The 1980 Izu-Hanto-Toho-Oki earthquake is studied in detail using near-field strong motion seismograms recorded at Japan Meteorological Agency stations. A seismogram inversion method is applied to deduce the dislocation distribution and the character of rupture propagation during this earthquake.

This earthquake involves left-lateral strike-slip motion on the almost vertical plane with a strike of N10°W. The fault plane is shallower than about 12 km in depth, and the length is about 20 km. The large dislocation (large seismic moment) occurs near the hypocenter and at the southern end of the fault plane. The rupture propagates southward from the central part of the fault plane and spreads to the shallow area of the northern part of the fault plane after a delay of about 5 sec relative to the initiation of this earthquake. The total seismic moment is about 7 × 1025 dyne·cm.

The aftershocks of magnitude equal to or greater than 4.0 take place in the areas where high stresses are expected to remain after this earthquake. The mechanical weakness of small submarine monogenetic volcanoes which are located above the source region seems to affect the rupture process of this earthquake.

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