Relation between Seismicity and Strain Rate in Japan

We examine the relationship between occurrence rates of small earthquakes and the crustal strain rates for the Japan Islands region. Recent GPS data from GEONET, operated by the Geographical Survey Institute, provide good measurements of the crustal deformation rates and the rates of small earthquakes are well recorded by the regional seismic networks. Therefore, there is good regional coverage of strain and seismicity rates across most of Japan. We want to look at the microearthquake occurrences associated with the secular rates of strain, so we eliminate locations of large earthquakes and high seismicity associated with volcanic events. For the crustal deformation rates, we calculated strain rate in a grid of even intervals of 0.1 degree. We used the method of Shen et al. (1996) which estimates horizontal displacement rate, strain rate and rotation rate at each grid point from observed displacement rates of the GPS stations. Then, we calculated dilatation rate and maximum shear strain rate using the estimated horizontal strain rate. For the seismicity rates, we used the JMA (Japan Meteorological Agency) hypocenter catalogue and estimated the number of earthquakes (M>2) that occurred within a 15km radius of each grid point at depths shallower than 20km. There is a large scatter in the plot of seismicity rate as a function of strain rate, but if values of seismicity rates are averaged over a range of strain rate values, some clear trends can be seen in the results. Comparing the numbers of earthquakes with the strain rates, we obtain the following results. Maximum shear strain rate and the number of earthquakes show a positive correlation for rates of 0 to 90 nanostrain/year. However, above 90 nanostrain/year, the number of earthquakes decreases with increase of the strain rate. Similarly, there is an increase in the number of earthquakes with increase of dilatation rate from 0 to -110 (negative dilatation rate indicates compression), and earthquake numbers decrease for values of compressive strain rate greater than -110. Thus, for both shear strain rate and dilatation rate, the number of earthquakes increased with increasing strain rate, but above some threshold strain rate, the numbers of earthquakes then decrease. The threshold above which seismicity rates decline, may be an important factor in the regional stress conditions that control the rates of small earthquakes.