HR: 08:30h
AN: T51C-03    [PDF]
TI: Implications of the 8/21/2003 Mw 7.2 Fiordland New Zealand Earthquake for the Development of the Alpine Fault Plate Boundary
AU: * Furlong, K P
EM: kevin@geodyn.psu.edu
AF: Penn State University, Geodynamics Research Group Department of Geosciences, University Park, PA 16802 United States
AU: Malservisi, R
EM: rmalservisi@rsmas.miami.edu
AF: RSMAS-MGG, 4600 Rickenbacker Causeway University of Miami, Miami, FL 33149-1098 United States
AB: The transition from subduction of the Australian plate beneath Fiordland to the transpressional Alpine fault represents an abrupt change in plate interactions. Beneath Fiordland, the Australian plate subducts to depths of at least 200 km, with the slip vectors of earthquakes inferred to represent plate interface events aligned in the direction of relative plate motion between the Australian and Pacific plates. To the north, along the Alpine Fault, the eastern edge of the Australian plate serves to define the position of the plate boundary, with the majority of transpressional deformation accommodated by the Pacific plate. Based on plate reconstructions, the eastern edge of the Australian plate that now defines the Alpine fault boundary was previously adjacent to the lithosphere that is currently subducted to depth beneath Fiordland. How this transition occurs has been a point of some debate. The recent Mw 7.2 earthquake (8/21/2003) provides evidence of the lithospheric deformation that allows this change in plate boundary structure. This event is characterized by two thrust planes: one shallowly dipping to the ESE, the other steeply dipping to the WNW. In contrast to subduction interface events further to the east, both fault planes have slip vectors that are nearly pure dip slip, oriented approximately 60 degrees away from the relative plate motion. Although the shallow eastward dipping plane is consistent with the inferred geometry of the slab, its slip vector is incompatible with other subduction events on the plate interface. If however the steeply dipping plane is selected, its orientation, slip vector, and the location of the earthquake are all consistent with it representing an event that occurs as the Australian plate is torn, forming a new eastern edge to that plate. Several other earthquakes with similar mechanisms have occurred on this southwest extension of the Alpine Fault; all with fault planes consistent with a tearing of the plate. This plate edge will translate northward becoming the west side of the Alpine Fault plate boundary. We have previously argued that a tearing of the slab in this way (essentially collinear with the Alpine Fault) was needed to match the patterns of slab deformation, uplift in Fiordland, and the substantial gravity anomaly in the region. This earthquake provides direct evidence of the tearing process and provides constraints on the location within the transition from subduction to translation that the new plate boundary is generated.
DE: 7209 Earthquake dynamics and mechanics
DE: 7215 Earthquake parameters
DE: 7230 Seismicity and seismotectonics
DE: 8107 Continental neotectonics
DE: 8123 Dynamics, seismotectonics
SC: Tectonophysics [T]
MN: 2003 Fall Meeting