A Safer Future with Clues from Earthquakes Past

Impact Objectives • Undertake detailed field observation and mapping of pseudotachylytes • Estimate the creep rates and the viscosity of mylonites to confirm them as geological signatures of transient post-seismic deformation in the lower crust • Conduct rock deformation experiments in the laboratory to reproduce shear zones observed in the field A safer future with clues from earthquakes past Dr Luca Menegon and Professor Iain Stewart are investigating fault lines in northern Norway to determine how evidence of past seismic shifts can teach us more about how the deep crust behaves during tectonic plate movement LM: In the context of the build-up and IS: Our results will help us understand release of tectonic stress and strain (the the controls on where earthquakes may earthquake cycle), our results will shed nucleate and propagate in the lower crust light on the earthquake source parameters and will inform earthquake hazard models in the lower crust and on the transient with direct observations of the architecture rheology that occurs after earthquake slip and the mechanical properties of a deep (post-seismic creep) in the lower crust. earthquake source. Direct comparison Dr Luca Menegon Professor Iain Stewart By characterising long-term rheology, our between active tectonics and the geological results will shed light on the strength record will help explain observations of Can you give us an overview of your project? evolution of the lower crust over multiple deformation acquired before and after large earthquake cycles, with co-seismic slip earthquakes. LM: This project focuses on an outstanding and associated fractures alternating with natural laboratory within a zone of ancient microstructural deformation related to Please tell us about the documentary fault roots exhumed from lower crustal viscous creep in the post-seismic period planned. depth and now dramatically exposed in and the longer inter-seismic periods Nusfjord (Lofoten, northern Norway). between earthquakes. This will improve our IS: As geologists, we go to the extremes of The shear zone at Nusfjord exposes two understanding of lower crust deformation the planet, climb a mountain, carefully collect different types of fault rock along the deep during major geodynamic processes such rock samples to take back to the laboratory crustal fault roots. Pseudotachylytes are as mountain-building processes and where we examine them under powerful quenched frictional melt produced during continental break-up during rifting and microscopes or crush them in high-pressure seismic slip and are considered the only the associated expected seismicity or lack machines. All to get some numbers that try unequivocal evidence of past seismicity in thereof. to define how the planet’s internal processes the geological rock record (they are scars work. It’s all a bit abstract. The documentary of past earthquakes). Mylonites are the What impact do you expect to have on the is an outsider’s view of how geologists work, product of the slow, steady, long-lasting world at large? almost like an anthropological perspective viscous creep. Documenting the changing on this strange scientific tribe. A tribe that fault rock character and architecture LM: Most people see earthquakes as natural has its reference frame in the physical reality showcases the switch in deformation disasters that occur at plate boundaries, of the field, alongside the experimental and process occurring in the lower crust along seismic fault lines such as the San laboratory world of other sciences. One between the rapid earthquake-related Andreas Fault in California. In fact, the whose timeframes switch back and forth deformation and the slow creep that planet’s continents are criss-crossed by from the hundreds of millions of years of accommodates the long-term movement ancient fault scars because earthquakes mountain building to the tens of seconds of the tectonic plates. are a fundamental part of how our planet of earthquake slip. A group whose spatial accommodates the large-scale movement scales of inquiry span the microscopic to the Why is it vital to understand both the long- of Earth’s outer skin. The geological study continental. In that sense, the documentary and short-term mechanical behaviour of the of faults is a critical part of modern Earth is about demystifying the nature of being a lower crust? science. geologist as much as demystifying the nature of earthquake faults. 06 www.impact.pub A window to the depths of the Earth Earthquakes can be devastating events but with further study of past seismic activity, we may better prepare for them in the future. Researchers at the University of Plymouth have undertaken an in- depth investigation into the fossil record of earthquakes past Amazingly, rock hundreds of millions of Norway is currently tectonically inactive break-up during rifting as well as any years old can tell us about the behaviours of and seismically quiet. However, 400 million seismicity associated with these processes. our modern-day planet. Our planet is filled years ago, its rocky terrain formed the with traces of its past, like scars on its body bottom of a Himalayan-scale mountain AN IDEAL SETTING and geologists can read these to uncover range plagued with earthquakes. The fossil The lower crust is not accessible to Earth’s physical history. Deciphering this traces of these earthquakes, exposed due to direct geological observation, and so its history can provide us with clues about hundreds of millions of years of erosion, can analysis must be instead constrained by how natural processes operate today and now be evaluated by Menegon and his team geodynamic numerical models, laboratory how they might continue to operate in the to assess the history of seismic events and experiments and large-scale geophysical future. Earthquakes, some of the most thus, predict future incidents. surveys. However, it is possible, yet rare, to jarring and active shapers of Earth’s physical find occurrences of exhumed lower crustal history, have the potential to destroy entire DIGGING DEEP earthquake sources providing the possibility communities. The lower crust of our Earth, the layer of direct observation of fossil lower crustal between the outer crust and mantle, processes. One of these exceptionally rare Dr Luca Menegon, Professor Iain Stewart typically tens of kilometres deep beneath sites is in Lofoten, Norway, the team’s and their colleagues at the University the surface, is a critical point of interest for designated site of exploration. There lies of Plymouth are working around these geologists studying tectonics. It is down exposed ancient, lower crustal rocks that limitations by studying fossil traces of there where we find some of the largest once hosted earthquakes, emerging from earthquakes at a unique site in northern earthquakes and their corresponding an opening in the Atlantic Ocean. ‘This Norway. Menegon, the project’s Principal devastating aftershocks, such as the outstanding natural laboratory makes this Investigator, is Associate Professor of earthquake of 26 January 2001 in India that project unique and further demonstrates Structural Geology and Tectonics. Stewart, killed up to 20,000 individuals. In fact, the importance of direct field observations his research partner, is Professor of these ‘intercontinental earthquakes’ have, of geological structures to understand Geoscience Communication and Director over the past century, killed significantly the dynamics of the Earth’s crust and of the Sustainable Earth Institute. He has more people than earthquakes striking at the mechanics of earthquakes,’ explains also had a 15-year partnership with BBC plate boundaries. Menegon. Science, generating mainstream television documentaries about the planet. Improved understanding of these It is not only the visible fossil earthquakes distressing and life-shattering earthquakes from the lower crust that make this site Their exceptional field site in northern requires deeper study into the rheology, or unique. During tectonic deformation, the the physics of flowing matter, of the lower lower crust is presumed to flow viscously, so crust and its relationship to surrounding the diagnostic scars of earthquakes in the earth as part of intracontinental earthquake geological record may be difficult to locate. cycles. Menegon and Stewart’s research The preservation of these particular scars is plans to elucidate this area of geology by exceptionally rare, yet have been discovered establishing lower crust earthquake source in Lofoten. parameters, rheology associated with post- seismic creep and the evolution of the lower CLEAR INTENTIONS crust over the course of multiple earthquake In analysing the Norwegian field site, the cycles. Their work will thus improve team has outlined well-defined objectives understanding of major geodynamic to document past earthquake activity and processes in the lower crust, including the subsequent viscous creep or slow mountain building and continental movement recorded in fault rocks that Electron backscatter diffraction map of the microstructure formed in the deep crust and are now of a mylonite from the study area. Field of view is 0.5 mm exposed at the earth’s surface. www.impact.pub 07 The earthquake cycle is a fundamental geological Project Insights process and one that has huge significance for FUNDING This project was supported by funding society from the Natural Environment Research Council. Firstly, they plan to undertake detailed multi-disciplinary team is at the core

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