Coulomb Stress Changes Following the 2012 Mw 7.8 Haida Gwaii, Canada, Earthquake: Implications for Seismic Hazard

This article provides a summary of the structure and tectonic history of the Queen Charlotte transform fault zone off western Canada, as background to understanding the 2012 M w 7.8 thrust earthquake off Haida Gwaii. There was margin subduction prior to the Eocene. The fault zone then became the mainly transcurrent Pacific-North America boundary. There was mid-Tertiary oblique extension, then 15°-20°oblique convergence from ∼6 Ma to the present that resulted in underthrusting and subduction initiation. The total underthrusting has been too small for BenioffWadati seismicity or arc volcanics but is indicated by (1) a trench, the Queen Charlotte Trough, into which the oceanic plate bows downward and an offshore flexural bulge, the Oshawa rise; (2) the Queen Charlotte terrace, an accretionary sedimentary prism;(3) seismic receiver function delineation of the underthrusting Pacific plate; (4) heat flow decreasing landward as predicted for underthrusting; (5) low gravity offshore and high onshore, consistent with underthrusting; and (6) late Tertiary uplift and erosion of the west coast of the islands. Oblique convergence is partitioned into nearly marginnormal underthrusting (i.e., M w 7.8 event) relative to the terrace, which is moving along the margin, and margin parallel on the Queen Charlotte strike-slip fault just off the coast that produced the 1949 M s 8.1 earthquake. Landward on the mainland, Global Positioning System data suggest slow coast-parallel shear with no historical seismicity. The convergence rate decreases to the north of Haida Gwaii off Dixon Entrance, but large thrust earthquakes are possible. To the south, underthrusting of the Winona basin margin also could generate large earthquakes.

Section: Tectonic Origin Of the 2012 Haida Gwaii Thrust Earthquakementioning

confidence: 87%

Tectonics and Structure of the Queen Charlotte Fault Zone, Haida Gwaii, and Large Thrust Earthquakes

Hyndman

2015

“…There are a few larger aftershocks with strikeslip mechanisms that seem to have occurred on the QCF in the later stages (Kao et al, 2015), implying that the QCF was involved in the overall seismogenic process after all. Also Hobbs et al (2015) show considerable Coulomb stress increase along the QCF, making these types of events more likely. Because none of these events had M w greater than 4.8, we conclude that most of the elastic strain accumulated along the QCF, especially the segment south of ∼52:5°N, remains in place.…”

Section: Discussionmentioning

confidence: 99%

Spatiotemporal Distribution of Events during the First Week of the 2012 Haida Gwaii Aftershock Sequence

Farahbod

Kao

2015

We systematically relocated the 2012 Haida Gwaii aftershock sequence by re-examining continuous seismic waveforms of the Canadian National Seismographic Network (CNSN). Because of the sparse station density in the source area and the offshore location of the majority of events, precise determination of hypocenters is extremely challenging. Constraints on the back azimuth and incident angle of ray paths were derived from the cross correlation of three-component waveforms and incorporated into the locating process to improve the results. We located 1228 aftershocks for the first week after the mainshock, effectively double the size of the CNSN routine earthquake catalog. The distribution of aftershocks tends to form two linear trends roughly parallel to the strike of the plate margin in the northwest-southeast direction. The trend located updip from the mainshock within the subducting Pacific plate appears to have three clusters spanning a lateral distance of ∼80 km. The other trend coincides with the surface trace of the Queen Charlotte fault (QCF), spanning the northern two-thirds of the main rupture zone. The overall area of aftershock distribution is ∼120 km × ∼40 km. Aftershocks overlap the southern part of the estimated rupture zone of the 1949 earthquake, with no activity in the seismic gap farther to the south. Therefore, it is likely that the 2012 earthquake only partially released the accumulated stress on the strike-slip transform fault system. In this scenario, the possibility of a major strike-slip earthquake along the southernmost part of the QCF should not be ignored.

“…The recurrence interval on either of these events is thought to be on the order of hundreds of years, which suggests there may be some relationship between these relatively rare earthquakes (Gomberg, 2013). Although the magnitude of the static stress changes from this event was too small to affect the seismicity rate across a distance of over 300 km (Hobbs et al, 2015), it has been suggested that oscillations in dynamic stress may have permanently altered the nucleation zone of the Craig earthquake (Gomberg, 2013). Corrected surface-wave waveforms recorded at stations in Alaska along strike to the northwest display clear amplification relative to stations at comparable distances along strike to the southeast (Gomberg, 2013).…”

Section: Introductionmentioning

confidence: 96%

“…Based on finite-fault inversions and aftershock distributions, this thrust earthquake ruptured more than 140 km along the continental margin beneath the Queen Charlotte terrace (QCT) (Fig. 1, reproduced from Hobbs et al, 2015) (Shao and Ji, 2012;Hayes, 2013;Lay et al, 2013;Farahbod and Kao, 2015;Kao et al, 2015;Nykolaishen et al, 2015; see Data and Resources for more information). This earthquake occurred on a previously unknown northeast-dipping potentially blind thrust fault with a strike of about 323°, dip of roughly 20°, and average rake approximately normal to the trench (Gomberg, 2013;Hayes, 2013;Lay et al, 2013;Nykolaishen et al, 2015;Wang et al, 2015).…”

Section: Introductionmentioning

confidence: 99%

See 1 more Smart Citation

Rupture Process of the 2012 Mw 7.8 Haida Gwaii Earthquake from an Empirical Green's Function Method

Hobbs¹,

Cassidy²,

Dosso³

2015

Self Cite

This article examines rupture processes of the 28 October 2012 M w 7.8Haida 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.