Depths of large earthquakes determined from long-period Rayleigh waves

Abstract

The depths and source mechanisms of nine large shallow earthquakes were determined from long-period (150 to 300 s) Rayleigh waves recorded by the Global Digital Seismograph Network (GDSN) and International Deployment of Accelerometers (IDA) networks. We inverted the data set of complex source spectra for a moment tensor (linear) or a double couple (nonlinear). By solving a least squares problem, we obtained the centroid depth or the extent of the distributed source for each earthquake. The depths and source mechanisms of large shallow earthquakes determined from long-period Rayleigh waves depend on the models of source finiteness, wave propagation, and excitation. We tested various models of source finiteness, Q, group velocity, and excitation in the determination of earthquake depths. In order to determine the depth of large earthquakes from long-period surface waves, source-finiteness effects must be corrected using adequate models. The depth estimates obtained using the Q model of Dziewonski and Steim (1982) and the excitation functions computed for the average ocean model of Regan and Anderson (1984) are considered most reasonable. Dziewonski and Steim's Q model represents a good global average of Q determined over a period range of the Rayleigh waves used in this study. Since most of the earthquakes studied here occurred in subduction zones, Regan and Anderson's average ocean model is considered most appropriate. Our depth estimates are in general consistent with the Harvard centroid-moment tensor (CMT) solutions. The centroid depths and their 90% confidence intervals (numbers in parentheses) determined by the Student's t test are Colombia-Ecuador earthquake (December 12, 1979), d = 11 km (9, 24 km); Santa Cruz Island earthquake (July 17, 1980), d = 36 km (18, 46 km); Samoa earthquake (September 1, 1981), d = 15 km (9, 26 km); Playa Azul, Mexico, earthquake (October 25, 1981), d = 41 km (28, 49 km); El Salvador earthquake (June 19, 1982), d = 49 km (41, 55 km); New Ireland earthquake (March 18, 1983), d = 75 km (72, 79 km); Chagos Bank earthquake (November 30, 1983), d = 31 km (16, 41 km); Valparaiso, Chile, earthquake (March 3, 1985), d = 44 km (15, 54 km); Michoacan, Mexico, earthquake (September 19, 1985), d = 24 km (12, 34 km).