"The Hawaii Earthquake of November 29, 1975: Low dip angle faulting due to forceful injection of magma""

Abstract

The mechanism of the Hawaii earthquake of November 29, 1975 (Ms = 7.1), which took place on the south flank of Kilauea Volcano, is discussed on the basis of a comprehensive set of body wave and surface wave data, the aftershock distribution, and tsunami and crustal deformation data. The aftershock distribution defines a gently dipping plane at about 10‐km depth beneath the south flank of Kilauea. This suggests that the shallowly dipping P wave nodal plane fits the fault of the Hawaii earthquake better than the nodal plane that has a nearly vertical dip angle. The fault length is fixed well by the aftershock distribution, which is also consistent with the tsunami and crustal deformation data. The fault width which is obtained from tsunami and crustal deformation data is, however, significantly greater than that obtained from the aftershock distribution. This discrepancy implies that about half of the main shock fault plane was not associated with aftershock activity. The source parameters are strike N70°E; dip angle 20°SSE; fault length 40 km; seismic moment 1.8×1027 dyne‐cm; fault width 20–30 km; fault movement is pure normal dip slip of 3.7–5.5 m, and stress drop is 43–93 bars. Results of geodetic surveys throughout the twentieth century and a history of volcanic activity on Kilauea imply that a north‐south compression due to magma injected into rift zones may have steadily increased on the south flank of Kilauea since the 1868 earthquake, an event comparable to the 1975 shock. This compressional stress was possibly released by the 1975 Hawaii earthquake. The long‐term eruptive activity of Kilauea may be affected by large earthquakes like the 1868 and 1975 events and may also have a similar 100‐yr recurrence interval.