Prantik Mandal

Passive‐Source Seismic Imaging of the Crust and Upper Mantle beneath the 2001 Mw 7.7 Bhuj Earthquake Region, Gujarat, India

Abstract Variations in crustal and lithospheric thicknesses below the Kachchh rift (KR) zone have been investigated employing the joint inversion of P receiver functions (RF) and Rayleigh‐wave and Love‐wave group velocity dispersion (5–70xa0s) on the broadband waveforms of teleseismic events. Estimated crustal and lithospheric thicknesses at 15 broadband sites led to the delineation of an updoming of 4–7xa0km in the Moho and 6–12xa0km in the asthenosphere beneath the central KR relative to the surrounding unrifted regions. The Moho depths vary from 35 to 42xa0km and lithospheric thicknesses range from 62 to 78xa0km. The observed marked shear velocity reduction (2%–6%) at the lithosphere–asthenosphere boundary (LAB) over an area of 120u2009u2009km×80u2009u2009km suggests the presence of patches of partial melt in the asthenosphere below the KR, evidencing the possible presence of imprints of the Deccan–Reunion mantle plume. The coincidence of the area of updoming of the Moho and asthenosphere, and the confined aftershock activity below the central KR implies that these patches could provide a high input of volatile CO 2 into the lower crust, which might play a key role in generating the continued aftershock activity in the region since the occurrence of the 2001 M w xa07.7 Bhuj mainshock.

Spatiotemporal Variation of Fractal Properties in the Source Zone of the 2001 Mw 7.7 Bhuj Earthquake

The Kachchh seismic zone (KSZ) in Gujarat, India, has been experiencing earthquakes since the occurrence of the 2001 M wxa07.7 Bhuj mainshock, which led to a homogeneous catalog of 2808 earthquakes of M w≥3.0 between 2001 and 2011. This enabled us to investigate the spatiotemporal variation of fractal dimensions ( D ), b ‐values, and seismic velocities in the KSZ, through 3D mapping of these three independent parameters. The estimated fractal dimensions for the Gedi fault (GF) events range from 0.7 to 1.0, whereas those of North Wagad fault (NWF) events vary from 1.2 to 2.4. The b ‐values for GF events also show smaller values in comparison to the NWF events. The D values associated with events occurring in the main rupture zone (MRZ) of the 2001 mainshock vary from 1.6 to 2.1, whereas those associated with events occurring in the surrounding unrifted zones show a wider range of variation (from 1.2 to 2.4). We also observe that events associated with the NWF satisfy the Aki’s relationship, D =2 b , (Aki, 1981) whereas events associated with GF do not follow this relationship. Both b ‐ and D ‐values tend to increase with an increase in the earthquake density, which could be connected with the process of disintegration of crustal rocks in the MRZ, where large material heterogeneities in terms of low‐ and high‐seismic velocity patches are being mapped through local earthquake velocity tomography. Through examining the temporal changes in b ‐ and D ‐values, we notice several episodes of abnormally low b ‐ and D ‐values associated with the occurrences of bursts of seismic activity in 2001, 2005, 2006, 2008, and 2010. This decrease in b ‐ and D ‐values is attributed to a reduction in rock friction in the causative fault zone at 10–35xa0km depths (probably due to the presence of metamorphic fluids or volatile CO2), thereby enhancing the probability of occurrence of stronger earthquakes.

Influence of Deccan volcanism/synrift magmatism on the crust–mantle structure and its implications for the seismogenesis of earthquakes occurring in the Kachchh rift zone

Abstract The present study focuses on the delineation of the crust–mantle structure underlying the Kachchh rift zone (KRZ), by modelling P-wave receiver functions (P-RFs) and P-wave teleseismic tomography. Our RF study delineates marked crustal and lithospheric thinning below the central KRZ relative to the unrifted surrounding regions. This thinning model receives further support from crust-corrected normalized P-residuals, which suggest dominant negative residuals associated with the central KRZ. Teleseismic tomography using these P-residuals reveals low velocity to a depth of 170 km below the central KRZ, while there are positive residuals associated with the surrounding unrifted zones. Such a complex heterogeneous crust–mantle structure, which could be related to K/T boundary Deccan mantle plume activity and rift-related magmatism, might play a crucial role in seismogenesis of lower crustal earthquakes that have occurred in the KRZ since 2001. Inverted crust–mantle models obtained from P-RFs suggest a low shear velocity zone extending from 70–110 to 170–220 km depth beneath the central KRZ. This receives further support from the presence of low P-wave velocity down to 170 km modelled using teleseismic tomography. This low Vp and Vs zone in the upper mantle could be explained by the presence of trapped melts related to Deccan volcanism at 65 Ma or older rift-related magmatism.