Abstract
There are three usable regional depth phases, sPg , sPmP , and sPn , and their corresponding reference phases, Pg , PmP , and Pn . The differential time between each depth phase and its reference phase can be used to estimate earthquake focal depth. We have developed a method to determine focal depth for moderate and small earthquakes by using a regional depth-phase modeling (RDPM) method. We used a default focal mechanism to generate the differential times for all earthquakes. To estimate the reliability of the modeled focal depths, we compared our solutions with those obtained by other methods and found the consistency is good. Because the focal depths estimated by RDPM are model dependent, we tested the extent of the dependency and found that a 10% error in the crustal model may generate a 10%–15% error in the modeled depth. The absolute error is determined by the error in the crustal model and the focal depth itself. We found that earthquake location errors have only a small effect on the modeled focal depths. By analyzing synthetic and observed waveforms, we found a distance window within which sPmP and PmP are well developed, and, within the window, the P portion of the waveform is relatively simple and sPmP and PmP are easy to identify. We also demonstrated that the assumptions of sPmP and PmP are correct, that regional depth phases are not developed or not discernible in some regions, and that regional depth phases have special features that can be used to identify those phases.
Highlights
Earthquake focal depth is a critical parameter for seismological research, seismotectonic study, seismic hazard assessment, and event discrimination
We previously reported some aspects of the regional depth-phase modeling (RDPM) method (e.g., Ma et al, 2003; Ma and Atkinson, 2006)
We have proved that the assumptions of depth phase sPmP and its reference phase PmP are correct, and by conducting several tests, found in which regions the regional depth phases are developed and in which they are not, and which factors contribute to errors in the modeled focal depths
Summary
Earthquake focal depth is a critical parameter for seismological research, seismotectonic study, seismic hazard assessment, and event discrimination. Regional depth phases (sPg, sPmP, and sPn) can be used to estimate focal depth if they and their reference phases (Pg, PmP, and Pn) can be correctly identified. We found that the contents of PmP and sPmP come from different interfaces beneath the source These findings are useful for researchers who want to use the RDPM method, and especially for identifying the regional depth phases and their reference phases. 2.1 Synthetics generated at different distances with a fixed focal depth To observe features of the regional depth phases that are displayed when the distance changes, we generated synthetic waveforms at distances ranging from 0.3o to 4.8° and plotted them The position of sPg shifts by about half a cycle when the depth changes by 1 km This means that the time difference sPg–Pg is very sensitive to focal depth. Because we do not use waveform shapes to estimate focal depth, we do not have to use a strike–slip focal mechanism to generate synthetics for earthquakes with strike–slip focal mechanisms
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