ALBERT

Description: The thrust mechanism of the 2012 M w  7.8 Haida Gwaii earthquake suggests convergence across the transpressive Pacific–North America plate boundary in the region is accommodated by underthrusting, with important consequences for seismic- and tsunami-hazard analysis. This article investigates the crustal structure and extent of subduction beneath Haida Gwaii by nonlinear inversion of receiver function data processed from teleseismic recordings at five land-based seismograph stations. Three of these stations were deployed since the 2012 earthquake to extend coverage to the southeast and have not been analyzed previously. The inversions provide estimates of the shear-wave velocity structure beneath much of Moresby Island. Results indicate a positive velocity contrast at approximately 18–26 km depth, interpreted as a shallow continental Moho. A 12–17 km thick low shear-wave velocity zone is also identified, which increases in depth from ~25 to 42 km along the direction of plate convergence, which is interpreted as subducting oceanic material. These results provide the first evidence that the subducting oceanic plate extends beneath the entirety of Moresby Island.

Description: This article examines rupture processes of the 28 October 2012 M w  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 M s  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 M w  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 the waveforms 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 2002 M w  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. Online Material: Figure of all relative source time functions used in directivity analysis.

Description: This article examines spatial changes to the local stress field resulting from the 28 October 2012, M w  7.8 Haida Gwaii earthquake, off the west coast of Moresby Island, British Columbia. This event occurred on a northeast-dipping, potentially blind-thrust 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 M s  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. Online Material: Figures of focal mechanisms and maximum Coulomb stress change, and table of aftershock moment tensor parameters.

Description: Using seismograms recorded at 66 Canadian seismic stations, coda Q was estimated from earthquakes in southwestern British Columbia and northern Washington State, employing the single backscattering approximation. A total of 580 earthquakes with magnitudes ranging from 1.2 to 6.4, depths from 0 to 67 km, and epicentral distances of 5–110 km were selected to obtain 3022 high signal-to-noise ratio traces for analysis. An average of all the data yields a relationship for coda Q of Q C =72 f 0.91 . There is little variation of this coda Q relationship when using either crustal or in-slab sources, which represent uniform sampling of the crust and upper mantle. Crustal earthquakes result in a relationship of Q C =73 f 0.89 , and for in-slab events Q C can be expressed as Q C =69 f 0.94 . In general, Q 0 ( Q C at 1 Hz) increases from the west coast of Vancouver Island to the east-southeast within the Coast belt. Stations on west-central Vancouver Island closest to the landward projection of the Nootka fault zone, and the location of the only two known large crustal earthquakes (1918 M ~7 and 1946 M ~7.3) on Vancouver Island, have the lowest Q 0 values in our study area, suggesting a contrast in Q between the north and south of the island. Online Material: Figure showing principal tectonic units and station locations, and tables of average Q 0 and alpha values with estimated uncertainties.