Correlation between earthquake b value and VP/VS ratio in Japan

Earthquake b value is primarily controlled by differential stress in the crust. Pore pressure has also been reported influencing b value locally. In nature, the influence can only be observed in the subsurface crust by injection wells. It remains unclear whether the influence of pore pressure on b value can be observed in the scale of the entire crust. To this end, we assume that pore pressure increases proportionally with V_P/V_S ratio, which is derived from seismic tomography studies, to examine correlation between V_P/V_S ratio and b value. We investigated this correlation in Japan because it is one of the most earthquake-prone countries with dense seismic networks and high-quality earthquake catalogs. We used an earthquake catalog from the Japan Meteorological Agency from 1998 to 2011 Feb to calculate the b values in the inland region of Japan above the 30 km depth. The selected period is based on a stable completeness of magnitude (M_c) since 1998 and the strong clustering effects by the both 2011 Tohoku and 2016 Kumamoto earthquakes. We then calculated M_c and b value by maximum curvature method and maximum likelihood method, respectively, in the grids of 0.1  0.1  10 km with a radius of 30 km from the center of the grids. The b value determination requires the number of earthquakes with magnitudes greater than the M_c over 150 within the radius. For the V_P/V_S ratios, we used the latest data derived from the National Research Institute for Earth Science and Disaster Resilience, Japan, to resample them to the same grids as b values. We simply resampled the V_P/V_S ratios by either averaging them into the grids of b values, or weighting them through a triangular function to the grids center of b values in depths. We analyzed b value as a function of V_P/V_S ratio and binned the b values within every 0.01 V_P/V_S interval to calculate the means and medians for liner regressions. Our preliminarily results show that there is little correlation between entire b values and V_P/V_S ratios among different depth ranges (0-10 km, 10-20 km, 20-30 km). We observed a linear negative relation in the binned data at the 10-20 km depth, however, this relation is not likely observed in the other depths. It may imply that the influence of pore pressure on b value could vary with depths. We’ll calculate the b values using entire magnitude range method and compare the results to the other localized geophysical observations.