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Selecting Empirical Green’s Functions in Regions of Fault Complexity: A Study of Data From the San Jacinto Fault Zone, Southern California

Deborah L. Kane, Debi L. Kilb, & Frank L. Vernon

Published April 2013, SCEC Contribution #1701

To constrain an earthquake’s source properties, the path- and site-effect contributions to the seismic waveform can be approximated using another earthquake as an empirical Green’s function (EGF). An ideal EGF earthquake is smaller in magnitude than the mainshock and shares a similar focal mechanism and hypocenter. Here, we quantify how to optimally select EGF events using data from the spatially complex San Jacinto Fault Zone (SJFZ) in southern California. The SJFZ’s high seismicity rate allows us to test the EGF method for 51 target 3<M<5 mainshock events over a range of potential EGFs (>200 for each mainshock). We purposely select a large population of inappropriate EGFs so we can identify thresholds and restrictions that optimize EGF selection criteria. For each mainshock/EGF pair we compute the spectral ratio, fit the mainshock corner frequency, and measure the variability of these corner frequencies across the network. We assume a suitable EGF event will produce similar corner frequency estimates at every station. We find that limiting hypocentral separation distances between mainshock and EGF events to <1 km (within ~1-3 mainshock fault lengths) is an effective criterion in EGF choice. Surprisingly, separation distances of 2-14 km produce negligible changes in corner frequency variability, suggesting that EGF events at 2 km distance may be as poor a choice as EGF events at much greater distances. When EGF events within 1 km are not available, we suggest limiting EGF events to those with highly similar waveforms to the mainshock waveforms to ensure source similarity.

Citation
Kane, D. L., Kilb, D. L., & Vernon, F. L. (2013). Selecting Empirical Green’s Functions in Regions of Fault Complexity: A Study of Data From the San Jacinto Fault Zone, Southern California. Bulletin of the Seismological Society of America, 103(2A), 641-650. doi: 10.1785/0120120189.