Evidence of Possible Precursor Events for Mega-thrust Earthquakes on the West Coast of North America

Megathrust earthquakes in western North America may be preceded by precursors several years ahead of the main event. For example on March 27, 1964, a 9.2 magnitude (on the Richter scale) earthquake occurred on the coast of Alaska. Changes in foraminifera and diatom biotas provided evidence of a precursor to this earthquake, thereby detailing a previously unknown sequence of events. We describe further evidence from marshes in Turnagain Arm, Alaska, USA and farther south in Netart's Bay, Oregon, USA; this is the first time that two widely spaced locations have been examined for precursors. The Alaska earthquake offers the possibility to compare a modern sequence (1964) of events with the geologic record. The Netart's Bay marsh has experienced no modern earthquake that could be used for comparison, but the nature of mega-thrust earthquakes implies that the modern and ancient events should be physically similar. The new cores examined from Turnagain Arm include both the 1964 earthquake and an event identified and dated at 1800yBP. The foraminifera and thecamoebian biota change from a forest sequence to a mildly brackish sequence just before the 1964 event; this zone was dated as being 15 years or less in length using Pb210 and Cs dating techniques. The event at 1800yBP was also associated with a similar transition indicating a small subsidence just before the major subsidence event. In Netart's Bay a new core was taken from a previously cored site because the chronology had already been determined by carbon-14 dating and also because at least four events were known to have occurred over the last 3000 years. The new core had four visual lithological transitions of which three were from marsh peat to mineralic deposits (possibly deposited by tsunamis) and back to a marsh deposit. Foraminifera indicated that these units represented high marsh transition to lower marsh, the earthquake event, and then rebounding back into marsh deposits. Sand deposits with either no or few foraminifera marked the tsunami/earthquake intervals. These transitions in two widely separated geographical areas indicate that similar mechanisms operate for large megathrust earthquakes from Alaska to Northern California in the Cascadia Subduction Zone thus implying that precursor events also occur and be detected by foraminiferal zonations all along this area as well. In a recent newspaper article scientists from the west coast suggested that "slow" or "silent" earthquakes they had measured with tiltmeters might be indicators of megathrust earthquakes. The transitions we document may be the prehistoric representations of these "silent" quakes. Foraminiferal evidence may help provide more accurate positioning of seismometers along the west coast of North America and therefore lead to more precise and timely earthquake predictions.