Rapid Estimates of the Source Time Function and M_w using Empirical Green’s Function Deconvolution

摘要

The U.S. Geological Survey National Earthquake Information Center (NEIC) uses a variety of classical network-averaged magnitudes (e.g., m_b and M_s) and waveform modeling procedures to determine the moment magnitude (M_w) of an earthquake from teleseismic observations. Initial magnitude estimates are often inaccurate because of poor azimuthal control (sampling of the focal sphere) and/or intrinsic limitation of each method to a specific range of event size. To provide faster and more accurate estimates of the moment magnitude, source duration, and source complexity, NEIC is exploring the use of a variation of the empirical Green’s function (EGF) deconvolution procedure. This approach uses a predicted focal mechanism derived from the Global Centroid Moment Tensor Catalog to compute teleseismic P-wave synthetic seismograms, which are then deconvolved from observed P and SH waveforms to determine station-specific M_w, source time function, and a networkaveraged M_w. Our EGF approach is validated using broadband waveforms from 246 earthquakes in the magnitude range M_w 6.0–9.1. Within approximately 13 min of earthquake origin time, our procedure using teleseismic P waves only computes an M_w that lies within ±0.25 of the final W-phase M_w in the magnitude range 6–8. Using later arriving teleseismic SH phases results in an M_w that lies within ±0.12 of the W-phase M_w. For magnitude 8 or larger earthquakes, we underestimated the moment magnitude by up to 0.8 magnitude units, primarily due to the initial P phase not containing the total seismic moment release. Long-period phases such as the W-phase and surface waves that better characterize total moment release can also be incorporated in the processing.