Abstract

The U.S. Geological Survey National Earthquake Information Center (NEIC) uses a variety of classical network‐averaged magnitudes (e.g., mb and Ms) and waveform modeling procedures to determine the moment magnitude (Mw) 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 Mw, source time function, and a network‐averaged Mw.

Our EGF approach is validated using broadband waveforms from 246 earthquakes in the magnitude range Mw 6.0–9.1. Within approximately 13 min of earthquake origin time, our procedure using teleseismic P waves only computes an Mw that lies within ±0.25 of the final W‐phase Mw in the magnitude range 6–8. Using later arriving teleseismic SH phases results in an Mw that lies within ±0.12 of the W‐phase Mw. 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.

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