Relocation, Vp and Vp/Vs Tomography, Focal Mechanisms and other related studies using aftershock data of the Mw 7.7 Bhuj earthquake of January 26, 2001

To comprehend the seismo-tectonic process of the aftershock zone of the 26 January 2001 Mw 7.7 Bhuj earthquake sequence of Mw 7.7, we relocated 999 aftershocks (M w 2.0-5.3) using HYPODD relocation technique and the data from a close combined network (NGRI, India and CERI, USA) of 8-18 digital seismographs during 12-28 February 2001. These precisely relocated aftershocks (ERH < 30 meter, ERZ < 50 meter) delineate an east-west trending blind thrust dipping (~ 45 o ) towards south (named as North Wagad Fault, NWF), about 25 km north of Kachchh Main Land Fault (KMF), as the causative fault for the 2001 Bhuj earthquake of Mw 7.7. The aftershock zone confines to a 60 km long and 40 km wide region lying between the KMF to the south and NWF to the north, extending from 10 to 45 km depth. The P- and S-wave station corrections determined from the JHD technique show a consistent and clear pattern of positive and negative values associated with the SW and NE part of the study area. The station corrections vary from 0.20 to –0.24 sec for the P-waves and from -0.55 to 0.95 sec for the S-waves. The tomographic inversion technique is used to invert 5516 P- travel times and 4061 S-P travel time differences from 600 aftershocks recorded at 8 to 18 stations. Tomographic results suggest a regional high velocity body (characterized by high Vp (7-8.5 km/s), high Vs (4-4.8 km/s) and small σ (0.24-0.55)) with a head extending 60 km in N-S and 40 km in E-W at 10-40 km depths. This high velocity anomaly is inferred to be a mafic pluton/rift pillow, which might have intruded during the rifting time (~135 Ma). Another important result of our study is the detection of a low velocity zone (low Vp (6.5-7 km/s), low Vs (3.6–4 km/s) and high σ (0.26-0.265)) within the mafic body at the hypocentral depth of mainshock (~18-25 km), which is inferred to be a fluid-filled (trapped aqueous fluid resulted from metamorphism) fractured rock mass. The depth-wise variation of anisotropic percentage, stress drops and b-values suggest an increase at 18-30 km depths corroborating the presence of a fluid filled fractured zone coinciding with low velocity zone (at 1825 km depths). Thus, the observed increase in anisotropy, stress drops and b-values at 18-30 km depths could be explained in terms of alignment of numerous fractures present in the fractured layer. The focal mechanism solutions of selected 444 significant aftershocks, which were recorded at 6-14 stations, suggest that the majority of the selected focal mechanisms ranged between pure reverse and pure strike slip; some pure dip slip solutions were determined. The stress inversion using the selected earthquake focal mechanisms in the aftershock zone of the 2001 Bhuj earthquake shows that the axis of maximum principal stress is oriented N181 o E with a shallow dip (=14 o ). Stress ellipsoid is oblate (R = 0.4). Further, the crustal shear wave anisotropy study suggests that leading shear wave polarization directions (LSPDs) over the aftershock zone vary from N-S to N-E with a delay of 0.07 to 0.14 sec. The delays in the N-S to N-E direction suggest cracks parallel to the direction of maximum horizontal regional compressional stress prevailing in the region.