OCEAN CABLE MEASUREMENTS OF THE TSUNAMI SIGNAL FROM THE 1992 CAPE-MENDOCINO EARTHQUAKE

The movement of the seawater across the earth's magnetic field produce s a large-scale motional electric field. Using the Point Arena, Califo rnia, to Hanauma Bay, Hawaii, unpowered HAW-1 cable, we have studied t he geopotential across this distance to look for possible tsunami-indu ced fields that might have been produced following the April 1992 Cape Mendocino earthquake. We have used a ten-day interval prior to and in cluding the earthquake as a reference for geopotential signals and for geomagnetic activity. We have also used geomagnetic data from Point A rena, Honolulu and Boulder as reference data. The results of the analy ses show that there are tsunami-related effects in the cable geopotent ial data. These are ( a) larger voltage prediction errors (residuals) For the interval following the main shock; (b) enhanced (compared to t he 10d reference interval) geopotential spectral power following the m ain shock: two enhancements are larger than geomagnetically-induced sp ectral power enhancements in the same time interval; and (c) strong ev idence For an similar to 30 min ''echo'' in the cable geopotential sig nal following the main shock.