For decades, millions of people have been waiting for the "big one" in either Tokyo, Istanbul or Los Angeles. No one is able to predict where and when such a disaster will happen first. People expect it as a self-evident fact, will experience it as a stroke of fate, and will speak of it as a “tragedy”.

Our aim cannot be (yet?) to predict where and when major earthquakes will occur, because that would amount to claiming to be able to announce in advance where and when the lightning strikes. Nevertheless, we believe that our understanding of the seismic process and associated risk should greatly benefit from the following question: what parameters control whether a dynamic rupture nucleates, grows or stops?

It is crucial to understand the processes and conditions causing the initial stages of catastrophic rock tearing under pressure, and the interplay between mineral- and tectonic-scale factors. Both fluid percolation events and transformation-driven stress transfers can trigger mechanical instabilities ultimately causing rupture nucleation. And once a rupture has nucleated, similar processes should also occur within the damage zone and modulate the ability of small ruptures to “self-propagate” towards large seismic events.

Our lack of understanding is considerable about the exact conditions for rupture nucleation and dynamic propagation. While observational methods help image mechanical instabilities, laboratory experiments provide insights on the physics of the lubrication processes enabling seismic faults to grow under pressure. Unfortunately, there remains a significant gap in scientific communication between researchers using different analytical methods or conceptual views.

Here we outline transdisciplinary connections between the contributions to the session. From seismology to electrical conductivity measurements in the laboratory, from field geology to numerical modelling, from machine learning to mineralogy, from geodesy to mineral physics, here we walk on the frontier of knowledge in order to reshape the central questions that we need to ask to further investigate the rupture phenomenon.

How to cite: Ferrand, T. P., Eberhard, L., Gilio, M., and Parameswaran, R.: Transdisciplinary connections help understand natural mechanisms behind major seismic ruptures, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-10076, https://doi.org/10.5194/egusphere-egu23-10076, 2023.

Although transformational faulting in the rim of the metastable olivine wedge is hypothesized as a triggering mechanism of deep-focus earthquakes, there is no direct evidence of such rim. Variations of the b value – slope of the Gutenberg-Richter distribution – have been used to decipher triggering and rupture mechanisms of deep earthquakes. However, detection limits prevent full understanding of these mechanisms. Using the Japan Meteorological Agency catalog, we estimate b values of deep earthquakes in the northwestern Pacific Plate, clustered in four regions with unsupervised machine learning. The b-value analysis of Honshu and Izu deep seismicity reveals a kink at magnitude 3.7–3.8, where the b value abruptly changes from 1.4–1.7 to 0.6–0.7. The anomalously high b values for small earthquakes highlight enhanced transformational faulting, likely catalyzed by deep hydrous defects coinciding with the unstable rim of the metastable olivine wedge, the thickness of which we estimate at ∼1 km.

How to cite: Li, J., Mao, G., Ferrand, T., Zhu, B., Xi, Z., and Chen, M.: Mechanisms of deep earthquakes unraveled thanks to unsupervised machine learning, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-9809, https://doi.org/10.5194/egusphere-egu23-9809, 2023.

Fluid flow in subduction zones is one of the essential factors of seismic activity in subduction zones. However, the timescale of fluid flow and fluid flow velocity in subduction zones is unclear. In this study, we report antigorite veins with brucite-rich reaction zones in the crust-mantle transition zone of the Oman ophiolite and estimate the timescale and fluid flow velocity during vein formation.

In this study, we observed 28 samples in the lower crust to upper mantle section obtained from the Oman Drilling Project Hole CM1A (Kelemen et al., 2020). The lithology of borehole CM1A consists of lower crust (0-160 m depth), crust-mantle transition zone (160-310 m depth), and mantle section (310-404 m depth), which were mainly composed of altered gabbroic rocks (olivine gabbro, troctolite: 5-95% altered) and completely serpentinized dunite and 70-100% serpentinized harzburgite, respectively. Antigorite-chrysotile (Atg-Ctl) vein network was found in dunite at 160-180 m in Hole CM1A. The matrix of the dunite (lizardite, brucite, and magnetite) is cut by antigorite-chrysotile (Atg-Ctl) vein network. Trace element analysis using LA-ICP-MS revealed that the Atg-Ctl vein is enriched in As and Sb compared to the matrix lizardite, suggesting that the Atg-Ctl veins were formed by fluids interacting with subducting sediments. Some of the Atg-Ctl veins are accompanied by brucite-rich reaction zones. The brucite-rich reaction zone was developed at both sides of the antigorite veins with widths of 0.5 – 4 mm. Elemental mapping of the reaction zone using EPMA and TEM show sharp reaction front at scale from micro- to nano- meter.

Mass balance calculations and thermodynamic considerations of the reaction zone suggest that the formation of the reaction zone was caused by the removal of silica from the host rock during the precipitation of antigorite in the veins. Based on a diffusion model, we estimated the fluid activity is short-lived (2.1 × 10^–1 to 1.1 × 10¹ yr), and the fluid flow velocity of 2.7 × 10^–3 to 4.9 × 10^–2 m s^-1, which is much faster than those observed for the intact mantle and crustal rocks. This fluid flow velocity along the fractures within the mantle wedge is similar to the observed propagation velocities of seismic events in subduction zones. These results suggest that fluid flow in the overlying plate occurs as episodic pulses as observed as the migration of seismicity in the present subduction zones.

How to cite: Yoshida, K., Oyanagi, R., Kimura, M., Plümper, O., Fukuyama, M., and Okamoto, A.: Transient fluid flow recorded in the Crust-Mantle transition zone of the Oman Ophiolite, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-8443, https://doi.org/10.5194/egusphere-egu23-8443, 2023.

The trigger mechanism of intermediate depth earthquakes (30 - 300 km) is a long-standing debate. Many studies showed that these seismic events nucleate along a double-seismic zone within the subducting slab. The seismic events of the lower plane coincide with the depth of major dehydration reactions in the lithospheric mantle. Consequently, it is thought that these events are related to the release of fluids. Several scenarios are currently discussed that might lead to brittle deformation. Among these are dehydration embrittlement and dehydration-driven stress transfer.

Antigorite is one of the most important candidates for fluid release due to its high H₂O content and stability limits within the lower Wadati-Benioff zone. The release of water through antigorite dehydration can be calculated by equilibrium thermodynamics and is mainly a function of temperature. This does, however, not account for deformation (e.g., stored internal strain energy) leading to local variations in the free energy of minerals. In this study we aim to explore the effect of shear stress on the stability of antigorite.

We performed high-pressure and high-temperature experiments in a Griggs rig. We used intact drill cores of two different starting materials for our experiments: a foliated and an isotropic antigorite-serpentinite. Both starting materials did not contain relict olivine and/or orthopyroxene. We run our experiments at 620 to 650 °C with a confining pressure of 1.5 GPa and a strain rate of 10^-6 s^-2. Subsequent analyses of the experimental runs revealed no dehydration products within the bulk sample. However, we observed the formation of ultra-fine grained (< 100 nm) olivine and orthopyroxene along narrow zones, which are orientated 30 to 40 ° with respect to the compression axis. These zones are similar in all runs and independent of the starting material microstructure. We thus propose that shear stress localization within our cylindrical sample triggered the dehydration. Within subduction zones local variations in stress field due to mineralogical or textural heterogeneities could promote dehydration, eventually leading to seismic events through stress transfer.

How to cite: Eberhard, L., Plümper, O., and Stünitz, H.: Experimental observation of antigorite dehydration triggered by shear stress at subduction zone pressure and temperature conditions, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-3360, https://doi.org/10.5194/egusphere-egu23-3360, 2023.

Geothermal energy is one of the renewable energy sources that can help mitigate climate change. Rocks are elasto-plastic materials at low pressure and temperature, and the deformation is accommodated along localized shear bands (brittle behavior); at high pressure and temperature rocks are elasto-visco-plastic and the deformation is homogeneous (ductile behavior); the transition between the two behaviors is called the brittle to ductile transition (BDT). Rocks capable of hosting geothermal fluids hot enough for electricity production (above 100°C) might be at or beyond the BDT.

The importance of the BDT of rocks stems from the fact that the largest earthquakes occur there, and it corresponds to a major decrease in the permeability of the crust. In the literature, the mechanical properties of rocks across the BDT are relatively well known, though only lately the effect of the presence of fluids and their chemical composition has been investigated by few research groups. However, certain fluid compositions might lead to mineral dissolution, precipitation, weakening and alteration, which in turn affect the mechanical properties of rocks.

To investigate the effect of water, fluid chemistry and alteration, triaxial experiments on a porous silicate sandstone (Adamswiller sandstone) with and without water at 100°C were done, to understand in detail the effect of water on deformation. To assess the evolution of the mechanical properties, complex electrical conductivity, permeability and ultrasonic seismic velocity were monitored in situ to fully describe the rock properties across the BDT.

The experiments resulted in a complete characterization of the failure and yield envelopes of Adamswiller sandstone across the BDT, together with the electrical conductivity, permeability and ultrasonic seismic velocities. The presence of water lowers both the peak stress and yield stress. Electrical conductivity decreases beyond the BDT due to porosity reduction, following the reduction of permeability.

How to cite: Lazari, F., Violay, M., and Begue, F.: Effects of water on the brittle to ductile transition of sandstones, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-6190, https://doi.org/10.5194/egusphere-egu23-6190, 2023.

The reactivation and stability of faults depend on a number of parameters, like rock composition, (effective) normal pressure, fault roughness, and loading rate. However, not much in known about the impact of temperature on faulting behavior. Using a Paterson-type gas deformation apparatus, triaxial compression experiments were conducted on dry Carrara marble samples containing a saw-cut oriented at about 40° to the axial stress direction. The tests were performed at constant axial strain rate of 1x10^-5 s^-1, confining pressures, P, between 30 and 150 MPa, and temperatures, T, in the range of 20 to 600°C. Under these conditions, intact Carrara marble deforms mainly in the semi-brittle regime and brittle, localized, deformation associated with strain weakening occurs only at room temperature and P < 100 MPa. At similar temperature, saw cut samples show formation of a new fracture zone inclined at 30° to the loading direction at P = 30 MPa and fault reactivation with stable sliding on the preexisting fault at P = 50 MPa. At higher temperatures up to 400°C and pressures < 100 MPa, we observed a mixture of matrix deformation and unstable (stick-slip) sliding on the fault. The peak stress at the onset of fault reactivation increased with P and T, resulting in higher associated peak strain. Also, the peak stress drop increased with increasing peak stress. At high T (>400°C) and P (>100 MPa) the fault remained locked and samples revealed ductile matrix creep with strain hardening, where the strength is almost similar to the strength of intact sample deformed under similar conditions. Microstructural observations reveal intense microcracking at the lowest P-T conditions. Samples exhibiting stick-slip behavior show a thin, discontinuous gouge layer and high twin density in specimens with late (high stress) fault reactivation. Bulk creeping samples reveal less damage and the fault appears to be partially sealed. Electron backscatter diffraction measurements suggest a slightly increasing crystallographic preferred orientation in the direct neighborhood to the fault compared the matrix under most conditions. Our results indicate that the fault reactivation stress of marbles increases with both, pressure and temperature, limited by the frictional strength. Above the brittle-ductile transition, the strength is limited by the bulk flow strength, which depends on total strain due to strain hardening, eventually leading to failure at high strain.

How to cite: Rybacki, E., Niu, L., Zhou, Y., and Xu, X.: Experimental reactivation of faults in marble across the brittle ductile transition, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-6440, https://doi.org/10.5194/egusphere-egu23-6440, 2023.

Faults can be reactivated by fluid injection and pore pressure increase in the rock volumes surrounding the fault zone. Induced earthquakes represent only one of the possible responses of active faults to pore pressure perturbations, since other strain transients characterize the spectrum of fault-slip behavior. A series of fluid injection experiments, designed and undertaken in the framework of the ERC Fault Activation and Earthquake Ruptures (FEAR) project, will be conducted in the Bedretto Underground Laboratory for Geosciences and Geoenergies (BULGG, Switzerland) to understand fault reactivation processes on a target well-identified fault zone. Small and accessible faults are to be instrumented to monitor deformation and seismicity during both fluid injection and fault reactivation.

The mineralogical, microstructural, and hydraulic properties of the target fault zone are investigated to characterize the fault-slip behavior. Characterization of the frictional response is achieved through a suite of laboratory rock-deformation experiments using both double-direct and rotary experimental apparatuses. Fault stimulation by fluid pressurization was also simulated in laboratory by using an injection protocol reliable for the in-situ hydraulic stimulation and consisting of stepwise pore fluid pressure increase. Experiments undertaken at low velocity with the double-direct apparatus (BRAVA) suggest that the selected fault, composed of mixed phyllosilicate-granular materials, is frictionally stable but yet can be dynamically reactivated by hydraulic stimulation.

Experiments were also performed on the fault gouge from the target fault with the rotary experimental apparatus (SHIVA). First, we apply half of the stresses measured at depth in the underground laboratory to accomplish the operating capability of the apparatus: 12 MPa normal stress, 7.5 MPa confining pressure and 1.5 MPa pore fluid pressure. Second, we imposed a slip rate of 10^-5 m/s for 0.01 m to have an equally compacted and textured layer. Third, we applied a shear stress so that an equivalent slip tendency of 0.35 is achieved (ca. 2.7 MPa), and kept it constant. We then increased stepwise the pore fluid pressure by 0.1 MPa every 150 s. This allows the spontaneous nucleation of slip events. After fault reactivation, the maximum slip velocity was set to 0.1 m/s. The fluid injection sequence results in a first reactivation (R1). Thanks to the nominally infinite slip available in SHIVA we run a second injection sequence up to a reactivation (R2).

Our experiments show two different styles of reactivation between R1 and R2. R1 reactivation is abrupt, with slip rate accelerating up to 0.1 m/s. Instead, R2 has a stage in which slip rate oscillates (0.5-3 mm/s) just before the last step of pore pressure increase leads to acceleration to 0.1 m/s. This would suggest a role for the shear fabric developed during the first reactivation, in which extensive grain size reduction might have led to stiffening of the fault, responsible of the oscillatory slip. This frictional behavior suggests the importance of considering the effect of texture development during multiple cycles of seismic slip. The generalization of our data and observations will contribute to shed light on the mechanics of faults and induced earthquakes by fluid pressure increase.

How to cite: Aretusini, S., Cornelio, C., Volpe, G., Pozzi, G., Spagnuolo, E., Di Toro, G., Collettini, C., Meier, M.-A., and Cocco, M.: Fault reactivation by fluid injection: insights from laboratory friction experiments with multiple reactivation sequences, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-13444, https://doi.org/10.5194/egusphere-egu23-13444, 2023.

On 11 November 2019, the Le Teil Mw4.9 earthquake occurred in southeast France, in the vicinity of a surface quarry. We focus this work on the effect of hydraulic recharge linked to the infiltration of meteoric water in the fault zones in the period preceding the earthquake. In the reference simulation, we used the in situ soil moisture at 30 cm depth (Berzème station) as surface boundary conditions.

We describe first the local 3D fault system from an updated geological model and the boundary conditions that are used to calculate the pressure variations at depth using a double permeability model.

The movement of moisture in partially-saturated media is then simulated by the Compass code (1) during the period 2015-2019. A maximum overpressure takes place near the junction of the three-fault system at around 1,200 m depth. Moreover, the calculated increase in pore fluid pressure is maximum during 2015-2019 just before the earthquake of 11 November 2019. Additionally, the surface soil moisture (SSM) data acquired by the SMOS satellite (2) are used to extend the study period between 2010 and 2015.

A sensitivity study carried out on the main hydraulic parameters allows us to estimate that the overpressure linked to the hydraulic recharge of the fault system is between 0.7 and 1 MPa at about 1200 m depth before the seismic event.

Finally, we compare this result with the maximum Coulomb stress change linked to the mass withdrawal from the surface quarry over the two past centuries (3). The conclusion is that the hydraulic effect is about two and a half times larger than the cumulative effect of the mechanical stress release due to the mass removal from the surface quarry.

(1) https://github.com/BRGM/ComPASS

(2) Li, X., Wigneron, J.-P., et al.: The first global soil moisture and vegetation optical depth product retrieved from fused SMOS and SMAP L-band observations, Remote Sensing of Environment 282, 113272, 2022. https://doi.org/10.1016/j.rse.2022.113272

(3) Maury, J., Guillon, T., Aochi, H., Bazargan, B., and Burnol, A.: Assessing the effect of mass withdrawal from a surface quarry on the Mw4.9 Le Teil (France) earthquake triggering, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-2742, https://doi.org/10.5194/egusphere-egu22-2742, 2022.

How to cite: Burnol, A., Armandine Les Landes, A., Raucoules, D., Aochi, H., Maury, J., Allanic, C., and Bazargan-Sabet, B.: Investigating the effect of a heavy rainfall episode on the Mw4.9 earthquake of 11 November 2019 at Le Teil (France), EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-8363, https://doi.org/10.5194/egusphere-egu23-8363, 2023.

Exhumed metamorphic rocks from fossil subduction zones represent a unique source of information on the microscale deformation mechanisms and stress history within the seismogenic domain in subduction zones. Microstructural analysis of these rocks yields insight into processes operating on length and time scales generally inaccessible for active systems due to limitations in surface-based geophysical and geodetic experiments.

We studied high-pressure – low-temperature blueschist facies metamorphic cherts, representative of the upper oceanic crust, exhumed from the Franciscan subduction complex and exposed at Mt. Diablo, California. These rocks underwent intense deformation at about 30 km depth. Their microstructural record reflects repeated superposition of different deformation stages, including long-term ductile deformation by viscous creep, short-term brittle failure followed by vein formation, and transient crystal plastic deformation. As such, the samples are taken to be a representative rock volume reflecting processes active in the seismogenic zone and are used as a gauge, recording the history of stress and fluid pressure near the plate interface at depth.

The microstructural record of episodic changes in the far field stress is to be isolated from small-scale heterogeneities in the stress field due to contrasting material properties. For instance, pure quartz veins record pronounced crystal plastic deformation at high stress, while embedded in a fine-grained polyphase matrix that undergoes viscous deformation by dissolution precipitation creep. In this case, the weak matrix causes stress concentration in the stiff veins.

Despite the limits given by the sample size and heterogeneity, the microstructural characteristics indicate distinct deformation stages at variable stress levels, repeatedly superimposed on each other. Here, the formation of tensile cracks is attributed to sudden stress changes at sufficient pore fluid pressure, widening and sealing of these cracks to transient deformation during stress relaxation, and stages of crystal plastic deformation, particularly distinctive in vein quartz, to high peak stresses attained by rapid loading. Based on these systematic observations, we present a conceptual generic model for the recorded episodic changes in the mode of deformation and the underlying cyclic stress history. We then discuss how stress changes as reflected by the microstructural record can be ascribed to the seismic cycle, with the respective seismic events having occurred at some time, and somewhere in the vicinity of the sample, along the plate interface. 

Interestingly, the number of recorded stress and deformation cycles is limited, generally not exceeding two or three cycles. When comparing the expected residence time of a HP - LT metamorphic rock in the given depth range with the present-day frequency of seismic events along the plate interface in a subduction zone, this observation indicates that only a small portion of the expected large number of seismic events has left a marked imprint, whereas the effects of the vast majority remain beneath the limits of detection. We suspect that the noticeable high-stress events are related to nearby fault propagation resulting in a vertical shift of the plate interface, presumably being prerequisite for the transfer of a rock from the subducted lower plate to the hanging wall, in order to become exhumed.

How to cite: Schwichtenberg, B., Wassmann, S., and Stöckhert, B.: Long-term viscous creep versus short-term brittle/plastic deformation in the seismogenic zone - the microstructural record of cherts from Mount Diablo, California, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-16761, https://doi.org/10.5194/egusphere-egu23-16761, 2023.

A sample of banded iron formation from a drill core from the Musselwhite Gold Mine, Ontario, Canada, shows millimeter-wide opaque veins at a low angle to the foliation with perpendicular offshoots. The veins are non-crystalline, structureless and chemically homogeneous, with oxide weight percentages of 35 % FeO, 40 % SiO2, 5 % MnO, 5 % MgO and 1 % CaO. The remaining ca. 15 % might be attributed to water content. Despite the water, the composition is very similar to the bulk rock composition, consisting mainly of grunerite and quartz. The overall geometry of the veins resembles the appearance of a pseudotachylyte with injection veins, partly sharp boundaries, and clasts of the host rock. However, no other indicative observations such as flow banding, quenched margins or corrosion of clasts can be made. Instead, the vein is overgrown by euhedral quartz, nucleating from wall rock quartz crystals. Some of the material is emplaced in a calcite vein where the calcite seems to be fragmented. EBSD orientation data show that the fragments have the same orientation as calcite crystals remaining attached to the wall rock. Calcite crystals are therefore not displaced, but rather dissolved or replaced by the vein filling material, but without leaving a chemical signature. We therefore favor an Fe-Si rich gel-like injection origin of the veins. It remains unclear, how the gel was produced and how it is mechanically possible to inject such a gel into a solid rock and replace calcite.

How to cite: Hawemann, F., Passchier, C., and Biczok, J.: Fe-Si-gel like injections in banded iron formations of the Musselwhite Mine (Canada), EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-5259, https://doi.org/10.5194/egusphere-egu23-5259, 2023.

Pseudotachylytes (frictional melts produced during seismic slip) in the metamorphosed anorthosites of the Lofoten archipelago preserve a record of seismic rupture in the dry lower crust at 650–750 °C, 0.8 GPa. Pyroxene deformation microstructures associated with preseismic loading and coseismic fragmentation reveal strongly localized transient stresses that presumably reached GPa-level magnitude. However, such transient high stresses have never been measured in exhumed seismogenic faults. In this work, we use high-angular resolution electron backscatter diffraction (HR-EBSD) on pyroxene grains to obtain spatial datasets of residual stresses retained in the crystal lattice. With these data, we aim to reconstruct the progressive build-up and release of transient high stress as it is recorded in the pyroxenes, and whether this recorded stress is related to preseismic loading or coseismic fragmentation.

The analysed anorthosite wall rock of a pseudotachylyte at the Nusfjord locality consists mostly of plagioclase, diopside, and enstatite, with diopside forming the main target minerals of this study. HR-EBSD maps were obtained in the wall rock at various distances from the pseudotachylyte interface along a 10 mm long transect, and on survivor clasts within the pseudotachylyte.

EBSD reveals that most diopside in the wall rock contains micron-scale deformation twins, except within 50 microns of the pseudotachylyte where it is fragmented. Residual stresses obtained via HR-EBSD vary along the transect, and are generally lower at greater distance from the pseudotachylyte. The highest values approximately coincide with the lithostatic pressure. The residual stresses are not in agreement with the very high transient stresses (>1 GPa) expected during the rupture propagation. Rather, the analysed diopside recorded the progressive build-up of stress during preseismic loading.

How to cite: van Schrojenstein Lantman, H. and Menegon, L.: Reconstructing the localized transient high stress state of seismogenic faults in the lower crust, Lofoten, Norway, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-9750, https://doi.org/10.5194/egusphere-egu23-9750, 2023.

Coseismic fracturing of the lower crust is an effective mechanism for creating permeable pathways for fluids to infiltrate and interact with the host rock, thus effectively altering the rheology of otherwise anhydrous and strong lower-crustal rocks. Most of the fracturing and fragmentation that facilitate fluid infiltration occurs in the damage zone of seismogenic faults. In this study, we have focused on characterizing the damage zone adjacent to a lower-crustal pseudotachylyte (solidified frictional melt produced during seismic slip) to understand the fracture generating and healing processes during a seismic event.

The Nusfjord East shear zone network (Lofoten, Norway) contains coeval pseudotachylytes and mylonitized pseudotachylytes that formed at lower-crustal conditions within anhydrous anorthosites. We present a micro- and nanostructural analysis of plagioclase grains in the damage zone of a natural pseudotachylyte using focused ion beam (FIB) prepared scanning transmission electron microscopy (S/TEM), electron backscatter diffraction (EBSD) analysis, electron microprobe analysis (EMPA), and SEM-cathodoluminescence (CL) imaging.

The damage zone of the host anorthosite is characterized by a network of fractures with minimal offset, consistent with a pulverization-style fragmentation process. CL intensities differentiate primary plagioclase (plagioclase₁), from secondary plagioclase neoblasts (plagioclase₂) filling some of the fractures. Plagioclase₁ grains often exhibit a diffuse CL intensity zonation from bright grain cores to a dark grey in healed cracks, while plagioclase₂ have a uniform mid-tone grey CL intensity with dark grain boundaries. CL zonation in the plagioclase₁ does not correlate with EMPA major element maps nor EBSD misorientation maps. TEM foils targeted key microstructure domains characterized by CL: (1) bright CL plagioclase₁ core, (2) dark CL plagioclase₁ in healed cracks, (3) transitional CL from bright to dark across a healed crack, and (4) plagioclase₂ neoblast. Results from S/TEM show that the dark CL spanning the healed cracks is associated with a high concentration of crystalline nanoparticles. In contrast, bright CL is associated with a few scattered dislocations, no nanoparticles, and numerous dispersed Ba-Ti-oxide nanograins. The mid-tone grey CL plagioclase₂ neoblast have the lowest dislocation density. Follow-up Transmission Kikuchi Diffraction (TKD) and NanoSIMS analyses on the nanoparticles in the healed cracks and the plagioclase₁ grains immediately next to these cracks will help further elucidate the origins of the nanoparticles and the CL intensity zonation.

How to cite: Michalchuk, S. P., Dunkel, K., Ohl, M., and Menegon, L.: Deformation and healing processes in the damage zone of a lower-crustal seismogenic fault, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-3414, https://doi.org/10.5194/egusphere-egu23-3414, 2023.

Earthquakes and stress distribution inside the earth depend on the rheology of rocks and their constituent crystals at variable conditions of temperature (T) and pressure (P). The contrast in the thermoelastic properties between a mineral inclusion and its surrounding host often leads to a positive inclusion residual pressure (P_inc) at ambient conditions, which can be used to retrieve the P and T of entrapment (e.g. Angel et al. 2015). In addition, the evolution of the inclusion residual strain and P_inc as function of P and T also provides the opportunity to explore the rheology of the crystals involved.

In this study, we explored the rheology of zircon inclusions within pyrope-rich garnet by in situ Raman spectroscopy at high T.

Because garnet has a larger thermal expansion than zircon, inclusions showing a positive P_inc at ambient conditions experience continuous relaxation upon heating until the P gradient disappears. Available equations of state (EoS) can predict the experimental results within uncertainties, implying that the system behaves purely elastically up to a certain temperature. At higher T, zircon inclusions have a negative P_inc, which increases in magnitude according to the EoS predictions. However, at T corresponding to P_inc of about -0.2(5) GPa, the residual strains deviate from those predicted by EoS and the inclusion approaches the strain of a free zircon crystal at the same T. We interpret such a deviation as the result of plastic relaxation of the system.  On cooling, a new stress gradient in the host and a positive P_inc in the inclusion developed within the same T range where a negative P_inc was observed upon heating. Importantly, the new residual strains can be predicted by the EoS only if the entrapment conditions correspond to the first T where P_inc = 0 after plastic deformation occurred. Thus, the system underwent resetting within the time-scale of laboratory experiments. In addition, multiple heating-cooling cycles carried out on the same inclusions show that the maximum negative P_inc attainable does not change within a T range of about 200 K. These results suggest that the resistance to plastic deformation (i.e. yield strength) of garnet decreases under tensile stress. Therefore, we conclude that the sign of the stress field affects the yield strength of crystals and may have important consequences on the overall rock rheology and related processes.

Financial support by the Alexander von Humboldt Foundation and the ERC grant agreement 714936 (ERC-STG TRUE DEPTHS) to M. Alvaro

Angel, R. J., Nimis, P., Mazzucchelli, M. L., Alvaro, M. & Nestola, F. (2015). Journal of Metamorphic Geology, 33(8), 801-813.

How to cite: Campomenosi, N., Angel, R. J., Alvaro, M., and Mihailova, B.: Rheology of host-inclusion mineral systems by in situ Raman spectroscopy, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-1837, https://doi.org/10.5194/egusphere-egu23-1837, 2023.

Earthquakes are generated through the brittle failure of rocks at depth. While earthquakes are generally caused by far–field tectonic stresses, the atomic–scale mechanisms that actually trigger brittle failure in dry ductile crustal rocks are still uncertain. Quartz, a widespread mineral in the lower crust, undergoes an instantaneous polymorphic transformation from the α to β phase at pressure and temperature conditions compatible with the estimates of several lower–crustal paleo–earthquakes recorded as pseudotachylytes. The α–β quartz transition is displacive, reversible and, as α–quartz approaches the transition temperature at constant pressure, its volume increases non–linearly but without sudden jumps. In contrast, near the phase–transition temperature, the bulk modulus of quartz drops from ~30 GPa to almost zero and then abruptly rises to more than 70 GPa within a temperature range of only 10 K (Lakshtanov et al., 2007).

Due to the confined space, near the α–β transition, a quartz inclusion in a garnet host is expected to develop strong differential strains and consequently will impose strong differential stresses on the surrounding host crystal. To check this hypothesis, we have applied in situ high–temperature Raman spectroscopy to quartz inclusions in garnet to monitor the development of structural deformation via the atomic dynamics at temperatures across the phase transition temperature T_c = 847 K for a free quartz crystal at atmospheric pressure. The temperature behaviour of the phonon wavenumbers ω of a quartz inclusion, in particular the hardening and disappearance of a minimum in ω(T) for the A modes near 208 and 464 cm^-1 (involved in the α-β phase transition) as well as the persistence of Raman activity of the modes at ~128 cm^-1 and ~355 cm^-1 above T_c, reveals the accumulation of abnormally high strain in the confined quartz grains in the vicinity of the expected phase transition. Consequently, the corresponding stored elastic energy in the inclusion is released through the inclusion-host boundary into the host matrix while crossing the α–β transition, causing the garnet around the quartz inclusion to fracture or even, in some cases, shatter due to the large differential stresses developing in the inclusion at its transition.  Inclusions of apatite and zircon in the same garnets remain unchanged at the same conditions, excluding the fracturing being caused by the host garnet itself.

We propose that this process can create sufficient fracturing in lower–crustal garnets, which can in turn accumulate into planar fractures along garnet-rich layers and thus trigger brittle failure and seismicity.

References

Lakshtanov, D.L., Sinogeikin, S.V., Bass, J.D., 2007. High-temperature phase transitions and elasticity of silica polymorphs. Physics and Chemistry of Minerals 34, 11-22.

How to cite: Gilio, M., Morana, M., Angel, R., Mihailova, B., and Alvaro, M.: Does the polymorphic transition in quartz trigger lower crustal earthquakes?, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-13491, https://doi.org/10.5194/egusphere-egu23-13491, 2023.

An intense and unusually seismic activity, occurred during September-October 2013, in Galati-Izvoarele region, situated in the central-eastern part of Romania, between two main crustal faults, Sf. Gheorghe and Peceneaga Camena fault. With several hundred (~400) earthquakes recorded in a short time, the activity was considered as seismic swarm. The magnitude M_L was always below 4, with three shocks of magnitude 3.9, accompanied by specific seismicity bursts and focal depths ranging from 1 and 40 km. The focal mechanism solutions of the studied earthquakes obtained from P-wave polarities generally show normal faulting, with an important strike-slip component in several cases. For the seismic source delineated in Galati-Izvoarele area, the stress field has an extensional stress regime (σ1 almost vertical), with maximum horizontal stress (SHmax) oriented in the NNW-SSE direction. The resulting SHmax orientation and normal fault regime with a radial component (R′=0.5) are consistent with the observed geological setting.

The results of this study are useful for revealing the crustal stress field, and, as such, for assessing past and current tectonic activities and potential future earthquake generation.

Also, transdisciplinary studies can trigger unexpected collaborations between researchers from diverse fields to understand the processes and conditions causing the initial stages of rock failure, and the interplay between mineral and tectonic scale processes.

How to cite: Craiu, A., Diaconescu, M., Craiu, M., Mihai, M., Armeanu, I., and Marmureanu, A.: Stress field in NW Galati seismotectonic area (Romania):  insights from the inversion of earthquake focal mechanisms, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-16133, https://doi.org/10.5194/egusphere-egu23-16133, 2023.

Seismic activity in Romania encounters a variety of tectonic domains, from crustal earthquakes, which occur along active faults, to intermediate-depth seismicity that extends down to 200 km depth and which is limited in a small subcrustal seismogenic volume beneath the SE bend of the Carpathians arc. Crustal depth seismicity in Romania is distributed throughout the territory, the areas with important seismic activity and which are also analyzed in this paper being: Vrancea, Fagaras-Campulung, Banat, Dobrogea zones.

Using the polarity of first arrivals methodology (FOCMEC software developed by Havskov et. al 2020), we calculate the focal mechanisms for crustal depth earthquakes that occurred in these areas between 2012 and 2022. We then derive the regional distribution of the stress field through a linear inversion using the focal mechanisms obtained in this study, supplemented by the solutions of the REFMC catalog (Radulian et al 2020). Inversion results vary from the compressive regime in the SE Carpathians bend zone, to strike-slip regime in Banat zone, and extensive regime in Dobrogea area. The stress field configuration is matching generally the configuration of the global stress pattern as shown by the World stress map with the exception of some significant deviations that reflect local conditions. 

The analysis of focal mechanisms as well as the stress field provides a basis for transdisciplinary discussions and collaborations between researchers from various fields.

How to cite: Mihai, M., Craiu, A., Craiu, M., Marmureanu, A., and Radulian, M.: Stress field evaluation in the earthquake-prone crustal zones of Romania, based on a comprehensive and updated focal mechanisms catalog, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-16503, https://doi.org/10.5194/egusphere-egu23-16503, 2023.