Earthquake energy budget and coseismic fault temperature from seismological observations

How do earthquakes happen? Understanding the nature of earthquakes is a key fundamental question in Geociences that holds many implications for society. Earthquakes are typically associated with a sudden release of energy that has slowly accumulated over hundreds to thousands of years, being strongly controlled by friction in faults buried several kilometers beneath our feet under quite extreme conditions. For example, the amount of heat produced in just a few seconds is such that it can dramatically change the nature of the fault zone near the sliding surface. Moreover, there is abundant evidence of substantial frictional weakening of faults (i.e., fault strength weakens with increasing slip or slip rate) during earthquakes. However, there are still many open questions related to earthquake source processes: How similar are earthquakes in different temperature-pressure conditions? What is the earthquake's energy budget, which controls the intensity of ground motions? What are the physical mechanisms responsible for fault weakening? Recent progress in seismological imaging methods, theoretical fracture mechanics and rupture dynamics simulations can help solve these questions. Huge volumes of freely available seismic and geodetic data from around the world now allow the routine calculation of earthquake models where earthquakes are typically described as single space-time points. Time is now ripe for systematically building robust, more detailed seismic models bearing information on earthquake's physics by using recently developed sophisticated modelling tools along with high-quality images of the 3-D Earth's interior structure enabled by high performance computing facilities. Moreover, it is now possible to model ruptures theoretically in detail using both analytical fracture mechanics calculations and numerical rupture dynamics simulations, and, for example, estimate the fault temperature during the rupture process, which is the most direct way to quantify friction. However, systematic quantitative links between these calculations and seismological observations are still lacking. This project addresses these issues through a coordinated effort involving seismology and rock mechanics aiming at estimating fault temperature rise during earthquakes from new macroscopic seismic source models. We will use advanced seismic source imaging methods to build a new set of robust kinematic, static and dynamic earthquake source parameters for a large selected set of global earthquakes (e.g., average fault length, width, rupture speed and time history, stress drop, radiated and fracture energy). These solutions will then be used as input parameters to estimate fault temperature using analytical and numerical rupture dynamics calculations. This will lead to an improved understanding of how local fault processes occurring at scales from few microns to tens of centimetres translate into macroscopic seismological properties, how energy is partitioned during earthquakes and which are the mechanisms responsible for fault weakening. Ultimately this project will shed new light on many basic questions in earthquake science such as the similarity of earthquakes in different P-T conditions and the potential geological record left by ruptures (e.g., melt). More broadly, this project will benefit hazard models and any studies relying on accurate earthquake source parameters such as studies in seismic tomography, active tectonics and microseismicity (e.g., associated with hydraulic fracturing).

The goal of this project is to substantially advance our fundamental understanding of how faults slip during global large magnitude earthquakes. New source models based on seismic and geodetic data will be constructed, which will be used together with rupture dynamics calculations to better constrain earthquake physics. Given the fundamental nature of this project, the main non-academic beneficiaries of this project will be:

(1) In the short-term: school children, teachers and the wider public. Earthquakes offer great potential to spark the interest of these users in Geosciences (and, more generally, in STEM subjects) and thus contribute to the UK's skillbase, notably in geophysics, an area with high industry demand and listed in the Home Office shortage occupation list (the high-level training of a PDRA in Geophysics within the project will also contribute to this).

(2) In the long-term: government agencies and private companies responsible for earthquake monitoring and for seismic hazard and risk assessment, as a key element for assessing seismic hazard is to accurately predict ground motions, which requires a good understanding of earthquake physics (see letter of support from our impact partner USGS). Moreover, the geophysics industry working on microseismicity may also benefit from this project as a clearer understanding of the relationships between macroscopic source parameters and the frictional behaviour of faults is useful for improved seismicity monitoring efforts in shale gas extraction, geological carbon dioxide sequestration, etc (see letter of support from our impact partner Pinnacle). Although this project will principally investigate global large-magnitude earthquakes, much of the learning achieved might be useful for smaller event regional studies.

A-level students and teachers will benefit from this project through the co-production of new learning materials, a short animated educational movie on the earthquake source processes investigated in the project, as well as from other activities (e.g., visits to UCL, notably to the rock mechanics laboratory, and focus group discussions). Beyond subject knowledge, they will also benefit by engaging with the scientific process, understanding how new data are gathered and interpreted, and how new ideas are proposed. More generally, the wider public will benefit from the project's participation in high-visibility popular science exhibits and shows, science cafes and debates.

Government agencies and private sector geophysics companies working on seismic hazard/risk and microseismicity will benefit from this project by actively following the project's development (see letters of support from our impact partners) and by participating in a final project workshop where the main scientific results will be presented and discussed, and where a reflection on the project's long-term impact for specific users will be undertaken. The conclusions of the workshop will be published in a journal accessible to the wide academic and non-academic community, such as the RAS journal 'Astronomy and Geophysics', the AGU Eos newspaper and/or journals of geophysics societies with wide visibility in industry (e.g., Society of Exploration Geophysicists (SEG) - The Leading Edge (TLE) or European Association of Geoscientists and Engineers (EAGE) - First Break (FB)).