Source finiteness of large earthquakes measured from long-period Rayleigh waves

Abstract

The source-finiteness parameters of 11 large shallow earthquakes were determined from long-period Rayleigh waves recorded by the Global Digital Seismograph Network and International Deployment of Accelerometers Networks. The basic data sets are the seismic spectra of periods from 150 to 300 s. In the determination of source-process times, we used Furumoto's phase method and a linear inversion method, in which we simultaneously inverted the spectra and determined the source-process time that minimizes the error in the inversion. These two methods yielded consistent results. The source-process times of the Sumbawa (Indonesia), Colombia-Ecuador, Valparaiso (Chile) and Michoacan (Mexico) earthquakes were estimated to be 79, 118, 69 and 77 s, respectively, from the linear inversion method. The source-process times determined from long-period surface waves were in general longer than those obtained from body waves. Source finiteness of large shallow earthquakes with rupture on a fault plane with a large aspect ratio was modeled with the source-finiteness function introduced by Ben-Menahem. The spectra were inverted to find the extent and direction of the rupture of the earthquake that minimize the error in the inversion. For a rupture velocity of 2.5 km s −1, the estimated rupture was unilateral, 100 km long and along the strike, N26°W, for the May 26, 1983 Akita-Oki, Japan earthquake; 165 km and S57°E for the September 19, 1985 Michoacan, Mexico earthquake; 256 km and N31°E for the December 12, 1979 Colombia-Ecuador earthquake; and 149 km and S15°W for the March 3, 1985 Valparaiso, Chile earthquake. The results for the August 19, 1977 Sumbawa, Indonesia earthquake showed that the rupture was bilateral and in the direction N60°E. These results are, in general, consistent with the rupture extent inferred from the aftershock area of these earthquakes.