Periodic Reporting for period 2 - MICA (Mechanics of slow earthquake phenomena: an Integrated perspective from the Composition, geometry, And rheology of plate boundary faults)
Reporting period: 2018-08-01 to 2020-01-31
The results of the project may inform seismic hazard evaluations by identifying faults and parts of faults that may or may not experience damaging earthquakes, as well as potential earthquake precursors. It is, for example, currently unknown how slow and fast earthquakes are related. Critical questions of societal importance include: If a fault experiences slow earthquakes, can it also experience earthquakes that are damaging? If parts of a fault experiences a slow earthquake, does this increase (or decrease) the probability of a damaging earthquake nearby? Can slow earthquakes accelerate and become fast and damaging?
In particular, the project seeks to identify the effects of fault zone thickness, internal geometry, and composition. This is done through detailed fieldwork and laboratory studies of rocks from fault zones exposed on the Earth's surface, or accessible through ocean drilling. Based on field and microscale observations, models are created for realistic geometry and deformation mechanisms in major fault zones. From these models, we identify variables that control slip speed, and allow slip at a range of speeds. These hypotheses are tested by numerical and laboratory deformation experiments.
The project has so far generated new geological data to test hypotheses for how a range of deformation styles may occur in a single location. An existing model for slow earthquakes in subduction zones (e.g. around the Pacific Rim) involves fluid-driven fractures within otherwise creeping, ductile shear zones. Two papers published in Geology describe such brittle-ductile deformation at very different depths. In the Damara belt, Namibia, rocks are now exposed that were deformed during ancient subduction as Gondwanaland amalgamated. These rocks retain a record of how that subduction was accommodated at depths of more than 30 km, and can be described as a ductilely sheared ocean floor package punctuated by numerous quartz-filled veins. We show that the vein-filled fractures and the ductile shear were coeval, and that the veins are chemically consistent with formation in a small depth range (Fagereng et al., Geology, 2018). We interpret these structures as a dominantly ductile shear zone which changes to a brittle behaviour episodically as fluid pressure builds and allows local and transient fracturing. At a very different depth, drill core from the Hikurangi margin, New Zealand, similarly show co-existing brittle and ductile structures, but at depths less than a kilometer (Fagereng et al., Geology, 2019). Here, the interpretation is also than transient changes in fluid pressure or strain rate can change, for a moment, the preferred deformation style of a fault.
A criteria for slow earthquakes seems to be that deformation occurs at low stresses and is sensitive to small perturbations. This may be the case in very weak faults in many locations. In addition to the subduction zone settings described above, we have shown that strike-slip faults in continents may also be very weak, as a consequence of alteration and growth of fine-grained, weak minerals (Stenvall et al., 2019, Geophysical Research Letters). In a general case, the combination of weak shear zones and rigid inclusions will generate force chains, where loss of continuous pathways of weak material locally and transiently leads to increases in stress, and potentially in strain rate as force chains are subsequently released. In Beall et al. (2019, Geophysical Research Letters), we have shown numerically that shear zones with more than 50 % strong material will spontaneously generate force chains, and hypothesise that slow earthquakes may therefore develop as a geometrical consequence of strength heterogeneity in any setting where viscosity contrasts are high.