THE GEOLOGICAL RECORD OF THE EARTHQUAKE CYCLE IN THE LOWER CRUST

Understanding the short- and long-term mechanical behaviour of the lower crust is of fundamental importance when trying to understand the earthquake cycle and related hazard along active fault zones. In some regions some 20% of intracontinental earthquakes of magnitude > 5 nucleates in the lower crust at depth of 30-40 km. For example, a significant proportion of seismicity in the Himalaya, as well as aftershocks associated with the destructive 2001 Bhuj earthquake in India, nucleated in the granulitic lower crust of the Indian shield. Earthquakes in the continental interiors are often devastating and, over the past century, have killed significantly more people than earthquakes that occurred at plate boundaries. Thus, a thorough understanding of the earthquake cycle in intracontinental settings is essential. This requires knowledge of the mechanical behaviour and of the strength (by which Earth scientists commonly mean the maximum stress that rocks can sustain before deforming) of the lower crust.
The most common conceptual model of the strength of the continental crust predicts a strong, seismogenic brittle upper crust (where the base of the seismogenic layer is typically considered to be at depth of 10-15 km), and a weak, viscous, aseismic lower crust. This model has been recently questioned by the finding that the lower crust can be seismic and, therefore, mechanically strong. The question arises, how thick is the seismogenic layer in the crust? Answering this question is crucial to determine the potential hazard caused by large earthquakes, which are also generally the deepest.
Our limited knowledge of the mechanical behaviour of the lower crust is largely due to the lower crust itself being poorly accessible for direct geological observations, so that most of our knowledge derives from indirect geophysical measurements (like the distribution of earthquakes). There are only a few well-exposed large sections of exhumed continental lower crust in the world. One of these is located in the Lofoten islands (northern Norway), which were exhumed from their original deep crustal position during the opening of the North Atlantic Ocean.
We propose an integrated, multi-disciplinary study of a network of brittle-viscous shear zones (i.e. zones of localized intense deformation of geological materials) from Lofoten, which records episodic rapid slip events (earthquakes) alternating with long-lasting aseismic creep. The study will link structural geology (analysis of geological faults and shear zones), petrology (analysis of the composition and textures of rocks), geochemistry (detailed chemical characterization of rocks and minerals) and experimental rock deformation (to reproduce in the lab under controlled conditions the deformation processes operative in the deep Earth's crust). This integrated dataset will provide a novel, clear picture of the mechanical behaviour of the continental lower crust during the earthquake cycle. Our direct geological and experimental observations will be tested against geophysical observations of currently active seismic deformation. The cumulative results of the projects will shed light on the currently poorly constrained mechanical behaviour of the lower crust during the earthquake cycle, and therefore on the sequence of inter-seismic slip (the period of slow accumulation of elastic deformation along a fault), co-seismic slip (the sudden rupture along a fault that is the earthquake) and post-seismic slip (the immediate period after an earthquake when the crust and the fault adjust to the modified state of crustal stress caused by an earthquake). This will greatly extend and complement existing efforts by the scientific community to understand and interpret the mechanical behaviour of rocks during the earthquake cycle recorded in the lower crust and the related hazard, and will provide key input for numerical models of continental dynamics.

Beyond the immediate academic community, our proposed research will also be of relevance to the education sector and the general public, and to industry.
1. Education sector and the general public. We recognise the great importance of communicating our work to the wider public and have already been involved in many outreach activities, including a number of public panel debates, interviews after recent large earthquakes, and Science Festivals. The earthquake cycle is a fundamental geological process, and one that has huge significance for society. Although our research uncovers the deep roots of the earthquake process, we are acutely aware that the societal impact of earthquakes arises largely from their calamitous effect on people and places. Thus, while the ultimate scientific goal of this research will be to constrain models of seismogenic fault behaviour, which, in time, can improve seismic hazard assessments, the immediate concern of earthquake scientists is to address far more basic challenges of earthquake education. For all the enigmatic aspects of the seismic process that we are intrigued by in this proposal, the rising lethality of earthquakes globally compel us to focus our wider attention on how we can communicate more imaginatively and effectively what we already know about why earthquakes happen and how individuals and communities can be protected. Getting that basic understanding of what drives seismic hazard out to the wider public will be a core element of the work programme, and one that will be achieved through a partnership with media professionals to generate imaginative and accessible digital visual content that 'makes public' how faults really work at depth. The externally-facing component of our work, therefore, is directed at creating high-quality educational content on the subject of earthquakes. That creative content will be disseminated via internet-based media networks, as we have already road-tested with the NERC-funded film 'Anatomy of an Earthquake' (https://www.youtube.com/watch?v=8QNigxTN384). With over 15,000 views, it is the most watched video on NERC's YouTube channel, combining a presenter-led narrative and high-end virtual reality animation to convey the bare essentials of seismic hazard to intermediate- and advanced-level secondary school children taking science and geography around the world. This project will involve the same animation team to co-produce a follow-up geo-visualisation of the earthquake process at a more advanced level, appropriate for introductory university-level geoscience students around the world. Educationalists and university teachers around the world will be able to use the geo-visualization to raise awareness of the manifestation of the earthquake cycles in the deep crust and on the hazard that it entails.
2. Industry. Our research is based upon quantitative microanalysis that will be conducted in two world-class electron microscopy centres at Plymouth and Liverpool. Microanalysis systems (EDS, WDS, EBSD) have recently developed important upgrades (in direct collaboration with EM at Liverpool and Plymouth) that have opened up new avenues for the microscale characterization of materials. Our systematic research on ultrafine-grained geological materials (which are challenging to analyse) will represent a benefit to the industry of microanalytical systems, in that it will enable optimization of data acquisition and of processing routines. To maximize the mutual benefit between academia and industry we plan to (1) disseminate part of our results in form of application notes that will be prepared in collaboration with industrial partners and distributed online; (2) organize UK-based workshops and training courses in electron microscopy techniques in collaboration with industrial partners. These workshops will be informed by our ongoing research and will be aimed to early career researchers working in material sciences and micro/nano-material characterization.