THE GEOLOGICAL RECORD OF THE EARTHQUAKE CYCLE IN THE LOWER CRUST

Lead Research Organisation: University of Plymouth
Department Name: Sch of Geog Earth & Environ Sciences

Abstract

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.

Planned Impact

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.

Publications

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Jamtveit B (2019) The Effects of Earthquakes and Fluids on the Metamorphism of the Lower Continental Crust in Journal of Geophysical Research: Solid Earth

 
Description How earthquakes nucleate deep in the continentatal crust has long been enigmatic, with several recent proposals drawing on the effects of deep fluid flow and metamorphic reactions to trigger deep seismicity. This work has provided an alternative explanation, using observations of faults made in the field, where earthquake rupturing is encouraged by the interaction of slow, viscously creeping shear zones. Around theses shear zones, stronger blocks of rock are loaded and stressed by the continuous creep along those shear zones, eventually accumulating enough stress to rupture via earthquake slip on a fault. The network of earthquake fault rocks observed during this project suggests that this loading was a repeating, cyclical process, with episodic earthquakes punctuating longer-term creep on the shear zones. The earthquakes are nonetheless significant in releasing large amounts of stress from across the wider region. This explanation for deep earthquake nucleation is applicable in many regions of the lower crust in the earth's continents, as viscous shear zones are the typical mode of deformation there.

An additional discovery is that the fault rocks produced by earthqakes (pseudotachylytes) may act as a significant structure in supporting rapidly changing stresses and strain rates in the lower crust. Microstructural examination of viscously-deformed pseduotachylytes found that a snapshot of high stress and high strain rates wee captures in the pseudotachylytes during this viscous deformation, before a return to slower, more stable strain rates. This transient high strain rate deformation can be linked to the relaxation of the lower crust following an earthquake, and is an important contribution to the discussion of the ability of the continental lower crust to support these post-seismic variations in stress and strain rate.
Exploitation Route The identification of a newly understood mechanism of earthquake nucleation extends our ability to asses where lower continental crustal seismicity may occur. This builds into a growing understanding of the frequency of seismic events in the lower crust. Future research may expand this understanding further by testing our model using a variety of methods, including experimental and numerical modelling. The understanding that localised faults in the lower continental crust can change the local rheology and support rapidly changing strain rates are important considerations for geodetic community modelling slip dristributions for observations of present day crustal-scale deformation.
Sectors Government, Democracy and Justice

 
Description There are two ongoing initiatives aimed at disseminating the project finding and its impact to the wide public. PSEUDOTACHYLYTE A long-form (50 min) dcoumentary film produced by Heidi Morstang (School of Art, Design and Architecture, University of Plymouth), entitled "Pseudotachylyte" focuses on the scientific work of the project team (Dr Menegon, Dr Campbell, Dr Mariani, Dr Fagereng, and Prof Pennacchioni - external collaborator), following their 2017 field capmapign in norther Norway. The premiere screening of "Pseudotachylyte' was at the Bergen International Film Festival, Norway (25th Sept - 3rd Oct 2019). The film has also been selected for the Norwegian Documentaries competition (https://www.biff.no/filmer/article1408158.ece?language=english) and the Golden Owl Competition (awarded to the best science documentary by the University of Bergen) ( https://www.biff.no/article1407657.ece?language=english). The Bergen International Film Festival 2019 announced: 'With PSEUDOTACHYLYTE, director Heidi Morstang (THINKING SPACE, BIFF 2016) returns with a beautiful and deeply fascinating documentary that shows how outstanding research creates the foundation for existential truths about time, humanity and the globe we live on. With striking black-and-white images of wild and beautiful Northern Norway scenery and a curious look at the people who are eagerly exploring it, this is an educational and thought-provoking meditation on research and the past's ability to give us captivating insights into our own time.' The Norwegian Documentaries competition reported: 'In this thought-provoking and visually stunning black and white film we follow a group of international geo-scientists in Lofoten as they are exploring the Arctic landscape, hoping to find reasons why the site is particularly vulnerable to deep and potentially devastating earthquakes. An exceptionally well-preserved field site gives the scientists a rare look into exposed rocks that were once deep below the surface of the earth. After millions of years of erosion, the rocks now show visible traces of ancient earthquakes as 'scars', so-called Pseudotachylytes.' The film has been included in a press release tied to the publication of the team's research paper in Nature Communications - https://www.plymouth.ac.uk/news/documentary-aims-to-reveal-the-human-side-of-earthquake-science https://vimeo.com/386040247 'A safer future with clues from earthquakes past' The second initiative consists of the publication of a dissemination article in the journal 'Impact'. This journal communicates research impact to key stakeholders such as national and regional funding agencies, research funding bodies, public sector organisations, local and regional governments, etc. The article has been approved by the research team and by the journal team, and published in December 2019. Menegon, L. & Stewart, I. 2019. 'A safer future with clues from earthquakes past'Impact, Volume 2019, Number 9, December, pp. 6-8(3). https://doi.org/10.21820/23987073.2019.9.6 The open-access article is available for download at: https://www.ingentaconnect.com/content/sil/impact/2019/00002019/00000009/art00003
First Year Of Impact 2020
Sector Education,Culture, Heritage, Museums and Collections
Impact Types Cultural,Societal

 
Description Gordon Research Conferences attendance support to Dr Lucy Campbell
Amount $1,095 (USD)
Organisation Gordon Research Conferences 
Sector Charity/Non Profit
Country United States
Start 08/2018 
End 08/2018
 
Description Film 'Pseudotachylytes' 
Organisation University of Plymouth
Department School of Art, Design and Architecture
Country United Kingdom 
Sector Academic/University 
PI Contribution Contextualization of the research topic (lower crustal earthquakes)
Collaborator Contribution Production of a film featuring the research team during the 2017 field work in Northern Norway
Impact Production of the film 'Pseudotachylytes' (finalization of the post-production planned for April 2019) featuring the research team during the 2017 field work in Norway. The film will represent one of the key products for the dissemination of the research to the wide public.
Start Year 2017
 
Description The peak strength of the lithosphere: New insights via micromechanical testing 
Organisation University of Oxford
Country United Kingdom 
Sector Academic/University 
PI Contribution Microstructural analysis of quartz and plagioclase deformed in the low-temperature plasticiy regime. Microstructural analysis of networks of coeval pseudotahylytes and mylonites.
Collaborator Contribution Nanoindentation tests of quartz, plagioclase, olivine and pseudocyahylytes. High-resolution electron backscatter diffraction analysis.
Impact Multi-disciplinary collaboration involving Earth Sciences, Material Sciences and Electron Microscopy. The partnership resulted in two grant applications and in ongoing mutual research visits at Plymouth and Oxford to investigate low-temperature plasticity of Earth's materials.
Start Year 2017
 
Description University of Oslo 
Organisation University of Oslo
Country Norway 
Sector Academic/University 
PI Contribution Invited submission of a review paper on lower crustal earthquakes to the Journal of Geophysical Research together with colleagues from the Department of Geosciences of the University of Oslo.
Collaborator Contribution They led the submission of the review paper.
Impact Review paper currently under review.
Start Year 2018
 
Description Joint conference of the UK Tectonics Studies Group and UK Metamorphic Studies Group, organised by the PI at Plymouth University in January 2018 
Form Of Engagement Activity Participation in an activity, workshop or similar
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Postgraduate students
Results and Impact The first joint conference of the UK Tectonics Studies Group (TSG) and Metamorphic Studies Group (MSG) was hosted by the University of Plymouth between the 3rd and 6th January 2018, and was organized by the Award PI, Dr Luca Menegon, in collaboration with members of staff of Earth Sciences at Plymouth University.
The meeting attracted 130 delegates from institutions across the UK, Europe, Australia, South Africa, and Canada, and showcased the latest developments in tectonics, structural geology and metamorphic geology. One of the key themes of the conference was a discussion on the contributions of structural and metamorphic geology to industrial projects on geo-resources and on the geological disposal of radioactive waste. The conference provided opportunities for networking and for fostering collaborations between the two groups and the industry, across all career stages.

Overall a total of 54 talks and 43 posters were presented by early career researchers, academics and representatives from the industry, across subjects including fault structure, geology of the earthquake source, structural geology and geo-resources, fluid flow in the lithosphere, interplays between metamorphism and deformation, and regional geology. The postdoc working on the Award (Dr Lucy Campbell) has presented an oral and a poster presentation on the ongoing research.
Year(s) Of Engagement Activity 2018
URL https://www.plymouth.ac.uk/whats-on/tsg-and-msg-joint-conference-2018
 
Description Press release 
Form Of Engagement Activity A press release, press conference or response to a media enquiry/interview
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Public/other audiences
Results and Impact Press release by Plymouth University on the Award, with a short interview to the Award PI.
Year(s) Of Engagement Activity 2017
URL https://www.plymouth.ac.uk/news/study-examines-causes-of-earthquakes-originating-deep-below-earths-s...