LOW FREQUENCY EARTHQUAKE DETECTIONS FROM TREMOR IN THE NORTHERN CASCADIA MARGIN

Accumulating evidence suggests that deep tremor in subduction zones consists entirely of swarms of low frequency earthquakes (LFEs) that occur as shear slip on the plate interface. Using a network autocorrelation approach, we have detected and located LFEs within three hours of tremor recorded in the northern Cascadia subduction margin during the September 2005 slow slip event. As in Japan, we find that LFEs are the primary constituent of tremor during this period and that they locate on the deep continuation of the plate boundary's seismogenic zone. In this study, we investigate the July 2004 tremor sequence in order to extend our understanding of tremor in northern Cascadia, and the mechanics of the deep continuation of the plate boundary. The deep continuation of the plate boundary in northern Cascadia is located between 30-45 km where modeled temperatures in the subducting oceanic crust reach 400-500 degrees C; conditions favorable for dehydration of hydrous minerals in oceanic basalt in subduction zones. Our results to date suggests that tremor in this region shares a common mechanism with tremor in other subduction zones (e.g. Nankai, Middle America, Mexico, Alaska).