Showing papers by "Ake Fagereng published in 2021"

Is complex fault zone behaviour a reflection of rheological heterogeneity

Abstract: Fault slip speeds range from steady plate boundary creep through to earthquake slip. Geological descriptions of faults range from localized displacement on one or more discrete planes, through to distributed shearing flow in tabular zones of finite thickness, indicating a large range of possible strain rates in natural faults. We review geological observations and analyse numerical models of two-phase shear zones to discuss the degree and distribution of fault zone heterogeneity and effects on active fault slip style. There must be certain conditions that produce earthquakes, creep and slip at intermediate velocities. Because intermediate slip styles occur over large ranges in temperature, the controlling conditions must be effects of fault properties and/or other dynamic variables. We suggest that the ratio of bulk driving stress to frictional yield strength, and viscosity contrasts within the fault zone, are critical factors. While earthquake nucleation requires the frictional yield to be reached, steady viscous flow requires conditions far from the frictional yield. Intermediate slip speeds may arise when driving stress is sufficient to nucleate local frictional failure by stress amplification, or local frictional yield is lowered by fluid pressure, but such failure is spatially limited by surrounding shear zone stress heterogeneity.

Geodetic Constraints on Cratonic Microplates and Broad Strain During Rifting of Thick Southern African Lithosphere

Abstract: Understanding strain distribution and kinematics during early stage continental rifting remains a fundamental challenge in plate tectonics. For example, the stresses responsible for amagmatic continental extension of the East African Rift System (EARS) are enigmatic (Kendall & Lithgow-Bertelloni, 2016), despite this being the best exposed active continental rift system on the Earth's surface (Corti, 2009; Ebinger, 2005). The enigma arises because plate boundary forces, dynamic topography, horizontal tractions, and gravitational potential energy do not provide sufficient stress to break the thick, cold continental lithosphere of cratonic Africa (Kendall & Lithgow-Bertelloni, 2016). Earthquakes are predominately confined to Proterozoic orogenic belts at the edges of Archean cratons (Figure 1a; e.g., Daly et al., 1989; Manzunzu et al., 2019; Midzi et al., 2018; Yang & Chen, 2008), but because global seismic networks are only able to detect events with M > ∼ 4.5, detailed kinematic information from seismicity is only available from limited local and regional studies (e.g., Craig & Jackson, 2021; Ebinger et al., 2019; Lavayssière et al., 2019).

Geological constraints on the mechanisms of slow earthquakes

Abstract: The recognition of slow earthquakes in geodetic and seismological data has transformed the understanding of how plate motions are accommodated at major plate boundaries. Slow earthquakes, which slip more slowly than regular earthquakes but faster than plate motion velocities, occur in a range of tectonic and metamorphic settings. They exhibit spatiotemporal associations with large seismic events that indicate a causal relation between modes of slip at different slip rates. Defining the physical controls on slow earthquakes is, therefore, critical for understanding fault and shear zone mechanics. In this Review, we synthesize geological observations of a suite of ancient structures that were active in tectonic settings comparable to where slow earthquakes are observed today. At inferred slow earthquake regions, a range of grain-scale deformation mechanisms accommodated slip at low effective stresses. Material heterogeneity and the geometric complexity of structures that formed at different inferred strain rates are common to faults and shear zones in multiple tectonic environments, and might represent key limiting factors of slow earthquake slip rates. Further geological work is needed to resolve how the spectrum of slow earthquake slip rates can arise from different grain-scale deformation mechanisms and whether there is one universal rate-limiting mechanism that defines slow earthquake slip.

A systems-based approach to parameterise seismic hazard in regions with little historical or instrumental seismicity: active fault and seismogenic source databases for southern Malawi

Abstract: Seismic hazard is commonly characterised using instrumental seismic records However, these records are short relative to earthquake repeat times, and extrapolating to estimate seismic hazard can misrepresent the probable location, magnitude, and frequency of future large earthquakes Although paleoseismology can address this challenge, this approach requires certain geomorphic setting, is resource intensive, and can carry large inherent uncertainties Here, we outline how fault slip rates and recurrence intervals can be estimated by combining fault geometry, earthquake-scaling relationships, geodetically derived regional strain rates, and geological constraints of regional strain distribution We apply this approach to southern Malawi, near the southern end of the East African Rift, and where, although no on-fault slip rate measurements exist, there are constraints on strain partitioning between border and intra-basin faults This has led to the development of the South Malawi Active Fault Database (SMAFD), a geographical database of 23 active fault traces, and the South Malawi Seismogenic Source Database (SMSSD), in which we apply our systems-based approach to estimate earthquake magnitudes and recurrence intervals for the faults compiled in the SMAFD We estimate earthquake magnitudes of MW 54–72 for individual fault sections in the SMSSD and MW 56–78 for whole-fault ruptures However, low fault slip rates (intermediate estimates ∼ 005–08 mm/yr) imply long recurrence intervals between events: 102–105 years for border faults and 103–106 years for intra-basin faults Sensitivity analysis indicates that the large range of these estimates can best be reduced with improved geodetic constraints in southern Malawi The SMAFD and SMSSD provide a framework for using geological and geodetic information to characterise seismic hazard in regions with few on-fault slip rate measurements, and they could be adapted for use elsewhere in the East African Rift and globally

Influence of Subduction Zone Dynamics on Interface Shear Stress and Potential Relationship With Seismogenic Behavior

Abstract: Whether tectonic convergence at subduction zones is accommodated predominantly through seismic or aseismic deformation, the former potentially generating large earthquakes, varies considerably between subduction margins. This margin‐scale variability has previously been linked to overriding plate deformation, trench migration and their influence on the plate interface stress state. While these processes are linked to mantle‐scale dynamics, it is unclear how such dynamics influence interface stress. We systematically analyse the interface stress state in a suite of 2‐D thermo‐mechanical subduction models, where slabs display a range of morphologies that arise from diverse multi‐scale interactions with adjacent mantle and the overriding plate. We demonstrate that the thickness of the interface layer varies dynamically, in response to Poiseuille flow induced by slab bending or unbending, leading to associated effects on interface shear stress at typical seismogenic depth. Lower shear stress occurs when slab unbending is significant, which is commonly associated with trench retreat and draping of the slab as it impinges on the higher‐viscosity lower‐mantle. Conversely, higher shear stress is associated with limited slab unbending, which is promoted by negligible trench migration and vertically subducting slabs. We conclude that the diversity of slab dynamics may cause large variations in interface stress state between and maybe within margins. This is an additional variable that potentially controls seismogenic behaviour, and we compare broad stress estimates for Circum‐Pacific margins to previous studies. Although predicted shear stress varies with observed seismogenic behaviour, more detailed constraints on stress state are needed to test for correlation.

Shear Zone Development in Serpentinized Mantle: Implications for the Strength of Oceanic Transform Faults

Abstract: Oceanic transform faults display fewer and smaller‐magnitude earthquakes than expected for their length. Several mechanisms have been inferred to explain this seismic slip deficit, including increased fault zone damage resulting in elevated fluid flow, and the alteration of olivine to serpentine. However, to date, these possible mechanisms are not supported by direct observation. We use micro‐ to kilometre scale observations from an exhumed oceanic transform fault in the Troodos Ophiolite, Cyprus, to determine mineral‐scale deformation mechanisms and infer likely controls on seismic behaviour of serpentinised lithospheric mantle in active oceanic transform faults. We document a range of deformation fabrics including massive, scaly and phyllonitic serpentinite, attesting to mixed brittle‐ductile deformation within serpentinite shear zones. The progressive development of a foliation, with cumulative strain, is an efficient weakening mechanism in scaly and phyllonitic serpentinite. Further weakening is promoted by a transition in the serpentine polytype from lizardite‐dominated massive and scaly serpentinites to chrysotile‐dominated phyllonitic serpentinite. The development of a foliation and polytype transition requires dissolution‐precipitation processes. Discrete faults and fractures locally crosscut, but are also deformed by, foliated serpentinites. These brittle structures can be explained by local and transient elevated strain rates, and play a crucial role in strain localisation by providing positive feedback for dissolution‐precipitation by increasing permeability. We propose that the evolution in structure and deformation style documented within the serpentinised lithospheric mantle of the Southern Troodos Transform Fault Zone is a viable explanation for the dominantly creeping behaviour and long‐term weakness of oceanic transform faults.

Variable in situ stress orientations across the Northern Hikurangi Subduction Margin

Abstract: We constrain orientations of the horizontal stress field from borehole image data in a transect across the Hikurangi Subduction Margin. This region experiences NW‐SE convergence and is the site of recurrent slow slip events. The direction of the horizontal maximum stress is E‐W at an active splay thrust fault near the subduction margin trench. This trend changes to NNW‐SSE in a forearc trench slope basin on the offshore accretionary wedge, and to NE‐SW in the onshore forearc. Multiple, tectonic, and geological processes, either individually or in concert, may explain this variability. The observed offshore to onshore stress rotation may reflect a change from dominantly compressional tectonics at the deformation front, to a strike‐slip and/or extensional tectonic regime closer to the Taupo Volcanic Zone, further inland. In addition, the offshore stress may be affected by topography and/or stress rotation around subducting seamounts, and/or temporal stress changes during the slow slip cycle.

Tectonic pressure gradients during viscous creep drive fluid flow and brittle failure at the base of the seismogenic zone

Abstract: Fluid-pressure cycles are commonly invoked to explain alternating frictional and viscous deformation at the base of the seismogenic crust. However, the stress conditions and geological environment of fluid-pressure cycling are unclear. We address this problem by detailed structural investigation of a vein-bearing shear zone at Sagelvvatn, northern Norwegian Caledonides. In this dominantly viscous shear zone, synkinematic quartz veins locally crosscut mylonitic fabric at a high angle and are rotated and folded with the same sense of shear as the mylonite. Chlorite thermometry indicates that both veining and mylonitization occurred at ~315–400 °C. The vein-filled fractures are interpreted as episodically triggered by viscous creep in the mylonite, where quartz piezometry and brittle failure modes are consistent with low (18–44 MPa) differential stress. The Sagelvvatn shear zone is a stretching shear zone, where elevated pressure drives a hydraulic gradient that expels fluids from the shear zone to the host rocks. In low-permeability shear zones, this hydraulic gradient facilitates buildup of pore-fluid pressure until the hydrofracture criterion is reached and tensile fractures open. We propose that hydraulic gradients established by local and cyclic pressure variations during viscous creep can drive episodic fluid escape and result in brittle-viscous fault slip at the base of the seismogenic crust.

Evidence of Seismic Slip on a Large Splay Fault in the Hikurangi Subduction Zone

Abstract: The Hikurangi subduction zone is capable of producing moderate to large earthquakes as well as regularly repeating slow slip events. However, it is unclear what structures host these different slip styles along the margin. Here we address whether splay faults can host seismic slip at shallow ( 1 m as observed in the 1947 Poverty and Tolaga Bay earthquakes.