Low-frequency earthquakes at the southern Cascadia margin

We use seismic waveform data from the Mendocino Experiment to detect low-frequency earthquakes (LFEs) beneath Northern California during the April 2008 tremor-and-slip episode. In southern Cascadia, 59 templates were generated using iterative network cross correlation and stacking and grouped into 34 distinct LFE families. The main front of tremor epicenters migrates along strike at 9kmd(-1); we also find one instance of rapid tremor reversal, observed to propagate in the opposite direction at 10-20kmh(-1). As in other regions of Cascadia, LFE hypocenters from this study lie several kilometers above a recent plate interface model. South of Cascadia, LFEs were discovered on the Maacama and Bucknell Creek faults. The Bucknell Creek Fault may be the youngest fault yet observed to host LFEs. These fault zones also host shallow earthquake swarms with repeating events that are distinct from LFEs in their spectral and recurrence characteristics.