RATIONALE: The primary objective of the teleseismic component of the B.C. array is to gain a better understanding of the hazard and structural controls on intraplate seismicity in the northern Cascadia subduction zone.

The motivation for this objective is:

i) Intraplate seismicity is historically responsible for more deaths in the Americas than megathrust earthquakes because they are located landward of the trench and often below population centers (e.g. Victoria, Vancouver). The most damaging earthquake in the Pacific Northwest this century was the intraslab 1949 Olympia earthquake causing 100 million dollars (about double that of the more recent intraslab 1965 Seattle M=6.5 event).

ii) The mechanisms and structural controls on intraplate seismicity are poorly understood. Intraplate earthquakes are thought to occur primarily within subducting oceanic crust as a result of prograde metamorphic reactions that occur as plate encounters higher temperatures and pressures. These reactions culminate in eclogitization and a dramatic increase in density which likely has profound influence on plate dynamics. They may also have implications for the development/evolution of continental crust.

iii) In addition to a better understanding of the mechanisms for intraplate seismicity, improved understanding of shallow (< 100 km depth) subduction zone structure will place further constraint on hazard from both megathrust and intraplate seismicity by allowing us to assess the importance/contributions of postcritical oceanic Moho reflections in focussing energy toward populations centers.

In southwestern British Columbia, intraplate seismicity extends from trench to Georgia Strait/westernmost lower mainland; with a concentration below the Gulf Islands. The recognition of i-iii) has spawned international interest in the topic of intraslab seismicity, as evidenced during an international workshop on the topic held on Vancouver Island this past September. Thus the POLARIS SWBC experiment is timely and results from our research should garner considerable attention from international circles (e.g through the new "SLAB Consortium").

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