Periodic Reporting for period 4 - MICA (Mechanics of slow earthquake phenomena: an Integrated perspective from the Composition, geometry, And rheology of plate boundary faults)
Reporting period: 2021-08-01 to 2023-06-30
Faults that accommodate tectonic deformation in the Earth's crust slip at speeds from mm/year, comparable to fingernail growth, to m/s, which generates earthquakes. Geophysical observations have detected events known as 'slow earthquakes’, which slip at rates intermediate between these end-members. This project is designed to determine the geological processes that lead to slow earthquakes.
The results may have societal impact by informing seismic hazard evaluations. It is, for example, unknown how slow and fast earthquakes are related. Critical questions 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?
The project has three main objectives:
(I) Create an observational model for fault zones that may host slow earthquakes, including constraints on fault zone thickness, internal geometry, and composition. This is done through studies of rocks from fault zones exposed on the Earth's surface, or accessible through ocean drilling.
(II) Determine the rheology (the grain-scale mechanism) that allows faults to slip at slow earthquake velocities. This objective uses field and microscale observations of natural faults, and microstructures developed in rock deformation experiments, to understand relationships between stress and strain rate.
(III) Develop numerical models of fault zones with realistic geometry and deformation mechanisms, to identify variables that control slip speed and allow slow earthquake generation.
The results may have societal impact by informing seismic hazard evaluations. It is, for example, unknown how slow and fast earthquakes are related. Critical questions 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?
The project has three main objectives:
(I) Create an observational model for fault zones that may host slow earthquakes, including constraints on fault zone thickness, internal geometry, and composition. This is done through studies of rocks from fault zones exposed on the Earth's surface, or accessible through ocean drilling.
(II) Determine the rheology (the grain-scale mechanism) that allows faults to slip at slow earthquake velocities. This objective uses field and microscale observations of natural faults, and microstructures developed in rock deformation experiments, to understand relationships between stress and strain rate.
(III) Develop numerical models of fault zones with realistic geometry and deformation mechanisms, to identify variables that control slip speed and allow slow earthquake generation.
From samples of ancient rocks on Kyushu, Japan, Tulley et al. (Sci. Advances, 2020) concluded that the subduction interface can flow viscously at low stress when temperatures exceed 300˚C. Furthermore, Tulley et al. (G-Cubed, 2022) found that transient, frictional slip occurred at temperatures where specific mineral dehydration reactions took place. Ujiie et al. (Geophys. Res. Lett., 2018) show that hat this transient slip can repeat at time intervals consistent with slow earthquakes. As such, this location is consistent with slow earthquakes being a consequence of local, fluid-driven weakening within a dominantly weak and viscous shear zone. Similarly, in the Kuiseb Schist, Namibia, we show that vein-filled fractures and viscous shear occurred at the same time, consistent with dehydration-triggered weakening by fluids at around 500˚C (Fagereng et al., Geology, 2018).
The active northern Hikurangi subduction margin, New Zealand, hosts some of the shallowest slow earthquakes recorded, and was target of scientific ocean drilling in International Ocean Discovery Program Expeditions 372 and 375. We found both brittle and ductile structures within a tens of metres thick fault zone, only ~ 300 m below the seafloor (Fagereng et al., Geology, 2019). We also studied the incoming seafloor sediments, and found that already before they are subducted, a mixture of brittle and ductile features have developed (Leah et al., Tectonics, 2020). The IODP expedition science team concluded that slow earthquakes in this location are an effect of geometrical and lithological heterogeneity (Barnes et al., Sci. Advances, 2020). We tested this idea using broadband seismic data, and concluded that roughness on the subducting plate is a key feature of this margin at least down to slow earthquake depths (Leah et al., Geology, 2022).
At Llanddwyn Island, Anglesey, UK, ancient oceanic crust records late Precambrian subduction. The shear zones here are dominated by basalt and volcanic sediments, and contain brittle and ductile structures developed simultaneously. In some carbonate rocks, crystal plastic flow was active despite relatively low temperatures (< 300˚C, Leah and Fagereng, Geophys. Res. Lett., 2022). In the other rocks, however, deformation was accommodated by competing frictional sliding and dissolution, transport, and precipitation of minerals - as in Japan, fluids promoted transient frictional sliding (Leah et al., J. Geophys. Res., 2022).
We show from the Kuckaus Mylonite, Namibia, that continental strike-slip faults can be very weak because of fine-grained mineral growth (Stenvall et al., Geophys. Res. Lett., 2019). This growth requires addition of small amounts of fluids from an external source (Stenvall et al., 2020; Geofluids). Oceanic transforms are thought to be weak based on their dominantly aseismic behaviour, and Cox et al. (J. Geophys. Res., 2021) show from the Troodos Ophiolite that this weakness may arise, at mantle depths, from alteration to serpentine. Serpentine may also host weak shear zones in the subduction zone mantle wedge (Tulley et al. Geophys. Res. Lett., 2022). At crustal depths, weakening occurs by alteration to frictionally weak and stably sliding chlorite, surrounding remnant, and potentially seismogenic, oceanic crust (Cox et al., Geophys. Res. Lett., 2021).
We generalise the slow earthquake source as a weak shear zone with relatively rigid inclusions. Interaction between these two components can generate force chains, where loss of continuous pathways of weak material leads to increases in stress. Beall et al. (Geophys. Res. Lett., 2019) show numerically that shear zones with more than 50 % strong material spontaneously generate force chains. Beall et al. (G-Cubed, 2019) suggest that if frictional yield is reached, then slip may accelerate within the rigid inclusions; depending on the length scale of the slipping zone, and the viscosity of the matrix, this could generate slow earthquakes. Fagereng and Beall (Phil. Trans. Royal Soc, 2021) make a further general case based on driving stress relative to frictional strength, and Lu et al. (in revision) use a micromechanical model to quantify when a shear zone scale, transient change from steady sliding to potentially fast, frictional slip can occur. Beall et al., (Geophys. Res. Lett., 2022) further apply a seismic cycle simulator to suggest that stress heterogeneity in a frictional-viscous shear zone may also control the size-distributions of earthquake magnitudes.
The active northern Hikurangi subduction margin, New Zealand, hosts some of the shallowest slow earthquakes recorded, and was target of scientific ocean drilling in International Ocean Discovery Program Expeditions 372 and 375. We found both brittle and ductile structures within a tens of metres thick fault zone, only ~ 300 m below the seafloor (Fagereng et al., Geology, 2019). We also studied the incoming seafloor sediments, and found that already before they are subducted, a mixture of brittle and ductile features have developed (Leah et al., Tectonics, 2020). The IODP expedition science team concluded that slow earthquakes in this location are an effect of geometrical and lithological heterogeneity (Barnes et al., Sci. Advances, 2020). We tested this idea using broadband seismic data, and concluded that roughness on the subducting plate is a key feature of this margin at least down to slow earthquake depths (Leah et al., Geology, 2022).
At Llanddwyn Island, Anglesey, UK, ancient oceanic crust records late Precambrian subduction. The shear zones here are dominated by basalt and volcanic sediments, and contain brittle and ductile structures developed simultaneously. In some carbonate rocks, crystal plastic flow was active despite relatively low temperatures (< 300˚C, Leah and Fagereng, Geophys. Res. Lett., 2022). In the other rocks, however, deformation was accommodated by competing frictional sliding and dissolution, transport, and precipitation of minerals - as in Japan, fluids promoted transient frictional sliding (Leah et al., J. Geophys. Res., 2022).
We show from the Kuckaus Mylonite, Namibia, that continental strike-slip faults can be very weak because of fine-grained mineral growth (Stenvall et al., Geophys. Res. Lett., 2019). This growth requires addition of small amounts of fluids from an external source (Stenvall et al., 2020; Geofluids). Oceanic transforms are thought to be weak based on their dominantly aseismic behaviour, and Cox et al. (J. Geophys. Res., 2021) show from the Troodos Ophiolite that this weakness may arise, at mantle depths, from alteration to serpentine. Serpentine may also host weak shear zones in the subduction zone mantle wedge (Tulley et al. Geophys. Res. Lett., 2022). At crustal depths, weakening occurs by alteration to frictionally weak and stably sliding chlorite, surrounding remnant, and potentially seismogenic, oceanic crust (Cox et al., Geophys. Res. Lett., 2021).
We generalise the slow earthquake source as a weak shear zone with relatively rigid inclusions. Interaction between these two components can generate force chains, where loss of continuous pathways of weak material leads to increases in stress. Beall et al. (Geophys. Res. Lett., 2019) show numerically that shear zones with more than 50 % strong material spontaneously generate force chains. Beall et al. (G-Cubed, 2019) suggest that if frictional yield is reached, then slip may accelerate within the rigid inclusions; depending on the length scale of the slipping zone, and the viscosity of the matrix, this could generate slow earthquakes. Fagereng and Beall (Phil. Trans. Royal Soc, 2021) make a further general case based on driving stress relative to frictional strength, and Lu et al. (in revision) use a micromechanical model to quantify when a shear zone scale, transient change from steady sliding to potentially fast, frictional slip can occur. Beall et al., (Geophys. Res. Lett., 2022) further apply a seismic cycle simulator to suggest that stress heterogeneity in a frictional-viscous shear zone may also control the size-distributions of earthquake magnitudes.
We defined characteristics of fault zones likely to have experienced slow earthquakes: (a) they are relatively thick (at least tens of metres, but up to kilometres) and not well described by a planar surface; (b) multiple, competing deformation mechanisms are active at the same time, so that the faults develop a range of internal structures, including both discrete fractures and structures indicating spatially distributed flow; and (c) fluids are present at least locally and/or intermittently.
We suggest a general mechanism for slow earthquakes where fast slip is initiated within a thick, overall weak, fault zone with variable internal strength. When such slip nucleates, it is because either (1) locally elevated stress lead to failure or (2) local weakening (for example by adding water) causes slip to start. This slip, however, is limited by the surrounding, weak fault zone, and therefore does not accelerate enough to become a damaging earthquake. This conclusion is very general, and not limited to a specific mechanism. This means that there are many scenarios that generate slow earthquakes, explaining how they are found at depths from the surface to over 60 km, and in most tectonic settings.
We suggest a general mechanism for slow earthquakes where fast slip is initiated within a thick, overall weak, fault zone with variable internal strength. When such slip nucleates, it is because either (1) locally elevated stress lead to failure or (2) local weakening (for example by adding water) causes slip to start. This slip, however, is limited by the surrounding, weak fault zone, and therefore does not accelerate enough to become a damaging earthquake. This conclusion is very general, and not limited to a specific mechanism. This means that there are many scenarios that generate slow earthquakes, explaining how they are found at depths from the surface to over 60 km, and in most tectonic settings.