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. 

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.

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. 

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.

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.

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.

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.