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TitleCoulomb stress changes following the 2012 Mw 7.8 Haida Gwaii, Canada, earthquake: implications for seismic hazard
AuthorHobbs, T EORCID logo; Cassidy, J FORCID logo; Dosso, S E; Brillon, C
SourceBulletin of the Seismological Society of America vol. 105, no. 2B, 2015 p. 1-12,
Alt SeriesEarth Sciences Sector, Contribution Series 20140149
PublisherSeismological Society of America (SSA)
Mediapaper; on-line; digital
File formatpdf
ProvinceWestern offshore region; British Columbia
NTS102O; 103B; 103C; 103F; 103G; 103J; 103K
AreaHaida Gwaii; Queen Charlotte
Lat/Long WENS-134.5000 -130.0000 54.5000 51.0000
Subjectsgeophysics; Health and Safety; earthquakes; earthquake mechanisms; earthquake studies; stress analyses; health hazards
Illustrationslocation maps; block diagrams; models
ProgramPublic Safety Geoscience Western Canada Geohazards Project
Released2015 04 07
AbstractThis article examines spatial changes to the local stress field resulting from the 28 October 2012, Mw 7.8 Haida Gwaii earthquake, off the west coast of Moresby Island, British Columbia. This event occurred on a northeast-dipping, potentially blindthrust fault rather than on the subvertical Queen Charlotte fault (QCF) that represents the Pacific-North American plate boundary. This was the largest earthquake along the Canadian portion of this plate boundary since the 1949 Ms 8.1 Queen Charlotte earthquake. The U.S. Geological Survey Coulomb software is used to quantitatively estimate the effect of the mainshock on the background stress field, the known aftershock nodal planes, and the nearby QCF. We use two different mainshock finite-fault models, both of which are seismologically derived (by Lay et al., 2013, and Hayes, 2013, separately) and subsequently adapted by K. Wang to account for the motion detected at four nearby Global Positioning System stations (see Nykolaishen et al., 2015, for more information). We also use the best-located set of aftershocks with information provided by a temporary array of ocean-bottom seismometers. Results indicate an apparent clustering of aftershocks slightly seaward of the main thrust, which is consistent with the modeled zone of promoted normal failure, likely related to extension in the footwall. Using existing models, we found a high number of aftershocks to be consistent with triggering by the mainshock, suggesting that static stress is a dominant control in the months following a large earthquake in this area. Further, we find loading greater than the triggering threshold on the QCF in an area interpreted as a seismic gap. This work improves understanding of the evolving seismic hazard along the Queen Charlotte margin and tests the usefulness of Coulomb modeling in this complex tectonic environment.
Summary(Plain Language Summary, not published)
The October 2012 magnitude 7.7 earthquake resulted in up to 4 m of movement along a previously unrecognised fault just off the west coast of Haida Gwaii. Movement along the fault extended more than 100-km's along the coast and extended about 50 km offshore. This very substantial movement of rocks has changed the stress field in the Haida Gwaii region (including along the Queen Charlotte Fault) and will likely influence the locations of aftershocks and, perhaps, future earthquakes. In this study we compute the changes in the stress field associated with the 2012 Haida Gwaii earthquake and find that most of the offshore aftershocks fall within the region of predicted maximum stress change. In addition, we found that additional stress has been added to the nearby Queen Charlotte Fault - including a segment that is considered a 'seismic gap' as it has not experienced an earthquake during historical times. This study helps to improve our earthquake hazard models of this region.

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