Large earthquakes in the macquarie ridge complex: Transitional tectonics and subduction initiation

• Published in: Pure and Applied Geophysics PAGEOPH
• Main Authors: Ruff, Larry J., Given, Jeffrey W., Sanders, Chris O., Sperber, Christine M.
• Other Authors: Department of Geological Sciences, University of Michigan, 48109, Ann Arbor, MI, USA, Sierra Geophysics, 98033, Kirkland, WA, USA, Seismological Laboratory, California Institute of Technology, 91125, Pasadena, CA, USA; USGS, 94025, Menlo Park, CA, Ann Arbor
• Format: Article in Journal/Newspaper
• Language: English
• Published: Birkhäuser-Verlag; Birkhäuser Verlag 1989
• Subjects: Geosciences; Geophysics/Geodesy; Earthquakes; Seismotectonics; Subduction Initiation; Soft Plate Boundary; Physics; Atmospheric; Oceanic and Space Sciences; Science; Pacific; New Zealand; Antarc*; Antarctica
• Online Access: http://hdl.handle.net/2027.42/43125; https://doi.org/10.1007/BF00874625

While most aspects of subduction have been extensively studied, the process of subduction initiation lacks an observational foundation. The Macquarie Ridge complex (MRC) forms the Pacific-Australia plate boundary between New Zealand to the north and the Pacific-Australia-Antarctica triple junction to the south. The MRC consists of alternating troughs and rises and is characterized by a transitional tectonic environment in which subduction initiation presently occurs. There is a high seismicity level with 15 large earthquakes ( M >7) in this century. Our seismological investigation is centered on the largest event since 1943: the 25 MAY 1981 earthquake. Love, Rayleigh, and P waves are inverted to find: a faulting geometry of right-lateral strike-slip along the local trend of the Macquarie Ridge (N30°E); a seismic moment of 5×10 27 dyn cm ( M w =7.7) a double event rupture process with a fault length of less than 100km to the southwest of the epicenter and a fault depth of less than 20km. Three smaller thrust earthquakes occurred previous to the 1981 event along the 1981 rupture zone; their shallow-dipping thrust planes are virtually adjacent to the 1981 vertical fault plane. Oblique convergence in this region is thus accommodated by a dual rupture mode of several small thrust events and a large strike-slip event. Our study of other large MRC earthquakes, plus those of other investigators, produces focal mechanisms for 15 earthquakes distributed along the entire MRC; thrust and right-lateral strike-slip events are scattered throughout the MRC. Thus, all of the MRC is characterized by oblique convergence and the dual rupture mode. The “true” best-fit rotation pole for the Pacific-Australia motion is close to the Minster & Jordan RM2 pole for the Pacific-India motion. Southward migration of the rotation pole has caused the recent transition to oblique convergence in the northern MRC. We propose a subduction initiation process that is akin to crack propagation; the 1981 earthquake rupture area is identified as ...