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.