Temporal fluctuations (2001–2013) in crack density, saturation rate, and seismic velocities and their implications for the occurrence of the 2001 Mw7.7 Bhuj earthquake sequence

The 2001 Mw7.7 Bhuj earthquake is the second deadliest intraplate earthquake of the twenty-first century. It caused the deaths of 20,000 people and resulted in the most extensive series of aftershocks within a tectonic plate to date. The seismic activity at Kachchh, Gujarat, is still ongoing, with occasional rare instances of Mw ≥ 3.0. In order to understand the genesis of this earthquake sequence, we performed simulations of crack density (ε) and saturation rate (ξ) using seismic velocity tomograms that we created between 2001 and 2013. Subsequently, we employed our discoveries to examine the mechanisms underlying the 17 Mw ≥ 5 earthquakes in the main rupture zone (MRZ) of the 2001 Bhuj mainshock. It is seen that the majority of Mw ≥ 5 earthquakes, including the 2001 mainshock, have taken place in areas with low velocity zones (characterised by low dVp, low dVs, high ε, high ξ, and high Poisson’s ratio (σ)). This indicates that these earthquakes were likely triggered by the presence of fluids. According to our analysis, earthquakes at shallow depths (0–10 km) were likely caused by meteoric water, while earthquakes at depths of 10–35 km were probably driven by aqueous/metamorphic fluids or volatile CO2 in the lower crust. The drop in seismic velocities in the MRZ is attributed to an increase in ε, ξ, and σ, indicating the prevalence of saturated cracks. Our modelling also shows that ε, ξ, and σ in the MRZ increases from 2001 to 2010 due to intense fracturing during the mainshock and post-seismic periods, but it decreases from 2011 to 2013 due to the perturbed Kachchh fault zone healing. Thus, our study suggests that the earthquake generating process in the MRZ is recreated using the principles of dilatancy, re-saturation, and subsequent dilatancy. In contrast, the process of generating earthquakes in the area beyond the MRZ involves dilatancy, re-saturation, and relaxation.