  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, 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. In this study, we extend the LFE catalog
obtained by Plourde et al. (2015) during an episode of high tremor activity in
April 2008, to the 8-year period 2004-2011. All of the tremor in the Boyarko et
al. (2015) catalog south of 42 degrees North has associated LFE activity, but
we have identified several smaller episodes of LFEs without associated tremor
activity, and extend their catalog forward and backward by a total of about 3
years. As in northern Cascadia, the down-dip LFE families have recurrence
intervals several times smaller than the up-dip families. For the April 2008
Episodic Tremor and Slip event, the best recorded LFE families exhibit a strong
tidal Coulomb stress sensitivity starting 1.5 days after the rupture front
passes by each LFE family. The southernmost LFE family, which has been
interpreted to be on the subduction plate boundary, near the up-dip limit of
tremor, has a very short recurrence time. Also, these LFEs tend to occur during
times when predicted tidal Coulomb stress is discouraging slip on the plate
boundary. Both observations suggest this LFE family may be on a different
fault.
