Modelling the effect of atmospheric pressure variations on gravity

Summary. The 1973 Hawaii earthquake occurred north of Hilo, at a depth of 40 to 50km. The location was beneath the east flank of Mauna Kea, a volcano dormant historically, but active within the last 4000 yr. Aftershocks were restricted to a depth of 55–35km. The event and its aftershock sequence are located in an area not normally associated with the seismicity of the Mauna Loa and Kilauea calderas. The earthquake was a double event, the epicentres trending NE-SW. The events were of similar size and faulting mechanism. The fault plane solutions obtained by seismic waveform analysis are a strike-slip fault striking EW and dipping 55° S, the auxiliary plane a NS vertical plane with a faulting plunge of 35°. The axis of maximum compressive stress is aligned with the direction of the gravity gradient associated with the island of Hawaii. The fault plane striking EW parallels a surface feature, the Mauna Kea east rift zone. The earthquakes were clearly not associated with volcanic activity normally associated with Mauna Loa and Kilauea and may indicate a deep seated prelude to a resumption of activity at Mauna Kea.