Low-Frequency Earthquakes in Southern Cascadia

English Abstract:

We use data from the EarthScope Flexible Array Mendocino Experiment to detect low-frequency earthquakes (LFEs) beneath northern California, in the southern Cascadia subduction zone, during the April 2008 tremor-and-slip episode. LFE templates were generated using iterative network cross-correlations and stacking, starting from a combination auto-detections and visually-identified initial templates. To improve the efficacy of detection via the network cross-correlation over this broad region, an approximate hypocenter is used to define station dependent template windows. By comparing final waveform cross-correlations and identifying shared detections, 58 templates were found to represent 33 distinct LFE families. For each family P- and S-arrivals were picked and used for precise hypocentral location, enabling a spatio-temporal analysis of tremor-and-slip evolution. The main slip front travels northward at ~9 km/day, while one instance of rapid tremor reversal (RTR) travels southward at 30-40 km/h. LFE depths vary from 30-35 km in the northern part of the study area to 35-45 km in the southern part, and are consistently shallower than recent plate-interface models. As these events are interpreted to occur on the plate-interface, this study suggests that the previous interface models have been biased deep.