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.

Description: Displacement, velocity, and velocity-squared records of P and SH body waves recorded at teleseismic distances are analyzed to determine the rupture characteristics of the Denali fault, Alaska, earthquake of 3 November 2002 (M (sub w) 7.9, M (sub e) 8.1). Three episodes of rupture can be identified from broadband ( approximately 0.1-5.0 Hz) waveforms. The Denali fault earthquake started as a M (sub w) 7.3 thrust event. Subsequent right-lateral strike-slip rupture events with centroid depths of 9 km occurred about 22 and 49 sec later. The teleseismic P waves are dominated by energy at intermediate frequencies (0.1-1 Hz) radiated by the thrust event, while the SH waves are dominated by energy at lower frequencies (0.05-0.2 Hz) radiated by the strike-slip events. The strike-slip events exhibit strong directivity in the teleseismic SH waves. Correcting the recorded P-wave acceleration spectra for the effect of the free surface yields an estimate of 2.8X10 (super 15) N m for the energy radiated by the thrust event. Correcting the recorded SH-wave acceleration spectra similarly yields an estimate of 3.3X10 (super 16) N m for the energy radiated by the two strike-slip events. The average rupture velocity for the strike-slip rupture process is 1.1beta -1.2beta . The strike-slip events were located 90 and 188 km east of the epicenter. The rupture length over which significant or resolvable energy is radiated is, thus, far shorter than the 340-km fault length over which surface displacements were observed. However, the seismic moment released by these three events, 4X10 (super 20) N m, was approximately half the seismic moment determined from very low-frequency analyses of the earthquake. The difference in seismic moment can be reasonably attributed to slip on fault segments that did not radiate significant or coherent seismic energy. These results suggest that very large and great strike-slip earthquakes can generate stress pulses that rapidly produce substantial slip with negligible stress drop and little discernible radiated energy on fault segments distant from the initial point of nucleation. The existence of this energy-deficient rupture mode has important implications for the evaluation of the seismic hazard of very large strike-slip earthquakes.