Quake4D - building physics-based, geologically-rich models for investigating earthquake interaction and seismic hazard

Earthquakes pose one of the greatest natural threats to vast populations. In the last century, earthquakes have caused 2.3 million deaths (1 million in the last 30 years alone) and US$820 billion of financial losses. Earthquakes are generated by movement along lines of geological weakness called "active faults" which, in some places, can be observed on the Earth's surface. Unlike other natural hazards, advances in scientific understanding have not yet led to a reduction in fatalities from earthquakes. Predicting the timing, location and magnitude of individual earthquakes is likely impossible, but estimating the spatial distribution of earthquake hazard is manageable, and of great importance to the global population and the insurance economy. However, there are difficulties in calculating the earthquake hazard because we are currently reliant on present-day measurements of the rates of movement of faults and historical records of past damaging earthquakes. We cannot simply observe earthquakes for longer, therefore we must develop 'geologically richer' numerical simulations to build synthetic earthquake records and seismic hazard models to improve our understanding of the fundamental processes that control earthquakes.

This project will develop a new, geologically-rich, fully integrated physics-based approach to modelling all aspects of the earthquake cycle. The earthquake cycle is the cyclical nature of earthquakes occurring, with tectonic stress building up and then releasing in a series of earthquakes over time. The physical processes that control the earthquake cycle operate on different time-scales, from seconds during the earthquake to millennia between earthquakes recurring on the same fault. The shape and spacing of faults also affect how earthquakes are generated, but it is not always easy to see the true shape of faults at the Earth's surface.

There are three stages of the earthquake cycle that are currently modelled separately. These are; 1. the dynamic process of fault slip occurring over seconds to minutes during the earthquake, 2. the resulting deformation and stress transfer onto surrounding faults and 3. the evolution and accumulation of tectonic stress between earthquakes. Each of these three stages can be modelled individually and are used to speculate on different aspects of the earthquake cycle. However, because they are presently not integrated, the effects of each one on the others are poorly understood.

Several active and inactive systems of extensional faults will be studied. The seismically active central and southern Italian Apennines will be studied because there is a wealth of data available; the faults are well-exposed at the surface and there is a 700 years record of damaging earthquakes and therefore high seismic hazard. The inactive fault systems that will be studied are offshore Norway, Australia and New Zealand. These inactive systems are important to study because we can use seismic reflection (like echo-location of the ground under the sea bed) to image the faults, to see their 3D shape and study how that has evolved with time. The slip rate on these faults can be quantified by studying the age and offset across these faults. It's important to study a range of different systems to synthesise the different data sets available in these regions.

In summary, earthquake hazard forecasting is currently lagging behind forecasting of other natural hazards. By combining three different physics-based modelling approaches and testing the resulting model on two data-rich natural fault systems, this project will generate a truly physical and geological model of a fault system - this has not been attempted before. These models will output synthetic earthquake catalogues that can be compared to historical records (hindcasting), used to speculate on the future locations of earthquakes (forecasting) and used to inform and understand uncertainty in seismic hazard mode

There are 3 key areas of impact identified for this project.

1. Seismic Hazard Assessment - this is the main non-academic impact of the project. Current state-of-the-art seismic hazard assessment (SHA) is undertaken using the "Cornell-approach" that uses earthquake recurrence intervals from historical records and calculates the expected Peak Ground Acceleration. This approach is flawed because it assumes that the historical catalogue is long enough to accurately characterise the seismic hazard, which is unlikely to be true for any seismically active region in the world. This project will generate a novel approach to simulating synthetic earthquake catalogues, by developing a physics-based numerical model that can simulate earthquake processes at a range of temporal scales and incorporate high-quality geological data. The synthetic earthquake catalogue produced for Italy will be used to probe (1) variability in earthquake recurrence intervals, (2) variability in slip rates on faults, (3) fault connectivity and the potential for multi-fault ruptures. These three factors are all considered in SHA, but are rarely quantified accurately due to short historical earthquake records.

This project will place constraints on these three data sets used in SHA and enable the incorporation of these uncertainties into existing hazard models. Other earthquake processes such as temporal clustering and static stress transfer are not currently considered in SHA, this project will provide quantifiable constraints on these processes and therefore enable them to be included in SHA. Probabilistic Seismic Hazard models for the Apennine regions of central and southern Italy (population 26 million) will be produced using the most advanced probabilistic methods and the simulated data sets. These will be disseminated to Italian hazard and civil protection agencies.

2. Earthquake Dynamics - the project will generate a new code for simulating all stages of the earthquake cycle in a fully coupled manner. This code will be open source and will be available to other academics interested in modelling the earthquake cycle on geometrically complex faults. Earthquake clustering is a commonly recognised phenomenon in all types of tectonic settings, but the mechanisms are currently poorly understood. The synthetic earthquake simulation will be used to probe the drivers of clustering, and whether differential stress changes driven by nearby earthquakes play a role. The opposite of clustering is faults with long recurrence intervals that have not ruptured during historical records. Precursors to earthquakes occurring after long elapsed time will be investigated using the synthetic catalogue, with a focus on the evolution of the state of stress. The connectivity of faults in the sub-surface is important to quantify, because this aids understanding the potential for multi-fault earthquakes occurring, using seismic reflection data from inactive fault systems, qualitative and quantitative constraints on this will be generated that can be applied to other extensional systems. These findings will be disseminated in relevant journals and conference sessions. The new code developed will be made available via GitHub.

3. Rift basin evolution and hydrocarbon exploration - the project will utilise seismic reflection data collected by the hydrocarbons industry; these datasets are essential to use to characterise potential sources of hydrocarbons. The extracted data sets will be used to characterise the geometry and evolution of faults, which are useful to understand the evolution of basins and sedimentary depocentres over time. These findings will be disseminated in relevant journals and conference sessions.