The study was conducted by Scott and Lipps, both micropaleontologists, along with Dalhousie undergraduate Andrea D. Hawkes and Rod Combellick of the Alaska Division of Geological and Geophysical Surveys in Fairbanks. A paper describing their findings will be published in the May/June issue of the Geological Society of America Bulletin.
The team surveyed salt marshes along the Pacific coast from Alaska to northern California, taking 12-foot deep cores to look for signs of past subsidence coinciding with evidence of a major quake, such as soil liquefaction and sand deposited by a tsunami. Subsidence is indicated by a temporary change in the types of microscopic organisms with shells specifically foraminifera and thecamoebians in the mud, as the formerly freshwater marshes sink and become brackish or saltwater marshes.
One marsh in Alaska apparently sank about half a foot starting perhaps 15 years before the devastating 1964 Alaska quake, which measured 9.2 on the Richter scale and generated a tsunami that killed dozens of people along the Pacific coast. Similarly, a core from a marsh in Oregon showed evidence of four large quakes over the past 3,000 years, each preceded by subsidence.
Seismologists estimate that the period between megathrust quakes along the Cascadia Subduction Zone, where the Pacific, Juan de Fuca and Gorda Oceanic plates creep under the North American plate, ranges from hundreds to thousands of years. This makes them hard to plan for, Scott noted. Precursory subsidence detected by tiltmeters would at least allow people along the coast to plan for a major quake and probable tsunami.
The indicator marshes are typically fresh water marshes within inches of the high tide mark, so that subsidence of a few inches increases slightly the saltiness of the march, either through salt spray or intrusion of salt water at high tide,
Contact: Robert Sanders
University of California - Berkeley