RISK CDT - Journey to the centre of the earthquake; what affects earthquake source properties and the variability of ground-shaking?

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This is a project within the multi-disciplinary EPSRC and ESRC Centre for Doctoral Training (CDT) on Quantification and Management of Risk & Uncertainty in Complex Systems & Environments, within the Institute for Risk and Uncertainty. The studentship is granted for 4 years and includes, in the first year, a Master in Decision Making under Risk & Uncertainty. The project includes extensive collaboration with prime industry to build an optimal basis for employability.

Earthquakes have the potential to cause enormous human, structural and economic devastation. However, the risks posed by these environmental hazards are poorly understood, primarily due to their significant, and seemingly random, variability and associated uncertainties. The frequency and magnitude of the seismic waves produced by earthquakes are a product of the source – the fault zone that is slipping – and the path along which the seismic waves travel. Little is known about the seismic source, particularly how the characteristics of a fault zone affect the slip kinematics of an earthquake. In principal both the frictional properties of the fault plus the elastic properties of surrounding rock, which will have been modified by fracture damage, will contribute to the nature of the rupture properties. These properties, such as stress drop and rupture velocity, will dictate the source character of the earthquake (e.g., how energetic the rupture is) and the resultant radiated wavefield.

The properties of earthquake ruptures affect the nature and variability of seismic radiation such as the frequency and amplitude, properties that must be known for accurate seismic hazard assessment. The rupture characteristics are a product of the strength evolution of the fault (the sliding interface) and also of the elastic and inelastic deformation that occurs in the surrounding country rock. Where elasticity is low, the rupture velocity will slow, and this will be exacerbated by any inelastic deformation of the surrounding rock that will act as an energy sink for propagation.

The variability of earthquake ground shaking for given scenarios (magnitude, distance) is large. Factors of well over 2 (at one standard deviation) are typical in modern ground motion models. This variability drives large uncertainties in seismic hazard (measured in terms of probability of exceedance of a threshold motion) at the long return periods (10,000 years and more) used for design of safety critical infrastructure such as dams, long span bridges, and nuclear facilities.

In recent years, a significant reduction in seismic hazard uncertainty has been achieved through adopting partially non-ergodic approaches to ground motion model development. In non-ergodic approaches, site-to-site aleatory variability in ground motion models is isolated and transferred to an epistemic component – which can be reduced through geophysical site investigations (increased scientific knowledge of the target site). Nevertheless, a significant uncertainty remains in the earthquake source. The aim of this project is to improve our understanding of the earthquake source to move toward fully non-ergodic ground motion models: an aleatory component of variability can then be moved to the epistemic domain – with reduction possible through geological and geophysical investigations of potential earthquake sources.

This project will use a combination of laboratory experiments and numerical modelling to understand how the properties of the earthquake source are affected by the country rock surrounding a fault. Simulated spontaneous earthquakes will be produced in the laboratory where key properties of the system can be carefully controlled and measured. The elastic properties of the blocks surrounding the slipping fault will be varied to observe the resultant properties of the rupture (velocity and amplitude). Numerical modeling of the ruptures will be developed so that upscaling of the results can be made to predict the larger-scale behaviour of earthquake ruptures.

We will study spontaneous ruptures in the laboratory. Stick-slip instability in laboratory experiments is recognized as an analogue for earthquakes and the properties of ruptures, such as the rupture velocity, and even the stress field surrounding the rupture tip, can be recorded. Acoustic sensors placed around a laboratory triaxial sample have been used to record the passage of an earthquake tip and have documented super-shear ruptures, where the rupture velocity exceeds the local shear wave velocity. The use of strain gauge rosettes attached close to the sliding surface can record the 2D strain field and the associated stress field can be computed from the elasticity of the slider block material.