SCEC Award Number 14115 View PDF
Proposal Category Individual Proposal (Integration and Theory)
Proposal Title A search for tremor-like seismic precursors in Southern California with a phase coherence method
Investigator(s)
Name Organization
Jean-Paul Ampuero California Institute of Technology
Other Participants Jessica Hawthorne
SCEC Priorities 1c, 5a, 5d SCEC Groups EFP, Seismology, FARM
Report Due Date 03/15/2015 Date Report Submitted N/A
Project Abstract
 We have searched for tremor-like precursors to M 2.5-6 earthquakes in southern California. We use a frequency-domain phase coherence technique to examine the 5 minutes before each of 10000 earthquakes. This approach allows us to identify signals coming from the same location as the earthquake, even if those signals come from a source with extended duration. This method successfully detects many previously identified foreshocks, as well as a few foreshocks that are not in the catalog. However, with the methods and data used here, we see no evidence for more emergent seismic signals. This suggests that emergent precursors are rare or small, which is consistent with the infrequent reports of them.
Intellectual Merit All of the outliers examined so far appear to result from uncatalogued foreshocks. Figure 4 shows one example. The phase coherence (panel c) identifies a coherent signal 5-10 seconds before the M 3.1 earthquake. In the seismogram (panels a and b) we see that this signal has an abrupt onset, suggesting that it is an earthquake. These earthquakes are located to the south of the US networks, which may be why the smaller earthquake is not present in the SCSN catalog.
Broader Impacts In our search, we use variable amounts of data for each earthquake. To identify the intervals that we are best able to resolve, we use the variance estimates from the multitaper coherence estimation. This provides an uncertainty on the unnormalized phase coherence (the numerator in equation 1), and we use only plot results from intervals where we should resolve a precursory signal that had energy comparable to that expected for a M 2 earthquake.
Exemplary Figure This procedure is illustrated with synthetic data in Figure 1. There are four observed signals: 2 from the earthquake and 2 from the synthetic precursor. In the first line, we cross-correlate the earthquake and precursor record at individual stations. This eliminates the Green’s functions’ phases. We then cross-correlate between stations to eliminate the phases of the source-time functions. Since the synthetic precursor has the same Green’s function as the earthquake, the final phase is zero.