How do earthquake ruptures propagate through clay-rich fault zones?

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On large tectonic faults movement can occur stably, producing fault creep, or by unstable slip where earthquakes occur. Fault creep has typically been associated with clay-rich fault gouges that accommodate slip across a fault. They have typically been thought to pose less seismic hazard than locked faults where earthquakes occur periodically. Recent studies have demonstrated that earthquakes can propagate through creeping sections of faults, with devastating consequences. This project will combine leading experimentalists and modellers to investigate under what conditions earthquake ruptures can propagate through 'creeping' faults. The work will utilize a unique new high-pressure rotary shear deformation apparatus to replicate and understand the physical response as an earthquake rupture passes and rupture models predict the large-scale response. Results from experiments and modelling will be used to develop new seismic hazard assessment for creeping faults, both in terms of how their potential for seismicity is viewed, and how the nature of a rupture would affect the radiated wavefield - which influences how destructive an earthquake will be.We know from slow-slip laboratory experiments that earthquakes are not expected to nucleate on clay-rich faults as they strengthen as slip starts to accelerate, thereby arresting any potential rupture. This is nicely illustrated by a lack of seismicity seen in the accretionary forearc clay-rich parts of subduction zones. However, recent events have suggested that large earthquake rupture, nucleated on a less clay-rich region of a fault zone can punch through clay-rich regions, and even greatly enhance slip, such as was seen in the Mw9.0 Tohoku-Oki earthquake in 2011, where the largest co-seismic slip ever recorded (~50m) occurred in the clay-rich accretionary forearc that produced a large offset of the seafloor leading to a devastating tsunami. Other examples of where earthquakes have propagated through creeping faults are The 1999 Mw7.6 Chi Chi earthquake in Taiwan, there the properties of the rupture were clearly modified (increase in the rupture velocity and slip speed), and the 1944 Mw7.4 North Anatolian Fault earthquake.This research will use unique laboratory equipment recently developed at Liverpool that can replicate the conditions during earthquakes and allow us to measure how the frictional strength of the fault develops, which will dictate whether or not an earthquake rupture will propagate or arrest in clay-rich faults. It will allow the approach of an earthquake ruptures to be simulated under fully confined conditions approximating to 15km depth. Experiments will be conducted where the strength and properties of the experimental fault zone is monitored under different imposed displacements and displacement rates. The peak acceleration and stress reduction will mimic earthquakes of different size and investigate the energy barrier required to promote unstable slip. In a different type of experiment, a stick-slip instability (laboratory earthquake) will be monitored as it propagates into clay-rich region of a laboratory fault zone. Results constraining the physical response of earthquake slip from the laboratory will be added into large-scale models to aid our understanding of (a) rupture propagation, which will dictate if a small earthquake will grow into large event and (b) what the properties will be, such as how fast it will travel and how much stress will be released, for use in probabilistic seismic hazard assessment.