5.4.7: M 8 or M 9? Where Do We Stand?

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Overview

Participants in a scientific conference held at Seaside, Oregon, in June 2000 were asked their opinions on M 9 vs. M 8 for the 1700 earthquake. The vote was overwhelming for an earthquake of M 9, indicating that another paradigm change had taken place. The evidence from the Japanese tsunami of A.D. 1700 and the constant number of post-Mazama turbidites from submarine channels from southern Oregon to Washington confirmed the tectonic model of Hyndman and Wang and carried the day.

However, this does not mean that all the earlier earthquakes recorded in the estuaries were M 9 as well. Atwater and his colleague, Eileen Hemphill-Haley, found that some of the paleoseismic sites at Willapa Bay recorded the drowning of a coastal forest, whereas others recorded the drowning of marsh grasses, evidence of less subsidence (Figure 4-21). If marsh grasses rather than forests were drowned, would this indicate a smaller earthquake, closer to M 8? So, even though scientists agree that the 1700 earthquake was M 9, earlier ones, and by inference, the next one, which is the main reason we are concerned about this, could be smaller. This is particularly true for that part of the subduction zone off the coast of northern California.