Analysis of a Decade-Long Low-Frequency Earthquake Catalog in Southernmost Cascadia

Abstract:

Low-frequency earthquakes (LFEs) are small magnitude earthquakes, with typical magnitude less than 2, and reduced amplitudes at frequencies greater than 10 Hz relative to ordinary small earthquakes. Their occurrence is often associated with tectonic tremor and slow slip events along the plate boundary in subduction zones and occasionally transform fault zones. They are usually grouped into families of events, with all the earthquakes of a given family originating from the same small patch on the plate interface and recurring more or less episodically in a bursty manner. They can be identified by looking for waveforms repeating simultaneously at several seismic stations. When enough (a few hundred) LFEs have been identified for a given family, a template waveform can be obtained by stacking all the waveforms corresponding to all the LFEs identified. Once a template is available, additional LFEs can be found by cross-correlating seismic data with the template, and assuming that an LFE is occurring whenever the value of the cross-correlation is higher than a chosen threshold. In this study, we use the templates obtained by Plourde et al. (2015, GRL) using data recorded by the FAME (EarthScope Flexible Array Mendocino Experiment) network in Northern California during an episode of high tremor activity in April 2008, and we extend their catalog to the whole two years (2007-2009) during which the seismic stations were installed. We then use the two-year-long catalog to create templates for stations from the permanent seismic networks, and use the seismic data recorded by these stations to extend the catalog before and after the FAME experiment. We observe LFEs every time that there is tectonic tremor on the tremor catalog from Boyarko et al. (2015, EPSL), and several additional smaller LFE episodes. Whereas the LFE families from the subduction zone are mainly active during the big Episodic Tremor and Slip events, the southernmost family is more active, with repeating events about once a month. LFE from the families on the crustal faults from the San Andreas Fault System are even more frequent.