An Earthquake-Like Magnitude-Frequency Distribution of Tectonic Tremor in Northern Cascadia

An Earthquake-Like Magnitude-Frequency Distribution of Tectonic Tremor in Northern Cascadia Figure 1. Log of number (N) of tremor swarms exceeding durations given on upper axis can be fit with a straight line indicating that N is proportional to τ ^-0.65 where τ is the duration of a tremor swarm. We assume that the seismic moment is proportional to tremor duration scaled by Mo (N-m) = 5.2x10^16 τ (hrs) [Aguiar et al., 2009] to equate duration (upper axis) to moment magnitude (lower axis). This allows a standard Gutenburg-Richter style analysis and produces a b-value of 1.0 ( logN = a − bMw ), which is within the range commonly seen for regular earthquakes.
Major episodic tremor and slip (ETS) events with Mw 6.4 to 6.7 repeat every 15 ± 2 months within the Cascadia subduction zone under the Olympic Peninsula. Although these major ETS events are observed to release strain, smaller “tremor swarms” without detectable geodetic deformation are more frequent. We employ a Waveform Envelope Cross-Correlation and Clustering (WECC) [Wech and Creager, 2008] algorithm to search every 50%-overlapping 5-minute window for tremor. The resulting 20,000 tremor epicenters from 2006 through 2009 in northern Washington naturally cluster in space and time into the 91 tremor swarms analyzed here. We find that 88 inter-ETS tremor swarms account for 45% of the total duration of tremor detection during the last three ETS cycles. Considering duration as proportional to moment release, the swarms follow a standard Gutenberg- Richter frequency-magnitude relation, with the major ETS events lying on the trend defined by inter-ETS swarms. This relationship implies that 1) inter-ETS swarms are fundamentally similar to the major events, just smaller and more frequent; and 2) despite fundamental differences in moment-duration scaling, the tremor magnitude-frequency distribution has the same power law trend of normal earthquakes with a b-value of 1.
</p><p>Aguiar, A. C., et al. (2009), Moment release rate of Cascadia tremor constrained by GPS, J. Geophys. Res., 114, 1-11.
</p><p>Wech, A. G., and K. C. Creager (2008), Automated detection and location of Cascadia tremor, Geophys. Res. Lett., 35, 1-5.
</p><p>Acknowledgements: This study was supported by the National Science Foundation/EarthScope (EAR-0545441) and the USGS (08HQGR0034, G09AP00024, G10AP00033). Primary data were supplied by PNSN, EarthScope, PBO and PGC seismometers.</p>


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