Spatial variation of crustal strain in the Kachchh region, India: Implication on the Bhuj earthquake of 2001

Journal of Geodynamics 61 (2012) 1–11

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Spatial variation of crustal strain in the Kachchh region, India: Implication on the Bhuj earthquake of 2001 Sushmita Sinha, S. Mohanty ∗ Department of Applied Geology, Indian School of Mines, Dhanbad 826004, India

a r t i c l e

i n f o

Article history: Received 30 September 2011 Received in revised form 26 June 2012 Accepted 5 July 2012 Available online 16 July 2012 Keywords: Intraplate seismicity Neotectonics Strike–slip zones Strain partitioning Strength envelope

a b s t r a c t The Kachchh province of Western India is a major seismic domain in an intraplate set-up. This seismic zone is located in a rift basin, which was developed during the early Jurassic break-up of the Gondwanaland. The crustal strain determined from the GPS velocity data of post-seismic time period following the 2001 Bhuj earthquake indicates a maximum strain rate of ∼266 × 10−9 per year along N013◦ . Focal mechanism solutions of the main event of 26 January 2001 and the aftershocks show that the maximum principal stress axis is close to this high strain direction. Maximum shear strain rate determined from the GPS data of the area has similar orientation. The unusually high strain rate is comparable in magnitude to the continental rift systems. The partitioning of the regional NE–SW horizontal stress (SHmax ) by the preexisting EW-striking boundary fault developed the strike–slip components parallel to the regional faults, the normal components perpendicular to the faults, NE-striking conjugate Riedel shear fractures and tension fractures. The partitioned normal component of the stress is considered to be the major cause for compression across the regional EW faults and development of the second-order conjugate shear fractures striking NE–SW and NW–SE. The NE-striking transverse faults parallel to the anti-Riedel shear planes have become critical under these conditions. These anti-Riedel planes are interpreted to be critical for the seismicity of the Kachchh region. The high strain rate in this area of low to moderate surface heat flow is responsible for deeper position of the brittle–ductile transition and development of deep seated seismic events in this intraplate region. © 2012 Elsevier Ltd. All rights reserved.

1. Introduction The distribution of earthquakes on the surface of the earth is mostly associated with the lithospheric plate boundaries and is less common in the intraplate regions. The intraplate regions account for the release of ∼5% global seismic energy (Schulte and Mooney, 2005). However, these regions can deform at a relatively rapid rate and attain the strain rate comparable in magnitude to that of the lithospheric plate boundaries (Liu and Zoback, 1997; Mohanty, 2011). The cumulative strength of the lithosphere in the intraplate regions is comparable to some of the plate margins and has the potential to cause major seismic hazards (Mohanty, 2011). Several hypotheses have been proposed to explain the seismicity of intraplate regions. These include the reactivation of the pre-existing fault zones due to the disturbances in local stress regime related to the plate boundary forces (Sykes, 1978; Hinze et al., 1988); stress concentrations around intersecting faults (Talwani, 1988); stress amplification near plutons (Campbell, 1978; Stevenson et al., 2006); lithospheric flexure (Bilham et al., 2003);

∗ Corresponding author. E-mail address: [email protected] (S. Mohanty). 0264-3707/$ – see front matter © 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jog.2012.07.003

localized strain in mid-lower crust (Zoback et al., 1985); stress amplification around regional gravity lows (Chandrasekhar et al., 2005); and stress concentration due to lateral variation of density (Sonder, 1990). Thus, the intraplate earthquakes occur due to localised stress build up in response to plate tectonic forces in the vicinity of intersecting faults, which act as stress concentrators within a pre-existing zone of weakness. The Kachchh province in the western part of India (Fig. 1) is a major seismic domain in the intraplate set-up with an historical seismicity of large intensity. The region has experienced two Mw > 7.7 earthquakes in a span of 182 years (Mandal et al., 2004a,b). While the 1819 Kachchh earthquake did cause surface rupture (Bilham, 1999), the causative fault of the 2001 Bhuj earthquake did not reach the surface (Mandal et al., 2004a; Bodin and Horton, 2004). Like all large (Mw > 7.5) intraplate earthquakes, both the 1819 Kachchh and the 2001 Bhuj earthquakes took place within an old rift that was tectonically reactivated under compression (Biswas, 1982, 1987; Biswas and Khattri, 2002; Talwani and Gangopadhyay, 2001; Bodin and Horton, 2004). The far-field stresses that could drive the Kachchh rift are influenced by the Indo-Eurasian collision, and by the push from the Mid-oceanic Ridges of the Indian Ocean (Cloetingh and Wortel, 1986; Gombos et al., 1995; Chandrasekhar and Mishra, 2002). The local stress

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Fig. 1. Map showing tectonic framework of the Kachchh area (marked by a box). Plate margins, and major fold-faults are shown by red and blue colours, respectively. CF = Chaman fault; HF = Herat fault; KF = Kathiawar fault; KR = Cambay rift; NPF = Nagar Parkar fault; OF = Owen fault; and SR = Sulaiman range. Velocity vector shown for the Indian plate is with respect to the Eurasian plate.

concentrations possibly took place along the previous structural fabrics by the lithospheric forces arising from the plate boundaries. The present work was undertaken to analyse the fault patterns and crustal strain in the Kachchh region, and to examine the tectonic controls on the seismicity of the region. 2. Regional geology of the Kachchh area The Kachchh region occupies a pericratonic rift basin at the western most part of the Indian craton (Fig. 1). The basin is located to the southeast of the Makran triple junction of the Arabian–Eurasian–Indian plates. The Owen fracture zone and the Chaman fault system define the Indian plate margin to the west of the basin. Two EW-striking faults, the Nagar Parkar fault and the Kathiawar fault, define the northern and southern boundaries of the Kachchh rift basin (Figs. 1–3). The Radhanpur arch in the east separates this EW-trending basin from the NW-trending Cambay–Barmer rift system (Fig. 1). The Kachchh basin was developed during the late Triassic–early Jurassic (∼200 Ma) break-up of the Gondwanaland, when the East Gondwana (India, Madagascar and Australia) was separated from the African east coast of West Gondwana (Biswas, 1982, 1987). The sediments of this rift basin were deposited over the Neoproterozoic granite porphyry and rhyolite forming the basement. The basement rocks are exposed in the Nagar Parkar area on the North, and the unconformable relationship between the granitic basement and Mesozoic sediments is found in a deep well at a depth

of 1718.5 m below the Banni plain of Kachchh (Singh et al., 1997). The separation of Madagascar from the western coast of India took place during the late Cretaceous time (at ∼80 Ma). Subsequently, the Kachchh basin and the Western Indian region were associated with volcanic eruption (Deccan Traps) at ∼65 Ma, when the Indian plate passed over the Reunion plume during late Cretaceous to early Palaeocene (Morgan, 1981). Sedimentation in the basin continued during Tertiary and Quaternary, and had more than 2000 m thick sediments during the Mesozoic and the Cenozoic Era. The basin scale unconformity surfaces between the Mesozoic sediments, the Deccan Traps and the Paleogene sediments indicate the response of the basin to the regional tectonic movements and to the global changes of the sea-level. The collision of India and Eurasia in the Eocene–Miocene time changed the stress regime of the Kachchh basin and resulted in basin inversion by the NS-compressive stress field. Besides the two bounding faults, the rifting developed other major regional EW-trending faults such as the Island Belt fault (IBF), the Kachchh Mainland fault (KMF) and the Katrol Hill fault (KHF), which gave rise to three major highlands or uplifts at the southern boundary of the faults: the Island Belt, the Kachchh Mainland Belt and the Katrol Hills Belt (Fig. 2). These topographic high areas are affected by several transverse faults with NE–SW and NW–SE strike (Biswas, 1987; Sohoni et al., 1999; Thakkar et al., 1999; Maurya et al., 2003). The NE-trending faults were mapped to the West of Wagad and Bela uplifts, West of Khadir and West of Pachcham Islands (Fig. 2). The NW-trending faults

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Fig. 2. Geological map of the Kachchh basin (modified from Biswas, 1987). Stars with numbers indicate the epicentres of three major earthquakes with year and magnitude. Focal mechanism solutions for two of these are also given. Inset: Location map of the Kachchh basin in India.

Fig. 3. Outline map of the Kachchh region, showing fault patterns and GPS sites (circles).

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Fig. 4. Map showing the position and attitude of the faults mapped around Bhuj.

are traced as the Allah bund fault, the Goradungar fault, the Banni fault and the Vigodi fault (Fig. 2). Most of these transverse faults were developed during Neotectonic activity associated with the earthquakes in the basin. We have mapped many such secondorder transverse faults close to the epicentres of the 1956 Anjar

earthquake and the 2001 Bhuj earthquake (Fig. 4). The overall distribution of the strike directions of the faults measured by us from the entire Kachchh area indicates dominance of four strike directions: N020◦ –N040◦ , N80◦ –N090◦ , N100◦ –N120◦ , and N170◦ –N180◦ (Fig. 5).

Fig. 5. Rose diagram of the distribution of strike directions of faults in the Kachchh area.

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3. Seismotectonic activities of the Kachchh area The geodynamics of the Kachchh basin is characterised by frequent earthquake episodes. This region falls under seismic zone V of the Indian subcontinent and has experienced two major earthquakes of Mw > 7.5. The area suffered the largest documented earthquake on June 6, 1819 (Oldham, 1926), which was associated with the development of one ∼EW trending fault of 100 km surface length forming a ridge near India–Pakistan border, known as the Allah bund (Figs. 2 and 3; Johnston, 1994; Bilham, 1999; Rajendran and Rajendran, 2001; Rajendran et al., 2001). This fault with 3–4 m uplift on the North cuts through the recent sediments and is parallel to the strike of the Nagar Parkar fault (Biswas, 1980). A palaeo-seismic investigation of the Allah bund feature has indicated an earlier event on the same fault about 800 years ago, which may correspond to the earthquake reported in 1030 ad (Rajendran and Rajendran, 2001). Additionally, the Indus delta to the northwest of the region had experienced another moderate size earthquake of 7.6 magnitude in 1668 (Rajendran and Rajendran, 2001). A damaging earthquake of Mw ≈ 6.5 (intensity IX) called the Anjar earthquake occurred in 1956, and had its epicentre at 23◦ 3 N:70◦ E (Fig. 2) south of the KMF (Karanth et al., 2001). The depth of focus and the fault plane solution of this event were obtained using waveform modelling of the teleseismic body waves (Fara, 1964; Chung and Gao, 1995). The focal depth was constrained to be 15 ± 3 km, and the fault plane solution (Fig. 2) suggested thrust fault mechanism with nodal planes striking 237◦ and 331◦ with dips of 65◦ westerly and 81◦ easterly, respectively (Fara, 1964; Schulte and Mooney, 2005). The Bhuj earthquake of January 26, 2001 (epicentre at 23◦ 4 N:70◦ 3 E; and Mw 7.7) was localized at a depth of 23 km (Biswas and Khattri, 2002) (Fig. 2). This earthquake was located close to the epicentre of the 1956 Anjar earthquake. The fault plane solution (Fig. 2) indicates a reverse oblique–slip displacement. One of the nodal planes strikes 292◦ and dips at 36◦ towards NW with a rake of 136◦ , and the other strikes 60◦ with a dip of 66◦ towards SE and rake of 62◦ (Biswas and Khattri, 2002). The main event was followed by large number of aftershocks, some of which were of Mw > 5. The isoseismals of all the three major earthquakes are similar with elongation parallel to the rift axis, suggestive of a genetic relationship between these geological features and the earthquakes. Chung and Gao (1995) compared the seismological characteristics of the two major earthquake events of Kachchh in 1819 and 1956 and concluded that these earthquakes were generated under similar tectonic stress field (i.e. N–S compression) and the reactivation of the KMF. The 2001 Bhuj earthquake was also related to the reactivation of the KMF. Geophysical investigations indicate a complex and heterogeneous crustal structure in the Kachchh region (Biswas, 1987; Gombos et al., 1995; Patriat and Achache, 1984; Kayal et al., 2002; Bodin and Horton, 2004; Mandal, 2006, 2007; Mandal and Pujol, 2006; Mandal and Chadha, 2008). The global positioning system (GPS) data suggest very slow strain accumulation in the Kachchh region (Jade et al., 2003; Mandal et al., 2004a,b).

Fig. 6. Diagram illustrating the method of strain analysis used in the present study. (a) Positions of two GPS sites X and Y, the northward and eastward velocity components and the resultant vector of each point. (b) The resultant vectors at oblique angles to the site vector XY are resolved into components parallel to the site vector (LX and LY ) and perpendicular to the site vector (SX and SY ). The longitudinal strain (e) = [(LY − LX )/XY] and the shear strain () = [(SY − SX )/XY]. The azimuth of e is the angle between XY and the north direction (i.e. Az), and the azimuth of the shear strain is the angle between the normal to XY and the north direction (i.e. 90 − Az).

4. Crustal strain patterns in the Kachchh region 4.1. Computation method Analysis of crustal strain in the Kachchh region was carried out to understand the cause of seismicity. Strain determination was done by analysing the global positioning system (GPS) measurements of the geodetic velocities at 11 sites (RAPR, RATN, DHAM, GAND, LODA, BIRN, BHUJ, MAND, NAKA, NALI, and NARA; Fig. 3). The GPS velocity data of the Kachchh region (Table 1) processed by the GAMIT/GLOBAK software and given in the International Terrestrial Reference Frame (ITRF2000) (Dong et al., 1998; McClusky et al., 2000; Altamimi et al., 2002) were obtained from Reddy and Sunil (2008). Strain determination was done from the velocity data using the method outlined by Mohanty (2011). For any point (e.g. X) the site velocity given in terms of North (NX ) and East (EX ) velocity components was converted to the resultant velocity (RX ) (Fig. 6a). For strain analysis between two points (e.g. X and Y) the resultant vectors (RX and RY ) were resolved into components (LX and LY ) parallel to the line joining the points and components (SX and SY ) perpendicular to the line (Fig. 6b). The longitudinal strain rate (e) ˙ was determined from the ratio of the differences of the components along the line and the distance between the two points ([LY − LX ]/XY). The shear strain rate () was determined from the ratio of the differences between the orthogonal components and the distance between the points ([SY − SX ]/XY). The azimuth of the

Table 1 GPS stations with North and East velocities (from Reddy and Sunil, 2008). Station code

Station name

Latitude

Longitude

VN (mm/yr)

RAPR RATN DHAM GAND LODA BIRN BHUJ MAND NAKA NALI NARA

Rapar Ratanpur Dhamadkapir Gandhidham Lodai Birandiyar Bhuj Mandvi Nakhatrana Naliya N. Sarovar

23.568 23.86 23.332 23.069 23.394 23.662 23.254 22.834 23.355 23.257 23.677

70.644 70.363 70.143 70.095 69.892 69.707 69.654 69.354 69.255 68.835 68.541

33.19 28.60 44.64 37.90 32.88 31.17 34.48 36.93 33.32 32.25 34.41

± ± ± ± ± ± ± ± ± ± ±

0.59 0.67 0.60 0.61 0.81 0.80 0.70 0.64 0.69 0.74 0.66

VE (mm/yr) 38.48 35.82 32.58 34.57 31.76 34.94 39.30 41.02 35.68 34.26 35.39

± ± ± ± ± ± ± ± ± ± ±

1.37 1.55 1.39 1.38 1.82 1.77 1.70 1.39 1.58 1.49 1.40

Reference frame ITRF2000 ITRF2000 ITRF2000 ITRF2000 ITRF2000 ITRF2000 ITRF2000 ITRF2000 ITRF2000 ITRF2000 ITRF2000

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Fig. 7. Spatial variation of the magnitude of (a) longitudinal strain rate (e) ˙ and (b) shear strain rate () in the Kachchh region.

longitudinal strain is the angle between the geographic north and the site vector (i.e. Az of XY in Fig. 6b) and the azimuth of the shear strain is obtained by adding ±90◦ to the azimuth of the site vector. As strain is a direction dependent quantity, the variations of longitudinal and shear strain with different geographic azimuths were graphically analysed from azimuth versus strain plots (Fig. 7). 4.2. Results The variations of longitudinal strain show a maximum extension rate of 203 × 10−09 per year towards N013◦ and also a maximum shortening rate of −266 × 10−09 per year towards N013◦ (Fig. 7a). This gives the idea of periodic accumulation and release of strain at this orientation. Therefore, this orientation is interpreted to be critical for the seismicity of the Kachchh basin. Two second order peaks at N061◦ (shortening) and N100◦ (extension) are also noted. These directions are interpreted to be parallel to the Riedel shear (N061◦ ), the conjugate Riedel shear (N13◦ ), and the P shear (N100◦ ) fractures formed due to a first-order compression along NE–SW direction, resolved into a master EW strike–slip fault. Similar results were also obtained from the variation pattern of shear strain, which shows two first order peaks at N015◦ (sinistral shear) and N146◦ (dextral shear) (Fig. 7b). These are symmetrical with respect to the North direction. This symmetrical pattern of shear with

opposite sense of movement indicates a regional shortening along NS direction, which is responsible for sinistral motion along the plane with NNE–SSW strike and dextral motion along the plane with NNW–SSE strike. The strain analysis of the GPS data by Reddy and Sunil (2008) had identified a maximum contractional rate of −300 × 10−09 per year towards N011◦ and a minimum extension rate of 60 × 10−09 per year towards N101◦ . The average principal strain rate of −70 × 10−09 per year contraction and 40 × 10−09 per year extension was reported by Reddy and Sunil (2008). Our analysis corroborates these reported high strain rates, and provides additional information about the variation in shear strain rate and presence of conjugate orientations of high shear strain rates with opposite signs symmetrically located around the maximum principal stress axes of the focal mechanism solution of the January 2001 Bhuj event. Our results also show that the direction having maximum longitudinal shortening rate has maximum longitudinal elongation rate, and both these high longitudinal strain rates are close to the direction of maximum shear strain rate. Therefore, we interpret this orientation (N013◦ ) to be critical and most vulnerable direction. 5. Proposed model for the seismicity of the Kachchh basin Investigation of the regional stress pattern indicates broadly uniform horizontal stress orientations and magnitudes over large

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Fig. 8. Conceptual model section of the Kachchh basin (from Biswas, 2005).

parts of the Earth (Zoback and Zoback, 1989). The orientation of the maximum horizontal compressive stress (SH) correlates reasonably well with both absolute and relative plate motions on a global scale. This suggests that the main source of stress is derived from the plate driving forces (Richardson and Reding, 1991). Seismicity in the continental interior is regionally non-uniform and is clustered locally along pre-existing weak zones. The Kachchh region is located in a rift environment that was developed by tensional stresses during the Mesozoic Era. However, the focal mechanism solutions of earthquakes during last few decades show a compressive stress regime with ∼N–S trending P-axes (Biswas, 1987; Chung and Gao, 1995; Antolik and Dreger, 2003). Thus, the present day seismological data suggest that the region is possibly undergoing a stage of inversion tectonics of an extensional basin (Gupta et al., 2001; Antolik and Dreger, 2003). On the basis of the intersections of the northern boundary fault near Nagar Parkar, and the South Wagad fault (SWF) in the inner part of the basin, both dipping towards south, with the northerly dipping Kachchh Mainland fault (KMF), and a mafic pluton near the intersection point (Fig. 8) were considered to be the main stress concentrators for the 26th January 2001 event (Mandal et al., 2004a,b; Biswas, 2005). On the basis of one-dimensional inversion of P and S wave data from the 600 aftershocks of 2001 Bhuj event, Mandal et al. (2004a,b) identified high crustal velocities of Vp (6.98 km/s), Vs (3.854 km/s) and Vp /Vs (1.81) in the depth range of 20.5–30 km. This zone was interpreted to be a mafic intrusive (with high crack density and probably fluid filled). Roy and Ram (2006) also confirmed such an interpretation from fractal analysis of the 2001 Bhuj event and its aftershocks. However, the gravity map of the Kachchh area shows two centres of mass deficiency under the southern part of the Kachchh Mainland and the Wagad uplift, separated by a NW–SE trending strike–slip fault (Chandrasekhar and Mishra, 2002). From the model isostatic regional pattern, the crustal thickness in this region is estimated to be 34–44 km (Chandrasekhar and Mishra, 2002). The magnetotelluric investigation along a profile across the epicentre of the 2001 Bhuj earthquake indicated presence of a low resistive zone at a depth of 5–20 km (Naganjaneyulu et al., 2010). However, no evidence of fluid activity and mafic pluton was found (Naganjaneyulu et al., 2010). The first-order stress pattern for the 2001 Bhuj earthquake is considered to be the plate margin stresses. The nearest plate boundary is 300–400 km towards West of Bhuj (Fig. 1). This boundary is dominantly a transform boundary with sinistral motion along the Chaman fault and Owen fault. The motion the Indian plate has slight obliquity with the boundary and has developed a zone of transpression in the Sulaiman region (Fig. 1). Therefore, this zone of transpression located to the North of Bhuj will have NS directed stress component acting on the pre-existing faults of the Kachchh region. The major component of the regional stress is considered to be the ridge-push from the Carlsberg Ridge in the

Indian Ocean and the collision resistance at the northern margin of the Indian plate along the Himalayan Mountains. The GPS data for the Indian Peninsula indicate its movement at a rate of approximately 52–63 mm/yr towards northeast and approximately 50% drop along the Himalayan orogenic belt (Fig. 1) along the northern margin of the Indian and Eurasian plates (Bilham and Gaur, 2000; Burgmann et al., 2001). This compressive force due to the collision controls the current pattern of seismotectonic activity inside India (Mohanty, 2011). In a region undergoing compression, two conjugate Riedel shear fractures develop symmetrically at an angle of ∼30◦ around the compression axis (Fig. 10a). This deformation pattern sometimes results in the development of a master shear at an angle of 45◦ from the compression axis. One of the Riedel shear fractures (at low angles to the master shear) shows sympathetic shear sense (R-shear) and the other Riedel shear (high angle to the master shear) develops antithetic shear sense (R -shear) with the master shear. The regional deformation pattern of the Kachchh region has been interpreted with this background. The Kachchh region lies in a mid-continental stress province where the mean orientation of the maximum compression is N030◦ (Gowd et al., 1992). However, analysis of plate motion and GPS data indicate maximum compression in the Indian plate to be ∼NE–SW (Burgmann et al., 2001; Mohanty, 2011). The major principal horizontal stress direction of the eastern Kachchh determined from the geogeneic electromagnetic radiation (EMR) is reported to be towards N060 ± 10◦ (Mallik et al., 2008). This maximum horizontal stress axis is oblique to the EW striking master faults in the Kachchh rift basin. The obliquity is responsible for the partitioning of the NE–SW compressive stress by the EW master faults to result in a NS component across the master faults, and an EW strike–slip component along the master faults. The regional stress is also responsible for the development two Riedel shears, oriented NNE–SSW (dextral R shear) and ENE–WSW (sinistral Rshear). Due to the stress partitioning, the region is undergoing EW stretching associated with strike–slip motion and the second order compression along NS. This accounts for the orientation of local maximum compressive stress along NS as determined from the seismicity data. It may be noted that the R -shear, which has dextral sense from the first-order NE–SW compression, tends to move in a sinistral sense by the second-order NS compression (Fig. 10b). Therefore, this orientation is expected to accumulate stress periodically and release the same during opposite motion. This pattern of movement is confirmed from the GPS strain measurements which gives N013◦ as the critical orientation for the seismicity of the Kachchh basin. An analysis of the aftershock data of 2001 Bhuj earthquake by Kayal et al. (2002), De et al. (2003) and Kayal and Mukhopadhyay (2006) recorded intense clustering of the aftershocks between two directions trending NE and NW, on both sides of the mainshock. The observed conjugate rupture propagation

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Fig. 9. Focal mechanism solutions of the mainshock (M.S.) and selected aftershock events (a–e) of the 2001 Bhuj earthquake (from Mishra and Zhao, 2003).

with two trends (i.e. NE and NW) suggests shear adjustments by fault interactions as the main cause for the main shock and the aftershocks (Kayal and Mukhopadhyay, 2006). The role of these two transverse fault systems in stress adjustments has been highlighted in the focal mechanism solutions (Fig. 9) of large number of aftershock events (Kayal et al., 2002; De et al., 2003; Kayal and Mukhopadhyay, 2006). A complex interplay between the movement patterns along the regional faults has given rise to periodic accumulation and release of stress along the antithetic Riedel shear planes striking NNE–SSW. Therefore, the fractures with NNE–SSW strike are considered to be critical for seismicity of the Kachchh region.

One of the major features of the 2001 Bhuj earthquake was development of the main dislocation at a depth of 23 km in the aseismic crust. More than 75% of 1200 aftershocks were generated at a focal depth of >25 km (Fig. 11; Mandal et al., 2004a). High magnitude of the main event also indicates higher rigidity of the crust at deeper level of the Bhuj area. Thermal stresses and presence of fluids, commonly considered to be the local stress concentrators, have the tendency to reduce the rigidity of the deforming crust. The MT investigations failed to detect fluids and any magmatic body below the epicentre of the 2001 Bhuj event (Naganjaneyulu et al., 2010). Therefore, the roles of high heat flow and presence of fluids to generate the high magnitude earthquake of the Kachchh area

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Fig. 10. (a) Relationship between the stress axes and shear planes in strike–slip zones and (b) the model for seismicity in the Kachchh region, showing the normal component (CN) across the regional (EW) strike–slip zone and the critical fault planes (FP1 and FP2).

become questionable. An enhanced strain rate model for the lithosphere with normal rate of the surface heat flow is proposed here to explain the deeper events (Fig. 12). The distribution of earthquakes inside the lithosphere has been explained by the variations in the crustal strength using a model of a strong upper crust, weak lower crust and a strong upper mantle (commonly referred as the “jelly sandwich model”; Ranalli and Murphy, 1987; Shudofsky et al., 1987; Burov and Diament, 1995; Watts and Burov, 2003; Afonso and Ranalli, 2004). The geometry of the strength envelope for the “jelly sandwich model” depends on the thickness of the crustal layers, the rate of surface heat-flow and the strain rate. Johnston (1994) analysed the seismicity data in the stable continental regions and found that the rate of deformation in these areas is about 10−10 to 10−09 per year whereas the plate boundaries have deformation rate of ≈10−07 per year. The strength envelope for a moderate surface heat-flow rate of

Fig. 11. Histogram of the focal depth distribution of the aftershocks of the 2001 Bhuj event (from Mandal et al., 2004a).

52 mW/m2 indicates that the upper crustal rigidity can extend to a maximum depth of ∼20 km at a low strain rate condition, and extend further downward at higher strain rate (Fig. 12a; Shudofsky et al., 1987). On the other hand, the upper crustal rigidity is reduced at high heat-flow rate condition; but low heat-flow rate can cause the whole crust to be a single rigid block (Fig. 12b; Shudofsky et al., 1987). Mandal and Pandey (2010) made an attempt to prepare a strength envelope for the Kachchh region using GNStress software version 2.16 (Robinson, 2005). This model had used a two-layered thin (34 km) crust with high heat flow (60 mW/m2 ) and weak upper mantle, and reported brittle–ductile transitions at 4 and 34 km depths, but another such transition beyond 20 km depth shown in the strength envelope was not reported (Mandal and Pandey, 2010). Another feature of the strength envelope prepared by Mandal and Pandey (2010) is of a very low differential stress value (maximum 150 MPa). Since a low differential stress value is unlikely to generate a high magnitude event (with high stress drop of ∼35 MPa; Copley et al., 2011), we have evaluated the crustal strength envelope in terms of high strain rate. It may be noted that the average thickness of the crust in the Kachchh region is ∼38 km (see discussions earlier) and is comparable to the crustal model shown in the analysis by Shudofsky et al. (1987). The surface heat flow measurements from the Kachchh region are not yet available. The heat flow atlas of India indicates the area to be in a zone with moderate heat flow value between 40 and 70 mW/m2 (Ravi Shanker et al., 1991; Thussu, 2002). The Curie isotherm depth estimated from the aeromagnetic data indicates the heat flow in the Kachchh region to be low and comparable to the stable shields (Rajaram et al., 2009). Therefore, the upper limit of heat flow value in the Kachchh region can be considered to be less than 50 mW/m2 . The present analysis has shown that the strain rate in the Kachchh area is relatively high (with a maximum value of ∼266 × 10−9 per year) and is nearly comparable to the plate boundary strain rate. A majority of the strain rate data have a magnitude of ∼50 × 10−9 per year (≈1.58 × 10−15 s−1 ). The high strain-rate of the area is considered to be the main cause for shifting of the brittle–ductile transition to the deeper level of the crust (Fig. 12). It may be noted that with a heat flow value of 40–50 mW/m2 , the deeper events in a 38 km thick continental crust require a strain rate of 10−15 s−1 to have a brittle crust. The strain rate, heat flow and crustal thickness of the Kachchh area match these requirements for the deeper events.

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Fig. 12. Lithospheric strength envelopes for (a) three different strain rates for Zambia using surface heat flow (qs ) of 52 mW/m2 , heat production (A0 ) of 2.4 ␮W/m3 and thermal conductivity (˛) of 3.0 W m−1 K−1 , and (b) three different surface heat flow for deep seismic events of East Africa using strain rate of 10−15 s−1 , heat production (A0 ) of 2.0 ␮W/m3 and thermal conductivity (˛) of 2.5 W m−1 K−1 (adapted from Shudofsky et al., 1987). Both the diagrams show the temperature variations for different rates of heat flow. The upper limit of temperature for crustal earthquakes is indicated.

6. Conclusions Stable continental region is no longer recognised as a continental interior of low seismic activity. The present study delineates interaction of the local heterogeneities with the regional tectonics as the causative factor for the seismicity of the Kachchh basin. Due to the appropriate orientation of the pre-existing boundary faults with respect to the regional horizontal stress direction (NE–SW) the SHmax is being partitioned into EW strike–slip component along the pre-existing faults and the NS component normal to the faults. This partitioning has caused the development of Riedel shears along NNE–SSW and ENE-WSW. The antithetic Riedel shears oriented towards N13◦ have opposite shear sense associated with the NE–SW first-order stress (resulting in dextral slip) and the second order maximum compressive stress along NS direction (causing sinistral slip). This orientation is considered to be critical for the seismicity of the Kachchh region and tends to accumulate stress and release the same during two opposite movement patterns. The inferences are substantiated by the strain analysis from the GPS data and observed distribution pattern of earthquake epicentres. A high strain-rate mechanism combined with low to moderate heat flow rate in the area can be invoked for explaining the deep events of the Kachchh area. Acknowledgements The work was carried out with financial assistance from the Ministry of Earth Sciences, Government of India. Helpful suggestions from Prof. P. Talwani and an unknown reviewer are thankfully acknowledged. The authors acknowledge help received from Dr. Rajni Singh. We also extremely thankful to Prof. R. Stephenson for constructive suggestions. References Afonso, J.C., Ranalli, G., 2004. Crustal and mantle strengths in continental lithosphere: is the jelly sandwich model obsolete? Tectonophysics 394, 221–232. Altamimi, Z., Sillard, P., Boucher, C., 2002. ITRF2000: a new release of the International Terrestrial Reference Frame for earth science application. Journal of Geophysical Research 107 (B10), 2214, http://dx.doi.org/10.1029/2001JB000561.

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