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

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.

This work aims to constrain fundamental aspects of the earthquake source and determine how this might affect seismic hazard assessment. These aims have the potential to inform and benefit a wide range of end users. Potential beneficiaries can be divided into several categories:

1. Policy makers that make use of probabalisitic seismic hazard assessment to inform planning regulations. There are two aspects to this; first hazard maps are based on the knowledge of the location of seismic faults. Creeping faults have typically been viewed as less hazardous, but information on when they could potentially allow seismic rupture will allow a re-assessment of risk maps in seismically active areas. The second aspect relates to the near-field shaking associated with rupture through clay-rich faults. This work will investigate the effect of source characteristics on the radiated wave-field that will help to establish the magnitude of peak ground acceleration and ground shaking associated with a rupture along a clay-rich patch of a fault.

2. Industry. Induced seismicity associated with injection into the sub-surface is a key issue for shale gas exploitation, geothermal energy and carbon dioxide storage. Shale gas is a potentially important future component of the UK's energy needs. However there are significant issues regarding induced seismicity in the UK where the population density is high. Induced seismic events during fracking occur within clay-rich horizons and understanding processes leading to propagation and arrest of ruptures will help to reduce risk. The same issues are faced by geothermal exploitation, where enhanced geothermal projects actively fracture the rock mass, potentially leading to induced events, such as occurred during the Basel and St Gallen geothermal projects in Switzerland. Low-enthalpy geothermal energy is becoming a key potential future energy source for Europe, for example, the Swiss committing to 30% of their power from geothermal sources by 2050. Finally, carbon storage projects are essential to the short-term reduction of carbon output. Key to this process is ensuring the integrity of top seals and fault seals to storage sites during injection and storage. Co-seismic damage to seals from induced events is an area what requires investigation. In summary, enhancing our understanding of rupture propagation through clay-rich sequences will allow better planning of safe injection rates for shale gas, geothermal and carbon dioxide storage, likely magnitude of induced events, and long-term management of storage facilities.

3. General public. Large-scale earth processes such as earthquakes captures the public imagination. It feeds interest in science and encourages younger people into careers that are essential for the future economic and environmental development of the UK and beyond. Additionally, education of the general public about the science behind processes such as fracking are essential in order to gain public acceptance for energy initiatives vital to the future economic well-being of the UK.