THE ORMOND, NEW-ZEALAND, EARTHQUAKE OF 1993 AUGUST 10 - RUPTURE IN THE MANTLE OF THE SUBDUCTED PACIFIC PLATE

Data from temporary seismographs installed immediately after the M-L 6 .3 Ormond earthquake of 1993 August 10 have been used to determine the nature of faulting which took place during the event. The rupture beg an at 37 km depth, within the mantle of the subducted Pacific plate, a nd aftershocks extended from near the base of the subducted crust to c . 20 km into the subducted mantle. Aftershocks in the mantle decayed e xceptionally rapidly compared with those in the crust of the subducted plate. This may reflect a hotter, more ductile mantle and/or relative ly homogeneous rupture within the mantle during the mainshock. Aftersh ocks within the mantle show a variety of thrusting mechanisms. Focal m echanisms of aftershocks within the subducted crust indicate that comp ression along strike dominates over slab pull, and that the down-dip s tress has a similar magnitude to the vertical stress. This suggests th at, at least after the Ormond earthquake, the tectonic stress coupled across the plate interface is rather low.