Earthquake fracture damage and feedbacks in the seismic cycle: a multidisciplinary study

Lead Research Organisation: University College London
Department Name: Earth Sciences

Abstract

Earthquakes are a very destructive and yet unpredictable manifestations of the Earth internal dynamics. They correspond to a rapid motion along geological faults, generating seismic waves as they propagate along the fault strands. The propagation of ruptures along faults induces dramatic stresses and deformation of the rocks hosting the fault, which become increasingly damaged (i.e, degraded) as multiple earthquakes occur along a fault over geological timescales. In turn, this damage of the off-fault rocks has an impact on the dynamic rupture processes: damage generation and earthquake rupture are coupled phenomena. A better knowledge of the dynamic damage processes can thus truly improve our understanding of the physics of earthquakes, and hence help to better predict strong motion and earthquake hazard.

It is the goal of this proposal to investigate how dynamic ruptures can induce damage in the surrounding rocks, the specific characteristics of this damage, how it affects the rocks properties, and finally to build an earthquake rupture model which includes the couplings between rupture propagation and off-fault damage.

The proposed approach is multidisciplinary, and includes: (1) field characterisation of naturally damaged samples around the San Jacinto fault in South California; (2) laboratory rock deformation experiments at very high deformation rates; and (3) the development of a numerical modeling approach, tested against experimental data, which will allow simulations of fully coupled earthquake rupture processes to be performed.

By far the most challenging aspect of the study of dynamic damage is to perform rock deformation experiments at deformation rates and pressure conditions relevant to earthquake ruptures. To achieve this, our proposal includes the design and construction of a novel deformation apparatus which will allow high speed compression and decompression tests to be performed on rock samples. This apparatus will be unique in Europe and will cover an unprecedented range of deformation conditions.

Planned Impact

Who could potentially benefit?
In addition to the academic beneficiaries outlined above, we see a potential for a wide range of beneficiaries. This includes governmental agencies tasked with assessing risks from earthquakes and related phenomena (e.g. British Geological Survey, United States Geological Survey and equivalent bodies in other countries). High strain rate damage in rocks is also an active field of research in mining engineering and Defence research, since it is involved in underground explosions monitoring and ballistic impacts. As outlined in the Pathways to Impact, there is also potential scope for outreach activities to the public. The University College London has an outstanding reputation for world-class science, and we will exploit our location in London to engage with public wherever possible. We intend to interact with local science museums, such at the Natural History or Science Museum in London, in order to create demonstrations of science related to this project using equipment from the laboratory.

How might they benefit?
Scientifically, the importance of the proposed research for the worldwide state-of-the-art knowledge in natural earthquake dynamics as well as induced fracture processes will enforce long-term multidisciplinary collaborations in applied areas of application. Scientifically, the results of the proposed research are expected to make a significant contribution to our understanding of damage feedback mechanisms of natural fault zones during the seismic cycle and will be of great interest to the European and worldwide scientific community. Our results will provide important inputs for rupture directivity and ground shaking models, and significantly enhance the predictions of seismic hazards, as quantification of these damage feedbacks will contribute to the risk assessment and forecast of seismic hazards, or even provide avenues to mitigate the effects of earthquakes in the long term. Such perspectives have obvious benefits to communities that live in the vicinity of large active faults, as well as financial implications for insurance companies and those with business interests in the region.

As a more long-term view, we envisage that our work on damage within the seismic cycle may add some fundamental results that should be added to the core academic teaching of both structural geology and seismology.
 
Description This is a 4 year grant currently in progress in the final year. After designing, developing and building a cutting edge bespoke apparatus to allow the reproduction of earthquake damage found around natural faults we are now finalising and publishing results. This includes high speed impacts data in experiments that recreates the high stresses and strain rates that rocks surrounding faults are subjected to during an earthquake. We have published work showing the importance of multiple earthquake loadings showing direct feedbacks on subsequent earthquake ruptures, indicating that cumulative fault damage is fundamentally important when trying to understand earthquake rupture dynamics and their changes with time. Field investigations have been complete active faults known to have slipped seismically and have allowed the comparison between experimentally and naturally deformed rocks, and will constrain the fate of damaged rocks across geological time and space scales. This work is currently being prepared for publication, and focusses on the cumulative fault damage that leads to complex fault heterogeneity and eventually pulverisation.

Large amounts of energy are released during earthquakes, part of which corresponds to seismic waves which can cause devastating ground shaking, and part of which is dissipated either into frictional heat on the fault (on-fault), or into creating new fracture damage around the fault in zones up to tens of meters wide (off-fault). One of the most longstanding issues in earthquake physics is quantification of the amount of energy dissipated by these vast volumes of damaged rock (discussions started in 1970s with Ida and Andrews' first earthquake models). This issue is a crucial step in understanding how much energy is available for ground shaking, and how fast earthquakes and in which direction earthquakes propagate - thus crucial towards improving earthquake hazard assessments. Solving this key problem has proven to be very challenging using seismological or field data alone. Current estimates of the contribution of off-fault dissipated energy span a very wide range (almost 2 orders of magnitude) and remain strongly debated.

In our most recent results in the laboratory, we tackle this problem by conducting laboratory rock fracture experiments using a novel 3D ultrasonic tomography method to retrieve, for the first time, an image of the evolving seismic structure around a growing rupture. We find very strong (-25%) localized drops in seismic velocity around the fault. Such strong variations correspond to off-fault strain energy variations, which we quantify to be around 3% of the total energy budget and of the order of 10% of the total fracture energy. These results provide an unprecedented and unequivocal constraint on off-fault energy dissipation, and show that the amount of off-fault dissipated energy is low - most earthquake energy is thus spent as ground shaking and friction. Nonetheless, we suggest that the minor amount of off-fault energy (for a M5 earthquake still equivalent to the energy of ~10 Hiroshima Atomic bombs) is crucial for the physics of earthquakes: Off-fault energy is mostly dissipated at the earliest stages of rupture, thereby changing the rock material and thus influencing the later stages of rupture and slip.

Our work solves a longstanding debate regarding the energy budget of ruptures. Our unique benchmark will have an impact on the understanding of earthquake physics, and for realistic earthquake simulations that predict ground motions. The spatio-temporal structure of rock properties revealed by our laboratory method bridges the gap between grain scale deformation processes and kilometer scale fault zone observations. This opens the way to quantitatively upscale well-established physics of fracture to complex geological structures. Our new experimental method and processing technique (which we make available as a free software) have a great potential to solve key problems in rock physics and geodynamics (e.g., fluid flow or induced fracturing), but is also directly applicable to other fields such as physics and engineering where problems such as fracture growth in complex materials (e.g., ceramics, granular or composite materials) can now be tackled.
Exploitation Route This study has created, for the first time, an important bridge between seismological source theory, rock and fracture mechanics, and the geological structure of natural fault zones which will allow improved predictions of earthquake rupture along fault, leading to an improved understanding of near-field ground motion and seismic hazard predictions.
Sectors Education,Financial Services, and Management Consultancy,Other