Sixteen unreported earthquakes associated with the Marianas subduction zone have been recorded during a 15-month period by an array of hydrophones in the Western Pacific near Wake Island at distances of about 2, 000 km. Thirteen of the events occurred along a relatively small section of the arc between 16 and 20°N latitude. During the same period of time only 12 shallow-focus events were reported by either the National Earthquake Information Service (NEIS) or the International Seismological Centre (ISC) for the same portion of the arc. Comparative studies show that the unreported events may range in body wave magnitudes from slightly less than 4 to as high as 5. Also, the data suggest that they may have very shallow focal depths. Many of the events appear to be located along the extreme oceanic edge of the subduction zone. These findings imply that, at least in some regions, the seismicity of ocean plate margins for shallow-focus earthquakes may not yet be accurately assessed.

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A fault model of the 1982 Urakawa-Oki earthquake is determined by analyzing seismic, geodetic, and tsunami data. Using Rayleigh waves with periods longer than 200s and P-wave first motions we obtain the source mechanism characterized by a reverse fault with the P-axis of the NE-SW direction. The seismic moment is 2.7×10²⁶ dyn ·cm. The fault model is constrained but not uniquely determined by aftershock distribution and leveling data. Tsunami simulation for actual ocean bottom topography helps us infer the fault parameters. Comparison of tide gauge records and the simulated tsunamis shows that the fault model inclined from the Pacific Ocean to the inland of Hokkaido provides the best fit to the observation. The fault model has a dimension of 12×22 km² on a plane dipping 39° toward N30°E. The average dislocation is estimated to be 2.5 m. The generation of the earthquake may be ascribed to the collision of the Kuril arc with the Tohoku arc.

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