Seismic imaging of the 2001 Bhuj Mw7.7 earthquake source zone: B-value, fractal dimension and seismic velocity tomography studies

摘要

We use precisely located aftershocks of the 2001 Mw7.7 Bhuj earthquake (2001-2009) to explore the structure of the Kachchh seismic zone by mapping the 3-D distributions of b-value, fractal dimension (D) and seismic velocities. From frequency-magnitude analysis, we find that the catalog is complete above Mw=3.0. Thus, we analyze 2159 aftershocks with Mw≥3.0 to estimate the 3-D distribution of b-value and fractal dimensions using maximum-likelihood and spatial correlation dimension approaches, respectively. Our results show an area of high b-, D- and Vp/Vs ratio values at 15-35km depth in the main rupture zone (MRZ), while relatively low b- and D values characterize the surrounding rigid regions and Gedi fault (GF) zone. We propose that higher material heterogeneities in the vicinity of the MRZ and/or circulation of deep aqueous fluid/volatile CO_2 is the main cause of the increased b-, D- and Vp/Vs ratio values at 15-35km depth. Seismic velocity images also show some low velocity zones continuing in to the deep lower crust, supporting the existence of circulation of deep aqueous fluid / volatile CO_2 in the region (probably released from the eclogitasation of olivine rich lower crustal rocks). The presence of number of high and low velocity patches further reveals the heterogeneous and fractured nature of the MRZ. Interestingly, we observe that Aki (1981)'s relation (D=2b) is not valid for the spatial b-D correlation of the events in the GF (D_2=1.2b) zone. However, the events in the MRZ (D_2=1.7b) show a fair agreement with the D=2b relationship while the earthquakes associated with the remaining parts of the aftershock zone (D_2=1.95b) show a strong correlation with the Aki (1981)'s relationship. Thus, we infer that the remaining parts of the aftershock zone are probably behaving like locked un-ruptured zones, where larger stresses accumulate. We also propose that deep fluid involvement may play a key role in generating seismic activity in the MRZ, therefore, a similar episode of deep fluid inflow could play a crucial role in causing the next strong earthquakes in the surrounding blocks of NWF.