Earthquake Magnitude and Hazard

Although there is little doubt that subduction earthquakes have occurred in the region and will occur in the future, there is uncertainty as to how much of the Cascadia subduction zone ruptures during such events. Conceivably, the entire 1000 km length of the subduction zone could rupture, producing an earthquake in excess of magnitude 9 (Rogers, 1988). Although unusual in the global experience, the great Chile earthquake in 1960 had such a rupture length. as happened, for example in Chile in 1960. Alternatively, sections several hundred kilometres in length might fail, producing more numerous magnitude 8 events. It is unlikely that plate convergence and underthrusting are accommodated by earthquakes smaller than magnitude 8 because so many would be required that some should have occurred during the 200-year historical period.

The issues of segmentation and the maximum magnitude of subduction earthquakes have not yet been resolved; however, the geophysical data indicate that the whole subduction zone is locked at present, and radiocarbon and tree-ring dating do not rule out the possibility that the last great earthquake ruptured the entire subduction zone. Historical evidence from Japan supports the idea that this was a single giant (magnitude 9+) earthquake rather than a series of lesser (magnitude 8) earthquakes. Satake et al. (1996) describe records of a tsunami at a number of sites along the Japanese coast on January 27, 1700. There is no indication of a local earthquake at this time, and, after eliminating other sources, Satake and his co-workers concluded that the tsunami was generated by an earthquake that ruptured much or all of the Cascadia subduction zone on the evening of January 26, 1700.