Microseismicity and geologic structure in the northern South Island, New Zealand

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Abstract
Locations, rates of occurrence, and composite focal mechanisms of microearthquakes in the Marlborough region are used to examine the tectonics of the region. Earthquakes in the upper crust reflect regional compression along an axis trending NW to WNW; their strike-slip and thrust mechanisms can be related to the regional geology. Seismicity deeper than about 20 km trends obliquely to the structural grain and appears to be intimately related to an underlying Benioff zone. Earthquakes in the 20–35 km depth range indicate a stress distribution markedly different from that in the upper crust and a predominance of normal faulting, suggesting either depth-varying stresses within continental crust or intraplate deformation within subducted oceanic lithosphere, depending on where the plate interface lies. Shallow microseismicity in the vicinity of the Clarence and Awatere Faults indicates that these currently are the most seismically active breaks in the Marlborough region. Activity along the Wairau Fault, the direct continuation of the Alpine Fault, is very low. North-west of the Alpine Fault, microearthquakes in two recognisable aftershock zones—that of the 1968 Inangahua earthquake (M L = 7.1) and that of a magnitude 5.9 event that occurred 5 months before the survey—dominate the observed microseismicity. The northern South Island straddles the Indian-Pacific plate boundary. Strike-slip mechanisms with ENE-trending slip vectors support the interpretation that the present plate boundary can be traced as a broad complex zone of transform faulting that connects the central Alpine Fault with the southern Hikurangi Trench.