Source Parameters and Tectonic Implications of Aftershocks of the Mw 7.6 Bhuj Earthquake of 26 January 2001

We present and discuss the spatial distribution of more than 1000 aftershocks of the largest continental intraplate earthquake to occur during the modern seismological period. The data were recorded on a network of eight portable digital seismographs deployed for 3 weeks starting 17 days after the mainshock. We have calculated high-quality single-event locations, based on a 1D velocity model determined for the region for earthquakes with magnitudes between ∼2 and 5. Aftershock locations reveal activity concentrated on a nearly east-striking, south-dipping plane, trapezoidal in outline. The active zone tapers from about 45 km along strike at the shallow end, which is about 5 km deep, to no more than 25 km long at a depth of 35 km. The total rupture area was about 1300 km 2 . We estimate the static stress drop of the mainshock at 16 ± 2 MPa. Aftershocks extend nearly through the entire crust, with concentrations in the lower crust at about 26 km and in the upper crust at about 10 km. The fault that ruptured was not mapped at the surface and not known to have been active prior to the 2001 earthquake. The aftershock data are consistent with the Bhuj earthquake resulting from reactivation in contraction of a fault formed under extension within a failed rift.

[1]  A. B. Roy.  Geological and Geophysical Manifestations of the Reunion Plume-Indian Lithosphere Interactions-Evidence from Northwest India , 2003 .

[2]  J. Pozzi,et al.  Palaeomagnetic estimates of crustal shortening in the Himalayan thrusts and Zangbo suture , 1984, Nature.

[3]  R. Beck,et al.  Stratigraphic evidence for an early collision between northwest India and Asia , 1995, Nature.

[4]  Vinod K. Gaur,et al.  Tectonic stress field in the Indian subcontinent , 1992 .

[5]  J. Achache,et al.  India–Eurasia collision chronology has implications for crustal shortening and driving mechanism of plates , 1984, Nature.

[6]  A. D. Miller,et al.  Rupture process of large earthquakes in the northern Mexico subduction zone , 1994 .

[7]  M. Ellis,et al.  Topography and tectonics of the central New Madrid seismic zone: Results of numerical experiments using a three‐dimensional boundary element program , 1994 .

[8]  J. Jaeger,et al.  Deccan flood basalts at the Cretaceous/Tertiary boundary? , 1986 .

[9]  S. Biswas.  Regional tectonic framework, structure and evolution of the western marginal basins of India , 1987 .

[10]  M. Zoback,et al.  Stress perturbation associated with the Amazonas and other ancient continental rifts , 1996 .

[11]  P. Talwani,et al.  Tectonic Framework of the Kachchh Earthquake of 26 January 2001 , 2001 .

[12]  R. S. Dattatrayam,et al.  Crustal and upper mantle structure of Peninsular India and source parameters of the 21 May 1997, Jabalpur earthquake (Mw = 5.8): Results from a new regional broadband network , 1999, Bulletin of the Seismological Society of America.

[13]  S. Biswas.  Rift Basins in Western Margin of India and Their Hydrocarbon Prospects with Special Reference to Kutch Basin , 1982 .

[14]  T. Hildenbrand,et al.  High-Resolution Aeromagnetic Study of the New Madrid Seismic Zone: A Preliminary Report , 1992 .

[15]  D. Dreger,et al.  Rupture Process of the 26 January 2001 Mw 7.6 Bhuj, India, Earthquake from Teleseismic Broadband Data , 2003 .

[16]  N. P. Rao,et al.  Deep lower crustal earthquakes in central India: inferences from analysis of regional broadband data of the 1997 May 21, Jabalpur earthquake , 2002 .

[17]  Kusala Rajendran,et al.  Characteristics of Deformation and Past Seismicity Associated with the 1819 Kutch Earthquake, Northwestern India , 2001 .

[18]  Robert Sivewright.  Cutch and the Ran , 1907 .

[19]  Gail M. Atkinson,et al.  The 26 January 2001 M 7.6 Bhuj, India, Earthquake: Observed and Predicted Ground Motions , 2002 .

[20]  P. Molnar,et al.  Focal depths of intracontinental and intraplate earthquakes and their implications for the thermal and mechanical properties of the lithosphere , 1983 .

[21]  Javed N. Malik,et al.  Modern and Historic Seismicity of Kachchh Peninsula, Western India , 1999 .

[22]  W. Chung,et al.  Source parameters of the Anjar earthquake of July 21, 1956, India, and its seismotectonic implications for the Kutch rift basin , 1995 .

[23]  R. N. Singh,et al.  Rheological stratification of the Indian continental lithosphere: Role of diffusion creep , 1999, Journal of Earth System Science.

[24]  E. Fielding,et al.  The 26 January 2001 “Republic Day” Earthquake, India , 2001 .

[25]  Naoshi Hirata,et al.  Size and orientation of the fault plane for the 2001 Gujarat, India earthquake (Mw7.7) from aftershock observations: A high stress drop event , 2002 .

[26]  J. Lahr.  HYPOELLIPSE; a computer program for determining local earthquake hypocentral parameters, magnitude, and first-motion pattern , 1979 .

[27]  P. Tapponnier,et al.  Paleontological view of the ages of the Deccan Traps, the Cretaceous/Tertiary boundary, and the India-Asia collision , 1989 .

[28]  G. Xie,et al.  Dip range for intracontinental reverse fault ruptures: Truth not stranger than friction? , 1998, Bulletin of the Seismological Society of America.

[29]  C. Scholz.  The Mechanics of Earthquakes and Faulting , 1990 .

[30]  R. Briggs,et al.  Eight Days in Bhuj: Field Report Bearing on Surface Rupture and Genesis of the 26 January 2001 Earthquake in India , 2001 .

[31]  C. Langston,et al.  East African earthquakes below 20 km depth and their implications for crustal structure , 1995 .

[32]  H. Kanamori.  Mechanics of Earthquakes , 1994 .

[33]  Mario Ordaz,et al.  A spectral analysis of the 21 May 1997, Jabalpur, India, earthquake (Mw = 5.8) and estimation of ground motion from future earthquakes in the Indian shield region , 1999, Bulletin of the Seismological Society of America.