ALBERT

Description: The rupture process of the M (sub W) 9.1 Sumatra-Andaman earthquake lasted for approximately 500 sec, nearly twice as long as the teleseismic time windows between the P and PP arrival times generally used to compute radiated energy. In order to measure the P waves radiated by the entire earthquake, we analyze records that extend from the P-wave to the S-wave arrival times from stations at distances Delta 〉60 degrees . These 8- to 10-min windows contain the PP, PPP, and ScP arrivals, along with other multiply reflected phases. To gauge the effect of including these additional phases, we form the spectral ratio of the source spectrum estimated from extended windows (between T (sub P) and T (sub S) ) to the source spectrum estimated from normal windows (between T (sub P) and T (sub PP) ). The extended windows are analyzed as though they contained only the P-pP-sP wave group. We analyze four smaller earthquakes that occurred in the vicinity of the M (sub W) 9.1 mainshock, with similar depths and focal mechanisms. These smaller events range in magnitude from an M (sub W) 6.0 aftershock of 9 January 2005 to the M (sub W) 8.6 Nias earthquake that occurred to the south of the Sumatra-Andaman earthquake on 28 March 2005. We average the spectral ratios for these four events to obtain a frequency-dependent operator for the extended windows. We then correct the source spectrum estimated from the extended records of the 26 December 2004 mainshock to obtain a complete or corrected source spectrum for the entire rupture process ( approximately 600 sec) of the great Sumatra-Andaman earthquake. Our estimate of the total seismic energy radiated by this earthquake is 1.4X10 (super 17) J. When we compare the corrected source spectrum for the entire earthquake to the source spectrum from the first approximately 250 sec of the rupture process (obtained from normal teleseismic windows), we find that the mainshock radiated much more seismic energy in the first half of the rupture process than in the second half, especially over the period range from 3 sec to 40 sec.

Description: We examine two closely located earthquakes in Japan that had identical moment magnitudes M (sub w) but significantly different energy magnitudes M (sub 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 (sub w) 6.7, M (sub e) 6.6) and the 6 October 2000 Tottori earthquake (M (sub w) 6.7, M (sub e) 7.4). To obtain regional estimates of radiated energy E (sub 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 (super 0.65) . For the strike-slip Tottori earthquake, the average regional attenuation is Q(f)=180f (super 0.6) . These attenuation functions are similar to those derived from studies of both California and Japan earthquakes. The regional estimate of E (sub S) for the Kyushu earthquake, 3.8X10 (super 14) J, is significantly smaller than that for the Tottori earthquake, E (sub S) 1.3X10 (super 15) J. These estimates correspond well with the teleseismic estimates of 3.9X10 (super 14) J and 1.8X10 (super 15) J, respectively. The apparent stress (tau (sub a) =mu E (sub S) /M (sub 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 (sub S) is particularly significant as teleseismic data for computing E (sub S) are routinely available for all large earthquakes whereas often there are no near-field data.