Estimating seismic moment magnitude (M_w) of tremor bursts in northern Cascadia: Implications for the 'seismic efficiency' of episodic tremor and slip

摘要

We develop a method to estimate the seismic moments of deep non-volcanic tremor bursts observed in northern Cascadia. For each tremor burst, the maximum amplitudes at individual stations within a time window 5 s around the predicted arrivals of the S phase are measured and compared to the maximum S amplitudes measured from synthetic seismograms. The proposed method is thoroughly calibrated using 464 local earthquakes and the results show excellent consistency between the reported M_L and the estimated M_w. We apply the method to northern Cascadia tremors and infer that most bursts have M_w～1.0-1.7. The corresponding b value appears to be 1, consistent with that of ordinary earthquakes but over a narrower M_w range. Comparison of cumulative tremor M_w and the M_w estimated from the accompanying slow slip suggests that the "seismic efficiency" of the Episodic Tremor and Slip (ETS) is of the order of 0.1% or less.