Seismic hazard assessment and understanding of continental deformation are hindered by unexplained slip-rate fluctuations on faults, associated with (a) temporal clusters of damaging earthquakes lasting 100s to 1000s of years, and (b) longer-term fault quiescence lasting tens to hundreds of millennia. We propose a new unified hypothesis explaining both (a) and (b), involving stress interactions between fault/shear-zones and neighbouring fault/shear-zones; however key data to test this are lacking. We propose measurements and modelling to test our hypothesis, which have the potential to quantify the processes that control continental faulting and fluctuations in the rates of expected earthquake occurrence, with high societal impact. Our aspiration is that cities and critical facilities worldwide will gain additional protection from seismic hazard through use of the calculations we pioneer herein. The background is that slip-rate fluctuations hinder understanding because they introduce uncertainty about whether specific faults are active or not. For example, a review in Japan of earthquake risk to critical facilities, such as the Tsuruga nuclear power plant (NPP), revealed a geological fault under a nuclear reactor (Chapman et al. 2014). The question that arose was whether the fault was active or not. Japan's Nuclear Regulatory Authority (NRA) has guidelines defining fault activity, and considered the fault under the reactor to be active, evidenced by faulting in sediments <~125,000 years in age. The Japan Atomic Energy Power Company (JPAC) disagreed, following study by an independent team of geoscientists. In 2014, the Tsuruga NPP remained closed due to ongoing debate between the NRA and JPAC, with similar debates ongoing for other NPPs. We suggest that defining fault activity as simply "active" or "inactive" is unsatisfactory because it is debatable even amongst experts. In fact a fault that has not slipped in many millennia may, in reality, not be inactive, but instead may simply have a low slip-rate, with the capability to host a damaging earthquake after a long recurrence interval. Our breakthrough is we think slip-rate fluctuations over both timescales (a and b) are a continuum, sharing a common cause involving interaction between fault/shear-zones. For the first time, we provide calculations that describe this interaction, quantifying slip-rate fluctuations and seismic hazard in terms of probabilities. We show that slip during an earthquake cluster on a brittle fault in the upper crust occurs in tandem with high strain-rate on the viscous shear-zone underlying the fault. This deformation of the crust produces changes in differential stress on neighbouring fault/shear-zones. Viscous strain-rate is known to be proportional to differential stress, so, given data on slip-rate fluctuations one can calculate changes in differential stress, and then calculate implied changes to viscous strain-rates on receiver shear zones and slip-rates on their overlying brittle faults. We provide a quantified example covering several millennia, but lack data allowing a test over tens to hundreds of millennia. If we can verify our hypothesis over both timescales, through successful replication of measurements via modelling, we will have identified and quantified a hitherto unknown fundamental geological process. We will study the Athens region, Greece, where a special set of geological attributes allows us to measure and model slip-rate fluctuation over both time scales (a and b), the key data combination never achieved to date. We know of no other quantified explanation that links slip-rate fluctuations over the two timescales; the significance and impact of accomplishing this is that it has the potential to change the way we mitigate hazard for cities and critical facilities. Chapman et al. 2014, Active faults and nuclear power plants, EOS, 95, 4

This project will investigate the behaviour of monolithic, adobe structures in low-gravity conditions, concluding to an optimal structural design framework for supporting future space exploration. Such structures will act as shielding to protect critical assets (such as robots, fuel tanks and power stations) and future inflatable structures (e.g. living quarters) from extreme conditions (radiation, sandstorms, temperature fluctuations) in an extraterrestrial environment. Linear and nonlinear numerical structural modelling and parametric static and dynamic analyses will identify the optimal design approach for utilising indigenous materials to produce such structures to minimise the weight burden on future manned explorations. The analytical work will be validated by experimental centrifuge tests which can simulate low gravity conditions at prototype scale. This will be the first systematic approach towards sustainable extraterrestrial structural design using methods and concepts developed for Earth. Until now, there have been isolated studies on conceptualising extraterrestrial structures given the challenges that needed to be addressed in such extreme environments, but there is not a systematic approach on how to realise these structures. Given the recent and ongoing research on the mechanical properties of regolith simulants, the proposed In-situ Resource Utilisation (ISRU) framework and the advances in 3D printing for extraterrestrial construction, it is timely to combine these fields with structural design strategies developed for Earth in order to identify optimal structural forms for use in low-gravity conditions, subject to extraterrestrial dynamic environmental actions. The first step for achieving this is the static approach and to identify from a wide class of monolithic vaults which is the optimal for long-span structures in low-gravity environments. The next step would be to identify engineering demand parameters from extraterrestrial natural hazards related to Lunar and Martian strong ground motions (shallow and deep moonquakes, marsquakes if available from the InSight mission and meteorite-impact generated ground motions). Subsequently, numerical models simulating different structural dynamic configurations (including soil-structure interaction, rocking and seismic isolation) will be implemented to conduct extensive linear and nonlinear dynamic analyses using the identified engineering demand parameters. This will result in the assessment of the dynamic performance of each different model and thus to the best structural option. However, a critical part is to validate the numerical models used in this project by centrifuge tests under the same extraterrestrial excitations. Nevertheless, it is out of the scope of this project to investigate the properties of regolith-based structural material since regolith simulants will be used for the experimental part of the project. The main benefit of this project is the establishment of a rigorous structural design framework regarding extraterrestrial structures and the identification and categorisation of the extraterrestrial strong ground motions as a first step for the quantification of the associated hazard. Aside from the aforementioned objectives related to space exploration and multi-planet colonisation, the potential applications of the results from this project can be: (a) the calibration and development of 3D-printing techniques incorporating regolith as a structural material; (b) the sustainable residential development of low- and middle-income countries using indigenous materials and (c) the optimal seismic design of submarine structures (arches in a global compression state under buoyancy/low gravity) that can prove useful for deep-ocean exploration and mining.

The breaking apart of a continent to form extended continental margins and ultimately ocean basins is a process that can last for 10s of millions of years. The start of this process of rifting is thought to contribute significantly to the structure and sedimentary layering of the continental margins that have formed by its end. Often the details of how rifting initiates and develops in the first few million years are lost in the complexities of deformation and thick sediment layers beneath the continent's edges. To understand the early phases, we have to study areas where rifting has only recently started, and the Gulf of Corinth, Greece, is a key example in its first few millions of years of history. Across the Gulf, the two sides of the rift are moving apart at up to 20 mm every year and this high rate of extension results in numerous earthquakes which historically have been very destructive. The rapid extension also results in a rapidly developing rift basin which is partially submerged beneath the sea and filling with sediments. Within the Gulf, a large volume of marine geophysical data has been collected, including detailed maps of the seabed, as well as seismic data that use sound sources to give cross-sections of material beneath the seabed. The seismic data allow us to directly image the accumulated sediment layers and to identify faults that offset the layers and create the basin. This project will integrate these data to make a very detailed interpretation of the sediment layers (and their likely age) and fault planes. Imaging and assigning ages to the layers, by comparing with models of climate and sea level change, allows us to determine how the basin has developed through time. The fault planes imaged by the data generate the extension and subsidence of the rift, and their history of activity controls how the basin develops. The results will be used to generate the first high resolution model of rift development over the initial few million years of a rift's history and will help to address some of the unanswered questions of how continents break apart. The model will be used by a range of scientists, including those trying to understand how tectonics, landscape morphology and climate all interact to cause sediments to move from one place to another: rift basins are one of the main sinks for sediments and we will calculate how the volume of sediment delivered to the Corinth basin has changed with time, as faults move and as climate changes. The majority of the world's petroleum resources are found in old rifts, but often details of how the rift developed and the detailed geometry of the rock units in which the oil is now found are masked by other geological processes and by shallower sediment layers. Understanding the early rift processes is important for determining where and what kind of sediments will be deposited in different parts of the basin with time. We will also analyse details of how individual faults grow and interact with other faults in the rift: this process affects where sediments enter a rift basin and is therefore also important for identifying petroleum reservoirs. The rift faults are responsible for the destructive earthquakes in central Greece, so this project's analysis of fault location and rate of slip will also help us to better understand the potential hazard, increasing the potential for reduction of associated risk. Ultimately, the project will be used to select sites for drilling and sampling the sediments of the rift zone, through the Integrated Ocean Drilling Program. These samples would provide: the actual age of sediment layers, and hence well resolved slip rates for each active fault and a test for the rift models generated here; and the types of sediments, that will tell us more about the regional climate of the last few millions of years and where sediments that typically form hydrocarbon reservoirs are located in this analogue for older rift systems like the North Sea.