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TitleRupture process of the 2012 Mw 7.8 Haida Gwaii Earthquake from an empirical Green's Function method
AuthorHobbs, T E; Cassidy, J F; Dosso, S E
SourceBulletin of the Seismological Society of America vol. 105, no. 2B, 2015 p. 1-12,
Alt SeriesEarth Sciences Sector, Contribution Series 20140187
PublisherSeismological Society of America
Mediaon-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; tectonics; earthquake studies; earthquakes; earthquake magnitudes; earthquake risk; earthquake mechanisms; earthquake foci; tectonic setting; aftershocks; landslides
Illustrationslocation maps; graphs; tables; plots
ProgramWestern Canada Geohazards Project, Public Safety Geoscience
AbstractThis article examines rupture processes of the 28 October 2012 Mw 7.8 Haida Gwaii earthquake off the coast of British Columbia, Canada, using an empirical Green’s function (EGF) technique. The Haida Gwaii earthquake was the largest event along the Canadian portion of the Pacific-North American plate boundary since the Ms 8.1 Queen Charlotte earthquake of 1949. It occurred along a potentially blind thrust fault dipping gently to the northeast rather than the main, subvertical Queen Charlotte fault. Surface waveforms from a 2001 Mw 6.3 event, located only 15 km from the 2012 epicenter and with similar mechanism, are used as an EGF and deconvolved from those of the 2012 mainshock. The resulting source time functions contain minimal path effects, focal mechanism effects, and instrument response, so thewaveforms display only properties of the 2012 mainshock rupture itself. By examining azimuthal variations in these source time functions, we constrain parameters such as average rupture velocity, extent, and directivity. In addition, information is obtained about the possible existence of major subevents and their relative locations. Results indicate two subevents within this rupture, the first 12 km south and updip of the epicenter and the second approximately 28 km from the first along a heading parallel to the Queen Charlotte terrace (?323°). Overall, the rupture front propagated roughly 50 km at an azimuth of 308.5°. This evidence for directivity to the northwest is important, given that earthquakes with strong directivity, such as the 2002Mw 7.9 Denali earthquake, have been shown to be capable of triggering earthquakes thousands of kilometers away. In this case, we suggest that northwest directivity of this earthquake is responsible for amplification of surface waves observed at seismic stations in Alaska (Gomberg, 2013) and may provide a potential link between this 2012 event and the 2013 Craig, Alaska, earthquake.
Summary(Plain Language Summary, not published)
The October 27, 2012 magnitude 7.7 earthquake resulted in up to 4 m of movement along a 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 earthquake was followed by thousands of aftershocks. In addition, another very large earthquake occurred 2 months later along the Queen Charlotte Fault, off the coast of Alaska, 330 km to the northwest. In this study we use global seismic datasets to examine the rupture directivity of the Haida Gwaii earthquake to help understand the potential for triggered earthquakes. We find that the Haida Gwaii earthquake initially ruptured towards the south, but that the overall rupture extended towards the northwest, towards Alaska. This study contributes to improved assessments of time-varying earthquake hazards in the region and other areas of the world.