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High-precision analysis of an aftershock sequence using matched-filter detection; the 4 may 2015 M (sub L) 6 Wanaka earthquake, Southern Alps, New Zealand

Emily Warren-Smith, Calum J. Chamberlain, Simon Lamb and John Townend
High-precision analysis of an aftershock sequence using matched-filter detection; the 4 may 2015 M (sub L) 6 Wanaka earthquake, Southern Alps, New Zealand
Seismological Research Letters (July 2017) 88 (4): 1065-1077

Abstract

The 4 May 2015 M (sub 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 (sub 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+ or -0.02 and a magnitude of completeness of M (sub 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 ( approximately 70 degrees ) fault striking at 250 degrees , 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 degrees ). Analysis of focal mechanism slip vectors shows that aftershocks principally occurred on secondary synthetic Riedel shears oriented 25 degrees 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.


ISSN: 0895-0695
EISSN: 1938-2057
Serial Title: Seismological Research Letters
Serial Volume: 88
Serial Issue: 4
Title: High-precision analysis of an aftershock sequence using matched-filter detection; the 4 may 2015 M (sub L) 6 Wanaka earthquake, Southern Alps, New Zealand
Affiliation: Victoria University of Wellington, School of Geography, Environment and Earth Sciences, Wellington, New Zealand
Pages: 1065-1077
Published: 201707
Text Language: English
Publisher: Seismological Society of America, El Cerrito, CA, United States
References: 47
Accession Number: 2017-080705
Categories: Seismology
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. sketch maps
S46°00'00" - S43°00'00", E167°00'00" - E171°00'00"
Country of Publication: United States
Secondary Affiliation: GeoRef, Copyright 2017, American Geosciences Institute. Abstract, Copyright, Seismological Society of America. Reference includes data from GeoScienceWorld, Alexandria, VA, United States
Update Code: 201742
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