S. Mazzotti

Tsunami hazard assessment of Canada

We present a preliminary probabilistic tsunami hazard assessment of Canadian coastlines from local and far-field, earthquake, and large submarine landslide sources. Analyses involve published historical, palaeotsunami and palaeoseismic data, modelling, and empirical relations between fault area, earthquake magnitude, and tsunami run-up. The cumulative estimated tsunami hazard for potentially damaging run-up (≥1.5xa0m) of the outer Pacific coastline is ~40–80xa0% in 50xa0years, respectively one and two orders of magnitude greater than the outer Atlantic (~1–15xa0%) and the Arctic (<1xa0%). For larger run-up with significant damage potential (≥3xa0m), Pacific hazard is ~10–30xa0% in 50xa0years, again much larger than both the Atlantic (~1–5xa0%) and Arctic (<1xa0%). For outer Pacific coastlines, the ≥1.5xa0m run-up hazard is dominated by far-field subduction zones, but the probability of run-up ≥3xa0m is highest for local megathrust sources, particularly the Cascadia subduction zone; thrust sources further north are also significant, as illustrated by the 2012 Haida Gwaii event. For Juan de Fuca and Georgia Straits, the Cascadia megathrust dominates the hazard at both levels. Tsunami hazard on the Atlantic coastline is dominated by poorly constrained far-field subduction sources; a lesser hazard is posed by near-field continental slope failures similar to the 1929 Grand Banks event. Tsunami hazard on the Arctic coastline is poorly constrained, but is likely dominated by continental slope failures; a hypothetical earthquake source beneath the Mackenzie delta requires further study. We highlight areas susceptible to locally damaging landslide-generated tsunamis, but do not quantify the hazard.

Crustal and Upper-Mantle Anisotropy Related to Fossilized Transpression Fabric along the Denali Fault, Northern Canadian Cordillera

We analyze crustal and upper‐mantle structure near the Denali fault (northern Canadian Cordillera) using 11 broadband seismic stations. Receiver functions at five stations within 5–30xa0km of the fault trace display a strong P ‐to‐ S conversion within the midcrust (10–25xa0km depth) that systematically varies with back azimuth. Stacking and velocity inversion along complementary northwest–southeast and northeast–southwest back‐azimuth ranges yield a strong anisotropy (>10%) in the midcrust, with complex crustal anisotropic behavior (at least two layers) close to the Denali and Duke River faults junction. Anisotropy occurs in a low‐velocity zone with the fast axis parallel to the Denali fault trend. Three additional stations close to the Denali fault (15–30xa0km), as well as two stations further away (>100u2009u2009km) show no significant crustal anisotropy. Shear‐wave ( SKS ) splitting analysis indicates similar upper‐mantle anisotropy, with a fast axis parallel to the trend of the Denali fault for all stations, except for station WHY located 125xa0km away. We relate this crustal and upper‐mantle anisotropy to fossilized structural and mineral fabrics due to Eocene transpression on the Denali fault. These results suggest the main transpression phase (∼400u2009u2009km displacement) was accommodated in a shear zone at least 50–60xa0km (less than 125xa0km) wide in the midcrust and upper mantle. Lack of clear anisotropy in the lowermost crust may relate to complex deformation within a detachment layer (orogenic float model).nnOnline Material: Table of SKS splitting information and figures of receiver function analysis.

Evidence of interseismic coupling variations along the Bhutan Himalayan arc from new GPS data

Although the first-order pattern of present-day deformation is relatively well resolved across the Himalayas, irregular data coverage limits detailed analyses of spatial variations of interseismic coupling. We provide the first GPS velocity field for the Bhutan Himalaya. Combined with published data, these observations show strong east-west variations in coupling between central and eastern Bhutan. In contrast with previous estimations of first-order uniform interseismic coupling along the Himalayan arc, we identify significant lateral variations: In western and central Bhutan, the fully coupled segment is 135–155u2009km wide with an abrupt downdip transition, whereas in eastern Bhutan the fully coupled segment is 100–120u2009km wide and is limited updip and downdip by partially creeping segments. This is the first observation of decoupling on the upper ramp along the Himalayan arc, with important implications for large earthquake surface rupture and seismic hazard.

Microseismicity and Tectonics of Southwest Yukon Territory, Canada, Using a Local Dense Seismic Array

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 Lxa03.0 to 1.0. We analyze 980 events ranging in magnitudes from M Lxa00.2 to 4.7, at depths from 0 to 35xa0km. Relocation analysis using the progressive multiple event location shows that seismicity is concentrated in four main areas: (1)xa0Yakutat block northern boundary–Fairweather fault, (2)xa0Duke River fault, (3)xa0southern Denali fault, and (4)xa0a 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.

Architecture of the crust and uppermost mantle in the northern Canadian Cordillera from receiver functions

The northern Canadian Cordillera (NCC) is an active orogenic belt in northwestern Canada characterized by deformed autochtonous and allochtonous structures that were emplaced in successive episodes of convergence since the Late Cretaceous. Seismicity and crustal deformation are concentrated along corridors located far (>200 to ~800 km) from the convergent plate margin. Proposed geodynamic models require information on crust and mantle structure and strain history, which are poorly constrained. We calculate receiver functions using 66 broadband seismic stations within and around the NCC and process them to estimate Moho depth and P-to-S velocity ratio (Vp/Vs) of the Cordilleran crust. We also perform a harmonic decomposition to determine the anisotropy of the subsurface layers. From these results, we construct simple seismic velocity models at selected stations and simulate receiver function data to constrain crust and uppermost mantle structure and anisotropy. Our results indicate a relatively flat and sharp Moho at 32 ± 2 km depth and crustal Vp/Vs of 1.75 ± 0.05. Seismic anisotropy is pervasive in the upper crust and within a thin (~10–15 km thick) sub-Moho layer. The modeled plunging slow axis of hexagonal symmetry of the upper crustal anisotropic layer may reflect the presence of fractures or mica-rich mylonites. The subhorizontal fast axis of hexagonal anisotropy within the sub-Moho layer is generally consistent with the SE-NW orientation of large-scale tectonic structures. These results allow us to revise the geodynamic models proposed to explain active deformation within the NCC.

GravProcess: An easy-to-use MATLAB software to process campaign gravity data and evaluate the associated uncertainties

We present GravProcess, a set of MATLAB routines to process gravity data from complex campaign surveys and calculate the associated gravity field. Data reduction, analysis, and representation are done using the MATLAB Graphical User Interface Tool, which can be installed on most systems and platforms. Data processing is divided into several steps: (1) Integration of gravity data, station location, and gravity line connection input files; (2) Gravity data reduction applying solid-Earth tide and instrumental drift corrections and, depending on the required processing level, air pressure and oceanic tidal corrections; (3) Automatic network adjustment and alignment to absolute base stations; (4) Free air and terrain corrections to calculate gravity values and anomalies, and to estimate the associated errors. The final step is dedicated to post-processing and includes graphical representations of data and an output text file, which can be used by Geographic Information System software. An example of this processing chain applied to a recent survey in northern Morocco is given and compared with previous available results.

Precision of continuous GPS velocities from statistical analysis of synthetic time series

We use statistical analyses of synthetic position time series to estimate the potential precision of GPS (Global Positioning System) velocities. The synthetic series represent the standard range of noise, seasonal, and position offset characteristics, leaving aside extreme values. This analysis is combined with a new simple method for automatic offset detection that allows an automatic treatment of the massive dataset. Colored noise and the presence of offsets are the primary contributor to velocity variability. However, regression tree analyses show that the main factors controlling the velocity precision are first the duration of the series, second the presence of offsets, and third the noise level (dispersion and spectral index). Our analysis allows us to propose guidelines, which can be applied to actual GPS data, that constrain velocity precisions, characterized as a 95 % confidence limit of the velocity biases, based on simple parameters: (1) series durations over 8.0 years result in low-velocity biases in the horizontal (0.2 mm yr−1) and vertical (0.5 mm yr−1) components; (2) series durations of less than 4.5 years are not suitable for studies that require precisions lower than mm yr−1; (3) series of intermediate durations (4.5–8.0 years) are associated with an intermediate horizontal bias (0.6 mm yr−1) and a high vertical one (1.3 mm yr−1), unless they comprise no offset. Our results suggest that very long series durations (over 15–20 years) do not ensure a significantly lower bias compared to series of 8–10 years, due to the noise amplitude following a power-law dependency on the frequency. Thus, better characterizations of long-period GPS noise and pluri-annual environmental loads are critical to further improve GPS velocity precisions.

Structural Inheritance Control on Intraplate Present‐Day Deformation: GPS Strain Rate Variations in the Saint Lawrence Valley, Eastern Canada

Structural inheritance is one of the key factors commonly proposed to control the localization of strain and seismicity in continental intraplate regions, primarily on the basis of a first‐order spatial correlation between seismicity and inherited tectonic structures. In this paper, we present new GPS (Global Positioning System) velocity and strain rate analyses that provide strong constraints on the magnitude and style of present‐day strain localization associated with the inherited tectonic structures of the Saint Lawrence Valley, eastern Canada. We analyze 143 continuous and campaign GPS stations to calculate velocity and strain rate patterns, with specific emphases on the combination of continuous and campaign velocity uncertainties, and on the definition of robustness categories for the strain rate estimations. Within the structural inheritance area, strain rates are on average 2–11 times higher than surrounding regions and display strong lateral variations of the style of deformation. These GPS velocity and strain rate fields primarily reflect ongoing glacial isostatic adjustment (GIA). Their comparison with GIA model predictions allows us to quantify the impact of the structural inheritance and the associated lithosphere rheology weakening. Outside of the major tectonic inheritance area, GPS and GIA model strain rates agree to first order, both in style and magnitude. In contrast, the Saint Lawrence Valley displays strong strain amplification with GPS strain rates 6–28 times higher than model‐predicted GIA strain rates. Our results provide the first quantitative constraints on the impact of lithospheric‐scale structural inheritance on strain localization in intraplate domains.