Testing the Performance of W-phase Source Inversion

We have recently developed a method to perform fast source parameter inversions for large earthquakes using the W-phase, a very long-period phase starting right after the P-wave arrival (Kanamori and Rivera, 2008). Here we report on the results of a systematic test on the performance of the method for seismic tsunami warning purposes. We tested it using a global data set of all Mw ≥ 7.0 earthquakes between 1990 and 2008 (240 events). We use the vertical VBB data from GSN and Geoscope stations (mainly STS-1 seismometers) filtered between 0.001 and 0.005 Hz. The window length, τ, used for inversion is distance dependent, τ= 15 Δ sec (Δ in degree). Typically we use stations up to 40°- 60° corresponding to a delay of 18-25 min after the origin time. To perform inversion, we need, in addition to the seismic traces, estimates of the hypocenter parameters and of the centroid delay. The PDE parameters, which are quickly available, can be used as a first approximation of the hypocenter location. The centroid source delay is first estimated from an approximate Mw determined from the W phase amplitudes. Having a database of pre-computed Green's functions, the actual inversion process is nearly instantaneous. We can improve the inversion using either grid search or parameter optimization by a non- linear least squares method. In the test reported here, we systematically use the Harvard or the Global CMT solution as reference. Specifically, we compare our solutions with the reference solutions, in terms of the seismic moment (i.e., Mw) and the orientation of the nodal planes. The results of comparison are encouraging. The differences in Mw between the W phase and the CMT inversions are 0.1 or smaller. The differences in the angular distance between the nodal planes are typically less than 5-10 deg. The solutions are in general robust. Because of the sparse station coverage, the solutions for older events (~ 1990-1992) are not constrained well, but are still adequate for tsunami warning purposes. Our tests indicate that the W phase inversion provides rapid and robust solutions useful for tsunami warning purposes. Also, since W phase covers the frequency band which has not been covered by the existing regional and global inversion methods, W phase solutions provide important additional information on long- period characteristics of earthquakes.