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A large, deep Hawaiian earthquake—The Honomu, Hawaii event of April 26, 1973

U.S. GEOLOGICAL SURVEY 760 NATIONAL CENTER, RESTON, VIRGINIA 22092
U.S. GEOLOGICAL SURVEY, 345 MIDDLEFIELD ROAD, MENLO PARK, CALIFORNIA 94025

Abstract

The largest subcrustal earthquake ever recorded from the Hawaiian Island chain (magnitude 6.2) occurred at a depth of 48 km on April 26, 1973. The proximity of the Hawaiian Volcano Observatory's extensive seismograph network and our knowledge of the crustal and upper mantle structure beneath the island made it possible to calculate accurate hypocenters for both the main shock and 57 aftershocks. The earthquake may have triggered swarms of small, shallow earthquakes at two different locations on the island: one 25 km and the other 50 km from the epicenter of the earthquake. The polarity of the P-wave arrivals for the main shock and most of the aftershocks, as recorded by the local network and worldwide stations, define nodal planes oriented N26°E dipping 77°W and N70°W dipping 61°S. Comparison of the inferred directions of the greatest and least principal stresses derived from these data with the stress direction within the Pacific plate assumed for various hypotheses of the formation of the Hawaiian island chain show closest agreement with the concept that the orientation of the archipelago is aligned parallel to the direction of the maximum shear stress and is not perpendicular to the orientation of the least principal stress.