The 7 March 1988 North Pacific Intraplate Earthquake

Abstract

A detailed analysis is made of an unusual North Pacific intraplate event 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 mb/Ms ratio (mb=5.8, Ms=5.1). The short‐period response recorded at the Washington Regional Seismograph Network (WRSN) contains strong water‐layer multiples that appear to suggest a source depth on or above the 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°, 69°, 72°) and (202°, 27°, 129°) (strike, dip, slip) and a source depth of 10 km below sea level. The observed low amplitude first arrival at WRSN and the fact that the signals observed at the other stations are modeled well indicates an almost pure double‐couple source for this strong intraplate event.