Differential Energy Radiation from Two Earthquakes in Japan with Identical MW

Teleseismic studies have found that in general the radiated energy ES of intraplate strike-slip earthquakes is elevated significantly for a given rupture size (as measured by moment) relative to the energies radiated by thrust earthquakes at subduction zones. We verify that this phenomenon can be observed for regional (< 400 km) data by applying the spectral technique of Boatwright et al. (2002) for estimating radiated energies from S, Lg and surface waves to two earthquakes in Japan which had identical teleseismic MW but very different Me (energy magnitude). For the thrust-fault earthquake of 1996 Oct 19 offshore of Kyushu (MW 6.7), strong motion data from 154 K-net stations were used. A frequency dependent attenuation Q(f) = 230f 0.65 was derived with a resulting ES of 3.9e14 J. For the strike-slip Tottori earthquake of 2006 Oct 6 (MW 6.7), the attenuation Q(f) = 180 (f)0.6 was determined from 217 K-net stations with a resulting ES of 1.3e15 J. The attenuation functions derived for these earthquakes are comparable to those derived from other studies of earthquakes in Japan as well as central California for f > 1.0 Hz. The ES of the strike-slip earthquake is more than three times larger than that of the thrust earthquake. Commensurate with the difference in energies, the macroseismic effects as reported for the strike-slip earthquake were more severe and widespread than effects reported for the thrust earthquake. However, from teleseismic data the energy of the strike-slip earthquake is a factor of 10 larger than that of the thrust earthquake. For the strike-slip earthquake, the acceleration spectra of teleseismic and regional analyses overlap smoothly. Good regional-teleseismic overlap has been seen in analyses of other strike-slip earthquakes such as the Hector Mines event. In contrast, for the Kyushu earthquake, the teleseismic analysis appeared to underestimate energy at a band of frequencies about 1.0 Hz relative to the regional data. A similar difference between regional and teleseismic spectra has been observed for the North Ridge and San Simeon, California thrust events.