High‐Precision Analysis of an Aftershock Sequence Using Matched‐Filter Detection: The 4 May 2015ML 6 Wanaka Earthquake, Southern Alps, New Zealand

Abstract

ABSTRACT The 4 May 2015 M L 6.0 Wanaka earthquake was the largest earthquake in the Central Otago region of New Zealand’s Southern Alps for 70 years and the largest shallow event in the region since national records began. The earthquake occurred within 50 km of the alpine resort towns of Wanaka and Queenstown (combined population 35,000), at 9.1 km depth, and was felt widely across South Island, although no significant damage was reported. We utilize waveforms recorded on a temporary broadband network and the national network to conduct matched‐filter analysis of the aftershock sequence. We use the mainshock and 99 aftershocks distributed through the sequence as template events for the detection of further aftershocks. We detect 2544 aftershocks over 26 days, 27 times more than reported by the national network alone. Magnitudes of detected aftershocks range from M L 5.8 (34 s after the mainshock) to 0.3 and follow a Gutenberg–Richter exponential relationship with a b ‐value of 0.807±0.02 and a magnitude of completeness of M L 1.4. We compute high‐precision relocations of detected events using cross‐correlation‐derived pick corrections. Our relocated aftershocks highlight a steeply northwest dipping (∼70°) fault striking at 250°, which allows us to identify the structure responsible for the earthquake in a region where no active fault traces are mapped. This orientation is consistent with the mainshock focal mechanism for a dextral strike‐slip earthquake on a subvertical plane subparallel to the plate convergence direction (249°). Analysis of focal mechanism slip vectors shows that aftershocks principally occurred on secondary synthetic Riedel shears oriented 25° clockwise from the principal slip zone, possibly controlled by local structures. Aftershock locations migrate in the down‐dip direction away from the mainshock, slowing with a 1/log(time) relationship, consistent with previous numerical models of afterslip within the fault plane.