Characterizing the distribution of temperature and normal stress on flash heated granite surfaces at seismic slip rates

We perform numerical simulations of sequences of earthquake and aseismic slip on planar rate and state faults separating dissimilar material within the 2-D plane strain approximation. We resolve all stages of the earthquake cycle from aseismic slip to fast ruptures while incorporating full inertia effects during seismic event propagation. We show that bimaterial coupling results in favorable nucleation site and subsequent asymmetric rupture propagation. We demonstrate that increasing the material contrast enhances this asymmetry leading to higher slip rates and normal stress drops in the preferred rupture propagation direction. The normal stress drop, induced by the bimaterial effect, leads to strong dynamic weakening of the fault and may destabilize the creeping region on a heterogeneous rate and state fault, resulting in extended rupture propagation. Such rupture penetration into creeping patches may lead to more frequent opening of earthquake gates, causing increased seismic hazard. Furthermore, bimaterial coupling may lead to irregular seismicity pattern in terms of event length, peak slip rates,and hypocenter location, depending on the properties of the creeping patches bordering the seismogenically active part of the fault . Our results highlight robust characteristics of bimaterial interfaces that persist over long sequence of events and suggest the need for further exploration of the role of material contrast in earthquake physics and models of seismic hazard.

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Hematite (U-Th)/He thermochronometry detects asperity flash heating during laboratory earthquakes

Geology ◽

10.1130/g46965.1 ◽

2020 ◽

Vol 48 (5) ◽

pp. 514-518 ◽

Cited By ~ 1

Author(s):

Gabriele Calzolari ◽

Alexis K. Ault ◽

Greg Hirth ◽

Robert G. McDermott

Keyword(s):

High Temperatures ◽

Temperature Rise ◽

Thick Layer ◽

Seismogenic Zone ◽

Experimental Conditions ◽

Slip Rates ◽

Seismic Slip ◽

Slip Surfaces ◽

Before And After ◽

Dynamic Weakening

Abstract Evidence for coseismic temperature rise that induces dynamic weakening is challenging to directly observe and quantify in natural and experimental fault rocks. Hematite (U-Th)/He (hematite He) thermochronometry may serve as a fault-slip thermometer, sensitive to transient high temperatures associated with earthquakes. We test this hypothesis with hematite deformation experiments at seismic slip rates, using a rotary-shear geometry with an annular ring of silicon carbide (SiC) sliding against a specular hematite slab. Hematite is characterized before and after sliding via textural and hematite He analyses to quantify He loss over variable experimental conditions. Experiments yield slip surfaces localized in an ∼5–30-µm-thick layer of hematite gouge with <300-µm-diameter fault mirror (FM) zones made of sintered nanoparticles. Hematite He analyses of undeformed starting material are compared with those of FM and gouge run products from high-slip-velocity experiments, showing >71% ± 1% (1σ) and 18% ± 3% He loss, respectively. Documented He loss requires short-duration, high temperatures during slip. The spatial heterogeneity and enhanced He loss from FM zones are consistent with asperity flash heating (AFH). Asperities >200–300 µm in diameter, producing temperatures >900 °C for ∼1 ms, can explain observed He loss. Results provide new empirical evidence describing AFH and the role of coseismic temperature rise in FM formation. Hematite He thermochronometry can detect AFH and thus seismicity on natural FMs and other thin slip surfaces in the upper seismogenic zone of Earth’s crust.

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Bridging the Gap Between Seismic and Sub-seismic Mirror-Slip Surfaces in Carbonate Fault Gouge

10.5194/egusphere-egu2020-7926 ◽

2020 ◽

Author(s):

Milo Trainor Moss ◽

Berend A. Verberne ◽

Miki Takahashi ◽

Andre R. Niemeijer

Keyword(s):

Normal Stress ◽

Fault Slip ◽

Shear Displacement ◽

Displacement Rate ◽

Structural Development ◽

Normal Stresses ◽

Seismic Slip ◽

Slip Surfaces ◽

Wide Range ◽

Triaxial Cell

Specularly light reflective fault plane interfaces known as Mirror-Slip Surfaces (MSS&#8217;s) are common in seismically active fault zones around the world and thus their role in controlling fault strength and stability is of great interest. MSS&#8217;s have been experimentally produced in simulated carbonate faults at relatively high (10-1-100 m/s) and low (10-7-10-5 m/s) sliding velocities (resp. HV and LV). However, their role in controlling fault mechanical properties at sub-seismic vs seismic fault-slip velocities remains enigmatic. With the aim to unravel the structural development of MSS&#8217;s with increasing shear displacement (rate) and effective normal stress, we conducted HV and LV shear deformation experiments on simulated faults composed of granular calcite. We employed a ring shear set-up in a HV rotary shear apparatus as well as a saw-cut assembly mounted in a triaxial cell, which enabled fault-slip tests under a wide range of slip velocities (v = 10-7 - 10-1 m/s) and effective normal stresses (&#963;n &#8776; 10 &#8211; 170 MPa). All experiments were carried out under room-dry conditions, at room temperature. Post-mortem microstructure analysis of recovered fragments was carried out through visual inspection, incident light and scanning electron microscopy, as well as using Raman spectroscopy.MSSs develop at sub-seismic slip velocities (v = 10-7 m/s) initially as visibly striated patches after 0.0062 m (&#963;n &#8776; 10 MPa), 0.004 m (&#963;n &#8776; 50 MPa) and 0.0026 m (&#963;n &#8776; 170 MPa) of shear displacement. The area covered by MSSs systematically increases with displacement to form continuous coatings after 0.042 (&#963;n &#8776; 10 MPa), 0.0062 m (&#963;n &#8776; 50 MPa) and 0.0036 m (&#963;n &#8776; 10 MPa).&#160; As displacement rate is increased (10-5 &#8211; 10-4 m/s) MSSs are no longer observed however continuous MSSs are visible again at seismic slip velocities (>10-1 m/s). Our microstructural analysis revealed that MSSs are layers of (nano)crystalline calcite some of which contain elongated nanofibrous structures. In addition, discrete, 3 - 20 micron-sized patches of amorphous carbon are produced at seismic slip velocities, and at sub-seismic velocities under high normal stresses (&#963;n > 160 MPa). We could not however identify any microstructural characteristics that are diagnostics of MSSs produced at certain slip rates or normal stress.Our interpretation is that MSSs form by sintering of nm-sized particles within ultrafine-grained shear bands. With increasing shear displacement, MSS patches connect into continuous veneers. The formation of (continuous) MSSs at low as well as high sliding velocities in our experiments implies that natural MSSs are unreliable indicators for palaeoseismicity.

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