saurashtra peninsula is considered as a cratonic horst surrounded by the three intersecting rifts namely Kachchh, Cambay and Narmada (Fig.1a) (Biswas, 1987). The major geological and tectonic events which affected the Saurashtra peninsula were of Mesozoic and Cenozoic periods and were related to (i) breakup of Africa from India along the western continental margin of India and (ii) eruption of large volume of Deccan flood basalt from the Reunion hotspot along the west coast of India (White and McKenzie, 1989). A major part of the Saurashtra peninsula is covered by Deccan volcanic flows, which is characterized by a large number of basic and acidic dykes as well as volcanic with various plutonic centers. The dyke swarms and lineaments present in the Saurashtra peninsula are related with the reactivation of three rifts (Kachchh, Cambay and Narmada) (Biswas, 1987). Also, there are

sedimentary sequences from Mesozoic to Quaternary ages in the northern and southern fringes of the Saurashtra (Merh, 1995). The Talala region in southern Saurashtra peninsula is also occupied by the Deccan volcanics. Major rock types are basalt, rhyolite, gabbro, and granophyres (Merh, 1995). The basalt is of tholeiitic type, highly fractured and jointed. The fracture density of the area is comparatively higher than that of the surrounding areas. The basement depth is ~ 3.5 km in Saurashtra (Chopra et al., 2014). Various volcanic plugs of acidic, alkaline and mafic/ultramafic (high-density) types have been reported in the entire Saurashtra peninsula (Biswas, 1987; Chandra, 1999; Merh, 1995). These plugs are the pipes like igneous intrusions and having limited horizontal dimensions from a few hundred meters to a few kilometers. One such prominent and well studied volcanic plug is

Please cite this article as: Mahesh, P., Gupta, S., The role of crystallized magma and crustal fluids in intraplate seismic activity in Talala region (Saurashtra), Western India: An ..., Tectonophysics (2016), http://dx.doi.org/10.1016/j.tecto.2016.05.025

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(a) Vp

Fig. 5. The distribution of seismic ray paths, in plan view. The seismic stations and events are represented by red triangles and yellow dots, respectively. Pink stars show the moderate magnitude Talala earthquakes (in years 2007 and 2011). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

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reported at Junagadh, which is ~40 km away and in the NE direction of the study region (Fig. 1a, b). This volcanic plug represents a layered complex with ring dykes of dolerite and granophyres. The three major lithological rock units present in this complex are pyroxenite/peridotite, gabbro and syenite/nepheline syenite (Chandra, 1999). The Bouguer anomaly and the total intensity magnetic anomaly maps show circular gravity high and a magnetic anomaly corresponding to this volcanic plug (Chandrasekhar et al., 2002). The deep resistivity sounding study (Singh et al., 2004) and magnetotelluric study (Sarma et al., 2004) also reported presence of this volcanic plug. In absence of any known major fault system near the epicenter zone of the Talala earthquakes, a ~N–S trending fault (Fig. 1a) was identified by the Geological Survey of India (GSI, 2000). Also, ~ 40 km away and in the NE

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Please cite this article as: Mahesh, P., Gupta, S., The role of crystallized magma and crustal fluids in intraplate seismic activity in Talala region (Saurashtra), Western India: An ..., Tectonophysics (2016), http://dx.doi.org/10.1016/j.tecto.2016.05.025

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of these earthquakes showed a left-lateral strike-slip faulting along a NE–SW trending fault (Fig. 4). 4. 3-D velocity tomography

Fig. 6. The trade-off curve between data variance reduction and model variance, used to select the optimal damping parameter for Vp (the number besides the arrow) to obtain the optimum 3-D velocity model.

direction to the Talala epicentral region (dotted line, Fig. 1a), a crustal crossing fault was reported in a deep seismic sounding study (Rao and Tiwari, 2005). With a thick intrusive magmatic body (Vp ~ 7.20 km/s) at the base of the crust, the moho depth in this region is about 32– 36 km (Rao and Tiwari, 2005).

3. Data We used local earthquakes recorded by 11 broadband seismographs of Gujarat Seismic Network (GSNet), which were being operated by the Institute of Seismological Research (ISR), Department of Science and Technology, Gujarat, India (Kumar et al., 2012). These 11 seismic stations fall in the vicinity of the Talala earthquake region (triangles in Fig. 1b). Seismic waveforms were recorded continuously at 50 samples/s for online stations and 100 samples/s for offline (temporary) stations. All the stations were equipped with Guralp CMG-3T broadband sensors and 24-bit Guralp CMG-DM24/REFTEK (RT 130-01) data recorders with 4 GB swappable hard disk and GPS. The arrival times of P- and S-phases of the earthquakes recorded by the network were picked by using the earthquake analysis software SEISAN (Havskov and Ottemoller, 2003, 2003). An example with analyzed waveforms and corresponding traveltime picks is given in Fig. 2. Reading accuracy of arrival times were estimated to be around 0.05–0.2 s for the P-wave and 0.1–0.3 s for the S-wave. During 2007– 2012, we could select 550 local earthquakes with at least 4 P- and 3 S-arrivals. The earthquake magnitudes varied between 1.0 and 5.1. A total of 550 local earthquakes were located using HYPOCENTER computer program (Lienert and Havskov, 1995). To determine the initial earthquake locations, we used the 1-D velocity model (Fig. 3a) from a deep seismic sounding study (Rao and Tiwari, 2005) and an average Vp/Vs ratio (1.73), which was abstracted from the P- and S-wave arrival-time data from our network (Fig. 3b). These initial locations had an average error of 2 km in latitude/longitude, and 2.5 km in depth. The travel time errors (RMS values) varied between 0.05–0.3 s for all the events. Fig. 4 shows the 3-D distribution of earthquakes used in this study. The majority of the hypocenters are showing following ~ NNE–SSW trend extending a distance of ~25 km and depth up to 15 km. We also plotted the fault plane solutions of the 2007 and 2011 Talala earthquakes (Rastogi et al., 2013; Yadav et al., 2011). The fault plane solutions

To understand the possible linkage between seismicity and crustal structure, we determined 3-D seismic P-wave velocity (Vp) and Vp/Vs perturbations using seismic tomography. We inverted 2470 P- and 2230 S-arrival-time data from our network using SIMULPS14 (Evans et al., 1994; Haslinger, 1998; Thurber, 1983). This method uses damped, iterative, least squares inversion to compute the 3-D variations of Vp and Vp/Vs. The Vp/Vs ratio is directly related to the Poisson's ratio and is a key parameter in studying petrologic properties of the crustal rocks (Christensen, 1996). This parameter is important for discussion on the seismogenic behavior of the crust, especially to understand the role of crustal fluids in the earthquake generation. The study region was parameterized in terms of velocities at the nodes of a 3-D grid and linear velocity gradients between the nodes. The velocity at any arbitrary point in the 3-D grid was calculated by linear interpolation between the surrounding nodes. The reliability of the inversion results depends on the initial reference model. We used the 1-D velocity model (Fig. 3a) obtained from a deep seismic sounding study (Rao and Tiwari, 2005) as the initial reference model; and used Vp/Vs (= 1.73) derived by us (Fig. 3b) as an initial Vp/Vs value. With 21.1°N, 70.7°E as the center of the grid, the model space was defined by 20.8°–21.4°N, 70.2°–71.2°E and 0–15 km in latitude, longitude and depth ranges, respectively. The seismic ray path distribution (Fig. 5) shows a sufficient ray coverage for tomography of the Talala earthquake region. To suppress large perturbations in the calculated parameters, we used the damping. The optimal damping parameter for the Vp and Vp/Vs were selected based on the empirical approach of Eberhart-Phillips (1986). To select the optimal value of damping factor, trade-off curve between the data and model variances was constructed by performing several one step inversions with damping factor values ranging from 5 to 200 (Fig. 6). The damping parameters for the P- and S-arrival times were set to be 8 and 10, respectively. We conducted several tomographic inversions with varying horizontal grid spacing (from 5 to 10 km), and varying depths (from 3 to 5 km). Though the patterns of tomographic images were largely similar, there were variations in amplitude of the velocity anomalies. We preferred the horizontal grid spacing of 6 km and vertical layering at 0, 3, 6, 9, 12 and 15 km depths. After inversion, the variance of the arrival time residuals was reduced to a value of 0.003 s2 from the starting value of 0.007 s2. The average root mean square residuals (rms) for the travel times decreased from 0.139 s to 0.109 s. In order to ascertain the adequacy of the ray coverage, spatial resolution and main features of the tomographic images, we carried out various resolution tests and closely examined several parameters which includes ray density, Derivative Weighted Sum (DWS) and resolution matrix (Evans et al., 1994). The ray density indicates the number of phase readings (KHIT) used for the inversion of a model parameter at a given grid node and depends on the number of ray paths passing near a given grid node. DWS describes the amount of data actually constraining the velocity at the node and is a better indicator of how the data is constraining the obtained results. It is similar to KHIT but, weighted by ray-node separation and ray path length in the vicinity of the node. The spread value (SV) of the resolution matrix estimates how well the velocity parameters are resolved (Michelini and McEvilly, 1991; Toomey and Foulger, 1989). It is a better estimator of the resolving power than the diagonal element of the resolution matrix, because it takes all the terms of the resolution matrix into consideration. A well-defined parameter has a small resolving width and thus a small spread function. The resolution tests of KHIT, DWS and SV for Vp and Vp/Vs are shown in Fig. 7 and Fig. 8, respectively. Fig. 9 compares the values of RDE (resolution diagonal element) and DWS for all the nodes of direct inversion for the finely gridded velocity model. The

Please cite this article as: Mahesh, P., Gupta, S., The role of crystallized magma and crustal fluids in intraplate seismic activity in Talala region (Saurashtra), Western India: An ..., Tectonophysics (2016), http://dx.doi.org/10.1016/j.tecto.2016.05.025

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nodes with similar DWS or RDE have different spread values, showing that neither of these two parameters can be used to evaluate the inversion quality. These plots were used to define a threshold SV, above which the results of the inversion would not be discussed as those correspond to unresolved nodes. We empirically chose the threshold SV as 2 for VP and VP/VS, because it was observed that the regions with larger values of SV showed the smaller values of DWS and RDE (Fig. 9). In order to further evaluate the adequacy of ray coverage, spatial resolution of the entire study area and the solution quality; we carried out a well-known synthetic test the Checkerboard Resolution Test (CRT). To make a synthetic input model, positive and negative velocity perturbations (± 10%) were assigned to the alternate 3-D grid nodes in the modeling space (Fig. 10a). Random errors in a normal distribution with a standard deviation of 0.1 s were also added to the theoretical

travel times, which were calculated for the synthetic models. The test results showed that the checkerboard images were recovered down to 12 km depth (Fig. 10b and c), as most of the earthquakes were located in the upper crust and the rays crisscrossed well from the surface down to about 12 km depth. Considering all resolution tests, we took all the nodes with KHIT N 250, DWS N 500, in general, and spread value b2, in particular, as fairly well resolved and identified the reliable features in the obtained tomographic images. Based on these resolution parameters, we believed that the Talala earthquake region was well resolved. 5. Results and interpretation Figs. 11 and 12 show P-wave velocity (VP) and VP/VS perturbations in plan views, the seismicity within 1.5 km of either side of each depth

Please cite this article as: Mahesh, P., Gupta, S., The role of crystallized magma and crustal fluids in intraplate seismic activity in Talala region (Saurashtra), Western India: An ..., Tectonophysics (2016), http://dx.doi.org/10.1016/j.tecto.2016.05.025

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Spread Value Fig. 8. (a) Total number of hit per cell (KHIT), (b) Derivative Weighted Sum (DWS), and (c) Spread Values (SV) of resolution matrix for Vp/Vs at different depths. KHIT, DWS and SV together are being used to identify the region(s) with reliable resolution in Vp/Vs perturbations (Figs. 12 and 13b).

layer is also projected. At shallower depths (b3 km), the lower Vp (4 to 10%) and lower Vp/Vs ratio (1 to 4%) may represent the volcanic rocks, which have lower velocity and higher density than that of the granites (Rao and Tiwari, 2005). This type of volcanic rock may be the basalt at low pressures, as observed in other parts of Deccan volcanic provinces (e.g., Dixit et al., 2014). The smaller magnitude earthquakes are mostly observed in the zone of lower Vp and lower Vp/Vs anomalies. Below 3 km depth, our preferred images of Vp and Vp/Vs perturbations indicate the possibility of granitic rocks of varying saturation and fracturing (e.g., Christensen, 1996; Wang et al., 2012). This possibility is in concurrence with earlier reports from this region which found that the basement was granitic (Rao and Tiwari, 2005) and the rocks were highly saturated and highly porous (Singh and Mishra, 2015). The moderate as well as smaller magnitude earthquakes in deeper crust are mostly concentrated in and around higher Vp and higher Vp/Vs anomalies. To understand the earthquake occurrence in this region, we plotted the Vp and Vp/Vs perturbations along a vertical cross-section (Fig. 13).

This cross-section (as shown in Figs. 11 and 12) is nearly perpendicular to the ~NNE–SSW trending fault as observed in the seismicity pattern in this region (Fig. 4). The moderate earthquakes and the background seismicity, within 10 km either side of the cross-section, were also plotted on these Vp and Vp/Vs perturbations (Fig. 13). All the moderate earthquakes along with many smaller magnitude earthquakes are falling in the zone of higher Vp and higher Vp/Vs. Whereas, at shallower depth (b4 km), the smaller magnitude earthquakes are in the zone of lower Vp and lower Vp/Vs (Fig. 13). The interpretation of Vp and Vp/Vs anomalies in terms of rock properties and geological structure is generally difficult and non-unique. Therefore, by taking constrains from available geological and geophysical studies; we attempted to interpret these anomalies and propose a possible model for the earthquake occurrence in this intraplate setting. In the study region, the lower crust is underplated by a thick and high velocity (Vp ~ 7.20 km/s) mantle derived magma intrusion (Rao and Tiwari, 2005), possibly due to extensive volcanism in this region

Please cite this article as: Mahesh, P., Gupta, S., The role of crystallized magma and crustal fluids in intraplate seismic activity in Talala region (Saurashtra), Western India: An ..., Tectonophysics (2016), http://dx.doi.org/10.1016/j.tecto.2016.05.025

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(White and McKenzie, 1989). The rocks are highly fractured and fluid filled at shallower crustal level (Singh and Mishra, 2015) as well as middle-to-lower crustal level (Chopra et al., 2014; Mandal, 2006). It was suggested that the faults in the Saurashtra region had become activated by stress perturbation caused by the 2001 Bhuj earthquake of Mw 7.7 (Rastogi et al., 2012). Considering these observations, we believe that the ~ NW trending zone of higher Vp and higher Vp/Vs perturbations, and having bigger earthquakes along with few smaller earthquakes (Fig. 13), may be representing a zone of hydrothermal fluid, which was possibly exsolution from the crystallizing magma routed from the deeper crust. Alternatively, this zone may be indicating crystallized mafic magma (solidified igneous material), where magma was transported from the deeper Earth through the ~ NNE–SSW trending deep crustal fault as observed in the seismicity pattern and was reported earlier (GSI, 2000; Rao and Tiwari, 2005). This possibility gets support with the presence of deep seated Junagardh volcanic plug; which approximately falls on the ~ NNE–SSW fault by extending this fault further in ~ NNE direction. In this volcanic plug, high density (~ 2880 kg/m3, Chandrasekhar et al., 2002) material, possibly mafic magma is intruded into the lighter density (~ 2700 kg/m3) rocks. We are of the opinion that the mantle-derived magma intruded into the crustal depths, via lower crust (as seen underplated lower crust), through the path with the observed higher velocity patterns (Fig. 13), got crystallized and by decompression/differentiation formed the rhyolite and granophyric like rocks as reported in the region (Sethna, 2003). Similar to our observation, the presence of mid-crustal solidified intrusive body (higher Vp and higher Vp/Vs) below the earthquake swarm focal zone has earlier been reported for a non-volcanic regions (e.g., Kato et al., 2010; Mousavi et al., 2015) as well as volcanic region (e.g., Chiarabba and Moretti, 2006; Patanè et al., 2003). We consider the imaged crystallized mafic magma as a pronounced heterogeneity within the crust; and by reducing the crustal strength it

provides a locale for the stress accumulation at its contact (McGinnis and Ervin, 1974). After the 2001 Bhuj earthquake (Mw 7.7) in the Deccan Volcanic Provinces, due to stress perturbation possibly the ~NNE– SSW trending fault got activated and the accumulated stress in and around the mapped crystallized mafic magma generated the bigger earthquakes in Talala (in years 2007, 2011). In Saurastra region, the influence of subvolcanic vents/plutons in either reactivating the near-by faults or the vents themselves acting as source of seismic activity has also been suggested (Reddy, 2005). Globally also, many researchers (e.g., Long, 1976; Mckeown, 1978) have suggested the spatial association between local seismicity and the unexposed mafic/ultramafic intrusions. Further, the seismic swarm-like earthquake activity at shallower depths may be due to the feeding of the crustal fluid, which was released from the mapped crystallized magma (Mousavi et al., 2015). The observed lower b-value (0.67, Rastogi et al., 2013) of the seismic swarm-like earthquake activity in this intraplate setting also indicates the possibility of increase in the pore pressure, possibly due to crustal fluids. However, as the smaller earthquakes at shallower crustal depths (b4 km) are in the zone of lower Vp and lower Vp/Vs (Fig. 13), possibly representing the zone containing water-filled cracks with several percent porosities (Lin and Shearer, 2009). Therefore, the swarm-like earthquake activities at shallower depths and as related with heavy rain/reservoir (Hainzl et al., 2015; Rastogi et al., 2013; Singh and Mishra, 2015) cannot be completely ruled out for this particular region. 6. Conclusions To understand the earthquake generation in the Talala region (Saurashtra, western India), an intraplate setting, we constrained the earthquake distribution pattern and, the crustal seismic P-wave velocity (Vp) and Vp/Vs variations using local earthquakes. We used first P- and S-

Please cite this article as: Mahesh, P., Gupta, S., The role of crystallized magma and crustal fluids in intraplate seismic activity in Talala region (Saurashtra), Western India: An ..., Tectonophysics (2016), http://dx.doi.org/10.1016/j.tecto.2016.05.025

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Fig. 10. (a) The input checkerboard model with 10% variation in Vp and Vp/Vs. Results of checkerboard resolution test for Vp (b) and Vp/Vs (c). The layer depths are shown on the upperright corner of each map.

wave arrivals from 550 crustal earthquakes recorded at 11 seismic stations during the period 2007–2012. The earthquakes distribution shows that the seismicity is following ~NNE–SSW trend, extending for a distance of ~ 25 km and up to a depth of 15 km. The swarm-type earthquake activities at shallower depths are mostly in the zone of lower Vp and lower Vp/Vs. Whereas, the moderate magnitude earthquakes are occurring in a ~ NW trending zone of higher Vp and higher Vp/Vs. We interpret the higher Vp and higher Vp/Vs as crystallized mafic magma, where magma was transported from the deeper Earth and got crystallized. This zone represents a pronounced heterogeneity within the crust, and by reducing the crustal strength provided locale for stress accumulation. After 2001 Bhuj earthquake (Mw 7.7), due to stress perturbation the ~ NNE–SSW trending fault got activated and the accumulated stress in and around the mapped crystallized mafic magma started releasing in the form of bigger

earthquakes in Talala region. Moreover, the crystallized mafic magma is possibly feeding fluids to the shallower depths and which might be causing the swarm-type earthquake activities as observed in this region.

Acknowledgments We thank Santosh Kumar for helping in the initial data preparation and G. Pavan Kumar for discussion. PM acknowledges the help and encouragement by DG and Director, ISR (DST, Govt. of Gujarat) to carry out this study. SG acknowledges support from CSIR-NGRI projects (MLP6505-28(SKG)) and INDEX. We are thankful to the guest editor Prof. Olivier Lacombe and two anonymous reviewers for providing us their very constructive comments to improve the quality of the manuscript.

Please cite this article as: Mahesh, P., Gupta, S., The role of crystallized magma and crustal fluids in intraplate seismic activity in Talala region (Saurashtra), Western India: An ..., Tectonophysics (2016), http://dx.doi.org/10.1016/j.tecto.2016.05.025

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70.2

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−10 −8

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−4

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Fig. 11. The plan view of P-wave velocity (Vp) perturbations. The black line encloses the better resolved regions (based on SV, DWS and KHIT as in Fig. 7). The layer depths are shown on the upper-right corner of each map. Red and blue colors indicate low and high Vp perturbations, respectively. The P-velocity perturbation scale (in %) is shown at the bottom. The green triangles represent the station locations. Small crosses show the grid nodes. Black dots show the earthquakes that occurred within 1.5 km of either side of each depth slice. The pink stars show the moderate magnitude Talala earthquakes (in years 2007 and 2011). The pink diamond shows the location of a volcanic plug at Junagadh. The A–A' line shows the location of vertical cross-section, used for showing P-wave velocity (Vp) perturbations in Fig. 13a. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

A

A'

−10 −8

−6

−4

−2

0

2

4

6

8

10

Fig. 12. The plan view of Vp/Vs perturbations. The black line encloses the well resolved regions (based on SV, DWS and KHIT as in Fig. 8). Red and blue colors denote low and high Vp/Vs perturbations, respectively. The Vp/Vs variation scale (in %) is shown at the bottom. The A-A' line shows the location of vertical cross-section, used for showing Vp/Vs perturbations in Fig. 13b. Other symbols are same as in Fig. 11. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Please cite this article as: Mahesh, P., Gupta, S., The role of crystallized magma and crustal fluids in intraplate seismic activity in Talala region (Saurashtra), Western India: An ..., Tectonophysics (2016), http://dx.doi.org/10.1016/j.tecto.2016.05.025

P. Mahesh, S. Gupta / Tectonophysics xxx (2016) xxx–xxx

F

A

D e p th (k m )

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2011(Mw 5.1) 2007(Mw 4.8) 2007(Mw 5.0)

(b) D e p th (k m )

A'

F

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0

0

10

20

30

40

Distance (km) Fig. 13. (a, b): Vertical cross-sections of the Vp and Vp/Vs perturbations along the profile AA' (as shown in Figs. 11 and 12 ). The dashed line encloses the better resolved regions (based on SV, DWS and KHIT as in Figs. 7 and 8). The Vp and Vp/Vs variation scales (in %) are shown in the side of respective plots. Small open circles show the small earthquakes that occurred within 10 km width in either side of the profile and recorded during 2007–2012. The pink stars show the moderate magnitude Talala earthquakes (in years 2007 and 2011). The arrow along with “F” shows the location of ~NNE–SSW trending fault. The H/V is 1:1. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

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Please cite this article as: Mahesh, P., Gupta, S., The role of crystallized magma and crustal fluids in intraplate seismic activity in Talala region (Saurashtra), Western India: An ..., Tectonophysics (2016), http://dx.doi.org/10.1016/j.tecto.2016.05.025