Microseismicity and stress in the vicinity of the Alpine Fault, central Southern Alps, New Zealand

Abstract

We investigate present‐day microseismicity associated with the central Alpine Fault and the zone of active deformation and uplift in the central Southern Alps. Using 14 months of data, robust hypocenter locations have been obtained for ∼1800 earthquakes of magnitudes between −0.3 and 4.2. We derived a magnitude scale with a frequency‐dependent attenuation factor, γ(f) = γ0f, where γ0 = 1.89 ± 0.02 × 10−3 s/km, that enables magnitudes to be calculated consistently for earthquakes of different sizes and frequency contents. The maximum depth of the seismicity varies systematically with distance from the Alpine Fault, from 10 ± 2 km near the fault to 8 ± 2 km within 20 km and 15 ± 2 km further southeast. This distribution correlates with lateral variations in crustal resistivity: earthquake hypocenters are concentrated in areas of strong resistivity gradients and restricted to depths of resistivities >100 Ωm. Rocks at greater depth are too hot, too fluid‐saturated, or too weak to produce detectable earthquakes. Focal mechanism solutions computed for 211 earthquakes (ML > 0.44) exhibit predominantly strike‐slip mechanisms. We obtain a maximum horizontal compressive stress direction of 115 ± 10° from focal mechanism inversion. This azimuth is consistent with findings from elsewhere in the central and northern South Island, and indicates a uniform crustal stress field despite pronounced variations in crustal structure and topographic relief. Our stress estimates suggest that the Alpine Fault (with a mean strike of 055°) is poorly oriented in an Andersonian sense but that individual thrust and strike‐slip segments of the fault's surface trace have close to optimal orientations.