High‐Precision Analysis of an Aftershock Sequence Using Matched‐Filter Detection: The 4 May 2015<i>M</i>_L 6 Wanaka Earthquake, Southern Alps, New Zealand

Abstract

Research Article| June 07, 2017 High‐Precision Analysis of an Aftershock Sequence Using Matched‐Filter Detection: The 4 May 2015 ML 6 Wanaka Earthquake, Southern Alps, New Zealand Emily Warren‐Smith; Emily Warren‐Smith aInstitute of Geophysics, School of Geography, Environment and Earth Sciences, Victoria University of Wellington, P.O. Box 600, Wellington 6140, New Zealand, [e-mail], [e-mail], [e-mail], [e-mail] at GNS Science, P.O. BOX 30‐368, Lower Hutt 5040, New Zealand. Search for other works by this author on: GSW Google Scholar Calum J. Chamberlain; Calum J. Chamberlain aInstitute of Geophysics, School of Geography, Environment and Earth Sciences, Victoria University of Wellington, P.O. Box 600, Wellington 6140, New Zealand, [e-mail], [e-mail], [e-mail], [e-mail] Search for other works by this author on: GSW Google Scholar Simon Lamb; Simon Lamb aInstitute of Geophysics, School of Geography, Environment and Earth Sciences, Victoria University of Wellington, P.O. Box 600, Wellington 6140, New Zealand, [e-mail], [e-mail], [e-mail], [e-mail] Search for other works by this author on: GSW Google Scholar John Townend John Townend aInstitute of Geophysics, School of Geography, Environment and Earth Sciences, Victoria University of Wellington, P.O. Box 600, Wellington 6140, New Zealand, [e-mail], [e-mail], [e-mail], [e-mail] Search for other works by this author on: GSW Google Scholar Author and Article Information Emily Warren‐Smith aInstitute of Geophysics, School of Geography, Environment and Earth Sciences, Victoria University of Wellington, P.O. Box 600, Wellington 6140, New Zealand, [e-mail], [e-mail], [e-mail], [e-mail] at GNS Science, P.O. BOX 30‐368, Lower Hutt 5040, New Zealand. Calum J. Chamberlain aInstitute of Geophysics, School of Geography, Environment and Earth Sciences, Victoria University of Wellington, P.O. Box 600, Wellington 6140, New Zealand, [e-mail], [e-mail], [e-mail], [e-mail] Simon Lamb aInstitute of Geophysics, School of Geography, Environment and Earth Sciences, Victoria University of Wellington, P.O. Box 600, Wellington 6140, New Zealand, [e-mail], [e-mail], [e-mail], [e-mail] John Townend aInstitute of Geophysics, School of Geography, Environment and Earth Sciences, Victoria University of Wellington, P.O. Box 600, Wellington 6140, New Zealand, [e-mail], [e-mail], [e-mail], [e-mail] Publisher: Seismological Society of America First Online: 15 Sep 2017 Online Issn: 1938-2057 Print Issn: 0895-0695 © Seismological Society of America Seismological Research Letters (2017) 88 (4): 1065–1077. https://doi.org/10.1785/0220170016 Article history First Online: 15 Sep 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Emily Warren‐Smith, Calum J. Chamberlain, Simon Lamb, John Townend; High‐Precision Analysis of an Aftershock Sequence Using Matched‐Filter Detection: The 4 May 2015 ML 6 Wanaka Earthquake, Southern Alps, New Zealand. Seismological Research Letters 2017;; 88 (4): 1065–1077. doi: https://doi.org/10.1785/0220170016 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietySeismological Research Letters Search Advanced Search ABSTRACT The 4 May 2015 ML 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 ML 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 ML 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. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.