Recording earthquakes on the sea floor
The vast majority of the earthquakes are located underneath the oceans where they are not recorded because of a lack of instruments. This is why the Woods Hole Oceanographic Institution (WHOI) has developed a new kind of ocean bottom seismometers (OBSs) to record both small and large earthquakes on the sea floor. Forty of them will be deployed at the beginning of 2007 in an area of the Eastern Pacific Ocean known to have large earthquakes. One goal of this one-year mission is to better understand earthquake processes, but this technology could soon be used to better monitor other parts of the oceans.
Jeff McGuire and John Collins at the Woods Hole Oceanographic Institution (WHOI) plan to deploy 40 ocean bottom seismometers, or OBSs, on the ocean floor along the East Pacific Rise in the Eastern Pacific Ocean. Their target area is a section of ocean about the size of the states of Connecticut and Rhode Island combined.
Because of their structure, these new OBSs will be able to record all the shocks happening before and during an earthquake.
Below is a photo of "John Collins (left) and Jeff McGuire with some of the OBSs at WHOI, part of the national OBS pool. The seismometer is the gray device at right." (Photo by Tom Kleindinst, Woods Hole Oceanographic Institution; larger version).
And below is a diagram showing the different elements of these new OBSs. "The four orange fiberglass 'hardhats,' mounted on a plastic grillwork, contain batteries and electronics. A differential pressure gauge measures earthquake-generated waves in the water. The seismometer is housed in a metal sphere attached to an swivel at right. When the OBS is deployed, a line corrodes, positioning the seismometer on the seafloor." (Credit: Jack Cook, WHOI; larger version).
What can we expect from such a mission? McGuire answers.
"We will be able to record large undersea earthquakes directly on top of the faults that generate them. Although our test area is in the middle of the Pacific Ocean, this technology will have broad application to other faults zones, including those of significant societal relevance such as the nearshore subduction zone off Oregon and Washington."
Previous WHOI missions have used other kinds of OBSs, for example to reveal the plume beneath Hawaii. Below is an illustration showing how 35 seismometers were used to record earthquake-generated seismic waves traveling through the plume in order to determine the plume's depth, width, and temperature. (Credit: Jayne Doucette, WHOI; larger version).
For more information about this previous mission, please read "Listening Closely to 'See' Into the Earth" (Oceanus, March 5, 2004).
Sources: Woods Hole Oceanographic Institution news release, via EurekAlert!, February 22, 2006; and various web sites
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