Displays

Small seismological events recorded in southern Norway in the period of 2017-2018 were used to calculate the sudden co-seismic temperature increase using a simple stress-drop model. In order to estimate the net production of thermal energy, both, industry explosions and natural events were included.  The range of resultant average temperature rise, with a maximum of ~ 143 ° C for a M_{w =}3.5, is proposed as an additional constituent that explain the weakness areas related to the high amount of intracrustal seismicity, mainly regarding the anisotropic thermal expansion of rocks and the flash heating thermal process produced by historical earthquakes with magnitudes over . The temperature values were subsequently used to estimate the thermal energy in 2D and 3D cumulative patterns in the area, as well as the total amount of energy that is available regarding seismic activity. The results were correlated with existing geological information, considering lineaments and heat flux data. Areas with high values of thermal energy seems to be spatially linked with both high heat flux zones and high density of lineaments, mainly to the south of the Trøndelag Platform as well as to the south of the Møre basin.

How to cite: Estay, R.: Thermal energy pattern related with the temperature increasing due to intraplate and induced seismicity, Southern Norway, 2017-2018, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10759, https://doi.org/10.5194/egusphere-egu2020-10759, 2020.

Frictional energy generating during an earthquake has been well studied in the last decades and quite a few laboratory experiments have been carried out recently with the objective to quantify and describe this type of energy in a better way.  In our research we modelled the temperature rise during a simulated seismic event and the consequent equivalent heat released using the ANSYS® Mechanical software. Our approach is using the Finite Element Method to model a symmetrical fault plane where several parameters such as density, pressure, structural and thermal material characteristics are set according the conditions of a compressional tri-axial test. Natural and forced models were explored applying the Mohr-Coulomb failure criteria. Using a temporal window similar to a realistic situation, we are capable to observe the differences that occur during the stick-slip behavior in the co-seismic rupture process. On the other hand, the time lapse allows us to observe model and infer how the heat is generated and transferred around the fault plane. As a preliminary result, a variation of approximately 1.5°C was obtained simulating the conditions for a laboratory induced micro-seismic event modelled as a tri-axial test under 10 MPa of confining pressure and 20 MPa as vertical pressure, with velocities in the order of 1.5 mm/s.

How to cite: Uslar, A.: Simulation of frictional heat generation due to underground motion, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10795, https://doi.org/10.5194/egusphere-egu2020-10795, 2020.

The heat transfer through the nappes of the Lepontine Dome (Central Alps, Ticino, Switzerland) produced metamorphic amphibolite-facies isogrades that locally dissect the tectonic contacts. This large-scale observation, suggesting a thermal amphibolite-facies event after thrusting and nappe formation, is however at odd with the extremely pervasive mineral and stretching lineation (NW-SE directed) that attests non-coaxial deformation during shearing at similar metamorphic conditions.

To solve this apparent paradox we performed 2D thermo-kinematic simulations in which we investigated the relationships between nappe geometry and the geometries of isogrades. The numerical simulations are based on the finite difference method. We evaluate the relative importance of velocity, thermal diffusion and advection, and geometry of the thrust sheets, on the geometrical relation between tectonic contacts and isogrades. We calculate the thermal evolution and peak temperatures in order to compare the numerical results with field and petrological data collected along the Simano and Cima Lunga nappes.

In the field, the alternation of lithotypes is parallel to the nappe boundaries and constant over their whole length (order of kms). Passing from the Simano to the Cima-Lunga nappe, the transition between the nappes is marked by a progressive change in the texture of gneisses, in which the porphyroblasts become more stretched from the bottom to the top, and by the change in the constituent lithotypes. In the studied area, the Simano nappe is formed mainly by metagranitoids and by minor paragneisses. The Cima Lunga nappe is made of metasediments, mainly quartz-rich gneisses intercalated with amphibolite-gneisses, peridotitic lenses and local calcschists and/or marbles. Finally, the widespread paragneisses forming both the nappes frequently contain garnets of different sizes and internal microstructure. Published and own petrological data of these garnet-bearing rocks will be used to restrict the physical parameters of the numerical results.

We intend to test multiple geological scenarios related to different sources of heat production, such as: internal heat sources (radiogenic heating); additional heat flux at the bottom of the nappes, such in the case of a magmatic underplating, slab break-off, lower crust delamination; and in situ-produced heat due to shear heating mechanisms at the tectonic boundary between the nappes (thrust surface).

How to cite: Tagliaferri, A., Schenker, F. L., Schmalholz, S. M., and Seno, S.: Heat transfer through the nappes of the Lepontine Dome, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-14142, https://doi.org/10.5194/egusphere-egu2020-14142, 2020.

A long standing debate in seismology revolves around the nonexistent heat flow anomaly across the San Andreas fault. Given the fault’s average slip rate and age, a strong San Andreas fault, i.e. characterized by a relatively high static friction coefficient of µ>=0.6, should produce a significant local heat flow anomaly across the fault [1]. Since the work of Lachenbruch and Sass [1], this anomaly has not been observed and although many possible causes for the lack of a heat flow anomaly have been explored, the static or dynamic weakness of the San Andreas fault remains a favorable explanation [2,3].

Recently, we have introduced the ENergy COnserving Seismicity (ENCOS) framework that relates elastic deformation energy loading rates to the long-term average energy release of the seismic process. Within the presented implementation of ENCOS for Southern California with an elastic loading rate between 300 MW and 1.9 GW, the two most significant parameters are the static friction coefficient and the average efficiency. In particular, they are the most significant sources of uncertainty in harnessing the GPS-derived strain rates and the stress data within the ENCOS framework.

Here, we show how ENCOS can be leveraged in combination with the constraints from heat flow measurements and observed seismicity to restrict the parameter space of the average efficiency and the static friction coefficient. This can help to reduce the uncertainty of the ENCOS model parameters, such as the elastic deformation energy loading rate, and opens a new viewpoint on the heat flow paradox.

[1] Lachenbruch, A. H., and Sass, J. H. (1980), Heat flow and energetics of the San Andreas Fault Zone, J. Geophys. Res., 85(B11), 6185–6222.

[2] Scholz, C. H. (2013). The Strength of the San Andreas Fault: A Critical Analysis. In Earthquakes: Radiated Energy and the Physics of Faulting (eds R. Abercrombie, A. McGarr, G. Di Toro and H. Kanamori).

[3] E. E. Brodsky et al. (2020), The State of Stress on the Fault Before, During, and After a Major Earthquake, Annu. Rev. Earth Planet. Sci. 48:2.1–2.26.

How to cite: Ziebarth, M. J., Anderson, J. G., von Specht, S., Heidbach, O., and Cotton, F.: From Elastic Deformation Loading Rates to Heat Flow Anomalies: Constraints on Seismic Efficiency and Friction Coefficient, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18763, https://doi.org/10.5194/egusphere-egu2020-18763, 2020.

We present a study on the paleoseismic Gole Larghe Fault Zone (GLFZ), composed of hundreds of sub-parallel faults hosted in tonalites of the Adamello Massif (Italian Southern Alps), where we collected a complete transect across the fault zone, including the background host rocks, over a thickness of >1km.

Along this transect, we studied the correlation between fracture spacing (for “fracture” here we mean joints, veins, faults, shear fractures, and all other brittle structures) and position with a robust non-parametric approach. This analysis, new for fracture distribution studies, allows detecting volumes of the fault zone with clustering or a trend in spacing, versus volumes where the spatial distribution is stationary. The analysis reveals that the GLFZ can be subdivided in “stationary volumes” where fractures shows stationary statistical properties. Each one of these volumes can be completely characterized with scanline and/or scanarea surveys to obtain a complete and statistically sound estimate of all fracture parameters (spacing, intensity, density, length, height, orientation, topology, etc.).

Within the GLFZ we have two main classes of structures: (i) “master” faults that are sub-parallel to the fault zone and are always characterized by pseudotachylytes and/or cataclasites, and (ii) minor “fractures” (e.g. Riedel fractures, joints, veins, etc.) that are oblique to the fault zone and interconnect the former. Out of the GLFZ we observe a background fracturing that is associated to the cooling of the Adamello tonalites under deviatoric tectonic stress (“cooling joints”).

By comparing fracture statistics within and outside the fault zone, we demonstrated that master faults within the GLFZ were almost completely inherited from the “cooling joints” of the host rocks. The cooling joints just grew in length and became completely interconnected at the scale of the seismic rupture. This means that, at least in the case of the GLFZ, the large faults and fractures along which seismic ruptures were running do not add significantly to the earthquake energy budget, because they were already present in the system before the onset of seismic activity. The only fractures to be considered in this budget are the minor interconnecting fractures (e.g. Riedel fractures, joints, veins, etc.) that are coated with pseudotachylytes.

These observations confirm once again the classical assumption that seismic ruptures propagate along pre-existing discontinuities and do not, in general, tend to fracture intact rocks.

How to cite: Bistacchi, A., Mittempergher, S., Smith, S. A. F., Di Toro, G., and Nielsen, S.: Detailed statistical analysis of the Gole Larghe Fault Zone fracture network (Italian Southern Alps) improves estimates of the energy budget for intraplate earthquakes in basement rocks, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13800, https://doi.org/10.5194/egusphere-egu2020-13800, 2020.

In the brittle part of the crust, deformation is usually perceived to be the result of displacement along fault planes, whose behaviors are controlled by their frictional properties. However, fault zones not only consist of a narrow fault core where slip occurs, they are also surrounded by a complex structure which is of key importance in the mechanics of faulting, hence in determining the overall energy budget. Indeed, as pointed out by the numerous field, geophysical, mechanical and laboratory observations, if the behavior of fault zones is intrinsically linked to the properties of the main sliding plane, it also depends on those of the surrounding medium.  In parallel, fault displacements may induce a substantial change in the physical properties of the surrounding medium. As a consequence, to improve our understanding of active fault zones, fault slip and the evolving physical properties must be studied as a unique system of stress accommodation and no longer as two distinct entities. To tackle this problem, we have developed a micromechanics-based constitutive model, thermodynamically argued, that can determine the inelastic behavior at macroscopic scale that arises from structural rearrangements at microscale. It is therefore the compulsory tool to emulate the strong coupling between the bulk and the fault that prevails during earthquakes. With this code, we can reproduce the strain rate sensitive, non-linear stress-strain relationship that leads to off-fault damage as a seismic event is propagating. We explore different scenarii and we show that there is a unique off-fault damage pattern associated with supershear transition of an earthquake rupture, that is also observed in the field.  We define, in return, the impact of damage on the propagation of the earthquake in itself and the generated waves. We conclude by assessing the kinetic energy, the dissipated energy and the radiated energy to define how energy is consumed within crustal systems during seismic events.

How to cite: Thomas, M. Y. and Bhat, H. S.: Impact of coseismic off-fault damage on the overall energy budget., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8030, https://doi.org/10.5194/egusphere-egu2020-8030, 2020.

Earthquakes correspond to a sudden release of elastic energy stored during inter-seismic period by tectonic loading around fault. The earthquake energy budget consists of four non-independent terms: the energy release rate (by unit crack length), the fracture energy, the heat term and finally the radiated energy. These terms depend on the rupture and sliding velocities, the amount of slip and the stress drop. Because of the impossibility to access to stress and strain conditions at depth, the earthquake energy budget cannot be fully constrained from seismological data, limiting our understanding of its influence on rupture propagation.

To address this issue, we conducted stick-slip experiments with large samples in a biaxial configuration apparatus. By imposing constant normal load and increasing shear load, seismic events were produced on a 20 cm long fault, for which the energy budget was estimated using different methods.

Fracture energy was estimated by recording the strain field around the crack tip through high frequency (2 MHz) strain gage rosettes array and comparing it to the theoretical LEFM strain field predictions obtained for same conditions (i.e. rupture velocity, distance from the fault). Fracture energies were then inverted and found to range in between 1 and 10 J/m². At the same time the energy partitioning was estimated through stress-slip evolution during rupture. The fracture energies obtained from this method are almost one order of magnitude larger than the ones inverted from LEFM and range in between 1 and 90 J/m². Moreover, the energy partitioning shows the radiated energy ranging between 80 and 300 J/m² and finally the heat/thermal energy as the largest fraction of the energy partitioning with values ranging from 200 to 2500 J/m². Our preliminary results highlight the importance of understanding the contribution of heat energy in frictional processes, since this term cannot be estimated from seismological data.

How to cite: Violay, M., Paglialunga, F., and Passelègue, F. X.: Energy budget of laboratory earthquakes: a comparison between linear elastic fracture mechanics approach and experimental approach., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-16230, https://doi.org/10.5194/egusphere-egu2020-16230, 2020.

Earthquake ruptures are driven by the dynamic weakening of frictional strength along faults. This drop of frictional stress toward a residual level is at the origin of the slip-weakening model, which became a well-established framework to study seismic ruptures and their energy budget. In this framework, the part of frictional energy associated to the rupture propagation (i.e. the fracture energy) corresponds to the excess of frictional dissipation on top of the residual stress, also referred as the breakdown work.

In this study, we test this energy partition for friction models that do not impose the magnitude of the residual stress. For example, rate-and-state models are a class of generic friction laws for which the residual stress after the rupture emerges from the interplay with the bulk elastodynamics. In this context, we simulate a frictional rupture at the interface between two linearly elastic solids and study the energy balance driving its propagation. Using dynamic fracture mechanics, we independently measure throughout the rupture the energy release rate from the bulk elastic fields and the frictional dissipation along the interface. From the comparison between these two quantities, we identify the part of interface dissipation corresponding to the fracture energy and show how the latter can be significantly smaller than the total breakdown work.

In a second phase, we test the generality of these results along another type of interface representative of mature fault zones filled with gouge.

This study shines new light on the energy budget of frictional ruptures and finds implications in the estimation of the fracture energy during earthquakes.

How to cite: Barras, F.: On the energy balance behind frictional ruptures, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18255, https://doi.org/10.5194/egusphere-egu2020-18255, 2020.

This study assesses the ability of work optimization to predict the spatial and temporal initiation of faults. We focus on the growth of flaws that develop into thrust faults at the toe of accretionary prisms because observations from physical laboratory accretion experiments provide rich data with which to validate the models, and the processes of accretionary thrust fault initiation remain unclear. In order to model these systems, we apply new implementations to the fault growth code GROW that improve its prediction of fault interaction using work optimization, including: 1) CPU parallelization, 2) a new growth algorithm that propagates only the most efficient fault in each growth increment, the single run mode, and 3) a new growth algorithm that only considers fault propagation from fault tips that host high sums of modes I and II stress intensity factors, K_G, the limiting mode. The single and limiting mode produce the geometries that best match the observed geometries, rather than the previous algorithm that allows all the faults to propagate simultaneously, regardless of K_G, the multiple and non-limiting mode. The single limiting models predict that frontal accretionary thrusts initiate at the midpack or shallower depths, consistent with findings of previous studies. The thrusts propagate upward, link with the surface, and then propagate downward and link with the detachment. The backthrust tends to propagate before the forethrust, and then influence the forethrust propagation. This temporal and spatial sequence of faulting arises from the lower compression, higher shear strain, higher Coulomb stress and higher strain energy density that develop near the wedge surface and the inflection of the wedge slope. The models reveal that the final slip distributions do not reliably indicate the initiation location of the faults, in contrast to the assumptions of previous analyses.

How to cite: Cooke, M., McBeck, J., and Fattaruso, L.: Predicting the propagation and interaction of frontal accretionary thrust faults with work optimization, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11253, https://doi.org/10.5194/egusphere-egu2020-11253, 2020.