Earthquake mechanics is controlled by a spectrum of processes covering a wide range of length scales, from tens of kilometres down to few nanometres. While the geometry of the fault/fracture network and its physical properties control the global stress distribution and the propagation/arrest of the seismic rupture, earthquake nucleation and fault weakening is governed by frictional processes occurring within extremely localized sub-planar slipping zones. The co-seismic rheology of the slipping zones themselves depends on deformation mechanisms and dissipative processes active at the scale of the grain or asperity. The study of such complex multiscale systems requires an interdisciplinary approach spanning from structural geology to seismology, geophysics, petrology, rupture modelling and experimental rock deformation. In this session we aim to convene contributions dealing with different aspects of earthquake mechanics at various depths and scales such as:
· the thermo-hydro-mechanical processes associated with co-seismic fault weakening based on rock deformation experiments, numerical simulations and microstructural studies of fault rocks;
· the study of natural and experimental fault rocks to investigate the nucleation mechanisms of intermediate and deep earthquakes in comparison to their shallow counterparts;
· the elastic, frictional and transport properties of fault rocks from the field (geophysical and hydrogeological data) to the laboratory scale (petrophysical and rock deformation studies);
· the internal architecture of seismogenic fault zones from field structural survey and geophysical investigations;
· the modeling of earthquake ruptures, off-fault dynamic stress fields and long-term mechanical evolution of realistic fault networks;
· the earthquake source energy budget and partitioning between fracture, friction and elastic wave radiation from seismological, theoretical and field observations.
· the interplay between fault geometry and earthquake rupture characteristics from seismological, geodetic, remote sensed or field observations;
We particularly welcome novel observations or innovative approaches to the study of earthquake faulting. Contributions from early career scientists are solicited.
Files for download
Download all presentations (88MB)
Chat time: Tuesday, 5 May 2020, 10:45–12:30
Due to the high confining pressures in the lower crust, the generating mechanisms of lower crustal earthquakes, occurring below the standard seismogenic zone, are puzzling. Their investigation is difficult because the records of such earthquakes, pseudotachylytes, are typically reacted and/or deformed. Here we describe exceptionally pristine pseudotachylytes in lower crustal granulites from the Lofoten Vesterålen Archipelago, Norway. The pseudotachylytes have essentially the same mineralogical composition as their host (plagioclase, alkali feldspar, orthopyroxene) and contain microstructures indicative of rapid cooling (microlites, spherulites, ‘cauliflower’ garnet). Neither the wall rock nor the pseudotachylytes themselves contain hydrous minerals, and no mylonites are associated with the pseudotachylytes. This excludes the most commonly suggested weakening mechanisms that may cause earthquakes below the brittle-ductile transition: dehydration- or reaction-induced embrittlement, plastic instability, thermal runaway, and downward propagation of seismic rupture from shallow faults into their deeper ductile extensions. Hence, we suggest that transient stress pulses caused by shallower earthquakes are the most likely explanation for the occurrence of fossil earthquakes in the analysed rocks from Lofoten.
Earthquakes are short events, but their effects on the tectonic and metamorphic development of their host can be long-lasting. The initial deformation features related to seismic events, which potentially determine these effects, are often overprinted by metamorphism driven by fluids infiltrating the rock along the seismic fault. Because of the anhydrous conditions in the present case, those structures are preserved. The wall rocks to the investigated pseudotachylytes appear undamaged in optical and backscatter electron observation; however, cathodoluminescence imaging of feldspar and quartz reveals healed fractures and alteration zones. Those areas are further investigated with electron backscatter diffraction and transmission electron microscopy to better understand the microstructural and chemical changes during and after the seismic event.
How to cite: Dunkel, K. G., Zhong, X., Morales, L. F. G., and Jamtveit, B.: Highly stressed lower crust: Evidence from dry pseudotachylytes in granulites, Lofoten, Norway, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2890, https://doi.org/10.5194/egusphere-egu2020-2890, 2020.
Strong dynamic weakening at seismic slip velocities in experiments on calcite has been attributed to a combination of grain-size reduction and nanoscale diffusion. However, these experiments were performed mostly dry and it is unknown how fluid-rock interactions affect the deformation mechanisms. The resulting physico-chemical interactions are key in deciphering deformation mechanisms and rheological changes during and after (seismic) faulting in the presence of a fluid phase. It is the interaction of the nanoscale of granular fault materials with fluids that may drive changes in rheological behaviour and fault stability. Considering that faults in the upper crust are major fluid pathways, there is a particular need for deformation experiments under wet conditions that focus on the nanoscale interaction between gouge material and pore fluid.
In order to track and quantify potential fluid – mineral interaction processes in carbonate faults, we have conducted deformation experiments on calcite gouge with water enriched in 18O (97 at%) as pore fluid. The fault gouge was deformed in a rotary shear apparatus at v = 1 m/s and a normal load of σn = 2 and 4 MPa. Raman spectroscopy and nanoscale secondary ion mass spectrometry (nanoSIMS) were used to analyse isotope distribution in the post-experiment samples. The nanostructure was characterised in electron transparent thin foils, prepared in a focused ion beam – scanning electron microscope (FIB-SEM), using transmission electron microscopy (TEM).
Raman analyses confirm the incorporation of 18O into the calcite crystal structure, as well as the presence of amorphous carbon. We identify three new band positions relating to the possible isotopologues of CO32- (reflecting 16O substitution by 18O). In addition, we detected portlandite (Ca(OH)2), pointing to the hydration reaction of lime (CaO) with water. Raman and NanoSIMS maps reveal that 18O is incorporated throughout the deformed volume, implying that calcite breakdown and isotope exchange affected the entire fault gouge.
Considering the oxygen self-diffusion rates in calcite (Farver, 1994) we conclude that solid-state 18O – isotope exchange cannot explain the observed incorporation of 18O into the calcite crystals during wet, seismic deformation. The hydration of portlandite and, calcite containing 18O implies the breakdown and decarbonation of the starting calcite and the nucleation of new calcite grains. Our results question the state and nature of calcite gouges during seismic deformation and challenge our knowledge of the rheological properties of wet calcite fault gouges at high strain rates. The observations suggest that the physico-chemical changes are a crucial part of the deformation mechanism and have implications for the development of microphysical models that allow us to quantitatively predict fault rheology.
John R. Farver, Oxygen self-diffusion in calcite: Dependence on temperature and water fugacity, Earth and Planetary Science Letters, Volume 121, Issues 3–4, 1994, Pages 575-587, doi:10.1016/0012-821X(94)90092-2.
How to cite: Ohl, M., King, H. E., Niemeijer, A., Chen, J., Drury, M., and Plümper, O.: Deciphering deformation mechanisms during seismic slip along wet carbonate faults, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7295, https://doi.org/10.5194/egusphere-egu2020-7295, 2020.
Specularly light reflective fault plane interfaces known as Mirror-Slip Surfaces (MSS’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’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’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 (σn ≈ 10 – 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 (σn ≈ 10 MPa), 0.004 m (σn ≈ 50 MPa) and 0.0026 m (σn ≈ 170 MPa) of shear displacement. The area covered by MSSs systematically increases with displacement to form continuous coatings after 0.042 (σn ≈ 10 MPa), 0.0062 m (σn ≈ 50 MPa) and 0.0036 m (σn ≈ 10 MPa). As displacement rate is increased (10-5 – 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 (σn > 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.
How to cite: Trainor Moss, M., Verberne, B. A., Takahashi, M., and Niemeijer, A. R.: Bridging the Gap Between Seismic and Sub-seismic Mirror-Slip Surfaces in Carbonate Fault Gouge, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7926, https://doi.org/10.5194/egusphere-egu2020-7926, 2020.
During shear failure in rock, fracture damage created within the failure zone causes localized dilation, which, under partially drained conditions, results in a localized pore fluid pressure drop. The effective normal stress within the failure zone therefore increases, and with it the fracture and frictional strengths. This effect is known as dilatancy hardening. Dilatancy hardening may suppress rupture propagation and slip rates sufficiently to stabilize the rupture and postpone or prevent dynamic failure. Here, we study the loading conditions at which the rate of dilatancy hardening is sufficiently high to stabilize failure. We do so by measuring the local pore fluid pressure during failure and the rate of dilatancy with slip at a range of confining and pore fluid pressures.
We performed shear failure experiments on thermally treated intact Westerly granite under triaxial loading conditions. The samples were saturated with water and contained notches to force the location of the shear failure zone. For each experiment, we imposed a different combination of confining pressure and pore fluid pressure, so that the overall effective pressure was either 40 MPa or 80 MPa prior to axial deformation at 10-6 s-1 strain rate. Dynamic shear failure was recognized by a sudden audible stress drop, whereas the stress drop during stabilized shear failure took longer and was inaudible. Local pore fluid pressure was measured with in-house developed pressure transducers placed on the trajectory of the prospective failure.
At effective pressures of 40 MPa and 80 MPa, we observe stabilized failure for a ratio λ (imposed pore fluid pressure over confining pressure) > 0.5. For λ < 0.5, we observe dynamic failure. Of two experiments performed at λ = 0.5 and 80 MPa effective pressure, one showed stabilized failure and one failed dynamically. For λ > 0.5, we observe pore fluid pressure drops in the failure zone of 30-45 MPa for 40 MPa effective pressure, and 60 MPa for 80 MPa confining pressure. The local pore fluid pressure during dynamic failure (λ < 0.5) is 0 MPa, strongly suggesting local fluid vaporization. Of the two experiments at λ = 0.5, the dilation rate with slip is higher for the dynamic failure relative to the stabilized failure.
We show that with increasing effective pressure, dilatancy hardening increases as the local pore fluid pressure drop during failure becomes larger. For λ < 0.5, dilatancy hardening is insufficient to stabilize failure because the local pore fluid pressure drop is larger than the absolute imposed pore fluid pressure. Near λ = 0.5, small variations in dilatancy control rupture stability. For λ > 0.5, dilatancy hardening is sufficient to suppress dynamic failure.
How to cite: Aben, F. and Brantut, N.: On the loading conditions for pore fluid stabilization of failure in crustal rock, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18129, https://doi.org/10.5194/egusphere-egu2020-18129, 2020.
Delayed failure or slip stabilization are expected outcomes of transient dilatancy and associated loss of pore pressure in isolated faults during rupture nucleation. Segall and Rice (1995), for example, developed relationships for pore pressure transients in the rate and state (r-s) friction formalism. They considered pore volume changes that were log(velocity) dependent and, depending on the hydraulic diffusivity and fault/fluid compressibility, could significantly change fault stability and rupture nucleation properties. Despite the theoretical importance of transient pore pressure effects, few laboratory experiments exist that show the effect of variable pore pressure in hydraulically isolated faults. This is due in large part to the difficulties involved in measuring pore pressure directly in isolated faults. We report on triaxial deformation of model sawcut faults in Westerly granite at normal stresses to 197 MPa. Samples were 76.2 mm-diameter cylinders with a fault inclined 30° to the sample axis. Tests were performed on bare surface granite and on faults containing 1 mm quartz gouge. Fault pore pressure was measured directly with a miniature pressure transducer with fast frequency response. Velocity-stepping experiments showed log(velocity) pressure drops as large as 4 MPa that are consistent with Segall and Rice (1995) and often larger than intrinsic r-s dependent strength changes. However, for large velocity steps the initial pressure response was a rapid increase that led to either slow slip or dynamic failure. We attribute this sudden pore pressure increase to rapid compaction as the open pore structure in the gouge became unstable and collapsed. Since this effect is only observed during rapid velocity increase, it is most likely to occur as a rupture front propagates along the fault. In this case, the pore collapse and associated weakening could contribute to an overall stress drop and is likely to slow rupture propagation. For example, a 4 MPa pore pressure rise on a fault with 40 MPa effective normal stress could result in a 2 to 3 MPa loss of shear strength, a strength loss much larger than would be predicted from typical r-s parameters.
How to cite: Lockner, D., Proctor, B., Kilgore, B., Mitchell, T., and Beeler, N.: Dilatancy hardening, rupture stabilization and instability in hydraulically isolated faults, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6217, https://doi.org/10.5194/egusphere-egu2020-6217, 2020.
At the field scale and in most laboratory studies the rupture nucleation mechanism of an earthquake, landslide or glacier stick-slip cannot be directly imaged. Near-field and source effects are thus difficult to observe. We use correlation of highspeed ultrasound images to track shear wave propagation at the rupture nucleation source and in its near-field. The particle velocity and accumulated displacement of the shear wave field emitted by the rupture are observed in-situ on a very dense grid. The grid consists of the ultrasonic imaging plane inside the frictional body and resolution is defined by the ultrasonic wavelenth (0.3 mm). The rupture process is generated by controlling a driving slab through a motor and a granular layer of sand or gravel constitutes the stick-slip behavior. The frictional body is a homemade Poly-Vinyl-Alcohol hydrogel. Although its properties are differing from those of classically investigated rocks, it constitutes a linear elastic material and reproduces rupture processes that are known from the field and rock physics. Through the elastic wave field we observe microslips which precede supershear rupture propagation along the frictional interface. We experimentally show that the source mechanism of a breaking asperity depends on the material contrast of the adjacent halfspaces. Neither a double-couple nor a single-force mechanism perfectly reproduce the experimental data of a rupturing asperity, while micro-slips are well reproduced by a singular shear point force.
How to cite: Aichele, J., Latour, S., Catheline, S., and Roux, P.: Imaging laboratory rupture nucleation at the source: A friction experiment using ultrafast ultrasound , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-22160, https://doi.org/10.5194/egusphere-egu2020-22160, 2020.
Supershear earthquakes are rare but powerful ruptures with devastating consequences. How quickly an earthquake rupture attains this speed, or for that matter decelerates from it, strongly affects high-frequency ground motion and the spatial extent of coseismic off-fault damage. Traditionally, studies of supershear earthquakes have focused on determining which fault segments sustained fully-grown supershear ruptures. Knowing that the rupture first propagated at subshear rupture speeds, these studies usually guessed an approximate location for the transition from subshear to supershear regimes. The rarity of confirmed supershear ruptures, combined with the fact that conditions for supershear transition are still debated, complicates the investigation of supershear transition in real earthquakes. Here, we find a unique signature of the location of a supershear transition: we show that, when a rupture accelerates towards supershear speed, the stress concentration abruptly shrinks, limiting the off-fault damage and aftershock productivity. First, we use theoretical fracture mechanics to demonstrate that, before transitioning to supershear, the stress concentration around the rupture tip shrinks, confining the region where damage & aftershocks are expected. Then, employing two different dynamic rupture modeling approaches, we confirm such reduction in stress concentration, further validating the expected signature in the transition region. We contrast these numerical and theoretical results with high-resolution aftershock catalogs for three natural supershear earthquakes, where we identify a small region with lower aftershock density near the supershear transition. Finally, using satellite optical image correlation techniques, we show that, for a fourth event, the transition zone is characterized by a diminution in the width of the damage zone. Our results demonstrate that the transition from subshear to supershear rupture can be clearly identified by a localized absence of aftershocks, and a decrease in off-fault damage, due to a transient reduction of the stress intensity at the rupture tip.
How to cite: Bhat, H., Jara, J., Bruhat, L., Antoine, S., Okubo, K., Thomas, M., Rougier, E., Rosakis, A., Sammis, C., Klinger, Y., and Jolivet, R.: Signature of supershear transition seen in damage and aftershock pattern, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7109, https://doi.org/10.5194/egusphere-egu2020-7109, 2020.
Shear ruptures propagating along natural faults or simulated faults in analog laboratory experiments present a wide range of rupture velocities. Most ruptures propagate at velocities below the Rayleigh wave speed and Linear Elastic Fracture Mechanics (LEFM) theory has been shown to predict quantitatively well the observed propagation speed. However, early theoretical and numerical work suggested that ruptures may surpass the shear wave speed and propagate at velocities that can reach the longitudinal wave speed. This was later confirmed in laboratory experiments and observed as supershear earthquakes in nature. While the transition from sub-Rayleigh to supershear propagation has been studied extensively, current knowledge of propagation speed in the supershear regime is limited to a couple of idealistic set-ups. Here, we analyse the propagation speed of supershear ruptures along various nonuniform interfaces using simulations and experiments. We show that an approximate fracture mechanics theory describes well supershear rupture speeds as observed in our experiments and simulations. Furthermore, the theory uncovers a critical rupture length below which supershear propagation is impossible. Beyond this critical length, a rupture can sustain supershear propagation for arbitrarily low prestress levels if local non-uniformities cause transition. The presented theory provides a tool to better understand the potential for supershear ruptures in more realistic heterogeneous systems.
How to cite: Kammer, D., Svetlizky, I., and Fineberg, J.: Rupture speed of supershear slip instabilities, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2213, https://doi.org/10.5194/egusphere-egu2020-2213, 2020.
Natural fault surfaces are interlocked, partly cohesive, and display multiscale geometric irregularities. Here we examine the nucleation of deformation and the evolution of shear in such interlocked surfaces using a closed-form analytical solution and a series of laboratory experiments. The analytical model considers an interlocked interface with multiscale roughness between two linear elastic half-space blocks. The interface geometry is based on three-dimensional fault surfaces imaging. It is represented by a Fourier series and the plane strain solution for the elastic stress distribution is represented as a sum of the constant background stress generated by a uniform far-field loading and perturbations associated with the interface roughness. The model predicts the critical stress necessary for failure and the location of failure nucleation sites across the surface, as function of the initial surface geometry.
A similar configuration is adopted in laboratory experiments as carbonate blocks with rough interlocked surfaces generated by tensional fracturing are sheared in a servo-controlled direct shear apparatus. Resistance to shear and surface roughness evolution are measured under variable normal stresses, slip distances and slip rates. We find that the evolution of surface morphology with shear is closely related to the loading configuration. Initially rough, interlocked, surfaces become rougher when normal stress and displacement rate are increased. Under a fixed, relatively low normal stress and fixed displacement rate however, the surfaces become smoother with increasing displacement distance.
The shear of the interlocked slip surfaces is associated with volumetric deformation, wear and frictional slip, all of which are typically observed across natural fault zones. We suggest that their intensities and partitioning are strongly affected by the initial surface roughness characteristics, the background stress, and the rate and magnitude of shear displacement.
How to cite: Sagy, A., Lyakhovsky, V., and Hatzor, Y. H.: Yield and shear of rough interlocked faults: analytical solution and experimental observations, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2565, https://doi.org/10.5194/egusphere-egu2020-2565, 2020.
Dynamic changes in the stress field during the seismic cycle of tectonic faults can control frictional stability and the mode of fault slip. Small perturbation in the stress field, like those produced by tidal stresses can influence the evolution of frictional strength and fault stability with the potential of triggering a variety of slip behaviors. However, an open question that remains still poorly understood is how amplitude and frequency of stress changes influence the triggering of an instability and the associated slip behavior, i.e. slow or fast slip.
Here we reproduce in the laboratory the spectrum of fault slip behaviors, from slow-slip to dynamic stick-slip, by matching the critical fault rheologic stiffness (kc) with the surrounding stiffness (k). We investigate the influence of normal stress variations on the slip style of a quartz rich fault gouge at the stability boundary, i.e. k/kc slightly less than one, by adopting two techniques: 1) instantaneous step-like changes and 2) sinusoidal variations in normal stress. For the latter case, modulations of normal stress were chosen to have amplitudes greater, less or equal to the typical stress drop observed during unperturbed experiments. Also, the period was varied to be greater, less or equal to the typical recurrence time of laboratory slow-slip events. During the experiments, we continuously record ultrasonic wave velocity to monitor the microphysical state of the fault. We find that frictional stability is profoundly affected by variation in normal stress giving rise to a variety of slip behaviors. Furthermore, during strain accumulation and fabric development, changes in normal stress permanently influence the microphysical state of the fault gouge increasing kc and producing a switch from slow to fast stick-slip. Our results indicate that perturbations in the stress state can trigger a variety of slip behaviors along the same fault patch. These results have important implications for the formulation of constitutive laws in the framework of rate- and state- friction, highlighting the necessity to account for the microphysical state of the fault in order to improve our understanding of frictional stability.
How to cite: Scuderi, M. M. and Collettini, C.: Stress triggering and the mechanics of fault slip behavior, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21610, https://doi.org/10.5194/egusphere-egu2020-21610, 2020.
On major plate-boundary fault zones, it is generally observed that large-magnitude earthquakes tend to nucleate within a discrete depth range in the crust known as the seismogenic zone. This is generally explained by the contrast between frictionally stable, velocity strengthening sediments at shallow depths and lithified, velocity-weakening rocks at seismogenic (10’s of km) depth. Thus, it is hypothesized that diagenetic and low-grade metamorphic processes are responsible for the development of velocity-weakening frictional behavior in sediments that make up fault gouges. Previous laboratory studies comparing the frictional properties of intact rocks and powdered versions of the same rocks generally support this hypothesis, however controlling lithification in the laboratory and systematically quantifying frictional behavior as a function of lithification and remains a challenge.
Here, we simulate the lithification process in the laboratory by using mixtures of halite and shale powders with halite-saturated brine, which we consolidate under 10 MPa normal stress and subsequently desiccate. The desiccation allows precipitation of halite as cement, creating synthetic rocks. We vary the proportion of salt to shale in our samples, which we use as a proxy for degree of lithification. We measure the frictional properties of our lithified samples, and equivalent powdered versions of these samples, with velocity-step tests in the range 10-7 – 3x10-5 m/s. We quantify lithification by two methods: (1) direct measurement of cohesion, and (2) measuring the porosity reduction of lithified samples compared to powders. Using these measurements, we systematically investigate the relationship between lithification and frictional slip behavior.
We observe that powdered samples of every halite-shale proportion exhibits predominantly velocity-strengthening friction, whereas lithified samples exhibit a combination of velocity strengthening and significant velocity weakening when halite constitutes at least 30 wt% of the sample. Larger velocity weakening generally coincides with friction coefficients of > 0.62, cohesion of > ~1 MPa, and porosity reduction of > ~50 vol%. Although none of our lithified samples exhibit strictly velocity-weakening friction, this is consistent with the frictional behavior of pure halite under our experimental conditions. Scanning electron microscopy images do not show any clear characteristics attributable to velocity-weakening, but did reveal that the shear surfaces for powders tends to exhibit small cracks not seen in the lithified sample shear surfaces. These results suggest that lithification via cementation and porosity loss may facilitate slip instability, but that microstructural indicators are subtle.
How to cite: Ikari, M. and Hüpers, A.: Quantifying effects of laboratory-simulated diagenetic sediment lithification on frictional slip behavior, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3396, https://doi.org/10.5194/egusphere-egu2020-3396, 2020.
Natural faults experience a variety of frictional, rheological, and stress heterogeneities. To investigate the effects of these heterogeneities on seismic stability and the mode of fault slip behavior, laboratory experiments were conducted using a biaxial shearing apparatus with a 0.76 m by 0.076 m simulated fault where 2.5 to 5 mm thick gouge layers were sheared at applied normal stresses of 7 to 12 MPa for 25 mm of cumulative slip. Laboratory faults consisted of uniform layers of gouge, homogeneous mixtures, and/or heterogeneous patches of talc, quartz, and gypsum minerals. Experiments with a uniform layer of velocity weakening fault gouge revealed the development of two asperities at the highly stressed ends of the fault that could fail independently, and creep fronts that facilitated interaction between asperities. This behavior was also reproduced with simple numerical simulations that employ rate- and state-dependent friction. In other experiments, the fault consisted of patches of alternating velocity strengthening and velocity weakening fault gouges. Patch size and location were varied to understand how earthquake ruptures accelerate or decelerate in this heterogeneous environment. These experiments revealed that a velocity weakening fault patch was more likely to remain stable if located next to a velocity strengthening fault patch. However, stability was dependent on the patch sizes and location relative to where the load is applied. In certain cases, some sections of the fault slipped unstably while others slid stably. These experiments, and matching numerical models, highlight the complexity that can arise on natural faults due to frictional, rheological, and stress heterogeneities.
How to cite: Ke, C.-Y., Leach Cebry, S. B., Shreedharan, S., Marone, C., Kammer, D. S., and McLaskey, G. C.: Laboratory observations of frictional stability and fault zone evolution under heterogeneous friction, rheology, and stress conditions, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4205, https://doi.org/10.5194/egusphere-egu2020-4205, 2020.
Earthquakes fail through a spectrum of slip modes ranging from slow slip to fast elastodynamic rupture. Slow earthquakes, or slow-slip events, represent fault slip behaviors that involve quasi-dynamic, self-sustained rupture propagation. To better understand the mechanisms that limit the slip speed and propagation rates of slow slip, we focus on a particular parameter: the critical frictional weakening rate of the fault surface, kc. When kc is approximately equal to k, the elastic loading stiffness of the fault, complex fault slip behaviors including slow-slip events are observed. If kc has a negative dependence on slip velocity, acceleration during the coseismic phase could decrease kc until it approximates k, terminating in a slow earthquake. Here, we describe the results of laboratory experiments designed to quantify the dependence of kc on frictional slip velocity. We conducted double-direct shear experiments in a biaxial shearing apparatus with 3 mm-thick fault zones composed of quartz powder to simulate fault gouge. We focus on step decreases in slip velocity from 300 to 3 m/s that were performed for a range of normal stresses, from 10 to 20 MPa, which we know to be near the stability transition from stable to unstable sliding defined by k/kc ~ 1.0. Under stable conditions, rate-state friction modeling was used to determine kc for each velocity step. Our data provide direct insight on the stability transition associated with kc(V), including experiments for which slow-slip instabilities grew larger and faster throughout velocity-step sequences. Ultimately, both numerical modeling and observational data indicate that the velocity dependence of kc is an important parameter when considering the mechanisms of slow earthquake nucleation.
How to cite: Krogh, J. and Marone, C.: Exploring frictional velocity dependence as a mechanism for slow earthquake rupture, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-16538, https://doi.org/10.5194/egusphere-egu2020-16538, 2020.
We model mechanics of an aseismic fault creep propagation and conditions when it may lead to the initiation of seismic slip. We do so by considering fault bounding medium to be elastically deformable and fault's interfacial strength to be slip rate- and state-dependent characterized by the steady-state rate-weakening. The fault is considered to be initially locked: a state of slip when interfacial slip velocity is considerably low and arbitrarily less than the steady-state sliding rate for given uniformly distributed prestress.
We find solutions for creep penetration into the fault under geologically relevant loading scenarios (e.g., that of a plate-bounding strike-slip faulting driven by the slip at depth, or that of a rate-weakening patch of a fault loaded by a creep on an adjacent rate-strengthening part due to, e.g., anthropogenic fluid injection). In all the cases, the creep makes its way as a self-similar traveling front characterized by high stress owed to the direct effect; however, the remaining creep profile exhibits a near steady-state sliding. This may imply that a choice from a set of rules for the evolution of state variable—with identical linearizations about steady-state sliding—has no bearing on the creep penetration. Further, we find that the prestress, close to or far from steady-state sliding stress, controls the rate and manner of the creep penetration.
We study slip propagation from an imposed dislocation accrued at a constant rate at one end of a homogeneous fault with the other end either at (1) the free surface of an elastic half-space or (2) strictly locked (buried) in the elastic full space. In both scenarios, no slip instability takes place over aseismic creep propagation distances relatable to the usual elasto-frictional nucleation lengthscale (e.g. Rubin & Ampuero 2005). Instead, in the first case creep propagation leads to the nucleation of the first and all subsequent dynamic events of the emerging cycle at/near the free surface after the creep traversed the entire length of the fault. In the second case, the creep front traverses nearly the entire length of the fault, but, instead of nucleating a dynamic event, the front arrests at some distance from the buried fault end, followed by the continual accumulation of aseismic slip without ever nucleating a dynamic event. These results may be owed to the physical and geometrical invariance of the considered homogeneous fault and may signal the essential role of fault strength heterogeneity, either that of the normal stress and/or frictional properties (Ray & Viesca, 2017, 2019), in defining its seismogenic character, i.e. under which conditions and where on the fault the earthquake slip instability can take place.
How to cite: Ray, S. and Garagash, D.: How fault creep makes its way!, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21286, https://doi.org/10.5194/egusphere-egu2020-21286, 2020.
Earthquakes are among the most catastrophic geological events that last only several to tens of seconds. During earthquakes, many processes may occur including rupturing, frictional sliding, pore fluid pressurization and occasionally frictional melting. However, little direct records of these fast processes remain preserved through geological time. During rapid shearing, frictional melt may form that lubricates the rocks and facilitates further sliding. The frictional melt layer may quench quickly within seconds to minutes depending on its thickness. After quenching, the product pseudotachylyte preserves valuable information about the conditions when the frictional melt was generated. Here, we study pseudotachylyte from Holsnøy Island in the Bergen Arcs of Western Norway, an exhumed portion of the lower continental crust. The investigated pseudotachylyte vein is ca. 1-2 cm thick and free of injection veins along the 2 m visible length of the fault. The pseudotachylyte matrix is made up of fine-grained omphacite (Jd38), sodic plagioclase (Ab83) and kyanite with minor rutile and sulphides. Many dendritic garnets are found within the pseudotachylyte showing gradual grain size reduction towards the wall rock. This suggests that the garnets crystallized during rapid quenching. The stability of epidote, kyanite and quartz in the wall rock plagioclase, and omphacite and albitic plagioclase together with quartz in the pseudotachylyte matrix constrains the ambient P ca. 1.5-1.7 GPa and T ca. 650-750°C. Using Raman elastic barometry, the constrained pressure condition from quartz inclusions in the dendritic garnets in the pseudotachylyte is > 2 GPa. Based on an elastic model (Eshelby’s solution), it is not possible to maintain 0.5 GPa overpressure within a thin melt layer by thermal pressurization or melting expansion. A potential explanation is that GPa level differential stress was present in the wall rocks and the melt pressure approached the normal stress when shear rigidity vanished during frictional melting. Our study illustrates how overpressure can be created within frictional melt veins under conditions of high differential stress, and offers a mechanism that facilitates co-seismic weakening during lower crustal earthquakes.
How to cite: Zhong, X., Petley-Ragan, A., Incel, S., Dabrowski, M., Andersen, N., Austrheim, H., and Jamtveit, B.: Lower crustal earthquake facilitated by overpressurized frictional melts, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5204, https://doi.org/10.5194/egusphere-egu2020-5204, 2020.
Pseudotachylytes (solidified friction melts produced during seismic slip) are considered to be rare in the geological record because they should be typical of particular seismogenic environments characterized by water-deficient cohesive rocks. Here we present field and experimental evidence that frictional melting can occur in “fluid-rich” faults hosted in the continental crust.
Pseudotachylytes were found in the >40 km long Bolfin Fault Zone of the Atacama Fault System (Northern Chile). The pseudotachylytes (1) are associated with a ~1 m thick ultracataclastic fault core which accommodated > 5 km of strike-slip displacement at 6-8 km depth and 280-350°C ambient temperature, (2) cut a ca. 50 m thick damage zone made of sub-greenschists facies hydrothermally altered diorites and gabbros, (3) cut and are cut by epidote+chlorite+calcite bearing veins. The microstructure of the pseudotachylytes include (1) tabular microlites of feldspar hosted in a glassy-like matrix and (2) vesicles filled by post-seismic sub-greenschist facies minerals hosted in a strongly altered matrix of albite, chlorite, and epidote crystals.
Experiments reproducing seismic slip in the presence of pressurized water and conducted with the rotary shear apparatus SHIVA on experimental faults made of the sub-greenschists (hydrothermally altered) facies damage zone rocks from the Bolfin Fault Zone, resulted in the production of vesiculated pseudotachylytes. In these experiments, fault weakening mainly occurred by melt lubrication rather than by pore fluid thermal pressurization.
The identification of pseudotachylytes and its association with intense pre- and post-seismic hydrothermal alteration challenges the common belief that pseudotachylytes are rare. Consistent with the experimental evidence, pseudotachylytes (1) could be a common coseismic fault product in the continental crust, (2) may easily be produced in fluid-rich hydrothermal environments but, (3) are easily lost from the geological record because they are prone to alteration.
How to cite: Di Toro, G., Fondriest, M., Mitchell, T., Gomila, R., Jensen, E., Masoch, S., Bistacchi, A., Magnarini, G., Faulkner, D., Cembrano, J., Mittempergher, S., and Spagnuolo, E.: Field and experimental evidence of frictional melting in fluid-rich faults, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7425, https://doi.org/10.5194/egusphere-egu2020-7425, 2020.
The Raman Spectroscopy of Carbonaceous Materials (RSCM) permits to quantify the degree of crystallinity of carbonaceous materials (CM), which increases upon geological heating. First believed to be a reliable indicator of metamorphic grade (Pasteris and Wopenka, 1991 ; Wada et al., 1996), the quantitative evolution of crystallinity of CM has been proposed as new geothermometers for a wide range of temperature between 200 and 650°C (Beyssac et al., 2002 ; Rahl et al., 2005 ; Lahfid et al., 2010 ; Kouketsu et al., 2014).
According to recent studies, RSCM approach has been used to detect evidence of frictional heating during seismic events from pseudotachylites (Ito et al., 2017) or on fault gouges and breccia (Furuichi et al., 2015 ; Kuo et al., 2018). This new application assumes that CM spectra reflect only the thermal record irrespectively of the potential impact of geological strain on CM crystallinity (Tagiri and Tsuboi, 1979 ; Bonijoly et al., 1982 ; Ross et al., 1991 ; Bustin et al., 1995).
The aim of this study is to reconsider this postulate by using RSCM method in order to understand the effects of seismic deformation on the structure of the carbonaceous material. For this purpose, we analyzed three pseuydotachylyte veins from the Shimanto Belt (Southwest Japan), one from a drilling in the Nobeoka Tectonic Line, another from Okitsu area and a last one from the Mugi area, with RSCM method through high-resolution cross-sections perpendicular to the structure. Samples are composed of weakly foliated tectonic melanges troncated by a millimetric shear plane filled by fine black vitreous material accompanied by injection veins. Filling material presents an important grain-size reduction and embayment structures of sandstones clasts, scattered iron sulfides while element maps show flow textures. These microstructure features are described as characteristics of melt-origin pseudotachylytes (Hasegawa et al., 2019) but could also be produced by an intense comminution along with fluids circulation. Area ratio show a large evolution of CM spectra inside the pseudotachylyte compare to the host rock. In addition, intensity ratio (i.e. R1 in Beyssac et al., 2002) drastically increases inside the pseudotachylyte as expected. However, intensity ratio values are higher than expected values at this temperature, from your own calibration on undeformed samples, and highest values are observed on each rim of the pseudotachylyte. This result suggest that structural evolution of CM is not only controlled by temperature, but also by deformation, in a broad sense. More importantly, these parameters shows a very sharp evolution in few microns along cross-sections, which is at variance with thermal diffusivity models applicable for others intrusive bodies (Aoya et al., 2010 ; Hilchie and Jamieson, 2014).
These observations of step-wise evolution of CM raman parameters suggest that deformation is the principal influencing factor of the evolution of CM crystallinity in fault cores. It therefore questions the maximum temperature reached fault zones, possibly much lower than previously estimated.
How to cite: Moris-Muttoni, B., Augier, R., Raimbourg, H., and Lahfid, A.: Raman Spectroscopy of Carbonaceous Material record in pseudotachylytes: heating or deformation? , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5568, https://doi.org/10.5194/egusphere-egu2020-5568, 2020.
Melting of fault gouge during fast co-seismic slip has been widely documented in laboratory studies. Because the real-time observation and local probing of this phenomenon is experimentally out of reach at the present time, the implication of melting on fault weakening are not yet fully understood,. Physics-based numerical modelling of a synthetic sliding interface could thus be a way to bring a better understanding of this physico-mechanical process.
In this study, we present a numerical work paving the way towards such an understanding. It is implemented in MELODY, a numerical tool combining Discrete Element Method (DEM) and a Multibody Meshfree Approach (i.e. highly deformable DEM). In this model, a small patch of seismic fault filled with granular gouge (composed of perfectly rigid and incrompressible grains with realistic angular shapes) is simulated. By shearing this simulated fault, we produce highly deformable gouge particles within a melted layer.
Numerical results show that melting processes have strong consequences on the fault rheology, by reducing shear stress and favouring the localization of the deformation on the sliding interface. Results are compared with experimental observations on saw-cut faults deformed in triaxial conditions in the laboratory. Future developments including thermal diffusion within the gouge and in the surrounding medium are described.
How to cite: Mollon, G., Aubry, J., and Schubnel, A.: Simulating melting of fault gouge at the local scale, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7928, https://doi.org/10.5194/egusphere-egu2020-7928, 2020.
Despite the numerous advantages of storing CO2 into basalts by dissolving carbon dioxide into water prior to its injection, the large amount of H2O required for this operation poses an increased risk of fluid overpressure into the fault/fracture networks, and renders the seismicity analysis pivotal to upscale this storage method to voluminous basaltic occurrences diffused worldwide.
To deepen our knowledge on the frictional strength, stability, and the healing properties of basalt-built faults, we carried out friction tests on basalts from Mt. Etna using the biaxial deformation machine, BRAVA, and the rotary-shear apparatus, SHIVA (HP-HT laboratory of INGV-Rome, Italy). Specimens were selected for their relative abundance of olivine and pyroxene crystals, i.e. the main sources of divalent cations in silicate rocks necessary to trap CO2 into basalts.
Experiments were performed both on synthetic powdered samples and bare surfaces, at room-dry and water drained-saturated conditions, at room temperature and pressure. Bare surfaces consisted in basalt slabs and hollowed cylinders, which were mounted on BRAVA and SHIVA apparatus, respectively. Samples were subjected to 5 to 30 MPa normal stress (σn) for powdered samples and in the range 5 to 10 MPa for bare surfaces.
At the investigated normal stresses, frictional sliding data obey Byerlee’s law for friction, with the friction coefficient µ = 0.59 – 0.78. Differences in μ mainly reflect sample variability, different experimental configurations, sample geometry, and, to a lesser extent, the boundary conditions (dry/wet). However, in detail, basalt slabs are generally characterized by the highest friction coefficient and hollow cylinders exhibit a slight increase in friction coefficient with increasing shear displacement, due to the progressive slip hardening resulting from gouge production during frictional sliding.
Velocity step increases were conducted on BRAVA after steady values of friction were attained (~ 6.5-7.5 mm for gouge and ~ 3 mm slip for bare surfaces) and consisted in velocity sequences from 0.1 to 300 µm s-1, with ~ 500 μm displacement for each step. Rate-and-state friction experiments show opposite mechanical behavior between bare surfaces and synthetic fault gouge: while bare surfaces experience a transition from rate-weakening at low sliding velocity (V) to rate-strengthening behavior at higher V without any clear dependence on the applied σn, gouge revealed a negative trend of (a-b) with shear velocity at σn > 5 MPa and a velocity-weakening behavior at V ≥ 30 µm s-1, regardless of experimental conditions. We ascribe this different behavior to shear delocalization owing to frictional wear production in bare surfaces, and to shear localization accompanied by grain size reduction along the Riedel R1 and boundary B shear zones in fault gouges, as also confirmed by microstructural analysis.
The velocity weakening behavior of fault gouge, coupled with the fast healing rates retrieved from slide-hold-slide experiments (500 µm displacement cumulated at V = 10 µm/s followed by hold times from 30 to 3000 s), define high strength zones that are potentially seismogenic. Conversely, velocity strengthening behavior of bare surfaces promotes aseismic creep at V ≥ 100 µm s-1, regardless of the faster restrengthening compared to fault gouge.
How to cite: Giacomel, P., Ruggieri, R., Scuderi, M., Spagnuolo, E., Di Toro, G., and Collettini, C.: Frictional strength, stability, and healing properties of basalt faults for CO2 storage purposes, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3771, https://doi.org/10.5194/egusphere-egu2020-3771, 2020.
The detailed characterization of internal fault zone architecture and petrophysical and geomechanical properties of fault rocks is fundamental to understanding the flow and mechanical behaviour of mature fault zones. The Goddo normal fault (Bømlo – Norway) accommodated c. E-W extension related to North Sea Rifting from Permian to Early Cretaceous times . It represents a good example of a mature, iteratively reactivated and thus long-lived (seismogenic?) fault zone, developed in a pervasively fractured granitoid basement at upper crustal conditions in a regional extensional setting.
Field characterization of the fault zone’s structural facies and analysis of background fracture patterns in the protolith have been integrated with in-situ petrophysical and geomechanical surveys of the recognized fault zone architectural components. In-situ air-permeability and mechanical directional tests (performed with NER TinyPerm III air-minipermeameter and DRC GeoHammer, L-type Schmidt hammer, respectively) have allowed for the quantification of the permeability tensor and mechanical properties (UCS and elastic modulus) within each brittle structural facies. Mechanical properties measured parallel to fault rock fabric of cataclasite- and gouge-bearing structural facies differ by up to one order of magnitude from those measured perpendicularly to it (~10 MPa vs. 100-200 MPa in UCS, respectively). Accordingly, permeability of cataclasite- and gouge-bearing facies is several orders of magnitude larger when measured parallel to fault-rock fabric than that perpendicular to it (10-0-10-1 D vs. 10-2-10-3 D, respectively). Virtual outcrop models (VOMs) of the fault zone were obtained from high-resolution UAV-photogrammetry. Field measurements of fracture orientations were used for calibration of the VOMs to construct a statistically robust fracture dataset. The results of VOMs structural analysis allowed for the quantification of fracture intensity and geometrical characteristics of mesoscopic fracture patterns within the different domains of the fault zone architecture.
Results from field, VOMs structural analysis, and in-situ petrophysical investigations have been integrated into a realistic 3D fault zone model with the software 3DMove (Petex). This model can be used to investigate the influence of mesoscopic fracture patterns, related to either the fault zone or the background fracturing, on the hydro-mechanical behaviour of a mature fault zone. In addition, the evolution of the hydro-mechanical properties through time can be assessed by integrating the progressive development of brittle structural facies and fracture sets developed during the incremental strain and stress history into the model. This contribution proposes a geologically-constrained method to quantify the geometry of 3D fault zones, as a possible tool for models to be adopted in stress-strain analysis, hydraulic characterization and in the mechanical analysis of fault zones.
 Viola, G., Scheiber, T., Fredin, O., Zwingmann, H., Margreth, A., & Knies, J. (2016). Deconvoluting complex structural histories archived in brittle fault zones. Nature communications, 7, 13448.
How to cite: Ceccato, A., Viola, G., Antonellini, M., Tartaglia, G., and Ryan, E. J.: In-situ petrophysical and geomechanical characterization and 3D modelling of a mature normal fault zone (Goddo fault, Bømlo – Norway), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6691, https://doi.org/10.5194/egusphere-egu2020-6691, 2020.
The seismicity of Peru is associated with the subduction process of the Nazca plate under South America and characterized by the occurrence of shallow, intermediate and deep earthquakes. In this study, we focus our attention on the rupture process of earthquakes (Mw>6.0 ) that occurred during the period 2018-2019 at intermediate depth (50<h<200 km) and deep depth (500<h<700 km). Focal mechanisms have been estimated from slip inversion of body waves at teleseismic distances (Kikuchi and Kanamori, 1991). We investigate possible differences in the moment rate functions at different focal depths using our results and those provided by SCARDEC database. Furthermore, an estimation of the radiated seismic energy (ER) was provided from the direct integration of the velocity P wave recorded at teleseismic and regional distances, getting values between 1015 to 1016 J. The data were corrected by geometrical spreading, anelastic attenuation, and free surface effect. These results are interpreted in terms of the seismotectonics of the region
How to cite: López Sánchez, C., Buforn, E., Mattesini, M., and Tavera, H.: The rupture process of the Peru intermediate and deep earthquakes, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7243, https://doi.org/10.5194/egusphere-egu2020-7243, 2020.
The release of elastic energy along an active fault is accommodated by a wide range of slip modes. It ranges from long-term slow slip events (SSEs) and creep to short-term tremors and earthquakes. They vary not only in their characteristic duration but also in their magnitude, spatial extend and slip velocities. As all slip modes are related to earthquakes, the understanding of the relationships between the different slip modes and the underlying mechanisms is crucial to assess earthquake hazards in various regions. The exact relation is unclear, as in some regions many slip modes occur simultaneously (e.g. Tohoku-Oki) and in others certain slip modes are completely absent (e.g. Cascadia).
One of the driving factors in the generation of this large variety of slip modes is the interplay of fault heterogeneity and geometrical complexity of the fault system. Using a scaled physical model we test various settings in terms of fault heterogeneity and geometrical complexity. The experimental results are then validated and benchmarked using multi-scale numerical simulations. We describe the system using the rate-and-state frictional framework and introduce the on-fault heterogeneity with variable frictional properties. All properties are the same for analogue and simulation as far as they can be determined or realized experimentally (a-b, vload, Shmax, Shmin, etc...). As analogue material we use segmented, decimetre sized neoprene foam blocks in multiple configurations (e.g. biaxial shear at forces <1 kN) to simulate the elastic upper crust. The contact surfaces are spray-painted with acrylic paint to generate velocity weakening characteristics in between the blocks. The major advantage of using neoprene over other materials, such as gelatine or polyurethane foams, is that it has closed pores and thus exhibits a more favourable Poisson’s ratio in comparison with rocks and shows better elastic strain propagation in the block. Furthermore, all used materials are inert and do not change their properties over time.
We are able to reliably generate frequent stick-slip events of variable size and recurrence intervals. The slip characteristics, such as slip distribution, are in good agreement with analytical solutions of fault slip in elastic media. In this contribution we will highlight the material properties, experimental results and used methodologies to monitor and process the experimental data. Additionally, we are going to give an outlook on the interaction behaviour of multiple faults in dependence of their geometric configuration and the generation of power-law type magnitude scaling relations.
How to cite: Rudolf, M., Podlesny, J., Rosenau, M., Kornhuber, R., and Oncken, O.: Slip modes and interaction in a simplified strike-slip fault system with increasing geometrical complexity, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9369, https://doi.org/10.5194/egusphere-egu2020-9369, 2020.
A scientific vertical borehole (borehole FDB) was rapidly drilled down to 691.7 m during September 2017 – March 2018 after the Mw 7.0 Kumamoto earthquake (mainshock), Japan occurred on 16th April 2016. This borehole penetrated the seismogenic fault called Futagawa Fault which ruptured during the mainshock. Temperature measurements across a newly ruptured fault enable us to detect the frictional heat induced by the high-speed fault slipping, and then to estimate the fault frictional resistance which controls earthquake dynamics. To investigate the frictional heat of its coseismic rupturing, we started temperature measurements in the borehole FDB from May 2018, i.e. two years after the mainshock. We are still repeating the temperature measurements once per two or three months; and have conducted seven times of the measurements until the end of November 2019.
At the drilling site located at Mashiki town, Kumamoto Pref, a ~2.5 m dextral strike-slip coseismic displacement which is the largest displacement of the mainshock was observed on the surface rupture in this area. The borehole FDB consists of a cased interval from the surface down to a depth of ~300 m, and an open hole interval below that, down to the bottom of the borehole. In this borehole, the groundwater level is ~42 m, we measure the water temperature below the groundwater level and assume that the water temperature is the same as that of the formation after they became an equilibrium state after several months after the drilling operation. We measured the temperature and pressure while putting down and pulling up high resolution temperature and pressure sensors at an approximately constant rate of 3 m/min. A positive temperature peak around a fault where resistivity and P-wave velocity obtained from borehole logging abruptly dropped. The temperature depth profile showed time variation possibly including dissipation of the coseismic frictional heat caused by the fault rupturing and the other natural reasons e.g. groundwater flow.
How to cite: Lin, W., Shibutani, S., Kamiya, N., Sado, K., Sugimoto, T., Yamamoto, Y., Yang, X., and Kinoshita, M.: Time variations of the temperature depth profile in a scientific-drilling borehole penetrated through the Futagawa Fault, Japan, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12831, https://doi.org/10.5194/egusphere-egu2020-12831, 2020.