To advance the understanding of the tectonic processes shaping the African continent, we construct the first continental-scale shear-wave velocity (Vs) model of the crust and uppermost mantle from joint analysis of ambient seismic noise and earthquake data recorded by ~1529 broadband seismic stations located in Africa, Arabia, and Europe from 1987 to 2018. We apply the widely used ambient noise cross-correlation and earthquake two-station methods to retrieve the fundamental-mode Rayleigh-wave group and phase velocity dispersions in the period range of 5 – 50 s which are jointly inverted using the neighbourhood algorithm to build a new three-dimensional Vs model with associated uncertainties. The inclusion of relatively short-period dispersion data from ambient seismic noise allows us to achieve better resolution at shallow depth and obtain a more accurate model than previous global and continental-scale studies, revealing lithospheric structures that correlate well with known tectonic features. In sparsely instrumented regions of north-central Africa, our model provides seismic evidence for the existence of cratonic remnants beneath thick sediments within the poorly imaged Sahara Metacraton and reveals unique mantle upwelling beneath hotspots, suggesting that they may be fed by unconnected plumes. The estimated crustal thickness varies among and within tectonic provinces and shows no clear evidence for the secular variation in crustal genesis. Our new model has the potential to serve as a preliminary reference velocity model for Africa and is useful for practical applications, including monitoring of the Comprehensive Nuclear-Test-Ban Treaty, geodynamic modeling as well as seismic hazard analysis.

To characterize the subsurface geomechanical response to hydraulic fracturing activities, we study the spatiotemporal changes of seismic velocity during the completion of four injection wells in the Fox Creek area, Alberta, Canada. We estimate temporal velocity changes (dv/v) from ambient seismic noise recorded during the Tony Creek Dual Microseismic Experiment (ToC2ME) by comparing a 5-day stacked noise correlation function with a reference noise correlation function stacked over the deployment period. In the frequency band (0.1 - 0.4 Hz) most sensitive to the injection depths (~3.4 km), we observe daily dv/v that revealed alternating gradual velocity decreases and increases with magnitudes in the range of ±0.9%. We found a strong temporal correlation between the onset of velocity decreases and periods of intense seismicity, suggesting that the observed dv/v reductions are likely caused by stress-induced subsurface deformation due to elevated pore pressures, increased crack density, and ground shaking. A period of dv/v increase observed between the beginning and end of different well stimulation is attributed to crustal healing. Comparing the dv/v time series with injection parameters, we observed a 272.66% increase in induced seismicity and 50% more reduction in dv/v during the second injection phase that are correlated with 90.53%, 169.64%, and 4.34% increase in the injection volume, rate, and pressure, respectively. Our study provides valuable new information on the changes in reservoir elastic properties within the Western Canadian Sedimentary Basin. It also demonstrates that coda wave interferometry using data from dense seismic arrays near injection sites can be an additional tool for monitoring hydraulic fracturing operations.

To advance the understanding of crustal deformation and earthquake hazards in Canada, we analyze seismic and geodetic datasets and robustly estimate the crust strain accumulation and release rate by earthquakes. We find that less than 20% of the accumulated strain is released by earthquakes across the study area providing evidence for large-scale aseismic deformation. We attribute this to Glacial Isostatic Adjustment (GIA) in eastern Canada, where predictions from the GIA model accounts for most of the observed discrepancy between the seismic and the geodetic moment rates. In western Canada, only a small percentage (< 20%) of the discrepancy can be attributed to GIA-related deformation. We suspect that this may reflect the inaccuracy of the GIA model to account for heterogeneity in Earth structure or indicate that the present-day effect of GIA in western Canada is limited due to the fast response of the upper mantle to the de-glaciation of the Cordillera Ice Sheet. At locations of previously identified seismic source zones, we speculate that the unreleased strain is been stored cumulatively in the crust and will be released as earthquakes in the future. The Gutenberg-Richter (GR) model predicts, however, that the recurrence interval can vary significantly in Canada, ranging from decades near plate boundary zones in the west to thousands of years in the stable continental interior. Our attempt to quantify the GIA-induced deformation provides the necessary first step for the integration of geodetic strain rates in seismic hazard analysis in Canada.

Identification of repeating earthquakes (repeaters) usually depends on waveform similarity expressed as the corresponding cross-correlation coefficient (CC) above a prescribed threshold, typically ranging from 0.70 to 0.98. However, the robustness and effectiveness of such a strategy have never been thoroughly examined. In this study, we examine whether CC is a valid proxy for repeater identification through both synthetic and real earthquake experiments. We reveal that CC is controlled by not only the inter-event distance but also many other factors, including station azimuth, epicentral distance, velocity structure, etc. Consequently, CC lacks the resolution in identifying true repeaters. We propose a physics-based approach that considers both inter-event separation and rupture radius. For an event pair to be true repeaters, their inter-event separation must be smaller than the rupture radius of the larger event. Our results imply that a systematic recheck of previously identified repeaters and associated interpretations/hypotheses may be important and necessary.

At the northern Cascadia subduction zone, the subducting Explorer and Juan de Fuca plates interact across a transform deformation zone, known as the Nootka fault zone (NFZ). This study continues the Seafloor Earthquake Array Japan Canada Cascadia Experiment to a second phase (SeaJade II) consisting of nine months of recording of earthquakes using ocean-bottom and land-based seismometers. In addition to mapping the distribution of seismicity, including an MW 6.4 earthquake and aftershocks along the previously unknown Nootka Sequence Fault, we also conducted seismic tomography that delineates the geometry of the shallow subducting Explorer plate (ExP). We derived hundreds of high-quality focal mechanism solutions from the SeaJade II data. The mechanisms manifest a complex regional tectonic state, with normal faulting of the ExP west of the NFZ, left-lateral strike-slip behaviour of the NFZ, and reverse faulting within the overriding plate above the subducting Juan de Fuca plate. Using data from the combined SeaJade I and II catalogs, we have performed double-difference hypocentre relocations and found seismicity lineations to the southeast of, and oriented 18° clockwise from, the subducted NFZ, which we interpret to represent less active small faults off the primary faults of the NFZ. These lineations are not optimally oriented for shear failure in the regional stress field, which we inferred from averaged focal mechanism solutions, and may represent paleo-configurations of the NFZ. Further, active faults interpreted from seismicity lineations within the subducted plate, including the Nootka Sequence Fault, may have originated as conjugate faults within the paleo-NFZ.

Riedel shear structures (RSS) are often observed in the embryonic stage of strike-slip fault development, which can be depicted in the field through outcrops and co-seismic surface ruptures. It is a critical concept linking the geomechanical behavior of individual earthquakes to structural geology at both local and regional scales. However, the influence of long-term fluid injections on the developing process of RSS, as manifested by the common occurrences of injection-induced earthquakes, has been rarely addressed. Here we document for the first time subsurface RSS expedited by long-term wastewater disposal injections in western Canada. We study an earthquake sequence consisting of 187 events (ML ranging 1.3–3.9) between 2018/01/01 and 2021/07/15 in an area without any previous seismic history. According to 31 well-constrained focal mechanism solutions, the injection-related earthquake sequence exhibits various faulting types with the vast majority (87%) being compatible with the background stress regime. The orientation of derived nodal planes collectively indicates a model of RSS that consists of four primary strike-slip structures striking 19º (R’), 79º (R), 94º (PDZ) and 109º (P), respectively. Moreover, six fault segments delineated from the relocated local seismicity are parallel to the sub-structures of RSS. Mohr-Coulomb failure analysis further suggests a cumulative stress perturbation of up to 10.0 MPa. Our observations suggest that long-term fluid injection can expedite the development of local fault systems. Therefore, it is probably important to consider the dimension of local/regional RSS in the assessment of the overall seismic hazard due to fluid injections.

Earthquakes resulting from hydraulic fracturing (HF) can have delayed triggering relative to injection commencement over a varied range of time scales, with many cases exhibiting the largest events near/after well completion. This poses serious challenges for risk mitigation and hazard assessment. Here, we document a high-resolution, three-dimensional source migration process with delayed mainshock triggering that is controlled by local hydrogeological conditions. Our results reveal that poroelastic effects might contribute to induced seismicity, but are insufficient to activate a non-critically stressed fault of sufficient size. The rapid pore-pressure build-up from HF can be very localized and capable of producing large, felt earthquakes on non-critically stressed fault segments. We interpret the delayed triggering as a manifestation of pore-pressure build-up along pre-existing faults needed to facilitate seismic failure. Our findings can deepen our understanding of the current stress state of crustal faults and also explain why so few injection operations are seismogenic.