Atwater and Hemphill-Haley analyzed an earthquake record spanning 3,500 years at Willapa Bay in southwestern Washington, identifying seven earthquakes, including the A.D. 1700 event. As shown in Figures 4-9 and 4-21, these earthquakes struck at irregular intervals. At the Redtail site, events S, U, and W struck within a five-hundred-year interval, whereas about nine hundred years elapsed between Event S and the previous Event N. In the Sixes River estuary in southern Oregon, just north of Cape Blanco (located on Figure 4-12), Harvey Kelsey, Eileen HemphillHaley, and their colleagues found evidence for eleven earthquakes in the past 5,500 years, with variation in time between events from seventy years to as much as nine hundred years. Goldfinger found turbidites triggered by thirteen earthquakes since the eruption of the Mazama Ash 7,700 years ago and eighteen earthquakes since the beginning of the Holocene about ten thousand years ago. The shortest time between post-Mazama turbidites is 215 years and the longest time nearly fifteen hundred years.
The average recurrence interval at Willapa Bay and Sixes River is 500-540 years, but it is not clear that there is a one-to-one correlation between earthquakes at the two sites. Kelsey and his colleagues suggested that an earthquake two thousand years ago at Sixes River lacks a counterpart at Willapa Bay. The average recurrence interval based on post-Mazama turbidites is six hundred years and, for the entire Holocene, it is 550-560 years. At Bradley Lake, north of Sixes River, the recurrence interval is about 440 years. But Alan Nelson of the USGS and his colleagues found that two earthquake-related deposits at Bradley Lake were separated by only forty years, suggesting that Bradley Lake was affected by smaller earthquakes to the north and south.
These studies reveal a paleoseismic record that is unparalleled anywhere else in the world. But until individual earthquakes are more closely dated by radiocarbon, about all that can be said at the present time is that north of California, the number of earthquakes is about the same for all earthquake sequences studied, and that repeat times for individual earthquakes are highly variable. As yet, we are far from being able to predict how long it will be until the next earthquake. One idea is that the longer the time since the last earthquake, the larger the next one is likely to be. If the next Big One happened tomorrow, a little more than halfway through the average recurrence interval, the earthquake would be smaller than the 1700 event because less strain would have accumulated. A suggestion that this might be the case can be inferred from the Willapa Bay record at the Redtail site (Figure 4-21). Events N and W were followed by long periods with no earthquakes, and the earthquakes that followed (Events S and Y, respectively) recorded maximum subsidence, a forest burial rather than a marsh burial. This seems like a great idea except that the 1960 Chilean earthquake, the largest of the twentieth century, was preceded by great earthquakes in 1835 and 1837, less than one hundred and thirty years earlier. Also, it is not clear that less vertical subsidence means a smaller earthquake. The presence of the same number of turbidites in channels below submarine canyons from Washington to southern Oregon suggests that all these events are very large, close to magnitude 9.