GIANT MEGATHRUST EARTHQUAKES ON THE WEST COAST OF NORTH AMERICA: MICROPALEONTOLOGICAL EVIDENCE FOR PRECURSORS OF MAJOR EARTHQUAKES

Large megathrust earthquakes on the west coast of North America have been shown to occur with repeat times of 200-500yrs at several localities from Alaska to N. California. Results are presented from two coastal marsh areas on the west coast of North America, Girdwood Flats, Alaska and Netarts Bay, Oregon, that suggest a different sequence of events than previously thought for megathrust earthquakes. There appears to be a subtle precursor event prior to the major rupture that causes megathrust earthquakes-that is a small subsidence in the order of 20-30cm that occurs 2-5yrs before the "main event". This precursor is detected by changes in thecamoebian and/or marsh foraminiferal assemblages where the fauna changes to reflect either a more brackish (closer to marine) as in Alaska or lower marsh level as indicated in Oregon. From the salt marsh deposits it is possible to determine with some confidence that the subsidence was in the order of 20-30cm. The 1964 Alaska sequence is particularly important since the exact (to the minute) timing is known and it is reasonable to assume that the same timing would occur for all these megathrust earthquakes. It was also established in the Alaska area that older events from Girdwood Flats exhibited the same precursor events, presumably with the same type of timing. In Oregon the setting was different where small changes in the marsh foraminiferal fauna indicated level changes within the salt marsh environment itself. Also in Oregon there were tsunami layers because this marsh was on the open coast. These were not present in Girdwood Flats because this area was inside a shallow inlet, protected from tsunamis. The significance of the precursor events is that for the first time there is a way of defining a shorter time frame for occurrences of these major earthquakes. Planners cannot plan on a 200 year return time but they can react to a two to three year warning-how they react is another issue but there at least does appear to be a reasonable time to react-i.e. more than a few seconds.