Statistical modelling of seismicity patterns before and after the 1990 Oct 5 Cape Palliser earthquake, New Zealand

The earthquake sequence for this study occurred off Cape Palliser at the southeastern tip of the North Island of New Zealand. Two main events occurred, on 1990 Oct 5 and 1990 Oct 6, both with local magnitude M(L)5.3. They were accompanied by a large number of aftershocks. The sequence is remarkable in that it broke the region of low seismicity in the area between the Hikurangi Trough and the main faults in the Wellington region. This paper studies the seismicity during the period 1978-96. The ETAS model is applied to the data. The whole period can be divided into four stages: early background period, relatively quiescent period, mainshock and the aftershock sequence, and active period of post-aftershocks. In order to detect the quiescence, residual analysis from the ETAS model was applied to minimise the effect due to previous aftershock clusters. To explain all features in this sequence, a seismicity phase hypothesis is proposed. The paper discusses aspects of the sequence including background, spatial-temporal analysis, ETAS model, relative quiescence, residual analysis, and its application to the data.