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Mechanics of slow earthquake phenomena: an Integrated perspective from the Composition, geometry, And rheology of plate boundary faults

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 project assesses the varied behaviour of faults that accommodate tectonic deformation in the Earth's crust. These faults slip at a range of speeds from millimetres per year, a rate comparable to fingernail growth, up to metres per second, a rate that generates earthquakes. In the last two decades, observations from GPS and seismometer networks have shown that faults also slip at rates intermediate between these end-members, in events now known as 'slow earthquakes'. The physical processes that control fault slip rate are poorly understood. This project is designed to explore the geological processes that control fault slip speed.

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.
An objective of the project is to create a catalogue of geological fault zone models and interpretations of how they deform under conditions where slow earthquakes occur. A starting point was the hypothesis that slow earthquakes occur in the transition zone between the 'seismogenic zone', where earthquakes occur, and adjacent areas that creep stably. A review paper was therefore written (Fagereng et al., 2018, Geological Society of America Special Papers) to summarise the processes that occur at the up- and downdip limits of the seismogenic zones in subduction zones, where most slow earthquakes have been observed to date. The broader geological and geodetic observational data on strain rates in Earth’s crust were also reviewed (Fagereng and Biggs, 2019, Journal of Structural Geology), to assess the range of speed at which the Earth deforms. These two reviews serve as a framework for the project by gathering the current state of the art, as well as new interpretations, of the rock record of deformation and its relation to geophysical data sets.

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.
A picture is emerging where locations of slow earthquakes are characterized by low effective stresses and a mixed brittle-ductile deformation style sensitive to small changes in local variables such as fluid pressure, strain rate, or driving stress. We are characterizing these variables for a range of tectonic settings, from direct field observations of composition and geometry, and geological inferences of active deformation mechanisms and stress/strain conditions. We have a numerical tool to test geological hypotheses in a state of the art geodynamic model that can describe the deformation of a polyphase shear zone (Beall et al., 2019, Geophysical Research Letters). In the next project period, we expect to use this model to further test behaviours of geologically constrained shear zone composition, geometry, and rheology, and also have plans to test hypotheses with laboratory deformation experiments.