DEEP TREMOR ALONG THE CASCADIA SUBDUCTION ZONE

On February 1, 2003, the Pacific Northwest Seismograph Network (PNSN) first recognized and recorded a single deep tremor burst in northern Puget Sound. In late February, a tremor episode began and continued over approximately two weeks, during which there were roughly three heightened periods of activity. These tremor signals have a frequency content between 1 and 6 Hz and apparent velocities consistent with shear waves (~4 km/s). On the PNSN, the tremor is seen as individual bursts that rise above background noise and last from minutes to hours. During this same time period, a slow earthquake was recorded by the regional geodetic network. We located 33 out of 60+ identified tremor bursts, the majority of which are in the vicinity of northern Puget Sound and southern Vancouver Island. This is consistent with the observations and locations made by Pacific Geoscience Centre. Errors in individual epicenters are on the order of 10 km, a distance significantly smaller than the distribution of tremor locations. The depths are less constrained because of the emergent nature and lack of clear phases in the signals. They lie between 30-40 km. While the majority of events progressed northward, 25+ bursts were observed on stations in the southern Puget Sound region, of which six could be located along the eastern edge of the Olympic Peninsula. The signal to noise ratio is much lower in this area, thus the events are even more difficult to detect and locate. There have been a few smaller bursts of tremor since the cessation of the Feb-March 2003 tremor swarm and silent slip event.