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: This paper presents direct-seismogram inversion (DSI) for receiver-side structure which treats the source signal incident from below (the effective source–time function—STF) as a vector of unknown parameters in a Bayesian framework. As a result, the DSI method developed here does not require deconvolution by observed seismogram components as typically applied in receiver-function inversion and avoids the problematic issue of choosing subjective tuning parameters in this deconvolution. This results in more meaningful inversion results and uncertainty estimation compared to classic receiver-function inversion. A rigorous derivation is presented of the likelihood function required for unbiased inversion results. The STF is efficiently inferred by a maximum-likelihood closed-form expression that does not require deconvolution by noisy waveforms. Rather, deconvolution is only by predicted impulse responses for the unknown environment (considered to be a 1-D, horizontally stratified medium). For a given realization of the parameter vector which describes the medium below the station, data predictions are computed as the convolution of the impulse response and the maximum-likelihood source estimate for that medium. Therefore, the assumption of a Gaussian pulse with specified parameters, typical for the prediction of receiver functions, is not required. Directly inverting seismogram components has important consequences for the noise on the data. Since the signal processing does not require filtering and deconvolution, data errors are less correlated and more straightforward to model than those for receiver functions. This results in better inversion results (parameter values and uncertainties), since assumptions made in the derivation of the likelihood function are more likely to be met by the inversion process. The DSI method is demonstrated for simulated waveforms and then applied to data for station Hyderabad on the Indian craton. The measured data are inverted with both the new DSI and traditional receiver-function inversion. All inversions are carried out for a trans-dimensional model that treats the number of layers in the model as unknown. Results for DSI are consistent with previous studies for the same location. The DSI has clear advantages in trans-dimensional inversion. Uncertainty estimates appear more realistic (larger) in both model complexity (number of layers) and in terms of seismic velocity profiles. Receiver-function inversion results in more complex profiles (highly-layered structure) and suggests unreasonably small uncertainties. This effect is likely also significant when the parametrization is considered to be fixed but exacerbated for the trans-dimensional model: If hierarchical errors are poorly estimated, trans-dimensional models overestimate the structure which produces unfavourable results for the receiver-function inversion.

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.