Missing: earthquakes in the oceans.

Missing: Earthquakes in the oceans

Last January, people living on the island of Kosrae in the Pacific Ocean felt a large earthquake. But neither the Tsunami Warning Center in Honolulu nor the National Earthquake Information Service in Boulder, Colo., had any record of the quake. Daniel Walker and his co-workers at the Hawaii Institute of Geophysics in Honolulu estimate that about 25 such oceanic earthquakes, many of which have magnitudes exceeding 5.0, go undetected every year by the conventional continental network of seismic stations.

"There's a lot more seismicity in the world's oceans that we currently think," says Walker. As a result, he says, scientists are missing an opportunity to learn about the tectonics of ocean plates, to improve tsunami warning capabilities and to bolster their ability to monitor nuclear explosions in the oceans.

Walker's group was in fact able to detect the January Kosrae earthquake and other large oceanic earthquakes because it has an array of 11 hydrophones situated near Wake Island in the center of the western Pacific. This array, perhas the deepest in the world, is the only seismic network covering a region comparable in size to the entire North American continent. Walker thinks it has detected oceanic earthquakes missed by the continental networks because the oceanic crust -- and perhaps the ocean, too -- acts as wave guide, preventing seismic waves from traveling very far into the continents. The Wake Island array also has the advantage of being on the ocean floor, where the crust, which Walker says "mucks signals up and attenuates their energies and frequencies," is thin.

Most of the earthquakes detected by the Wake array since October 1982 have occurred in and along the western Pacific basin. On the basis of a string of earthquakes detected near Kosrae Island in the southwestern Pacific by the Wake array, together with the geology of the region and other reported earthquakes to the north and south of the Kosrae string, Walker and Loren W. Kroenke, also at the Hawaii Institute of Geophysics, have proposed a new subduction zone, which they call the Micronesian trench. As discussed a paper to be published in an upcoming issue of EOS, the researchers believe that this trench results from a northward shift in the convergence of the Indo-Australia and Pacific plates. Walker and Kroenke say that if their hypothesis is correct, the Micronesian trench would be the longest single continuous-arc segment in the world and may also be a "heretofore unrecognized tsunami-generating zone within the Pacific basin."