Sinking coastlines early indicators of large subduction quakes
20 January 2005, CIESM News

A new study concludes that slight sinking along nearby coastlines may precede by two to five years the actual rupture and subsequent earthquakes such as the one that generated the recent massive tsunami in South Asia. If coastal subsidence is common before subduction zone quakes, areas such as those ringing the Pacific Rim could be on the lookout for subsidence as a warning for future megathrust quakes.

Subduction zones are areas where one tectonic plate slips under another, raising mountain ranges along the margin sprinkled with volcanoes. Quakes occur when the edge of the overlaying plate sticks to the subducting plate, causing a slight dip nearest the zone and a slight bulge farther away. Eventually, the stuck edge lets go in a massive quake, after which the margin relaxes to pre-quake levels. Because these plates thrust over one another in these zones, the quakes are referred to as megathrust earthquakes and often generate tsunamis.

The team surveyed salt marshes along the Pacific coast from Alaska to northern California, taking 4-meter 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 former freshwater marshes sink and become brackish or saltwater marshes.

One marsh in Alaska apparently sank about a half a meter some 15 years before the devastating 1964 Alaska quake, which ranked 9.2 on the Richter scale and generated a tsunami that killed dozens of people along the Pacific coast. The large 1964 Alaska quake temporarily dropped some coastal land 2 meters, drowning towns and forests.

The researchers' hunch that foraminifera and thecamoebians (basically amoebas with microscopic shells) might be able to give more precise dates for precursory subsidence, since they are more sensitive than diatoms to elevation changes, proved correct. They saw a remarkable change in the fauna, about a centimeter below the quake area itself: going from a mix of species that live in the forest to a slightly brackish fauna with only one species. It was not a single event, as they were able to detect a 1,800-year old event from the same area, then went to other affected areas around the Pacific Rim and found similar situations.