Understanding the deformation and processes that link ductile flow in the deep continental crust with frictional processes in the upper crustal seismogenic zone

Subduction zones-regions of Earth's crust where one tectonic plate dives beneath another-are usually but not always associated with frequent, violent earthquakes. Occasionally, the downgoing plate slides slowly, smoothly and almost silently under the overriding plate. Canadian scientists have contributed to the research of seismic slow signals with geophysical observations of the Cascadia subduction zone. However, field observations, rock experiments and theoretical developments are also needed to advance our understanding of these newly identified fault slip phenomena. The proposed research program, with implications for future earthquakes offshore western Canada, aims to (a) expand our knowledge of mechanical processes that link mega earthquakes in the upper crust to steady and continuous slip in the lower crust via the brittle-ductile transition zone, which hosts the seismic slow signals, and (b) test a set of innovative methods to determine the ages of pre-instrumental earthquakes. (a) The importance of this research lies in the potential relationship between the fault transition zone and the parts of faults that generate destructive earthquakes. Some calculations find that the probability of a large earthquake occurring during a slow-slip event is 30-100 times greater than background probabilities. My research program involves interdisciplinary studies of exemplary exposed megathrust systems that record different stages of their development. Continental megathrust systems will be investigated via integration of observations across a range of scales and approaches, including field studies, microanalyses, thermochronology, geochronology, and numerical modelling. (b) Deformation data obtained from faulted rocks should be interpreted in the context of temporal or spatial variations in seismic processes. Thus, precise and accurate dating is crucial and entails developing innovative techniques that directly link age data to evidence of seismic slip in the rock record. This goal is achievable because of recent developments in our understanding of the physics of signals from minerals released via exposure to heat produced by seismic slip, and improvements in related analytical methods. To test the new techniques, we selected several locations where a surface earthquake rupture of known age provides access to samples suitable for attaining this goal. In addition to taking a leadership role in the discipline, Canada will benefit from this research by further understanding the processes responsible for Earth's major evolutionary steps, which can then feed into policy development for geohazard and environmental protection. Program goals will involve Canadian and international collaborators and train 2 PhD and 2 MSc graduate students, 1 PDF and undergraduates who will become knowledgeable in science communication and international and earthquake research, collaborate with world-leading research groups and gain access to world-class infrastructure.