Relationship Between Regional Strain and Microseismicity in Japan

It is generally thought that the level of activity of small earthquakes is related to the regional stress or strain field. In this study, we compare the observed strain field in Japan with the rates of shallow crustal earthquakes, to see how well the microearthquake activity correlates with the regional strain field. With the installation of over 1000 continuous GPS stations of GEONET in Japan, we are able to observe the deformation field to a resolution of several tens of kilometers. We used the average horizontal displacement rates measured over the last 5 to 7 years at each station. The average rates were calculated by fitting linear trends to the data with periods of large earthquakes or other large deviation removed. These displacement rates were interpolated onto a 50 km grid and horizontal strains were calculated for the region of the Japanese Islands. Using earthquake locations from the Japan Meterological Agency catalog, we calculated yearly rates of shallow earthquake activity for the same grid and same time period that was used for the GPS observations. For the comparisons with the strain field, we tested several depth ranges from 10 to 30 km and magnitude thresholds of M1.0 to M3.0. We compared the rates of earthquake activity with the rates of maximum shear strain for the region covering the 4 major islands of Japan. Areas that had the very highest strain rates 1x10^-6, such as the region of inferred dike injection near Kozushima had the highest rates of seismicity. However, in the strain rate range of 10^-8 to 10^-7, which includes over 90% of the data, there were no clear correlations between strain rate and earthquake activity. Using different depth ranges and magnitudes of earthquakes or varying the grid the size did not produce significantly different results.