THE 7 MARCH 1988 NORTH PACIFIC INTRAPLATE EARTHQUAKE

A detailed analysis is made of an unusual North Pacific intraplate eve nt that occurred on 7 March 1988 approximately 500 km off the coast of the Japanese island of Hokkaido. It is the largest event recorded in the seismically quiet area by nearly an order of magnitude and has an anomalous m(b)/M(S) ratio (m(b) = 5.8, M(S) = 5.1). The short-period r esponse recorded at the Washington Regional Seismograph Network (WRSN) contains strong water-layer multiples that appear to suggest a source depth on or above die ocean floor. This can be explained, however, as the sampling of a node of the P-wave radiation pattern. The modeling of short-period seismograms recorded worldwide gives a focal mechanism with fault planes (340-degrees, 69-degrees, 72-degrees) and (202-degr ees, 27-degrees, 129-degrees) (strike, dip, slip) and a source depth o f 10 km below sea level. The observed low amplitude first arrival at W RSN and the fact that the signals observed at the other stations are m odeled well indicates an almost pure double-couple source for this str ong intraplate event.