1 Introduction

Seismicity and seismotectonics are studied for the assessment of earthquake hazard of Gujarat region, where even moderate earthquakes seem to have higher hazard potential in larger areas due to dense population, non-engineered structures and efficient transmission of wave energy. The study is important as large-scale development of the region is planned. Moreover, the knowledge gained here will go a long way in understanding earthquakes in other SCR regions, where they are less frequent.

Geographically the region is divided into three parts: The Kachchh Peninsula which is westernmost, the Saurashtra Peninsula south of it and the Mainland which is east of both these regions. The Kachchh region is seismically one of the most active intraplate regions of the World. It falls in zone V of the seismic zoning map of India with potential of M8. The other two regions mostly fall in zone III with M ≤ 6 potential. The Kachchh region was known to have low seismicity but high hazard in view of the occurrence of several large earthquakes but fewer moderate or smaller shocks. Besides the aftershocks, seismicity in Kachchh and Saurashtra has increased multifold in the first decade of the twenty-first century but not increased along Narmada zone or in other parts of the Mainland. Paper describes seismotectonics, earthquake monitoring and estimated return periods of large earthquakes in Gujarat state, western India as well as gives a catalogue of earthquakes of the region.

2 Tectonics

The Gujarat region is at the tri-junction of three failed rifts: Kachchh, Cambay and Narmada, with several active faults (Biswas 1987). These rifts were formed by rifting along major Precambrian trends. The rifting occurred at successive stages during the northward movement of the Indian plate after the breakup from Gondwanaland in the Late Triassic or Early Jurassic (Mesozoic era at about 184 Ma). The Kachchh rifting took place in the Late Triassic-Early Jurassic, Cambay rifting in Early Cretaceous and Narmada rifting in the Late Cretaceous. The rifting ceased in the Late Cretaceous era during the pre-collision stage of Indian plate. Post-collision, the Kachchh and Narmada rifts became zones of compression giving strike-slip and thrusting. The Kachchh and Narmada rifts have E–W trending major faults that are active, while the Cambay basin has N to NNW trending marginal faults that are less active. There are some smaller transverse strike-slip faults in Kachchh. South of Kachchh, in the Deccan volcanics of Saurashtra, the NW and NE trending smaller strike-slip faults are also activated in the form of moderate earthquakes in response to the plate-tectonics stress.

3 Seismic monitoring

Starting 1970s, seismicity was monitored in Gujarat with several analog seismographs in the mainland and at Bhuj (Fig. 1; Table 1). It is believed that this network has not missed any earthquake of M > 3 in Gujarat in last four decades. Bhuj was upgraded to digital broadband seismograph in 1999 by India Meteorological Department, and Dharoi broadband seismograph station was started since 2000. From 2001, a network of up to ten broadband seismograph stations was operated in Kachchh. From mid-2006, a dense network has been installed by ISR, which currently has 60 broadband seismographs and 49 accelerographs in and around Gujarat (Fig. 2; Tables 2, 3, Chopra et al. 2008a). Thirty-six stations are connected via VSAT to ISR data center, which works round the clock. The autolocation program enables the estimation of earthquake epicenter and magnitude within minutes of the arrival of seismic waves. The detectability is M2.0 in Kachchh active area and M2.5 in other areas.

Fig. 1
figure 1

Seismograph stations in and around Gujarat from 1977 to 2000

Full size image
Table 1 List of Seismograph stations in and around Gujarat since 1971 and their present status
Full size table
Fig. 2
figure 2

Seismograph stations in Gujarat Net including 36 online broadband seismographs, 24 off-line broadband seismographs and 49 accelerographs

Full size image
Table 2 List of present online seismographic stations in and around Gujarat
Full size table
Table 3 List of presently off-line seismograph stations in Gujarat
Full size table

4 Earthquake catalogue

The earthquake catalogue of Gujarat and the adjoining region bound by 20°–25.5°N and 68°–75°E (Table 4; Fig. 3) has been prepared from the earliest times to 2010 with the help of all available earthquake catalogues (historical and recent) pertaining to the region. Historical earthquakes of this region before 1900 were taken from the catalogue prepared by Oldham (1883). The catalogues prepared by Tandon and Srivastava (1974), Chandra (1977), Srivastava and Ramachandran (1985), Ramachandran and Srivastava (1991), Srivastava and Rao (1997) and Malik et al. (1999) for this region and the Indian subcontinent have also been taken into consideration for some other historical and modern earthquakes. The intensity values given for historical earthquakes have been converted to magnitude using relation between magnitude and intensity. The sources of modern seismicity database are India Meteorological Department (IMD), Geological Survey of India (GSI), NEIC-USGS, International Seismological Centre (ISC), Gujarat Engineering Research Institute (GERI), National Geophysical Research Institute (NGRI) and Institute of Seismological Research (ISR). Aftershocks have been removed from the whole catalogue.

Table 4 Catalogue of earthquakes in Gujarat region from earliest time to December 2010
Full size table
Fig. 3
figure 3

Epicenters of earthquakes of M2 or greater from 1684 to 2010 excluding aftershocks

Full size image

The earthquake catalogue is homogenized with moment magnitudes as generally recommended (Johnston 1994). The moment magnitudes have been taken from different sources and estimated by us from isoseismals for past large earthquakes as well as digital data for smaller recent earthquakes. For other earthquakes, m b or equivalent ML or the M estimated from intensity value is considered to be M w as the following empirical relation derived by Jaiswal (2010) indicates only a minor average difference of <0.1:

$$ M_{\text{w}} = m_{\text{b}} + \, 0.0 7\quad R^{ 2} = \, 0. 7 8\quad {\text{ for 4}}. 8 { } \le m_{\text{b}} \le { 7}.0 $$

For this study, data are taken from Harvard catalogue from 1976 onward and for earlier period (1411–2005) from some publications that includes 19 earthquakes of m b 4.8–6.9 and M w 5.0–7.6 for the period 1967–2005.

For the earthquakes prior to 1960, instrumental determinations of magnitude are not generally available. In such cases, magnitude, M, has been estimated from maximum intensity and Io by using the relation,

$$ M = { (2}/ 3 ) {\text{ Io }} + 1. $$

This formula gives a magnitude of 5 for intensity VI and a magnitude of 5.7 for intensity VII. The intensity data are taken from several Indian publications as well as from Martin and Szeliga (2010) and Szeliga et al. (2010).

As most of the regions have been populated for centuries, it is expected that for the last 200 years, no earthquake of magnitude ≥4 is missed as, such earthquakes are found to be felt strongly in wide areas. However, the catalogue completeness has been also assessed with respect to time, and it is observed that for the earthquake of magnitudes M4.0–4.5, it is complete for the period 1918–2009; for M4.6–5.0, it is complete for 1908–2009; for M5.1–5.5, it is complete for 1890–2009; and for M ≥ 5.5, it is complete for 1802–2009.

Earthquake catalogue of Gujarat and the adjoining region bound by 20°–25.5°N and 68°–75°E prepared for the period 1684 through 2010 contains a total of 242 earthquakes (Table 4) including 2 of M7.7 and 7.8, 2 earthquakes of M6 and 6.3, 27 earthquakes of M5–5.9, 68 earthquakes between M4.0 and 4.9, 89 earthquakes between 3.0 and 3.9, and 18 earthquakes of M2–2.9 (Table 5). The catalogue completeness has been assessed with respect to time, and it is observed that for the earthquake of magnitudes M ≥ 5.5, it is complete from 1802; for M5.1–5.5, it is complete from 1890; for M4.6–5.0, it is complete for 1908–2009; and for M4.0–4.5, it is complete from 1918. However, most of the regions being populated for centuries, it is expected that for the last 200 years, no earthquake of magnitude ≥4 is missed as such earthquakes are found to be felt strongly in wide areas. Out of these, five shocks of M ≥ 5.0 and about 25 strongly felt (M < 5) occurred in the decade after 2000.

Table 5 Magnitudewise no. of earthquakes in Gujarat from 1686 to 2010 excluding aftershocks
Full size table

5 Recurrence rates

Stochastic analysis of earthquakes of Gujarat region using Weibull, Gamma and Lognormal models indicated recurrence intervals of earthquake of M ≥ 5.0 in Saurashtra, Mainland Gujarat and Kachchh as 40, 20 and 13 years, respectively (Yadav et al. 2008).

6 Seismicity patterns and seismotectonics

The Kachchh region is considered seismically one of the most active intraplate regions of the World. It was known to have high hazard but low seismicity in view of the occurrence of several large earthquakes but fewer moderate or smaller shocks. Normally, the number of earthquakes increases ten times for every unit of lower magnitude. In Kachchh for two earthquakes of M7.8 and 7.7, earthquakes of M < 7 are fewer. In Saurashtra as part of regional seismicity, earthquakes of M < 6 have occurred on the eastern part and around West Cambay Fault near Bhavnagar. Narmada rift zone has experienced shocks of magnitude M5.4 at Bharuch in 1970, but M6 or more at east of Gujarat. The Cambay rift has shown less seismicity, which is confined more to its southern part and to M5.7 level.

Prior to 2001, Kachchh had experienced three large earthquakes: M7.8 Allah Bund (24.00°N 69.00°E) earthquake in 1819, M6.3 Lakhpat (23.80°N 68.90°E) earthquake in 1845 and M6 Anjar (23.30°N 70.00°E) earthquake in 1956. Smaller shocks include seven earthquakes of magnitude 5–5.6, and only 71 of M3.5–4.9. Other areas of Gujarat have experienced a few damaging earthquakes of magnitude <6, for example, the 1970 M5.4 earthquake in Bharuch (21.625°N 72.96°E) along the South Narmada fault. In Saurashtra, the two significant earthquakes were 1919 M5.7 Ghogha (22°N 72°E) and 1938 M5.7 Paliyad (22.40°N 71.80°E).

6.1 Seismotectonics of the Kachchh rift

Earthquakes (up to M w 7.8 level) occur in about 250 km × 150 km size Kachchh rift along E–W faults. The entire crust is brittle as evidenced by focal depths of earthquakes from near surface to 36 km. Sometimes hidden faults are active. As most parts of the basin experience seismicity, it is critically stressed. The pre-existing faults get reactivated due to strain accumulation or fault weakening. Several studies (monitoring of seismicity through 25 or more local broadband seismographs, active fault investigations, geophysical surveys and GPS network) for the 2001 Bhuj earthquake and aftershocks have given insights into seismogenesis of earthquakes in intraplate regions. The main shock and most of the aftershocks occurred along a south dipping (~45°) hidden fault from 10 to 40 km depth. Three-dimensional velocity structure obtained from inversion of travel times of P and S waves of aftershocks indicates a high Vp magmatic body at regional scale and in 10–40 km depth range (Singh et al. 2011). This body is probably acting as a stress concentrator due to density and rigidity contrast. A low-velocity fluid-filled patch in 20–30 km depth range in the hypocentral zone might have acted as barrier. In the epicentral region of 2001 earthquake, lithosphere is inferred to be hot and thin (only 70 km as compared to normal 100 km) and crustal thickness is also thin (34 km as compared to 40 km in the surrounding region) caused due to rifting at around 184 Ma. Restructuring of this warm and thin lithosphere might have occurred due to thermal plume at 65 Ma (Mandal and Pandey 2010).

6.2 Seismicity of Saurashtra

Saurashtra region is bounded on all the sides by boundary faults. However, no seismicity has been noticed along these faults except the West Cambay Fault that borders the eastern side that has experienced earthquakes of magnitude up to 5.7. Interior part of Saurashtra is either covered by volcanic Deccan Traps or Quaternary deposits due to which no clear fault has been identified and neither any significant earthquake has occurred. The earthquakes in this region are likely to be associated with small faults corresponding to Aravali trends in NW direction or Narmada trend in ENE direction. The focal depths are determined to 15 km.

6.3 Seismicity of mainland Gujarat

The two important tectonic features in the mainland region of Gujarat are the Narmada and Cambay rifts. The third tectonic feature, south of Narmada rift and considered part of it, is the Tapi rift zone that has low seismicity. Seismicity of magnitude about 6 has been associated with the ENE trending Narmada rift zone while of magnitude up to 5 along the NW trending Cambay and ENE trending Tapi rifts. The significant earthquake in this region was Bharuch M5.4 earthquake in 1970. It was followed by some aftershocks (Gupta et al. 1972). During 1986 in Valsad, south Gujarat, a sequence of foreshocks-aftershocks was experienced for about a year associated with M4.6 earthquake on April 30 (Rao et al. 1991). There is no other long sequence known to have occurred in Mainland Gujarat.

6.4 Enhanced seismicity during the 1st decade of the twenty-first century in Kachchh and Saurashtra

M w = m b + 0.07 R 2 = 0.78 for 4.8 ≤ m b ≤ 7.0w7.7 great Bhuj earthquake of 2001 is followed by damaging aftershocks over a decade. The aftershock activity includes over 10,000 shocks of M1–2.9, 2401 shocks of M3–3.9, 357 shocks of M4–4.9 and 20 shocks of M ≥ 5.0. In this region, M5.5 level seismicity continued until mid-2006. From mid-2006, there is no shock of M ≥ 5.0; however, shocks of M < 5.0 are continuously occurring once every 2 months or so. Until 2003, the activity concentrated along the 2001 rupture zone of 40 km × 40 km (Fig. 4) but migrated subsequently (Fig. 5). During 2006–2009, several nearby faults in Kachchh up to 100 km distance from Bhachau are activated with more than ten mainshocks of M4–5.7 occurred along different faults like South Wagad Fault, Gedi Fault, Gora Dungar Fault and Island Belt Fault. Most of these mainshocks are associated with their own foreshock-aftershock sequences. There is no such report that so many faults were active simultaneously in the known 200-year history. Hence, it can be said that during the first decade of the twenty-first century besides the aftershocks of 2001 earthquake, the seismicity (in terms of mainshocks alone along different faults) is substantially high in Kachchh compared to last twenty decades. No doubt that the detectability of earthquakes has improved with time, the space–time changes inferred here are gross and not much influenced by it.

Fig. 4
figure 4

Epicenters and focal depths of Kachchh mainshock and aftershocks during 2001 show that they were confined to NWF and 40 km × 40 km area. Nearly the same pattern continued for the next 2 years of 2002–2003 (NGRI data)

Full size image
Fig. 5
figure 5

Epicenters of earthquakes of M~1 or greater in Gujarat during 2001–2010 including aftershocks

Full size image

In 2000, Bhavnagar, Saurashtra, experienced damaging earthquake of M4.2 and several earthquakes associated with a months-long sequence (Bhattacharya et al. 2004). After 2006, more than twenty mainshocks of M3.0–5.0 with long sequences of foreshocks and aftershocks have occurred in the Saurashtra region along small faults (Fig. 3, Chopra et al. 2008b) at (1) Khankotda, Kalavad, Sanala, Moti Khavdi and Bhanvad areas in Jamnagar district, (2) Talala in Junagadh district, and (3) Sayala-Chotilla in Surendernagar district. The activity of these areas was monitored by local broadband seismograph (BBS) networks. The areal extent of the epicenters is 2–30 km. The shocks are shallower than 10 km and are associated with subterranean sounds.

The unusually high seismicity in Kachchh and Saurashtra since 2006 is suggested to be due to stress increase by stress perturbation due to 2001 Bhuj M7.7 earthquake up to 200 km distance in the south (ISR annual reports, isr.gujarat.gov.in). The sudden rise in water table by 30 m from pre- to post-monsoon in Saurashtra is inferred to cause 3 bars stress change that triggers small to moderate earthquakes.

7 Discussion

Seismicity in Gujarat is monitored from 1970s, with several analog seismographs. It is believed that this network has not missed any earthquake of M > 3 in Gujarat in last four decades. Digital broadband seismographs were deployed at Bhuj in 1999 and subsequently at 60 locations. Thirty-six stations are connected online and the autolocation program installed during 2011 gives information of earthquake epicenter and magnitude within seconds of the arrival of seismic waves.

A catalogue prepared for Gujarat indicates four severely damaging earthquakes of magnitude 5.4–7.8 (Kachchh 1819, Anjar 1956, Bharuch 1970 and Bhuj 2001) and two lightly damaging earthquakes of magnitude about 5 (Valsad 1986 and Bhavnagar 2000). For two earthquakes of M7.7 and 7.8, one expects a large number of M6 and 5 earthquakes according to Gutenberg-Richter relation. Missing of such earthquakes defines the region of high hazard but low seismicity.

Aftershocks of 2001 earthquake are continuing for over a decade. Post-seismic deformation can be large and long lasting. For example, the post-seismic deformation for 1964 Alaska earthquake is 15–20 mm/year even 46 years after and for 1967 Koyna earthquake even after 42 years.

Seismicity has increased several fold in Kachchh after the Bhuj (23.44°N 70.31°E) earthquake of M w = m b + 0.07 R 2 = 0.78 for 4.8 ≤ m b ≤ 7.0w7.6 in 2001 as some ten small to moderate mainshocks with associated foreshocks and aftershocks occurred along a number of faults besides the continuing damaging aftershocks of M4 and 5 for over a decade in the 2001 rupture zone. In Saurashtra, south of Kachchh and a region of relatively much low seismicity, and also the number of small to moderate earthquakes increased substantially (Chopra et al. 2008b) when over 20 felt/damaging shocks of M3–5 occurred at a dozen different locations. Long sequences of shocks for months/years were experienced at three locations. Even though the detectability has improved since 2006, the increase in seismicity is anomalous.

The seismicity in Kachchh was found to have been extended first eastward and north-eastward and then in the north along newly activated faults from 2006 to 2008. Further, it extended to the Saurashtra region, in the south since 2006. The seismicity seems to be affected by the 2001 Great Bhuj Earthquake.

In Saurashtra, at unusually large number of places, swarm type of activity is associated with earthquakes of M3.5–5. This is unusual comparing the catalogue of last 200 years. Though the stress pulse may propagate horizontally through lower crust and upper mantle, it is transferred vertically to faults at near surface. The seismicity in Saurashtra is very shallow with 5–6 km focal depth. Seismicity in Kachchh too is shallower than 10 km.

The increase in seismicity in Gujarat after 2001 earthquake appears to have been triggered by critically stressed preexisting faults.

8 Conclusions

Gujarat has active tectonics along E–W faults in the Mesozoic rift zones of Kachchh and Narmada with earthquakes of M8 and M6, respectively. Cambay rift is less active and has experienced earthquakes in its southern part to M5.7 level. Seismicity is monitored for last four decades, and earthquakes of M > 3 are not likely to be missed during this period. Catalogue of earthquakes for last 325 years includes independent 242 mainshocks of magnitude 2–7.8. The three earthquakes of Allahbund 1819 (M7.8), Anjar 1956 (M6) and Bharuch 1970 (M5.4) have been severely damaging, while 2001 Bhuj earthquake of M7.7 has been devastating. Twenty-seven other earthquakes were moderate with M5–6. In the first decade of the twenty-first century, several faults in Kachchh and Saurashtra have become active due to stress increase caused by 2001 Bhuj earthquake. In Saurashtra, 30 m rise in water table during monsoons adds additional 3 bars stress triggering earthquakes during September to November.