2012 Haida Gwaii Quake: Insight Into Cascadias Subduction Extent

Eos, Vol. 94, No. 9, 26 February 2013

VOLUME 94 NUMBER 9 26 FEBRUARY 2013

EOS, TRANSACTIONS, AMERICAN GEOPHYSICAL UNION PAGES 85–96

meaning that the plates are converging 2012 Haida Gwaii Quake: with each other as well as translating past each other—with approximately 19 millim- eters per year of convergence expected. Insight Into Cascadia’s The Haida Gwaii earthquake provides evidence for how this convergence is Subduction Extent accommodated, with a hypocenter east of the mapped Queen Charlotte fault at a depth of about 20 kilometers and a dominantly reverse focal mechanism, a signature of convergence P A G E S 8 5 – 8 6 motion is primarily strike slip, with a smaller (Figure 1 ). component of convergence. This finding is The limits of Cascadia were first defined to supported by the 22 August 1949 M 8.1 Queen contain nearly the entire margin of the Pacific Charlotte Island earthquake (Figure 1 ), which How Does the QCF Accommodate Motion? Northwest, from Cape Mendocino through displayed a right‐lateral first motion and the Alaska Panhandle [Schuchert , 1 9 1 0; presumably occurred along the main trace of The oblique nature of plate motion along Schuchert and Barrell, 1914 ]. Since that time, the QCF just offshore of the northern island of the QCF has led to two end‐member models the boundary of Cascadia has shrunk to Haida Gwaii. to describe how transpressional strain is become essentially synonymous with the South of the 1949 epicenter the trace of the accommodated: through distributed compres- region where the Juan de Fuca plate subducts QCF forms a restraining bend where the sion along crustal faults in both the Pacific beneath the North American plate. relative motion between the North American and North American plates [Rohr and Tryon, As a consequence, seismic hazard assess- and Pacific plates is more transpressional— 2010 ] and through subduction of the Pacific ments in the Pacific Northwest have conven- tionally focused on the potential for large megathrust earthquakes along the interface of the Juan de Fuca and North American plates. The 28 October 2012 M 7.8 Haida Gwaii earthquake (Figure 1 ), however, suggests that the northern terminus of subduction along the Cascadia margin may need to be redefined; such a redefinition has ramifica- tions for seismic hazard analysis along the entire west coast of the United States and Canada.

Tectonic Setting of Northern Cascadia

Off the coast of northern British Columbia, subduction of the Juan de Fuca plate beneath the North American plate gives way to a transform fault between the Pacific plate and North American plate: the Queen Charlotte fault system (QCF). This fault system extends northward from the Explorer plate, a microplate chipped off the Juan de Fuca plate, to Baranof Island, Alaska, where it Fig. 1. Map of the Queen Charlotte fault system showing the general tectonic layout. Earth- transitions into the Fairweather fault and quake focal mechanisms (beach balls) are drawn with compressive quadrants filled; unla- related structures (see Figure 1 ). Over the past beled focal mechanisms are from the 2012 sequence, and those with dates show large century, these fault systems have hosted historical earthquakes. Aftershocks from the 2012 sequence are marked by gray circles; several large earthquakes, including the 1899 aftershocks from the 1949 and 1972 earthquake are not shown. Earthquake epicenters are M8 Yakutat, the 1949 M 8.1 Queen Charlotte, from the International Seismological Centre catalog. Red arrows show relative plate motion the 1958 M 7.7 Lituya Bay, and the 1972 M7.6 velocities according to the Mid‐Ocean Ridge Velocity (MORVEL) model’s reference frame for Sitka. Recent geodetic velocity estimates North America [DeMets et al ., 2010 ]. The Brooks Peninsula (BP) on Vancouver Island and the along the QCF suggest that relative plate Nootka fault (dashed line) mark the current northern limit of the Cascadia subduction zone. The boundary between the North America plate and Explorer plate is indicated with question marks and does not appear to be actively subducting. The locations of the spreading centers BY W. SZELIGA are from Wilson [ 2002 ] and are schematic. JdF is Juan de Fuca.

© 2013. American Geophysical Union. All Rights Reserved. Eos, Vol. 94, No. 9, 26 February 2013 plate beneath the North American plate (USGS) suggest a large (150 kilometer × 40 need to reassess the implications for seismic [Yorath and Hyndman, 1983 ; Hyndman and kilometer) along‐strike and downdip region of hazard not only along the margin itself but Hamilton , 1993 ]. Other interpretations fault rupture. These fault dimensions indicate also to the rest of the Pacific Basin. intermediate to these two end‐member an unusually large downdip faulting dimen- models exist, and additional references may sion, which is more consistent with a be found in these papers. shallowly dipping fault and would be unusual Acknowledgments In the distributed crustal fault model the for a vertically dipping fault. QCF acts as the plate boundary and serves to Additionally, the locations, focal mecha- “Did You Feel It?” data are courtesy of accommodate strike‐slip motion between the nisms, and strikes of the two largest after- USGS. Focal mechanisms and magnitude for Pacific and North American plates, while shocks (M 6.5 and M 6.3; Figure 1 ) are the 2012 earthquake were contributed by convergence is accommodated along smaller suggestive of outer‐rise normal faulting USGS and the National Earthquake crustal thrusts in both plates. This model is earthquakes, shown to follow other much Information Center in Golden, Colo. supported by the occurrence of M 8 strike‐slip larger subduction earthquakes, such as the Earthquake locations were contributed by the earthquakes, the occurrence of M6 thrust 1960 Chile, 2004 Sumatra, and 2011 Tohoku International Seismological Centre. Tim earthquakes but a historical lack of thrust earthquakes [e.g., Lay et al. , 2 0 1 1, a n d Melbourne and Craig Scrivner deserve special earthquakes with a magnitude larger than M6, references therein]. In general, outer‐rise thanks for fruitful discussions, as do Anne and the absence of a seismically detectable normal faulting earthquakes are indicative of Egger, Chris Mattinson, and Lisa Ely for keen slab north of the Brooks Peninsula on tensional plate‐bending and slab‐pull stresses, proofreading. Vancouver Island (Figure 1 ). and their occurrence is related to temporal In contrast, the subduction model suggests changes in plate coupling following subduc- References that the Queen Charlotte fault is not the plate tion. All of these observations provide boundary but is instead a strike‐slip fault evidence for the oblique subduction model. B u s t i n, A . M . M ., R . D . H y n d m a n, H . K a o, a n d within the fore arc of the North American Similar to the 1949 Queen Charlotte J. F. Cassidy ( 2007 ), Evidence for underthrust- plate. In this scenario, convergence is earthquake, the October 2012 main shock was ing beneath the Queen Charlotte Margin, British accommodated along the plate boundary widely felt throughout the coastal Pacific Columbia, from teleseismic receiver function interface between a subducting Pacific plate Northwest, with reports from as far away as analysis, Geophys. J. Int., 171 , 1198 – 1211 , and an overriding North American plate. The Juneau, Alaska (650 kilometers), and Seattle, doi: 10.1111/j.1365 ‐246X.2007.03583.x . subduction model is supported by a down- Wash. (900 kilometers). In addition, a little D e M e t s, C ., R . G . G o r d o n, a n d D . A r g u s (2 0 1 0) , G e o - ward flexural bulge in the Pacific plate, more than an hour after the main shock, logically current plate motions , Geophys. J. Int., parallel low and high free‐air gravity Deep‐ocean Assessment and Reporting of 181 , 1 – 80 , doi: 10.1111/j.1365 ‐246X.2009.04491.x . H y n d m a n, R . D ., a n d T . S . H a m i l t o n (1 9 9 3) , Q u e e n anomalies centered along the QCF, and a Tsunami (DART) buoys in the northern Charlotte area and Cenozoic tectonics and vol- seismically imaged, 10‐kilometer‐thick, slab‐ Pacific Ocean recorded a small open ocean canism and their association with relative plate like structure beneath Haida Gwaii inter- tsunami, and an alert was issued for the motions along the northeastern Pacifi c margin , preted to be the eastward dipping partially Hawaiian Islands. While the subsequent J. Geophys. Res., 98 ( B8 ), 14,257 – 14,277 . subducted Pacific plate [Bustin et al. , 2 0 0 7] . tsunami wave heights proved to be smaller L a y, T ., C . J . A m m o n, H . K a n a m o r i, M . J . K i m, a n d Oblique subduction such as this occurs in than anticipated (1.2 meters at Wailoa Harbor, L. Xue ( 2011 ), Outer trench‐slope faulting and the other, more tectonically mature, regions, such Hawaii, and 0.76 meters at Kahului, Maui), the 2011 M w 9.0 off the Pacifi c coast of Tohoku earth- as Sumatra, the western Aleutian Islands, and fact that this section of the plate margin was quake, Earth Planets Space, 63 , 713 – 718 . the Philippines. In these regions, strain is able to generate a distal tsunami at all R o h r, K . M . M ., a n d A . J . T r y o n (2 0 1 0) , P a c i fi c– North America plate boundary reorganization partitioned between large strike‐slip struc- suggests that subduction along the QCF in response to a change in relative plate motion: tures in the overriding plate and thrust presents an underappreciated hazard to the Offshore Canada , Geochem. Geophys. Geosyst., faulting along the subduction interface. In all Pacific Basin. 11, Q06007 , doi:10.1029/2009GC003019 . of these regions, focal mechanisms in any S c h u c h e r t, C . (1 9 1 0) , P a l e o g e o g r a p h y o f N o r t h single earthquake typically show either pure Hazard Reassessments and the Need America, Bull. Geol. Soc. Am., 20 , 427 – 606 . Schuchert , C. , and J. Barrell (1914 ), A revised strike‐slip or pure thrust mechanisms and for Further Research only rarely a blend. geologic time‐table for North America , Am. J. Sci., 38( 223 ), 1 – 27 . The occurrence of the Haida Gwaii Wilson , D. S. (2002 ), The Juan de Fuca plate and Which Model Is Correct? New Insight From earthquake suggests that subduction in the slab: Isochron structure and Cenozoic plate the 2012 Haida Gwaii Earthquake Pacific Northwest resumes north of the motions , in The Cascadia Subduction Zone and Explorer plate, with the Pacific plate descend- Related Subduction Systems, e d i t e d b y S . K i r b y, K . The 28 October 2012 M 7.8 main shock is ing beneath North America. Further research W a n g, a n d S . D u n l o p, U.S. Geol. Surv. Open File the first instrumentally recorded major thrust will be needed to verify the extent of Rep., 02‐328, 9 – 12 . Y o r a t h, C . J ., a n d R . D . H y n d m a n (1 9 8 3) , S u b s i - earthquake along this segment of the QCF. subduction along the QCF and fault systems dence and thermal history of Queen Charlotte The aftershocks of the 2012 earthquake span farther north. Pending the outcome of this basin , Can. J. Earth Sci., 20, 135 – 159 . approximately 140 kilometers in the along‐ research, a reanalysis of what is defined as strike direction, and finite fault inversions the extent of subduction along the Cascadia —W ALTER SZELIGA, Geological Sciences from both the California Institute of margin may be in order. With this new Department, Central Washington University, Technology and the U.S. Geological Survey definition the scientific community may also Ellensburg; E-mail: [email protected]