ALBERT

Description: This study applies Bayesian inversion to receiver functions (RFs) to estimate local shear-wave velocity ( V S ) structure of the Juan de Fuca (JdF) plate beneath the northern Cascadia subduction zone (CSZ) offshore and onshore Vancouver Island, British Columbia, Canada. We use passive seismic data recorded on NEPTUNE (NorthEast Pacific Time-series Undersea Networked Experiments) Canada ocean-bottom seismometers (OBSs), on temporary autonomous KECK Foundation OBSs, and on two land-based seismometers on Vancouver Island that are part of the Canadian National Seismograph Network (CNSN). Three-component, broadband recordings of large ( ), distant (30°–100°) earthquakes are used to compute RFs dominated by locally generated P -to- S converted waves. These are subsequently inverted using a nonlinear Bayesian approach that yields optimal profiles of V S , V P (compressional-wave velocity), and strike and dip angles, as well as quantitative uncertainty estimates for these parameters. The introduction of NEPTUNE Canada helps fill a gap in offshore seismic monitoring. Results from OBS stations indicate a thin oceanic crust at the JdF Ridge which thickens to ~10 km at the continental slope where sediment thickness also increases to ~5 km. At OZB, a coastal station, a 6–8 km thick, two-part low-velocity zone (LVZ) is imaged at 19 km depth. An LVZ of similar thickness is also observed 34 km beneath PGC, a south-central Vancouver Island station. The thickness of the LVZ imaged at these two land-based stations indicates that the oceanic sediments are not subducted but are scraped off the JdF plate and accreted to the North American plate. Determining these V S models at various stages of the CSZ provides a more detailed image of the subducting plate, and therefore contributes valuable new information useful for seismic-hazard analysis.

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: Finite-difference modeling of 3D long-period (〉2 s) ground motions for large ( M w  6.8) scenario earthquakes is conducted to investigate effects of the Georgia basin structure on ground shaking in Greater Vancouver, British Columbia, Canada. Scenario earthquakes include deep (〉40 km) subducting Juan de Fuca (JdF) plate earthquakes, simulated in locations congruent with known seismicity. Two sets of simulations are performed for a given scenario earthquake using models with and without Georgia basin sediments. The chosen peak motion metric is the geometric mean of the two orthogonal horizontal components of motion. The ratio between predicted peak ground velocity (PGV) for the two simulations is applied here as a quantitative measure of amplification due to 3D basin structure. A total of 10 deep subducting JdF plate earthquakes are simulated within 100 km of Greater Vancouver. Simulations are calibrated using records from the 2001 M w  6.8 Nisqually earthquake. On average, the predicted level of average PGV at stiff soil sites across Greater Vancouver for an M w  6.8 JdF plate earthquake is 3.2 cm/s (modified Mercalli intensity IV–V). The average increase in PGV due to basin structure across Greater Vancouver is 3.1. Focusing of north-northeast-propagating surface waves by shallow (〈1 km) basin structure increases ground motion in a localized region of south Greater Vancouver; hence, scenario JdF plate earthquakes located ≥80 km south-southwest of Vancouver are potentially the most hazardous. Online Material: Depth slices of 3D velocity model, peak ground velocity maps, and snapshots and videos of wave propagation.

Description: On 9 October 2007, an unusual sequence of earthquakes began in central British Columbia about 20 km west of the Nazko cone, the most recent (circa 7200 yr) volcanic center in the Anahim volcanic belt. Within 25 hr, eight earthquakes of magnitude 2.3-2.9 occurred in a region where no earthquakes had previously been recorded. During the next three weeks, more than 800 microearthquakes were located (and many more detected), most at a depth of 25-31 km and within a radius of about 5 km. After about two months, almost all activity ceased. The clear P- and S-wave arrivals indicated that these were high-frequency (volcanic-tectonic) earthquakes and the b value of 1.9 that we calculated is anomalous for crustal earthquakes but consistent with volcanic-related events. Analysis of receiver functions at a station immediately above the seismicity indicated a Moho near 30 km depth. Precise relocation of the seismicity using a double-difference method suggested a horizontal migration at the rate of about [IMG]/medium/1732eq1.gif" ALT="Formula "〉, with almost all events within the lowermost crust. Neither harmonic tremor nor long-period events were observed; however, some spasmodic bursts were recorded and determined to be colocated with the earthquake hypocenters. These observations are all very similar to a deep earthquake sequence recorded beneath Lake Tahoe, California, in 2003-2004. Based on these remarkable similarities, we interpret the Nazko sequence as an indication of an injection of magma into the lower crust beneath the Anahim volcanic belt. This magma injection fractures rock, producing high-frequency, volcanic-tectonic earthquakes and spasmodic bursts.

Description: The objective of this paper is to provide a better understanding of the relationship between the microseismicity, active tectonics, and crustal structures in the southwest Yukon Territory, Canada, in order to improve seismic-hazard assessments in this region. We utilize data from a new dense seismic array that was deployed in the southwest Yukon in the summer of 2010. Data from the new seismic array significantly improve the magnitude completeness level in the region from M L  3.0 to 1.0. We analyze 980 events ranging in magnitudes from M L  0.2 to 4.7, at depths from 0 to 35 km. Relocation analysis using the progressive multiple event location shows that seismicity is concentrated in four main areas: (1) Yakutat block northern boundary–Fairweather fault, (2) Duke River fault, (3) southern Denali fault, and (4) a previously unrecognized northeast trending cluster that may highlight a previously unknown active fault. This cluster may contribute to stress and strain transfer inland from the Yakutat block region.

Description: Finite-difference modeling of 3D long-period (〉2 s) ground motions for large ( M w  6.8) scenario earthquakes is conducted to investigate the effects of the Georgia basin structure on ground shaking in Greater Vancouver, British Columbia, Canada. Scenario earthquakes include shallow blind-thrust North America (NA) plate earthquakes, simulated in locations congruent with linear clusters of shallow seismicity, that is, potential active faults. A slip distribution model of the M w  6.7 Northridge, California, blind-thrust earthquake, with the hypocenter modified to 5 km depth, is used to characterize the source rupture process. Two sets of simulations are performed for a given scenario earthquake using models with and without Georgia basin sediments. The ratio of predicted peak ground velocity (PGV) for the two simulations is applied here as a quantitative measure of amplification due to 3D basin structure. A total of eight shallow blind-thrust NA plate scenario earthquakes are simulated within 100 km of Greater Vancouver. Overall, predicted ground motions are higher in the down-dip direction of each epicenter due to the source radiation pattern; hence, scenario earthquakes located south of Vancouver produce the highest motions in the city. The average maximum PGV at stiff soil sites across Greater Vancouver considering all eight scenario earthquakes is 17.8 cm/s (modified Mercalli intensity VII); the average increase in peak motion due to the presence of Georgia basin sediments is a factor of 4.1. The effective duration of moderate-level (≥3.4 cm/s) shaking within Greater Vancouver is an average of 22 s longer when Georgia basin sediments are included in the 3D structure model. Online Material: Snapshots and videos of wave propagation, and peak ground velocity maps.

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.