New Zealand Journal of Geology and Geophysics abstracts

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The Mw 6.2 Cass, New Zealand, earthquake of 24 November 1995: reverse faulting in a strike-slip region

KEN GLEDHILL
RUSSELL ROBINSON
TERRY WEBB
RACHEL ABERCROMBIE
JOHN BEAVAN
JIM COUSINS

Institute of Geological & Nuclear Sciences
Gracefield Centre
P.O. Box 30 368
Lower Hutt, New Zealand

DONNA EBERHART-PHILLIPS

Institute of Geological & Nuclear Sciences
Private Bag 1930
Dunedin, New Zealand

Abstract  On 24 November 1995 an earthquake of moment magnitude MW 6.2 struck near the small settlement of Cass in the Southern Alps, South Island, New Zealand. Body-wave modelling using teleseismic arrivals gives an oblique reverse focal mechanism for the mainshock, with the fault plane striking approximately north-south, and a shallow centroid depth of 3-6 km. Aftershock recordings at the station SNZO near Wellington were used as empirical Green's functions to estimate a source time function duration of 7 s. A joint inversion for velocity and location of 169 selected events was used to derive a one-dimensional velocity model with station terms, and this velocity model was then used to relocate all recorded aftershocks. A subset of the best 803 events was then selected for further analysis. The apparent trend of the aftershock zone is NNW-SSE, with the mainshock near the centre. However, projections of the aftershocks on north-south and east-west cross-sections show a band of activity shallowing to the south and dipping to the west. The north-striking, west-dipping nodal plane of the mainshock focal mechanism is therefore most likely to be the fault plane. Early aftershocks occurred mainly to the south of the mainshock location, suggesting rupture to the south, a feature supported by the mainshock modelling. The aftershock focal mechanisms are mixed but reflect the regional stress field (NW-SE compression).

Keywords  Cass earthquake; focal mechanisms; aftershocks; strong ground motion

New Zealand Journal of Geology & Geophysics, 2000, Vol. 43: 255-269

0028-8306/00/4302-0255 $7.00/0 (c) The Royal Society of New Zealand 2000

PDF file of entire paper: medium quality (1778K); (scanned from paper original: notes about this process)

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