Exploring the geological signature of Slow Earthquakes through legacy experiments and field analysis
Lead Research Organisation:
University of Leeds
Department Name: School of Earth and Environment
Abstract
Earth's outer layer is made up of plates that slide past each other, move into one another or pull apart. A large percentage of the World's population is living close to such boundaries. The last two decades of remote sensing has shown that our view of how plates deform along these boundaries needs to be revised. We now recognise Slow Earthquakes (SEs), which slip more slowly than regular earthquakes, but significantly faster than normal plate movements. Intriguingly, these SEs seem to be triggered by very low forces equivalent to the weight of a filled bathtub and their observed characteristics cannot, at present, be reconciled with our current understanding of how rocks deform. Plate-scale deformation is governed by processes active at the microscopic scale, however the nature of the processes enabling observed SE slip rates remains enigmatic. Because SEs can influence the likelihood of damaging earthquakes, they need to be understood if we are to better assess hazards and risks associated with plate boundary motions.
The current approach to this challenge relies heavily on data from sensitive instruments detecting remotely very subtle ground movement. Because the SEs are occurring in rocks that are many kilometres beneath our feet, there is no direct way to test the theories that have been put forward to explain SEs. This reliance on inferences rather than direct evidence poses a major barrier to our understanding of SEs.
Luckily, the Earth presents us with another set of independent observations. SEs should leave an imprint in the geological record; they accommodate most of the deformation over 100-km-wide regions in many subduction zones, where one plate dives into the Earth's interior beneath another. Thus, in principle, in-depth analyses of exhumed rocks found in regions that were once subject to SEs should hold the signature of the processes responsible. But to date there is no consensus on these signatures.
We hypothesise that SEs have remained invisible in the geological record because we currently do not know what to look for. In our exploratory project, we use
(1) geophysics of SEs to help us to narrow down the processes likely to produce them
(2) laboratory experiments that can reproduce SE behaviour - the resulting samples can show and teach us what rocks would look like following SEs.
(3) exhumed SE rocks from suitable field locations, allowing us to test if our theory is correct (namely that the identified processes are indeed responsible for SEs)
Geophysical data tells us that the process responsible for SE must enable rocks to flow like a dough and to suddenly crack and move fast. Interestingly, decades of laboratory deformation experiments have shown exactly this behaviour. Such "Slow Fractures" closely resemble SE behaviour and occur when deforming a fluid-rich rock. However, such experiments have been regarded as "failed" as they could not be used to assess the slow flow behaviour of rocks interpreted to govern plate deformation. We will test our hypothesis by utilising these so far largely neglected experiments. We will perform analyses down to the nanometer scale using only-now available analytical techniques. Equipped with our newly trained eye to recognise the predicted SE process, we will carry out field work in an exhumed area currently presumed to hold Slow Earthquake fingerprints and perform analyses on natural samples. If our hypothesis is right, we will for the first time be able to use the geological record to help constrain processes that have previously only been visible to geophysical data. Close discussion with the team's slow slip, remote sensing expert will ensure inferred geological processes can explain observed SE remote sensing data.
If we are correct, our "proof of concept" project will lay the foundation for fundamental understanding of SEs and allow the team to build-up the knowledge-base to propose a well-founded, larger SE focused research project.
The current approach to this challenge relies heavily on data from sensitive instruments detecting remotely very subtle ground movement. Because the SEs are occurring in rocks that are many kilometres beneath our feet, there is no direct way to test the theories that have been put forward to explain SEs. This reliance on inferences rather than direct evidence poses a major barrier to our understanding of SEs.
Luckily, the Earth presents us with another set of independent observations. SEs should leave an imprint in the geological record; they accommodate most of the deformation over 100-km-wide regions in many subduction zones, where one plate dives into the Earth's interior beneath another. Thus, in principle, in-depth analyses of exhumed rocks found in regions that were once subject to SEs should hold the signature of the processes responsible. But to date there is no consensus on these signatures.
We hypothesise that SEs have remained invisible in the geological record because we currently do not know what to look for. In our exploratory project, we use
(1) geophysics of SEs to help us to narrow down the processes likely to produce them
(2) laboratory experiments that can reproduce SE behaviour - the resulting samples can show and teach us what rocks would look like following SEs.
(3) exhumed SE rocks from suitable field locations, allowing us to test if our theory is correct (namely that the identified processes are indeed responsible for SEs)
Geophysical data tells us that the process responsible for SE must enable rocks to flow like a dough and to suddenly crack and move fast. Interestingly, decades of laboratory deformation experiments have shown exactly this behaviour. Such "Slow Fractures" closely resemble SE behaviour and occur when deforming a fluid-rich rock. However, such experiments have been regarded as "failed" as they could not be used to assess the slow flow behaviour of rocks interpreted to govern plate deformation. We will test our hypothesis by utilising these so far largely neglected experiments. We will perform analyses down to the nanometer scale using only-now available analytical techniques. Equipped with our newly trained eye to recognise the predicted SE process, we will carry out field work in an exhumed area currently presumed to hold Slow Earthquake fingerprints and perform analyses on natural samples. If our hypothesis is right, we will for the first time be able to use the geological record to help constrain processes that have previously only been visible to geophysical data. Close discussion with the team's slow slip, remote sensing expert will ensure inferred geological processes can explain observed SE remote sensing data.
If we are correct, our "proof of concept" project will lay the foundation for fundamental understanding of SEs and allow the team to build-up the knowledge-base to propose a well-founded, larger SE focused research project.