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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

Field geology

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.

Microscopy

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.

Numerical models

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).

Laboratory experiments

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.

Findings

Natural faults

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:

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.

Numerical models

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.

Laboratory experiments

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.

Synthesis

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.

Publications

The project team

Lead

Ake Fagereng

Professor Ake Fagereng

Professor in Structural Geology

Team

Support

This research was made possible through the support of the following organisations: