Our group has been active in studying the seismic structure of subduction zones in Cascadia and Costa Rica. Using data from earthquakes around the world propagating through the forearc region of subduction zones, our group has helped establish that the crust of the subducting oceanic plate is over pressured, and that lateral variations in the properties of the subducting plate control the seismic behaviour of the plate interface. Recently we have begun investigating the seismic anisotropy of subducting plates to put constraints on the source of mantle fabrics. In a different project we identify low-frequency earthquakes occurring within sequences of anomalous slow earthquake seismicity in the deep portion of subduction zones.
Structure and tectonics of the northern Canadian Cordillera
The northern Canadian Cordillera is a vast region encompassing most of the Yukon and the western part of the Northwest Territories. Even though it is one of the most seismically active regions in Canada, its seismicity and subsurface structure are poorly constrained due to the spare distribution of seismic stations. Our group has installed an array of 7 new seismograph stations across the Cordillera in order to refine seismic models of the subsurface as well as to improve the detection and characterization of earthquakes. Results from our first comprehensive studies indicate that the Cordilleran crust is flat at 35 km, in agreement with thermal isostasy models. In the uppermost mantle, the lithosphere of the North American Craton appears to extend to the Tintina Fault, a result with important implications for the origin and evolution of the Mackenzie Mountains and seismicity. Seismic anisotropy data suggest that upper mantle fabrics are due to long-term shearing along the major transcurrent faults, or reflect upper mantle flow associated with slab window opening.
Crustal deformation from human activity
There is increasing evidence that human activities are not only affecting Earth’s outer fluid layers (oceans, atmosphere, ice sheets), but also the topmost solid layer resulting in shallow crustal deformation (e.g., basin subsidence from oil and water pumping, increased seismicity following gas extraction, etc.). In a recent study, my colleagues and I document a broad zone of rock uplift surrounding the San Joaquin Valley in Central California, the site of historical and ongoing groundwater depletion for irrigation, and infer that the water mass loss can cause detectable growth of the southern Sierra Nevada and affect seismicity rates along the adjacent San Andreas Fault. This study is the first of its kind to show a link between human activity and large-scale vertical motion of the solid Earth. Such unnatural causes of crustal deformation will likely become increasingly important as climate scenarios forecast higher demands for agricultural, urban and environmental groundwater uses. These studies will contribute to characterize the extent of human influence on solid Earth deformation and evaluate their impact on active crustal processes.
Deformation and rheology of the crust from seismic anisotropy
Tectonic forces and processes leave their imprints on rock deformation and textures. In turn, these fabrics affect seismic wave fields propagating through such media by producing seismic anisotropy: the variations in seismic properties that depend on the direction of propagation. By studying how rock textures produce elastic (and therefore seismic) anisotropy, it becomes possible to map rock deformation to depth using seismic data. Our group is interested in the seismic anisotropy of subducting oceanic plates as well as crustal shear zones in northern California and the western Cyclades in order to study otherwise inaccessible imprints of tectonic processes.
Wavelet analysis of potential field data
Rheology of the lithosphere can be studied using gravity and topography data and a model for the flexural support of lithospheric loads. Methods for estimating the thickness of the effectively elastic part of the lithosphere (or elastic thickness) have long been used to study the rheology of the Earth and terrestrial planets, but the two communities (Earth versus planetary) use different assumptions and methods that are not always consistent. In our most recent work we apply analysis tools to study gravity and topography data using a spherical wavelet analysis and implement the models and techniques to characterize the elastic thickness of the Earth and terrestrial planets in a consistent manner. In a recent project we apply the wavelet transform to magnetic anomaly data to estimate the depth to the Curie point in northwestern Canada, which can be used to put constraints on lithospheric geotherms.