An earthquake‐like magnitude‐frequency distribution of slow slip in northern Cascadia

Abstract

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. An automatic search from 2006–2009 reveals 20,000 five‐minute windows containing tremor which cluster in space and time into 96 tremor swarms. The 93 inter‐ETS tremor swarms account for 45% of the total duration of tremor detection during the last three ETS cycles. The number of tremor swarms, N, exceeding duration τ follow a power‐law distribution N ∝ τ−0.66. If duration is proportional to moment release, the slip inferred from these swarms follows a standard Gutenberg‐Richter logarithmic frequency‐magnitude relation, with the major ETS events and smaller inter‐ETS swarms lying on the same trend. This relationship implies that 1) inter‐ETS slip is fundamentally similar to the major events, just smaller and more frequent; and 2) despite fundamental differences in moment‐duration scaling, the slow slip magnitude‐frequency distribution is the same as normal earthquakes with a b‐value of 1.