Systematic Determination of Earthquake Rupture Directivity and Fault Planes From Analysis of Long-Period P-Wave Spectra

If an earthquake has a primarily unilateral rupture, the pulse width observed on seismograms will vary depending on the angle between the rupture direction and the takeoff vector to the station. We have developed a method to estimate the amount of pulse broadening from the spectrum and apply it to a long-period database of large, globally-distributed earthquakes that occurred between 1988 and 2000. We select vertical-component P waves at epicentral distances of 20^o--98^o. We compute the spectrum from a 64-s-long window around each P wave arrival. Each spectrum is the product of source, receiver, and propagation response functions as well as local source- and receiver-side effects. Since there are multiple receivers for each source and multiple sources for each receiver, we can estimate and remove the source- and receiver-side terms by stacking the appropriate P log spectra. For earthquakes deeper than ∼200~km, directivity effects dominate the residual spectra. We use our pulse-width estimates to determine the best rupture direction and to identify which nodal plane of the Harvard CMT solution is most consistent with this rupture direction for 66~events. In about 40% of the cases, one of the two nodal planes produces a much better fit to the data and can be identified as the true fault plane. Our results show good agreement with the known rupture directions and slip planes of recent earthquakes.