Seismic Energy for Shallow Earthquakes in Southwest Japan

Abstract

We estimated the radiated energy for 88 earthquakes with M w ≥4.0 that occurred in southwest Japan by analyzing direct S waves recorded by the Freesia broadband seismic network (F-net) at local/regional distances. The attenuation factors for each source–receiver group are estimated by the coda method using the recently established high-sensitivity and high-density seismic network (Hi-net) data. The site amplification for each F-net station is calculated by using the coda normalization method. We find good agreement between the source spectra calculated by using these propagation effects and those determined by using an empirical Green’s function method for 4 M w >5 earthquakes. The station variance of the energy estimates is a factor of 2 for earthquakes with M w ≥5, and it is a factor of 3 for the smaller ones. The resultant energy estimations show that: (1) The overall ratios between the radiated energy and seismic moment ( E s / M 0 ) lie in a band of 5 × 10 −7 to 5 × 10 −5 for 88 earthquakes with M w 3.9–6.8. For the Tottori, 2000, earthquake and its 16 aftershocks with M 0 10 14 –10 19 N m, we found a very slight increase of E s / M 0 with increasing seismic moment. For the rest of earthquakes E s / M 0 are almost constant for earthquakes occurring in the same source area. The constants corresponding to each source area are slightly but systematically different, indicating a location dependence of E s / M 0 . Earthquakes that occur in volcanic areas show lower E s / M 0 values relative to earthquakes occurring in nonvolcanic areas. However, the weak to non-scaledependence is not certain because of the scantiness of large earthquakes in the data set. (2) For earthquakes in the volcanic source areas, there appears to be a trend of increasing E s / M 0 with focal depth. We believe this trend is a result of, at least in part, the decreasing attenuation with depth in volcanic areas. For the nonvolcanic area earthquakes, however, E s / M 0 tends to be a constant in a depth range of 12–50 km. Since the Akinada earthquake, 2001 ( M w 6.8), took place in the western part of subducting Philippine Sea slab and the two deeper earthquakes occurred in the upper mantle below the same subducting slab in its eastern part, the constancy of E s / M 0 may indicate that the apparent stress at the brittle patches in the ductile zone can be as high as in the brittle part of the crust. (3) Estimated E s / M 0 are inversely related to the surface heat flow, approximately. Such a relationship suggests that the geothermal structure may be the dominating tectonic factor that controls the level of apparent stress.