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A New Method for Determining Small Earthquake Source Parameters Using short-period P waves

Donald V. Helmberger, & Ying Tan

Published 2007, SCEC Contribution #1601

We developed a new technique of inverting short-period (0.5-2 sec) P waveforms for de¬termining small earthquake (M < 3.5) focal mechanisms and moments, where magnitude ~4 events with known source mechanisms are used to calibrate the “unmodeled” struc¬tural effect. The calibration is based on a waveform cluster analysis, where we show that clustered events of different sizes, e.g., M ~ 4 vs. M ~ 2, display similar signals in the short-period (SP, 0.5-2 Hz) frequency band, implying propagational stability. Since both M ~ 4 and M ~ 2 events have corner frequencies higher than 2 Hz, they can be treated as point sources, and the “unmodeled” structural effect on the SP P waves can be derived from the magnitude 4 events with known source mechanisms. Similarly, well-determined magnitude 2’s can provide good calibrator for studying even smaller events at higher fre-quencies, e.g., 2-8 Hz. In particular, we find that the “unmodeled” structural effect on SP P waves is mainly an amplitude discrepancy between data and 1D synthetics. The sim¬ple function of “Amplitude Amplification Factor” (AAF) defined as the amplitude ratio between data and synthetics provides useful calibration, in that the AAFs derived from different clustered events appear consistent, hence stable and mechanism-independent. We take a grid search approach to determine source mechanisms by minimizing the misfit error between corrected data and synthetics of SP P waves. The validation tests with calibration events demonstrate the importance and usefulness of the AAF corrections in recovering reliable results. We introduce the method with the 2003 Big Bear sequence. However, it applies equally well to other source regions in southern California, since we have shown that the mechanism-independence and stability of the AAFs for source re¬gions of 10 km by 10 km are typical. By definition, the AAFs contain the effects from the station site, the path and crustal scattering. Although isolating their contributions proves difficult, the mechanism-independence and stability of the AAFs suggest that they are mainly controlled by the near-receiver structure. Moreover, the ratios between the AAFs for the vertical and radial components from various events at different locations

Helmberger, D. V., & Tan, Y. (2007). A New Method for Determining Small Earthquake Source Parameters Using short-period P waves. Bulletin of the Seismological Society of America, 97, 1176-1195.