The 1987–1992 Gulf of Alaska earthquakes

Abstract

We present a study of the 1987–1992 Gulf of Alaska earthquake sequence using relocated seismicity data together with body wave analysis of selected larger events. Most of the sequence is located on a N-S-trending fault, directly south of the Yakataga seismic gap and consists of four main events of M w = 7.2, 7.8, 7.7 and 6.8 and associated aftershocks. The first earthquake is of left-lateral strike-slip type on an ENE-WSW-trending fault. The second event is a right-lateral strike-slip earthquake on the N-S-trending fault. The fault plane of the third strike-slip event was identified as an ENE-WSW-trending fault, located to the south of that for the first event, in a previous study using body wave analysis. We show that though the body wave study cannot unambiguously identify the fault plane, the temporal development of the seismicity together with the pattern of aftershock distribution on the conjugate fault suggests that this event also occurred on the N-S-trending fault and is of right-lateral type. The seismicity on the conjugate fault is interpreted as being triggered by the increase of the shear stress in the direction of the normal to the fault plane due to the main shock. The occurrence of the fourth main shock, a right-lateral strike-slip event in 1992, which itself can be considered an aftershock of the 1987–1988 sequence with epicentres distributed along a N-S-trending fault, favours this conclusion. The first three events of the sequence have been described in earlier studies as having too short a rupture length for their seismic moment. If the rupture lengths inferred from the aftershock zones in this study are used we find that this is not the case. The events in the 1987–1992 sequence lie along magnetic anomaly 13 and related conjugate fracture zones, indicating that the oceanic crust is rupturing along pre-existing zones of weakness in response to plate boundary stresses. At the southern end, the seismic activity terminates near a seamount, which rises 1 km above an otherwise relatively low relief ocean floor, and close to a fracture zone perpendicular to the N-S trend of the earthquake sequence, suggesting that the N-S-striking rupture along the N-S-trending fault is terminated here by a barrier. At the northern end, the sequence terminates at the Alaska trench near the Yakataga seismic gap, where there is a right-lateral offset in the bathymetry along the Transition fault, this offset also being visible on the gravity map of the region. Thus, this offset has the same right-lateral sense as the N-S-trending strike-slip fault planes of three of the main shocks and suggests that though there was no significant seismic activity in this region of the Gulf of Alaska since 1964, faulting here has been active in the past.