麻豆传媒

The MacEachern - Ponsford Memorial Award - 1999

In May 1964 a 9.2 magnitude (on the Richter scale) earthquake occurred on the coast of Alaska. Since then, research has shown evidence of precursors to the earthquake using both foraminifera and diatoms, thereby detailing a previously unknown sequence of events. This project focuses on a marsh environment lying farther south in Netarts Bay, Oregon, USA. Netarts Bay was also reported to have precursor earthquake events that were discovered using only diatom analysis. The precursor events are small subsidence events prior to the actual earthquake, that occur 2-5 years before the seismic event.

The Alaska earthquake offers the possibility to compare a known sequence of events with the geologic record. However, the Netarts Bay marsh has experienced no modern earthquake that could be used for comparison, but the nature of mega-thrust earthquakes means that the modern and ancient events should be physically similar. One of the previously cores sites was re-cored to resample and re-examine transition zones using foraminifera and thecamoebians. The benefit of using a previously sampled site is that the core has already been dated and zones of transition have already been identified lithologically.

The core used in this study has four visually distinguishable transitions. Previous dating indicates that such events have occurred over a period of ca. 3000 years. Of the four transition zones examined, transitions 2 and 3 display strong precursor evidence, based on marsh foraminifera and thecamoebians, to mega-thrust earthquakes. Transition 1, the only transition that displayed an emergence precursor may be seismic but is most likely non-seismic and the result of sediment infilling. Transition 4 is a weaker version of transitions 2 and 3, which all depict a subsidence precursor event.

Keywords:
Pages: 90
Supervisor: David Scott