SCEC Award Number 11020 View PDF
Proposal Category Individual Proposal (Data Gathering and Products)
Proposal Title Empirical Green's Function (EGF) Methods: Identifying How to "Get It Right" by Studying How We "Get It Wrong"
Investigator(s)
Name Organization
Debi Kilb University of California, San Diego
Other Participants SIO Graduate Student Deborah Kane
SCEC Priorities A4, B5, A10 SCEC Groups Seismology, FARM
Report Due Date 02/29/2012 Date Report Submitted N/A
Project Abstract
 To constrain the source properties of a mainshock earthquake one can use another earthquake as an empirical Green’s function (EGF) to approximate the path- and site-effect contributions to the seismic waveform. An ideal EGF earthquake is smaller in magnitude than the mainshock and has a similar focal mechanism and hypocentral location to the mainshock. Here, we quantitatively define optimal EGF event selection using data from the spatially complex San Jacinto Fault Zone (SJFZ) in southern California. The high seismicity rate within the SJFZ allows us to test the EGF method for 51 target M>3 mainshock events using a large range of potential EGFs (>200 for all mainshocks; >1000 for 29 mainshocks). We estimate a mainshock corner frequency from the spectral ratio of each mainshock/EGF waveform pair. The suitability of an EGF event is determined by quantifying the variability of these corner frequencies across the network, assuming an ideal EGF event produces similar corner frequencies at every station. We find optimal EGF events are at least one unit of magnitude smaller than the mainshock, but that selecting EGFs with larger than one magnitude differentials does not produce significantly better results. Negligible differences are found in our results for EGF events located 2-14 km from the mainshock, suggesting an EGF event ~2 km from the mainshock may be as poor a choice as an EGF event ~14 km from the mainshock. We suggest also limiting mainshock/EGF pairs to those with high waveform cross-correlations at all stations (median >0.5 for this dataset).
Intellectual Merit A key aspect of our analysis is that we use a large suite of possible empirical Green’s function EGF earthquakes representing a broad range of event choice characteristics. This large dataset of trial EGF earthquakes ranges from those within the common limits for EGF selection to events falling far outside the typically accepted limits. The purpose in allowing such a wide range is to identify the transition separating these two populations.
Broader Impacts This project included work by graduate student Deborah Kane.
Exemplary Figure Figure 1: Median corner frequency variability versus hypocentral separation distance using distance bins of 0.1 km width. Negligible differences are found in our results for EGF events located 2-14 km from the mainshock, suggesting an EGF event ~2 km from the mainshock may be as poor a choice as an EGF event ~14 km from the mainshock.