Understanding earthquakes using the geological record

Fault-zone rheology governs the mechanics of faults and the earthquake cycle and determines the hazards arising from fault slip in the Earth’s crust. Our knowledge of the frictional and bulk rheology of crustal fault zones has traditionally been based on laboratory rock mechanics experiments. However, such experiments have to be carried out at spatial and temporal scales that are very far from those found in nature. It is also possible to probe the mechanical properties of fault zones using geodetic and seismological observations of fault zones during transient periods of postseismic afterslip, fault slip and microseismicity modulated by tides and seasonal loads, and spontaneous slow slip events. These deformation episodes can thus serve as natural laboratory experiments that improve understanding of the mechanics of fault slip. Recent advances in space geodetic observations and seismological techniques have helped better illuminate fault-zone properties. As the data improve, increasingly physical models should be developed to determine fault zone properties. To better understand fault-zone structure and properties, it is also important to consider geological field observations of fault zones exhumed from varying tectonic settings and depths. Thus, to further this type of research it is essential to optimize and integrate a wide variety of observations. A number of recent examples are used to illustrate the promises and challenges of geophysical probing of fault-zone behaviour and rheology. 

Professor Roland Bürgmann, University of Berkeley, USA

Professor Roland Bürgmann, University of Berkeley, USA

Roland Bürgmann received his Vordiplom in Geology at the Universität Tübingen, Germany, in 1987, his MS at the University of Colorado, Boulder, in 1989, and his PhD at Stanford University, in 1993. He is currently Professor at the Department of Earth and Planetary Science at UC Berkeley. His research interests are in active tectonics, crustal deformation and lithosphere rheology. Bürgmann’s research group uses space-geodetic measurements, seismic data, and field observations to constrain crustal deformation associated with active faults, volcanoes, fluid reservoirs, and landslides. He is a Fellow and 2013 Birch lecturer of the American Geophysical Union (AGU). He is currently chairing the National Earthquake Prediction Evaluation Council (NEPEC) and is a member of NASA’s Earth Science Advisory Committee (ESAC). See http://seismo.berkeley.edu/~burgmann/  for more information about Bürgmann’s research and publications. 

A first-order feature of the geological history of continents is the contrast between the long-lived stability of the ancient continental interiors and the widespread deformation in Phanerozoic orogenic belts; displayed most obviously in the asymmetry of the India-Asia collision. Through advances in seismic tomography, we can now make increasingly detailed maps of the variations in lithosphere (plate) thickness on the continents. The variations are dramatic, with some places up to 300 km thick, and clearly relate to the geological history of the continents as well as their present-day deformation. Where the lithosphere thickness is about 120 km or less continental earthquakes are generally confined to upper crustal material that is colder than about 350^oC. On the edge of thick lithosphere, the entire crust may be seismogenic, with earthquakes sometimes extending into the uppermost mantle if the Moho is colder than 600^oC; but the continental mantle is generally aseismic. In such regions, earthquakes in the continental lower crust at 400-600^oC require the crust to be anhydrous (granulite facies) and are a useful guide or proxy to both composition and strength. These correlations have important implications for the geological evolution of the continents. They can be seen to have influenced features as diverse as: the location of post-collisional rifting; intracratonic basin formation; the location, origin and timing of granulite metamorphism; and the formation, longevity and strength of cratons. In addition, they have important consequences for earthquake hazard assessment on the slowly deforming edges of continental shields or platforms, where the large seismogenic thickness can host very large earthquakes.

Professor James Jackson CBE FRS, University of Cambridge, UK

Professor James Jackson CBE FRS, University of Cambridge, UK

James Jackson is a Professor in the Department of Earth Sciences in the University of Cambridge.  He was born and raised in India, which established his interest in all aspects of Asia, where much of his research has been concentrated.  He uses seismology, geodesy, and Quaternary geology, combined with observations of the landscape in the field, to study how the continents develop and deform on all scales, from the movement that occurs in individual earthquakes to the evolution of mountain belts.  His field work has taken him to many parts of Asia, the Mediterranean, Africa, New Zealand and North America.  He is increasingly involved in how to use earthquake science to reduce the appalling risk from earthquakes to populations in developing countries, and co-led the project ‘Earthquakes Without Frontiers’ (www.ewf.nerc.ac.uk).

Earthquakes are deformation increments that must contribute to build geological structures and topography in the long run. The Himalaya is one place where this process can be observed at play. Crustal shortening is active and has produced a well-expressed thrust system and the highest topography on Earth today. It might have come as a surprise that, a result of both coseismic subsidence and intense mass wasting by landslides, the high Himalaya went down during the 2015, Mw7.8 Gorkha earthquake. Modelling shows that, in the Himalayan context, the topography actually builds up in the time period between large earthquakes due to thermally enhanced aseismic deformation. This kinematics results from the ramp-and-flat geometry and thermal structure of the thrust system, which in turn are the result of coupling between crustal deformation and erosion over the long-term. This framework reconciles the geological and topographic expression of the Himalaya with its current activity. Within this framework, various types of observations can be used to inform rheological properties. A low effective friction is necessary to allow slip with little heat production on the sub-horizontal décollement beneath the lesser Himalaya. Given that the décollement was locked before the Gorkha earthquake and didn’t produce any significant afterslip, the low friction is likely due to dynamic weakening during seismic sliding. Observations of post-seismic relaxation, crustal rebound following lake regression in south Tibet, and gravity can be used to constrain further the rheology of the crust and mantle lithosphere across the Himalaya and southern Tibet. 

Professor Jean-Philippe Avouac, California Institute of Technology, USA

Professor Jean-Philippe Avouac, California Institute of Technology, USA

Jean-Philippe Avouac is the Earle C. Anthony Professor of Geology and of Mechanical and Civil Engineering at the California Institute of Technology, and director of the NSF center for Geomechanics and the Mitigation of Geohazards. He received his PhD in from the Institut de Physique du Globe de Paris, France, in 1991. He worked at the Commissariat à l’Energie Atomique, France, from 1991 to 2001. He was the director of the Tectonics Observatory at the California Institute of Technology from 2004 to 2014. He worked as the BP-McKenzie Professor of Earth Sciences at the University of Cambridge from 2014 to 2015. 

Jean-Philippe Avouac studies crustal deformation, earthquakes in particular, and geomorphic processes. He has contributed to method developments in remote sensing, geodesy, geomorphology and seismology. Jean-Philippe Avouac has published over 190 articles in international peer-reviewed journals (Google scholar profile) and holds patents in remote sensing.

It is now possible with satellite-based systems to monitor deformation of the Earth’s surface at high spatial resolution over periods of several decades and a significant fraction of the seismic cycle. The relation between deformation at this short timescale and long-term geological faulting, over 10s to 100s kyrs, is examined for subduction, continental shortening and rift settings, using examples from the active New Zealand and Central Andean plate boundary zone. Simple models of locking on a deep-seated megathrust or decollement, or mantle flow, provide excellent fits to the short-term observations without requiring any information about the geometry and rate of surface faulting. The short-term deformation in these examples cannot be used to determine the long term behaviour of individual faults, but instead places constraints on the forces that drive deformation. Thus, there is a fundamental difference between the stress loading and stress relief parts of the earthquake cycle, with failure determined by dynamical rather than kinematic constraints; the same stress loading can give rise to widely different modes of long-term deformation, depending on the strength and rheology of the deforming zone, and the role of gravitational stresses. The process of slip on active faults may have an intermediate timescale of kyrs to 10s kyrs, where faults fail piecemeal without any characteristic behaviour. Physics-based dynamical models of short-term deformation may be the best way to make full use of the increasing quality of this type of data in the future.

Dr Simon Lamb, Victoria University of Wellington, New Zealand

Dr Simon Lamb, Victoria University of Wellington, New Zealand

Simon Lamb completed his undergraduate and postgraduate degrees at Cambridge University. His doctoral thesis, supervised by Dr Alan Smith, was on the geological evolution of some of the oldest known supracrustal rocks in the Barberton Mountain Land of Swaziland. He switched to the other end of the geological timescale for his postdoctoral research, studying the mechanisms of tectonic rotations about a vertical axis in New Zealand, working with Professor Dick Walcott.  Subsequently he was in the Department of Earth Sciences at Oxford University for over 20 years, first as a Royal Society University Research Fellow, then University Lecturer, leading a multidisciplinary project studying the tectonics of the Central Andes. Since 2010, he has been Associate Professor in Geophysics at Victoria University of Wellington, New Zealand, focusing on the tectonics of the New Zealand plate-boundary zone and integrating short term GPS observations with the longer term geological evolution.

Episodic tremor and slow slip (ETS) is observed in several subduction zones down-dip of the locked megathrust, and may provide clues for preparatory processes before megathrust rupture. Exhumed rocks provide a unique opportunity to evaluate the sources of rheological heterogeneity on the subduction interface and their potential role in generating ETS-like behaviour. Data is presented from two subduction interface shear zones representative of the down-dip extent of the subduction megathrust: the Condrey Mountain Schist (CMS) in northern CA (greenschist to blueschist facies conditions) and the Cycladic Blueschist Unit (CBU) on Syros Island, Greece (blueschist to eclogite facies). Both complexes highlight the propensity for fluid-mediated metamorphic reactions to produce strong rheological heterogeneities. In the CMW, hydration reactions led to partial serpentinization of peridotite bodies and development of highly localized strain on their margins and distributed frictional-viscous deformation in their interiors. In the CBU, dehydration reactions in MORB-affinity basalts led to progressive development of strong eclogitic lenses within a weaker blueschist and metasedimentary matrix. Brittle deformation in the eclogites is coeval with ductile deformation in the surrounding blueschist and metasedimentary matrix, indicating concurrent frictional-viscous flow. Although transient deformation processes cannot easily be distinguished in exhumed rocks, three approaches (scaling analysis, comparison to experiments, and numerical modelling) are used to assess whether these heterogeneities could have generated deformation behaviours similar to deep ETS. All three approaches suggest that frictional-viscous heterogeneities of the types and length-scales observed in the exhumed rock record are compatible with ETS as documented in modern subduction zones.

Dr Whitney Behr, ETH Zürich, Switzerland

Dr Whitney Behr, ETH Zürich, Switzerland

Whitney Behr has been Associate Professor and Chair of the Structural Geology & Tectonics Group in the Geological Institute at ETH Zürich since summer 2018. Whitney completed her Bachelor’s degree at California State University Northridge in 2006 and her PhD at the University of Southern California in Los Angeles in 2011. She then spent 11 months at Brown University as a Postdoctoral Fellow in the Geophysics Research group. Prior to arriving at ETH, from 2012–2018, Whitney was an Assistant Professor in the Jackson School of Geosciences at the University of Texas at Austin. Whitney’s research focuses on the rapidly deforming zones that define Earth’s tectonic plate boundaries and generate many of the planet’s geohazards. Her group is interested in the rates and directions in which faults and shear zones move; their geometries, widths and mechanical behaviours at depth; and the processes that shape them over geologic time.

The deformation of the dry lower continental crust is commonly characterized by a cyclic interplay between viscous creep (mylonitisation) and brittle, seismic slip associated with the formation of pseudotachylytes. Seismic slip may trigger fluid infiltration, weakening, and a transition to viscous creep along faults initially characterized by frictional melting. The cyclical interplay between seismic slip and viscous creep implies transient oscillations in stress and strain rate during the activity of faults and shear zones steered by the earthquake cycle. This talk will discuss (i) the ability of shear zone microstructure to preserve evidence of transient high strain rate creep, and (ii) models for earthquake generation in the lower crust. Results from the study of exhumed networks of lower crustal mylonites and pseudotachylytes from different localities will be presented, with a focus on Nusfjord (Lofoten, Norway). In Nusfjord, granulite facies ductile shear zones dissect an anorthosite intrusion. The shear zones localize on precursor pseudotachylytes and developed at ca 700°C and 0.8 GPa. Mylonitised pseudotachylytes locally preserve microstructures (ie layers of fine grained, ca 10 μm, dynamically recrystallized quartz) suggestive of transient high strain rate deformation that are interpreted as episodes of accelerated post-seismic creep. The shear zones occur in three main sets of parallel structures that separate undeformed blocks of anorthosite, within which pristine (non mylonitised) pseudotachylyte fault veins link adjacent or intersecting shear zones. These pristine pseudotachylytes represent direct identification of transient earthquake nucleation as a consequence of local stress amplification during ongoing aseismic creep in ductile shear zones.

Dr Luca Menegon, University of Oslo, Norway

Dr Luca Menegon, University of Oslo, Norway

Luca Menegon is Associate Professor of Structural Geology and Tectonics at the University of Plymouth, UK, and Associate Professor of Deformation and Metamorphism at the University of Oslo, Norway. Luca’s academic roots are in structural and metamorphic geology. After completing a PhD in structural geology at the University of Padua, Italy (2006), he has been postdoctoral researcher at the University of Padua (2006–2009), at the University of Stockholm (2008) and at the University of Tromsø, Norway (2009–2012). In 2013 he joined the University of Plymouth, and in 2019 he joined the University of Oslo. 

Luca’s research investigates the brittle deformation and the viscous flow of rocks in the Earth’s interior, which controls the formation of tectonic plate boundaries and the dynamics of the deepest portions of earthquake-generating faults. The relationships between fluid-rock interaction, metamorphism, deformation processes and rock rheology represent a core theme of his current research, which is motivated by the interest in understanding how grain-scale deformation mechanisms control lithosphere dynamics and the earthquake cycle. 

https://www.plymouth.ac.uk/staff/luca-menegon

Much of the Earth’s crust is composed of metamorphic rocks that have experienced ancient orogenesis. During these orogenic events the rocks likely underwent continuous progressive metamorphism, with the presence of H₂O-CO₂ fluid or silicate melt inferred. However, as both fluid and silicate melt are mobile phases, they can leave the source rocks resulting in a distinct asymmetry to metamorphism. This asymmetry results in relatively dry metamorphic rocks. Such rocks are able to experience subsequent tectonic activity, but are likely to behave differently during any subsequent event. In particular, such dry rocks show a greater tendency to undergo deformation and metamorphism along discrete shear and fault zone structures which may be linked to seismic activity, with surrounding rocks resisting deformation and metamorphism and preserving the earlier metamorphic assemblages. This subsequent metamorphism also occurs over short, discrete timescales resulting in a non-progressive metamorphic reworking. A consequence of reworking under fluid/melt-absent conditions is the existence of typically high-temperature anhydrous minerals at conditions well below where they occur in fluid/melt-bearing systems, making the extraction of P–T conditions of formation difficult. Thus, the high-temperature, open system metamorphic processes that typify the first metamorphic event predispose the rocks to more brittle behaviour in subsequent metamorphic events.

Professor Richard White, University of St Andrews, UK

Professor Richard White, University of St Andrews, UK

Professor White currently works at the University of St Andrews having been previously employed at the University of Mainz in Germany and the University of Melbourne in Australia. Professor White’s main interests are in constraining how metamorphic processes operate and shape the Earth’s crust during continental plate collision. He develops and applies thermodynamic models for minerals to predict the stability of different phases at elevated pressure and temperature, with a particular interest in high-temperature open system processes such as devolatilisation or melting. He has worked on rocks spanning geological time in areas ranging from Antarctica to northern Scotland. He is currently involved in several projects aimed at better integrating thermodynamic and geodynamic simulations.