Numerical modeling of seismicity and geodynamics of the Kachchh rift zone, Gujarat, India

• Block-and-fault dynamics model of the Kachchh rift zone(KRZ) is developed.
• A NNW-SSE trending compression is found to be a principal driving force in the KRZ.
• The above compression explains basic features of the regional seismicity in Kachchh.
• This compression also explains the direction of block motions in the KRZ.
• The FMD for both synthetic and observed seismicity shows a similar slope.

The numerical block-and-fault model of lithosphere dynamics and seismicity (BAFD) is used to understand crustal motion and features of the observed seismicity in the Kachchh rift zone, Gujarat, Western India. The block-model allows simulating seismicity and geodynamics simultaneously unlike other modeling approaches for studying seismicity or geodynamics. The model structure of Kachchh rift zone is composed of seven major crustal blocks separated by fault planes. Based on the orientation of boundary crustal block movements, we develop a set of numerical experiments to analyze the spatial distribution of earthquakes, frequency-to-magnitude relationships, earthquake focal mechanisms, velocity field, and fault slip rates in the model. The main results of our modeling suggest that an NNW–SSE trending compression is a principal driving force in the Kachchh rift zone that explains basic features of the regional seismicity, direction of block motions, and the presence of an extensional stress regime associated with the Cambay rift zone. Large synthetic events occur on the fault segments associated with the Allah-Bund fault, Katrol hill fault and north Wagad fault which have been causative faults for the 1819 Mw7.7 Allah-Bund, 1956 Mw6.0 Anjar and 2001 Mw7.7 Bhuj earthquakes. The frequency–magnitude distribution for both synthetic seismicity and observed seismicity shows a similar slope. The focal mechanisms of the synthetic events are found to be consistent with those of earthquakes in the region. A special attention has been paid to study long-term and post-seismic deformations. Our results are in a qualitative agreement with the GPS post-seismic observations in the Kachchh rift zone. We infer that the observed seismicity and crustal block motions are a consequence of the dynamics of the entire regional fault and block system rather than that of a single causative fault only.