Premonitory seismicity patterns near Vancouver Island, Canada

Abstract

The search for premonitory patterns for large seismic events requires an a-priori threshold to define them. In the area of the Explorer and Juan de Fuca plates (the area of Vancouver Island, Canada) this threshold appears to be M ≥ 6.5. The three strongest earthquakes ( M > 6.0) which have occurred since the seismicity catalog has been acceptably complete for M >- 2.5 are preceded by activation of the earthquake flow in the lower magnitude range, in the generalized sense depicted by the algorithm “M8” suggested by Soviet scientists. One event of magnitude 6.0 may have been preceded by activation, but two others were not. The times of increased probability (TIP) of a strong earthquake occupy about 30% of the total time-space domain considered and there are no “false alarms”. This implies a significant similarity between the earthquake flow here and in other regions of the world where algorithm M8 was tested. There is a possibility, which remains to be tested, of anticipating future strong earthquakes in this area with an accuracy of 5 years and 100 km. Such anticipation could justify some precautionary measures and could be useful for attempts to make short-term predictions. Analysis using patterns of quiescence restrict the location of the anticipated earthquake. There is evidence for the coupling of seismicity in the subduction zone with that above it. Such interaction is a natural consequence of the model of a lithosphere as an assemblage of interacting blocks.