Physics of the Earth and Planetary Interiors, 72(1992)99—121 99 Elsevier Science Publishers B.V., Amsterdam The April 5, 1990 Mariana Islands earthquake and subduction zone stresses Jiajun Zhang and Thorne Lay Institute of Tectonics and CF. Richter Seismological Laboratory, University of California, Santa Cruz, CA 95064, USA (Received 30 October 1991; accepted 1 November 1991) ABSTRACT Zhang, J. and Lay, T., 1992. The April 5, 1990 Mariana Islands earthquake and subduction zone stresses. Physics Earth Planet. Inter., 72: 99—121. The April 5, 1990 Mariana Islands earthquake (M~ = 7.5) involved normal faulting at shallow depth in the Pacific plate under the Mariana trench, about 200 km east of the central Mariana Islands. The earthquake is significant for its tectonic setting, focal mechanism, and large size. The event occurred in the vicinity of a proposed slab segment boundary and may in part owe its unusually large size to lateral variation in intraplate strain, as appears to be the case for other large, normal faulting events near trenches. Detailed analysis of the seismic source parameters of the earthquake and of the relationship between seismicity and tectonics in the Mariana Islands is conducted to improve our understanding of the mechanical behavior and seismotectonics in island arcs with aseismic convergence. The 1990 Mariana earthquake rupture initiated with very low levels of energy release for an interval of about 7.2 s, followed by a large, single, impulsive event with a duration of about 6 s. Body wave inversion yields a strike = 215°, dip = 490, rake = —95°, and a moment = 1.2 x 1020 Nm with a centroid time of 11—12 s. The source mechanism is further constrained by the long-period Rayleigh and Love wave data, with inversions yielding a strike = 184 (±5)°, dip = 67 (±)°, rake = —90 (±5)°, and moment 2.2 (±0.5)x1020 Nm. The long-period centroid depth is 23 (±)km, with a centroid time of 20 (±3) s. 1. Introduction members: the Mariana type and the Chilean type. The Mariana subduction zone is characterized by The April 5, 1990 Mariana Islands earthquake distinctive tectonic features such as an actively (2112:35.50 UT, 15.125°N, 147.596°E, M~ = 7.5, opening back-arc basin, almost purely aseismic National Earthquake Information Center (NEIC)) plate motion at the interplate boundary, an ex- is the largest recent event to occur in the central traordinarily steeply dipping Wadati—Benioff Mariana subduction zone (Fig. 1). This arc is the zone, and absence of outer rise topography and boundary between the Pacific and Philippine gravity highs. Along the Mariana subduction zone, plates in the western Pacific ocean. The 1990 the very old (about 160 Ma) Pacific plate is steeply event is a normal-fault earthquake, with a larger subducting toward the west with the dip of the moment than any known thrust event in the re- Wadati—Benioff zone increasing to almost 90° at gion, reflecting the unusual character of the Mar- large depths. The convergence rate varies along iana subduction zone. Uyeda and Kanamori the arc from 2.5 cm year 1 near the Caroline (1979) classified subduction zones into two end Ridge to 5 cm year1 near the Marcus—Necker Ridge, and is about 4 cm year’ near the 1990 Correspondence to: Jiajun Zhang, Institute of Tectonics and hypocenter. Ruff and Kanamori (1980) used the C.F. Richter Seismological Laboratory, University of Califor- magnitude of the largest earthquake in a given nia, Santa Cruz, CA 95064, USA. arc to characterize the strength of coupling be- 0031-9201/92/$05.00 © 1992 — Elsevier Science Publishers B.V. All rights reserved