Source Inversion of the W-Phase: Real-time Implementation and Extension to Low Magnitudes

Abstract

We assess the use and reliability of a source inversion of the W-phase in real-time operations at the U.S. Geological Survey National Earthquake Information Center. The three-stage inversion algorithm produces rapid and reliable estimates of moment magnitude and source mechanism for events larger than M_w 7.0 within 25 minutes of the earthquake origin time, often less, and holds great promise for vastly improving our response times to such earthquakes worldwide. The method also produces stable results (within ±0.2 units of Global Centroid Moment Tensor project estimates) for earthquakes as small as M_w 5.8 when using stations out to distances of 90°. These applications extend the use of W-phase far beyond the higher magnitude events for which the inversion was originally intended, facilitating its use as a complementary approach to traditional body- and surface-wave methods for assessing the source properties of an earthquake. Kanamori and Rivera (2008) introduced the use of W-phase as a reliable method to rapidly assess the source properties (M_w and mechanism) of earthquakes greater than ~M_w 7.5. They showed that the W-phase inversion method has important real-time applications for tsunami warning purposes (and indeed for the calculation of earthquake moment), particularly for very large earthquakes such as the 2004 Sumatra–Andaman Islands event for which traditional methods may suffer due to the clipping of seismograms and/or because they do not deliver accurate results quickly enough to be useful for tsunami warning.