In-Slab Earthquakes at the North End of the Cascadia Subduction Zone

At the north end of the Cascadia subduction zone, in-slab earthquakes occur within the Juan de Fuca plate up to depths of 100 km and in two distinct concentrations. One concentration, ranging in depth from 25 to 35 km, lies beneath the west coast of Vancouver Island. First motion focal mechanism solutions from this group of earthquakes illustrate a complex pattern, indicating no prominent regional tectonic stress pattern. The second concentration lies beneath Georgia Strait and Puget Sound, Washington. These earthquakes are deeper than the first concentration. Most events are in the 45 to 65 km depth range, as they occur farther from the margin where the angle of the subducting plate increases from approximately 15 degrees to approximately 30 degrees. Most of the in-slab earthquakes in the Georgia Strait/Puget Sound area are dominated by normal faulting and the larger, deeper events indicate predominately down-dip tension axes, as displayed by first motion and moment tensor focal mechanism solutions from the recent Nisqually and Satsop earthquakes. There appears to be some systematic differences in magnitudes calculated for in-slab earthquakes by the Geological Survey of Canada (GSC) and the University of Washington (UW). For earthquakes larger than approximately M=2 in the Puget Sound area, M_L from the GSC are consistently lower than coda magnitude (M_C) by the University of Washington. For the few larger events where moment magnitude (M_W) is available, M_W is comparable to M_C. However, farther north, in the concentration beneath Vancouver Island, UW's M_C is generally lower than the GSC's M_L. There are not sufficient M_W calulations in this region to compare.