Self-Consistent Retrieval of Source Parameters Using Mantle Waves

The large number of high-quality stations from global networks (e.g., IRIS, GEOSCOPE) allows for rapid and robust recovery of source parameters of large earthquakes. We present a self-consistent analysis of surface waves in terms of directivity parameters. The boxcar is often used to recover source parameters but is rarely a faithful representation of an event' s time function. We demonstrate that more reliable estimates of directivity parameters can be obtained when we take into account the source time function (STF) of the event. A low-frequency STF is obtained from first (R1) and second (R2) orbit Rayleigh waves in the range 2 to 15 mHz, with a spectral synthesis and inversion method. We measure the differential amplitudes between data and 3D synthetics. We use the R1/R2 ratio of differential amplitudes to invert, in a single narrow-frequency band, for the event's apparent rupture azimuth and velocity. For the M w 8.1 Chile 1995 event, data are consistent with unilateral southward propagation, with a velocity of about 2.4 km/sec. For the Mw 7.9 Mexico 1995 earthquake, the rupture propagated toward the NW at about 2.0 km/sec. Our low-frequency estimates agree with broadband studies of both events and can be retrieved rapidly on a routine basis after a large earthquake.