Inversion of Source Parameters for Moderate Earthquakes Using Short-Period Teleseismic P Waves

In this paper, we introduce a new method for estimating the source parameters of moderate earthquakes (M (w) similar to 5.0) by modeling short-period teleseismic waveforms. This method uses a grid-search algorithm to minimize misfits between observed data and synthetic seismograms in depth, magnitude, and mechanism domain in a relative high-frequency range of 0.8-2.0 Hz, similar to the traditional cut-and-paste method used in regional modeling (Zhu and Helmberger, Bull Sesimol Soc Am 86:1634-1641, 1996). In this frequency range, a significant challenge is determining the initial P-wave polarity because of a low signal-to-noise ratio (SNR). Therefore we first determine source properties for a master earthquake with a relative strong SNR. Both the travel time and amplitude corrections are developed relative to the reference 1D model along each path used in inverting the master event. We then applied these corrections to other earthquakes clustered in the same area to constrain the initial P polarities. Thus the focal mechanisms can be determined reasonably well. We inverted focal mechanisms for a small set of events beneath Qeshm Island in southern Iran and demonstrate the importance of radiation pattern at short periods.