A Granular Jamming Model For Low-Frequency Earthquakes

A catalog of low frequency events (LFEs) beneath Vancouver Island is analyzed in the context of a granular flow model. The catalog contains origin-times and magnitudes of 269,423 LFEs grouped within 130 families and recorded between 2003 and 2013. Each family represents a distinct location within the boundary between the subducting Juan de Fuca and overriding North American plates. The LFEs occurred during 10 episodic tremor and slip (ETS) events that recurred at similar to 14-month intervals and lasted for about a week. With one exception, each family was active in all 10 ETS episodes. Our analysis suggests that LFEs do not follow Gutenberg-Richter statistics, but are normally distributed with respect to magnitude and, therefore, log-normally distributed with respect to moment. The Kostrov strain associated with the moments in a given family is used to estimate its size as L = 350 +/- 15 m. These observations are explained through a model of granular flow in a channel that accommodates displacement along the plate boundary. In this model the LFE families correspond to granular jams in flow that persist over many ETS episodes. Each LFE is a slip between two grains in the jam. The log-normal distribution of LFE moments can be ascribed to a theoretically predicted log-normal distribution of grain sizes in each jam. This model explains the weak dependence of seismic duration on LFE moment because frictional slip between grains in an over-pressured environment need not scale like a growing rupture in an elastic medium.