Description: Frictional behavior of experimental faults during a simulated seismic cycle Elena Spagnuolo (1), Stefan Nielsen (2), Marie Violay (3), Fabio Di Felice (1), Giulio Di Toro (4,5) (1) Istituto Nazionale di Geofisica e Vulcanologia, Rome, Italy (elena.spagnuolo@ingv.it), (2) Durham University, Durham, UK (stefan.nielsen@durham.ac.uk), (3) ENAC, LEMR, Lausanne, Switzerland (marie.violay@epfl.ch ), (4) University of Manchester, Manchester, UK (giulio.ditoro@manchester.ac.uk), (5) Università degli Studi di Padova, Padua, Italy Laboratory friction studies of earthquake mechanics aim at understanding complex phenomena either driving or characterizing the seismic cycle. Previous experiments were mainly conducted on bi-axial machines imposing velocity steps conditions, where slip and slip-rate are usually less than 10 mm and 1 mm/s, respectively. However, earthquake nucleation on natural faults results from the combination of the frictional response of fault materials and wall rock stiffness with complex loading conditions. We propose an alternative experimental approach which consists in imposing a step-wise increase in the shear stress on an experimental fault under constant normal stress. This experimental configuration allows us to investigate the relevance of spontaneous fault surface reworking in (1) driving frictional instabilities, (2) promoting the diversity of slip events including the eventual runaway, and (3) ruling weakening and re-strengthening processes during the seismic cycle. Using a rotary shear apparatus (SHIVA, INGV, Rome) with an on-purpose designed control system, the shear stress acting on a simulated fault can be increased step-wise while both slip and slip-rate are allowed to evolve spontaneously (the slip is namely infinite) to accommodate the new state of stress. This unconventional procedure, which we term “shear stress-step loading”, simulates how faults react to either a remote tectonic loading or a sudden seismic or strain event taking place in the vicinity of a fault patch. Our experiments show that the spontaneous slip evolution results in velocity pulses whose shape and occurrence rate are controlled by the lithology and the state of stress. With increasing shear stress and cumulative slip, the experimental fault exhibits three frictional behaviors: (1) stable behavior or individual slip pulses up to few cm/s for few mm of slip in concomitance to the step-wise increase in shear stress; (2) unstable oscillatory slip or continuous slip but with abrupt changes in slip rate (lower than 10 cm/s) under about constant imposed shear stress; (3) fault dramatic weakening or continuous slip with gradually increasingly slip rates up to 6.5 m/s (an imposed upper bound limitation). The shear stress-step loading experimental technique proposed here provides new hints on the behavior of pre-existing faults during the seismic cycle and, for instance, reproduces precursory slip events observed in some large in magnitude earthquakes (e.g., Izmit, Mw 7.6, 1999).

Description: Published

Description: Vienna

Description: 1T. Deformazione crostale attiva

Description: Tectonic pseudotachylytes are solidified frictional melts produced on faults during earthquakes and are robust markers of seismic slip events. Nonetheless, pseudotachylytes are apparently uncommon fault rocks, because they are either rarely produced or are easily lost from the geological record. To solve this conundrum, long-lasting (18–35 days) hydrothermal alteration tests were performed on fresh pseudotachylytes produced by sliding solid rock samples at seismic slip rates in the laboratory. After all tests, the pseudotachylytes were heavily altered with dissolution of the matrix and neo-formation of clay aggregates. Post-alteration products closely resemble natural altered pseudotachylytes and associated ultracataclasites (i.e., fault rocks affected by fracturing in the absence of melting), demonstrating that the preservation potential of original pseudotachylyte microstructures is very short, days to months, in the presence of hydrothermal fluids. As a consequence, pseudotachylytes might be significantly underrepresented in the geological record, and on-fault frictional melting during earthquakes is likely to occur more commonly than generally believed

Description: ERC CoG NOFEAR 614705

Description: Published

Description: e2020GL090020

Description: 3T. Sorgente sismica

Description: JCR Journal

Description: All’interno dei percorsi forma vi di Alternanza Scuola­Lavoro un gruppo di ricercatori dell’Istituto Nazionale di Geofisica e Vulcanologia ha proposto e sviluppato con una terza classe del Liceo Scientifico Cavour di Roma un progetto riguardante l’avvicinamento degli studenti al mondo della ricerca scientifica, allo scopo di affrontarla e svilupparla in tu e le sue fasi. Dopo aver spiegato e condiviso l’obiettivo della ricerca con gli alunni, la prima fase del progetto ha riguardato la raccolta dei da tramite l’utilizzo di strumenti di misura; si è in seguito proceduto all’analisi dei dati ottenuti , attraverso un foglio elettronico di calcolo, la produzione di grafici e mappe tema che e infine la scrittura di un articolo scientifico (parte essenziale del lavoro del ricercatore) con la descrizione della ricerca svolta e dei risultati ottenuti . La ricerca ha riguardato la distribuzione del radon indoor in alcuni rioni di Roma. Gli studenti hanno avuto la possibilità di misurare il radon indoor tramite strumenti specifici nelle loro abitazioni e nella loro scuola, di analizzarne la distribuzione spaziale e temporale e di individuare dei casi particolarmente anomali. Sono stati quindi evidenziati i legami tra i valori misura , la geologia del sottosuolo e le tipologie abitative, illustrate per mezzo di mappe tematiche, facendo confluire il tutto o nella scrittura di un articolo scientifico. Il presente rapporto tecnico è stato introdotto dai ricercatori e sviluppato dagli studenti .

Description: Published

Description: 1-24

Description: 1TM. Formazione

Description: N/A or not JCR

Description: Smectite clays are the main constituent of slipping zones found in subduction zone faults at shallow depth (e.g., 〈1-km depth in the Japan Trench) and in the decollements of large landslides (e.g., 1963 landslide, Vajont, Italy). Therefore, deformation processes in smectite clays may control the mechanical behavior from slow creep to fast accelerations and slip during earthquakes and landslides. Here, we use (1) laboratory experiments to investigate the mechanical behavior of partly water-saturated smectite-rich gouges sheared from subseismic to seismic slip rates V and (2) nanoscale microscopy to study the gouge fabric. At all slip rates, deformation localizes in volumes of the gouge layer that contain a "nanofoliation" consisting of anastomosing smectite crystals. "Seismic" nanofoliations produced at V = 0.01, 0.1, and 1.3 m/s are similar to "subseismic" nanofoliations obtained at V = 10-5 m/s. This similarity suggests that frictional slip along water-lubricated smectite grain boundaries and basal planes may occur from subseismic to seismic slip rates in natural smectite-rich faults. Thus, if water is available along smectite grain boundaries and basal planes, nanofoliations can develop from slow to fast slip rates. Still, when nanofoliations are found highly localized in a volume, they can be diagnostic of slip that occurred at rates equal or larger than 0.01 m/s. In such a case, they could be markers of past seismic events when found in natural fault rocks.

Description: European Research Council Consolidator. Grant Number: 614705 NOFEAR

Description: Published

Description: 6589-6601

Description: 3T. Sorgente sismica

Description: JCR Journal

Description: The rapid and accurate information about the ground shaking following an earthquake is necessary for emergency response planning. A prompt strategy is contouring the real data recorded at the stations. However only few regions, i.e. Japan and Taiwan, have an instrumental coverage as good as needed to produce shaking maps relying almost entirely on real data. ShakeMap has been conceived in order to “fill” the data gap and producing stable contouring using the ground motion predictive equations (GMPEs) and site effect. Thus for regions where the data coverage is sparse, the interpolation plays a crucial role and the choice of the GMPE can affect strongly the goodness of the ground shaking estimation. However the GMPEs derive from an empirical regression describing the averaged behavior of the ground shaking and tend to mask, when present, specific trends due to multidimensional effects like the asymmetry of the rupture process (directivity effect). Thus, ShakeMaps for large events may not reproduce faithfully the ground motion in the near source if determined without the introduction of rupture related parameters. One way to improve the ShakeMap prediction is to modify the ground motion modeling in order to better explain the ground motion variability. To this purpose, the empirical model can be refined with information about the rupture process (Spagnuolo PhD2010), in this case using the directivity term defined by Spudich and Chiou (Earthquake Spectra 2008). The aim of this work is to quantify the effectiveness of refined GMPEs in improving the performance of ShakeMap. We quantify the agreement of this new GMPE with the real recorded data, and make inference about the reliability of this new ShakeMap. The test is focused on the study of the ShakeMap degradation when the number of the observations is reduced, and on the quantification of the improvements due to the directivity term. In order to conduct properly the test, we investigate two well- recorded events from Japan: the 2008 Iwate-Miyagi (M7) and the 2000 Tottori (M6.6) events. This work is part of the DPC-INGV S3 project (2007-09), as described in the companion abstract Ameri et al. (ESC2010).

Description: Published

Description: Montpellier, France

Description: 4.1. Metodologie sismologiche per l'ingegneria sismica

Description: open