Detailed fault structure highlighted by finely relocated aftershocks, Arthur's Pass, New Zealand

Abstract

More than 6000 aftershocks were recorded following the 1994 Arthur's Pass Mw6.7 earthquake in South Island, New Zealand. Here we relocate over 3500 of the aftershocks using double-difference tomography to map detailed fault structures, and obtain a 3-D model of P-wave velocity and Vp/Vs ratio in the volume around the aftershock region. Waveform cross-correlation methods are used to calculate high quality differential times for event pairs. Two parallel clusters in particular are very well defined after the double-difference relocation, with a dip of ?70–80° and ?N60E ±10° strike. A third NW-dipping cluster aligns very closely with one of the preferred fault planes for the mainshock, corroborating previous interpretations of the mainshock source mechanism. Results from the double-difference tomography indicate P-wave velocities and Vp/Vs change markedly immediately above the upper bound of the seismicity, with a Vp of 4.5 to 5.5 km/s and elevated Vp/Vs found in the top 2 km.