Differential Energy Radiation from Two Earthquakes in Japan with Identical Mw: The Kyushu 1996 and Tottori 2000 Earthquakes

We examine two closely located earthquakes in Japan that had identical moment magnitudes M-w but significantly different energy magnitudes M-e. We use teleseismic data from the Global Seismograph Network and strong-motion data from the National Research Institute for Earth Science and Disaster Prevention's K-Net to analyze the 19 October 1996 Kyushu earthquake (M-w 6.7, M-e 6.6) and the 6 October 2000 Tottori earthquake (M-w 6.7, M-e 7.4). To obtain regional estimates of radiated energy E-S we apply a spectral technique to regional (< 200 km) waveforms that are dominated by S and Lg waves. For the thrust-fault Kyushu earthquake, we estimate an average regional attenuation Q (f) = 230f(0.65). For the strike-slip Tottori earthquake, the average regional attenuation is Q(f) = 180f(0.6). These attenuation functions are similar to those derived from studies of both California and Japan earthquakes. The regional estimate of ES for the Kyushu earthquake, 3.8 x 10(14) J, is significantly smaller than that for the Tottori earthquake, ES 1: 3 x 1015 J. These estimates correspond well with the teleseismic estimates of 3: 9 x 1014 J and 1: 8 x 1015 J, respectively. The apparent stress (tau(a) = mu E-S/M-0, with mu equal to rigidity) for the Kyushu earthquake is 4 times smaller than the apparent stress for the Tottori earthquake. In terms of the fault maturity model, the significantly greater release of energy by the strike-slip Tottori earthquake can be related to strong deformation in an immature intraplate setting. The relatively lower energy release of the thrust-fault Kyushu earthquake can be related to rupture on mature faults at a subduction environment. The consistence between teleseismic and regional estimates of E-S is particularly significant as teleseismic data for computing E-S are routinely available for all large earthquakes whereas often there are no near-field data.