Variations of seismic velocities in the Kachchh rift zone, Gujarat, India, during 2001–2013

Abstract

We herein study variations of seismic velocities in the main rupture zone (MRZ) of the Mw 7.7 2001 Bhuj earthquake for the time periods [2001–05, 2006–08, 2009–10 and 2011–13], by constructing dVp(%), dVs(%) and d(Vp/Vs)(%) tomograms using high-quality arrival times of 28,902 P- and 28,696 S-waves from 4644 precise JHD (joint hypocentral determination) relocations of local events. Differential tomograms for 2001–05 reveal a marked decrease in seismic velocities (low dVp, low dVs and high d(Vp/Vs)) in the MRZ (at 5–35km depths) during 2001–10, which is attributed to an increase in crack/fracture density (higher pore fluid pressure) resulted from the intense fracturing that occurred during the mainshock and post-seismic periods. While we observe a slight recovery or increase in seismic velocities 2011–13, this could be related to the healing process (lower pore fluid pressure due to sealing of cracks) of the causative fault zone of the 2001 Bhuj mainshock. The temporal reduction in seismic velocities is observed to be higher at deeper levels (more fluid enrichment under near-lithostatic pressure) than that at shallower levels. Fluid source for low velocity zone (LVZ) at 0–10km depths (with high d(Vp/Vs)) could be attributed to the presence of meteoric water or soft alluvium sediments with higher water content, while fluid source for LVZ at 10–35km depths could be due to the presence of brine fluids (released from the metamorphic dewatering) and volatile CO2 (emanating from the crystallization of carbonatite melts in the asthenosphere), in fractures and pores. We also imaged two prominent LVZs associated with the Katrol Hill fault zone and Island Belt fault zone, extending from shallow upper-crust to sub-crustal depth, which might be facilitating the deeper circulation of metamorphic fluids/volatile CO2, thereby, the generation of lower crustal earthquakes occurring in the Kachchh rift zone.