|Scherwath, M. George Spence, Koichiro Obana, Shuichi Kodaira, Kelin Wang, Michael Riedel, Jeffrey McGuire, and John Collins,, Monitoring Earthquakes in Northern Cascadia With a Japan-Canada-US Seafloor Seismometer Array,, EOS, 2011|
The magnitude Mw=9.0 earthquake of 11 March 2011 at the Japan Trench and its devastating tsunami again reminded us of the importance of understanding seismogenic behavior of subduction faults and realistically estimating the potential size of future earthquakes and tsunamis. For the Cascadia subduction zone (Figure 1a), a critical knowledge gap is determining the level of microseismicity offshore, especially near the megathrust. In 2010, the first detailed seafloor earthquake monitoring campaign along the northern Cascadia subduction zone recorded nearby earthquakes in the local magnitude (ML) range from possibly around zero to 3.8 (Figures 1b, 1c) and larger earthquakes from outside this region.
Preliminary analysis indicates that the network appears to have yielded a fairly complete catalogue for events with ML>1.2, and only a few tens of these events occurred beneath the continental shelf and slope (Figure 1a), with the majority of the earthquakes located along the margin-normal Nootka transform fault zone. This relatively low seismicity away from the Nootka fault is consistent with a fully locked megathrust. Land-based GPS measurements cannot resolve if the offshore part of the megathrust seismogenic zone is narrow but fully locked or wider but only partially locked (slowly creeping). If it were only partially locked, we would expect many more small earthquakes along the interface, readily detectable with seafloor instruments.
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