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TitlePossible interactions between the 2012 Mw 7.8 Haida Gwaii subduction earthquake and the transform Queen Charlotte Fault
AuthorHobbs, TORCID logo; Cassidy, JORCID logo; Dosso, S
SourceAmerican Geophysical Union Annual Fall Meeting, abstracts; by American Geophysical Union; 19192, 2014 p. 1
LinksOnline - En ligne
Alt SeriesEarth Sciences Sector, Contribution Series 20140160
PublisherAmerican Geophysical Union
MeetingAmerican Geophysical Union Annual Fall Meeting; San Francisco; US; December 15-19, 2014
Mediadigital; on-line
File formathtml
ProvinceBritish Columbia; Western offshore region
AreaQueen Charlotte Islands; Haida Gwaii
Lat/Long WENS-136.0000 -128.0000 56.0000 52.0000
Subjectsgeophysics; structural geology; earthquakes; earthquake mechanisms; earthquake studies; subduction; subduction zones; structural analyses; structural features; faults; faults, transform
ProgramPublic Safety Geoscience Western Canada Geohazards Project
AbstractThis paper examines the effect of the October 2012 Mw 7.8 Haida Gwaii earthquake on aftershock nodal planes and the neighboring Queen Charlotte Fault (QCF) through Coulomb modeling and directivity analysis. The Haida Gwaii earthquake was the largest thrust event recorded in this region and ruptured an area of ~150 by 40 km on a gently NE-dipping fault off the west coast of Moresby Island, British Columbia. It is particularly interesting as it is located just to the west of the QCF, the predominantly right-lateral strike-slip fault separating the Pacific and North American plates. The QCF was the site of the largest recorded earthquake in Canada: the 1949 Ms 8.1 strike-slip earthquake whose rupture extended as far south as this 2012 event and roughly as far north as an Mw7.5 strike slip event at Craig, Alaska, which occurred just two months later in January 2013. The 75 km long portion of the QCF south of the 1949 rupture has not had a large (M = 7) earthquake in over 116 years, representing a significant seismic gap.
Coulomb stress transfer analysis is performed using finite fault models which incorporate seismic and geodetic data. Static stress changes are projected onto aftershock nodal planes and the QCF, including an inferred southern seismic gap. We find up to 86% of aftershocks are consistent with triggering, and as high as 96% for normal faulting events. The QCF experiences static stress changes greater than the empirically-determined threshold for triggering, with positive stress changes predicted for roughly half of the seismic gap region. Added stress from the mainshock and a lack of post-mainshock events make this seismic gap a likely location for future earthquakes. Empirical Green's function and directivity analyses are also performed to constrain rupture kinematics of the mainshock using systematic azimuthal variations in relative source time functions. Results indicate rupture progressed mainly to the northwest within 15o of the direction of the 2013 Craig epicenter, with at least two sources of significant moment release. These results explain observed surface wave amplification at Alaskan seismic stations and support the idea that strong surface wave shaking may be linked to the possible delayed triggering of the Mw 7.5 Craig event, through an unknown intermediate mechanism that accounts for the two-month hiatus.
Summary(Plain Language Summary, not published)
The October 2012 magnitude 7.8 subduction 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). 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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