Mechanics of slow earthquakes (MICA)
The MICA project uses the unique natural laboratory of exhumed and active faults to build numerical models constrained by observed fault geometry and microstructurally-defined deformation mechanisms. The primary aim of this work is to determine the factors that control how fast faults can slip – in other words, what controls whether faults slip slowly or accelerate to generate earthquakes.
In this project, funded by the European Research Council’s Horizon 2020 programme, we assess the varied behaviour of faults that accommodate tectonic deformation in the Earth's crust. Major tectonic faults have, until recently, been thought to accommodate displacement by either continuous creep or episodic, damaging earthquakes. High-resolution geophysical networks have now detected ‘slow earthquakes’. Slow earthquakes are transient modes of displacement that are faster than plate boundary creep but slower than earthquakes. The physical processes that control fault slip rate are poorly understood, and 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?
Activities and methods
Observations made in the field form the backbone of the project, and areas of interest include both active and ancient fault zones. Close to home, we study the exceptional ancient plate boundary preserved in the GeoMôn UNESCO Global Geopark on Anglesey in north Wales. Elsewhere in the world we have studied rocks from ancient subduction zones in Japan and Namibia, active subduction offshore in New Zealand, ancient transforms from continental crust in Namibia and oceanic crust preserved on Cyprus.
We use the microscopy and imaging and electron microbeam facilities in the Cardiff School of Earth and Environmental Sciences, to image and analyse microscale textures in deformed rocks. From such analysis, we infer the stresses and strain rates rocks have deformed at, as in [this example from the Kuckaus Mylonite, Namibia.
Led by post-doctoral fellows Adam Beall and Lucy Lu we have used the numerical codes Underworld and MOPLA (MultiOrder Power-Law Approach) to quantify effects of various variables on fault behaviour. For example, we have developed methods for analysing the strength of two-phase shear zones. This work is supported by ARCCA,(Advanced Research Computing at Cardiff)
Collaborating with rock deformation labs at the Universities of Utrecht and Bremen, we use experiments to test hypotheses developed in the field, and obtain parameters for inclusion in models.
A picture is emerging where locations likely to host slow earthquakes are weak enough to deform under relatively small force, and the style of deformation (slow or fast) is sensitive to small changes in local variables such as fluid pressure, strain rate, or driving stress. We see such conditions at a range of depths and tectonic settings, and a strength of this project is having looked at a large variety of locations. Examples of geological evidence include:
- Intense vein networks formed in a narrow temperature range, corresponding to where the mineral chlorite dehydrates, in nominally viscous shear zones in the Damara Belt, Namibia.
- Led by our Japanese colleagues, we demonstrated that vein systems preserved on the island of Kyushu are consistent with forming incrementally at low stress, and can be considered a geological example of episodic tectonic tremor.
- Quartz grain size, sensitive to stress, implies very low tectonic driving stresses below the earthquake-generating zone, in exhumed subduction and continental strike-slip zones.
- At very shallow depths where stress is low by definition, subduction-related faults can switch between distributed and localized deformation and carbonates can host both viscous and brittle deformation.
- Oceanic transforms have variable strength and slip speed controlled by degree and type of serpentinisation.
With colleagues, we have also considered the published geological evidence from well-studied faults around the world, comparing this to the geophysical observations of slow earthquakes. We came to a range of common features, published in Nature Reviews.
Two-component viscous shear zones with more than 50% strong material will spontaneously generate force chains. We therefore suggest that local increases in stress build up and break these chains generating temporary increases in flow speed.
At a larger scale, plate boundary shear stresses may also be affected by the plate-scale dynamics leading to some margins being more capable of generating larger earthquakes.
We know from observations in natural faults, that small amounts of hydration can promote weaker, more ductile behaviour.
In the lab at MARUM (Zentrum für Marine Umweltwissenschaften), University of Bremen, we tested this concept at low pressure and temperature. We found that increasing chlorite-content, which simulates increased hydration, led to weaker, stably-sliding faulting. Additional experimental work is ongoing in collaboration with Utrecht University'.
We reviewed natural and numerical evidence for how heterogeneity affects deformation. We find that the most basic control, that can explain a whole range of behaviours, is the combined effect of firstly, the strength contrast between coexisting rocks and minerals, and secondly how far the driving stress is from the stress required for the strongest material to break.
- Cox, S. , Fagereng, Å. and MacLeod, C. J. 2021. Shear zone development in serpentinised mantle: Implications for the strength of oceanic transform faults. Journal of Geophysical Research: Solid Earth 126 (5) e2020JB020763. (10.1029/2020JB020763)
- Kirkpatrick, J. D. , Fagereng, Å. and Shelly, D. R. 2021. Geological constraints on the mechanisms of slow earthquakes. Nature Reviews Earth & Environment 2 , pp.285-301. (10.1038/s43017-021-00148-w)
- Fagereng, Å. and Beall, A. 2021. Is complex fault zone behaviour a reflection of rheological heterogeneity?. Philosophical Transactions A: Mathematical, Physical and Engineering Sciences 379 (2193) 20190421. (10.1098/rsta.2019.0421)
- Beall, A. et al. 2021. Influence of subduction zone dynamics on interface shear stress and potential relationship with seismogenic behavior. Geochemistry, Geophysics, Geosystems 22 (2) e2020GC009267. (10.1029/2020GC009267)
- Leah, H. et al. 2020. Mixed brittle and viscous strain localisation in pelagic sediments seaward of the Hikurangi margin, New Zealand. Tectonics 39 (8) e2019TC005965. (10.1029/2019TC005965)
- Stenvall, C. A. et al. 2020. Sources and effects of fluids in continental retrograde shear zones: Insights from the Kuckaus Mylonite Zone, Namibia. Geofluids 2020 3023268. (10.1155/2020/3023268)
- Tulley, C. J. , Fagereng, Å. and Ujiie, K. 2020. Hydrous oceanic crust hosts megathrust creep at low shear stresses. Science Advances 6 (22) eaba1529. (10.1126/sciadv.aba1529)
- Fagereng, Å. and Ikari, M. J. 2020. Low‐temperature frictional characteristics of chlorite‐epidote‐amphibole assemblages: implications for strength and seismic style of retrograde fault zones. Journal of Geophysical Research. Solid Earth 125 (4) e2020JB019487. (10.1029/2020JB019487)
- Barnes, P. M. et al., 2020. Slow slip source characterized by lithological and geometric heterogeneity. Science Advances 6 (13) eaay3314. (10.1126/sciadv.aay3314)
- Beall, A. , Fagereng, A. and Ellis, S. 2019. Fracture and weakening of jammed subduction shear zones, leading to the generation of slow slip events. Geochemistry Geophysics Geosystems 20 (11), pp.4869-4884. (10.1029/2019GC008481)
- Fagereng, Å. and Biggs, J. 2019. New perspectives on 'geological strain rates' calculated from both naturally deformed and actively deforming rocks. Journal of Structural Geology 125 , pp.100-110. (10.1016/j.jsg.2018.10.004)
- Fagereng, A. et al. 2019. Mixed deformation styles on a shallow subduction thrust, Hikurangi margin, New Zealand. Geology 47 (9), pp.872-876. (10.1130/G46367.1)
- Stenvall, C. , Fagereng, Å. and Diener, J. 2019. Weaker than weakest: on the strength of shear zones. Geophysical Research Letters 46 (13), pp.7404-7413. (10.1029/2019GL083388)
- Beall, A. , Fagereng, A. and Ellis, S. 2019. Strength of strained two-phase mixtures: Application to rapid creep and stress amplification in subduction zone mélange. Geophysical Research Letters 46 (1), pp.169-178. (10.1029/2018GL081252)
- Fagereng, A. and MacLeod, C. 2019. On seismicity and structural style of oceanic transform faults: A field geological perspective from the Troodos Ophiolite, Cyprus. In: Duarte, J. S. ed. Transform Plate Boundaries and Fracture Zones. Elsevier Books. , pp.437-459. (10.1016/B978-0-12-812064-4.00018-9)
- Fagereng, A. et al. 2018. Fluid-related deformation processes at the up- and downdip limits of the subduction thrust seismogenic zone: What do the rocks tell us?. In: Byrne, T. et al., Geology and Tectonics of Subduction Zones: A Tribute to Gaku Kimura. Vol. 534, GSA Special Papers Geological Society of America(10.1130/2018.2534(12))
- Ujiie, K. et al., 2018. An explanation of episodic tremor and slow slip constrained by crack-seal veins and viscous shear in subduction mélange. Geophysical Research Letters 45 (11), pp.5371-5379. (10.1029/2018GL078374)
- Fagereng, A. et al. 2018. Quartz vein formation by local dehydration embrittlement along the deep, tremorgenic subduction thrust interface. Geology 46 (1), pp.67-70. (10.1130/G39649.1)
The project team
This research was made possible through the support of the following organisations: