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

Abstract

The Kachchh seismic zone (KSZ) in Gujarat, India, has been experiencing earthquakes since the occurrence of the 2001 M w 7.7 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 10–35 km depths (probably due to the presence of metamorphic fluids or volatile CO2), thereby enhancing the probability of occurrence of stronger earthquakes.