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

On 24 November 1995 an earthquake of moment magnitude M-W 6.2 struck near t he small settlement of Cass in the Southern Alps, South Island, New Zealand . Body-wave modelling using teleseismic arrivals gives an oblique reverse f ocal mechanism for the mainshock, with the fault plane striking approximate ly north-south, and a shallow centroid depth of 3-6 km. Aftershock recordin gs at the station SNZO near Wellington were used as empirical Green's funct ions 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-d imensional velocity model with station terms, and this velocity model was t hen used to relocate all recorded aftershocks. A subset of the best 803 eve nts was then selected for further analysis. The apparent trend of the after shock zone is NNW-SSE, with the mainshock near the centre. However, project ions 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 the refore 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 mechani sms are mixed but reflect the regional stress field (NW-SE compression).