KALAPANA EARTHQUAKE OF NOVEMBER 29, 1975 - INTRA-PLATE EARTHQUAKE AND ITS RELATION TO GEOTHERMAL PROCESSES

By use of teleseismic and local data, the P-wave source mechanism of the Kalapana, Hawaii, earthquake of November 29, 1975 was found to have a common strike of N64°E for the two nodal planes. One plane dipped 4° to the NW; the other dipped 86° to the SE. After consulting subsurface geological data obtained by the recent Hawaii geothermal exploration program, it was decided that the plane dipping to the NW at 4° was the preferred solution. Seismic moment obtained from body-wave data and surface-wave data averaged 1.2 · 1027 dyn cm. Fault area from P-wave, surface wave and tsunami data amounted to about 2200 km2. Stress drop was on the order of tens of bars. The earthquake appears to be of volcanic origin. When magma pressure in the dike complex of the east rift of Kilauea exceeded the fracture point, the southern flank of the east rift was pushed across the ancient sea floor upon which the volcanic edifice rests. The result was a low-angle overthrust, which also produced a tsunami. The hypothesis of forceful intrusion of magma into the east rift is consistent with the mechanism of the earthquake. The low stress drop (in relation to other intra-plate earthquakes) is probably due to the occurrence of the earthquake in a hot-rock regime. © 1979.