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Verifying low frequency earthquake detections from tremor

BROWN, J.R., Department of Geophysics, Stanford University, Stanford, CA  94305,; BEROZA, G.C., Department of Geophysics, Stanford University, Stanford, CA  94305,; SHELLY, D.R., Berkeley Seismological Laboratory, UC-Berkeley, Berkeley, CA 94720,

Recent studies have shown that deep tremor in the Nankai Trough under western Shikoku consists of a swarm of low frequency earthquakes (LFEs) that occur as slow shear slip on the down-dip extension of the primary seismogenic zone of the plate interface. The similarity of tremor in other locations suggests a similar mechanism, but the absence of cataloged low frequency earthquakes prevents a similar analysis. We present a method for identifying LFEs within tremor using a running autocorrelation of tremor seismograms. Our method employs a matched-filter algorithm, similar to the technique used to infer that tremor in parts of southwest Japan is comprised of LFEs. In our case, however, we do not know the origin times and locations of the LFEs a priori. We search for LFEs using a running autocorrelation of the tremor waveforms for a network of stations in the vicinity of the tremor source. We apply this method to recordings of tremor and demonstrate that this allows us to extract both known and previously unidentified LFEs. Lags showing a high degree of similarity in the autocorrelation are either repeats, or near repeats, of LFEs within the tremor. Once LFEs are identified, we apply waveform cross-correlation to measure differential times and locate the events using the SVD option of hypoDD. We will also show that making use of the data residuals as a proxy for verifying LFEs allows us to validate the detection scheme. Residuals for events located from the lags of false detections are anomalously high. Likewise, the false detections will not locate in vicinity of shear slip on a fault.


Last Modified: 2011 Aug 10