Description: Tectonic faults typically break in a single rupture mode within the range of styles from slow slip to dynamic earthquake failure. However, in increasingly well‐documented instances, the same fault segment fails in both slow and fast modes within a short period, as in the sequences that culminated in the 2011 Mw = 9.0 Tohoku‐Oki, Japan, and 2014 Mw = 8.2 Iquique, Chile, earthquakes. Why slow slip alternates with dynamic rupture in certain regions but not in others is not well understood. Here, we integrate laboratory experiments and numerical simulations to investigate the physical conditions leading to cycles where the two rupture styles alternate. We show that a bifurcation takes place near the stability transition with sequences encompassing various rupture modes under constant loading rate. The range of frictional instabilities and slip cycles identified in this study represents important end‐members to understand the interaction of slow and fast slip on the same fault segment.

Description: Published

Description: e2020GL087985

Description: 3T. Sorgente sismica

Description: JCR Journal

Description: Understanding the physical mechanisms at the origin of slow‐slip events has been proven a very challenging task. In particular, little is known on the role of fault heterogeneity during slow slip. In this study, we provide evidences that fault fabric controls slip velocity time histories during slow‐slip events generated in the laboratory. We performed experiments using a double‐direct biaxial shear apparatus and two different fault gouges, homogeneous quartz powder, and heterogeneous anhydrite/dolomite mixture. We measure details of fault slip to resolve the slip velocity function and volumetric deformation that, coupled with an analysis of the resulting microstructure, allow us to infer the mechanical processes at play. Our results show that slow‐slip events can be generated for both fault gouges when k ~ kc with similar values of breakdown work. The shear fabric exerts a strong influence during the coseismic breakdown stage. In quartz, where most of the slip occurs on a very localized slipping surface, the peak slip velocity is attained near the final stage of friction breakdown and therefore a relevant amount of the mechanical work is absorbed during slip acceleration. In anhydrite/dolomite mixture, the peak slip velocity is suddenly reached after a relatively small drop in friction, accompanied by fault dilation, implying that most of the mechanical work is absorbed during slip deceleration. For anhydrite/dolomite mixture these results are likely related to heterogeneous slip distribution along the observed foliation. Taken together, these observations suggest that the mechanics of slow‐slip events depends on shear zone fabric.

Description: Sapienza Grant Ateneo 2018 to C. C. and Horizon 2020 innovation program under the Marie Sklodowska‐Curie 656676 FEAT to M. M. S.

Description: Published

Description: e2020JB020405

Description: 1T. Struttura della Terra

Description: JCR Journal

Description: Giant rockslides are widespread and sensitive to hydrological forcing, especially in climate change scenarios. They creep slowly for centuries and then can fail catastrophically posing major threats to society. However, the mechanisms regulating the slow-to-fast transition toward their catastrophic collapse remain elusive. We couple laboratory experiments on natural rockslide shear zone material and in situ observations to provide a scale-independent demonstration that short-term pore fluid pressure variations originate a full spectrum of creep styles, modulated by slip-induced undrained conditions. Shear zones respond to pore pressure increments by impulsive acceleration and dilatancy, causing spontaneous decel- eration followed by sustained steady-rate creep. Increasing pore pressure results in high creep rates and eventual collapse. Laboratory experiments quantitatively capture the in situ behavior of giant rockslides and lay physically-based foundations to understand the collapse of giant rockslides.

Description: ERC grant Nr. 259256 “GLASS”

Description: Published

Description: 1352

Description: 1T. Struttura della Terra

Description: JCR Journal

Description: Empirical scaling relationships between fault or slip dimensions and earthquake magnitudes are often used to assess the maximum possible earthquake magnitude of a territory. Upon the assumption of the reactivability of any fault, the earthquake magnitudes derived from the surface fault length (FLEM) are compared at the national scale in Italy against catalogued magnitudes. FLEMs are obtained by considering a comprehensive fault dataset regardless of fault age, stress field orientation, strain rate, etc. In particular, (1) a comprehensive catalogue of all known faults is compiled by merging the most complete databases available; (2) FLEM is then derived from fault length; and (3) the resulting FLEMs are compared (i.e. the mathematical difference) with catalogued earthquake magnitudes. Results show that the largest FLEMs as well as the largest differences between FLEMs and catalogued magnitudes are observed for poorly constrained faults, mainly inferred from subsurface data. It is suggested that these areas have to be further characterized to better estimate fault dimension and segmentation and hence properly assess the FLEM. Where, in contrast, the knowledge of faults is geologically well constrained, the calculated FLEM is often consistent with the catalogued seismicity, with the 2 value of the distribution of differences being 1.47 and reducing to 0.53 when considering only the Mw 6:5 earthquakes. Our work highlights areas, in Italy, where further detailed studies on faults are required.

Description: Published

Description: 1555–1579

Description: 4T. Sismicità dell'Italia

Description: JCR Journal

Description: On natural faults that host repeating slip events, the inter‐event loading time is quite large compared to the slip event duration. Since most friction studies focus on steady‐state frictional behavior, the fault loading phase is not typically examined. Here, we employ a method specifically designed to evaluate fault strength evolution during active loading, under shear driving rates as low as 10−10 m/s, on natural fault gouge samples from the Waikukupa Thrust in southern New Zealand. These tests reveal that in the early stages of loading following a slip event, there is a period of increased stability, which fades with accumulated slip. In the framework of rate‐ and state‐dependent friction laws, this temporary stable phase exists as long as slip is less than the critical slip distance and the elapsed time is less than the value of the state variable at steady state. These observations indicate a minimum earthquake recurrence time, which depends on the field value of the critical slip distance and the background slip rate. We compare estimates of minimum earthquake recurrence times with the recurrence times of repeating large earthquakes on the Alpine Fault in southern New Zealand and repeating small‐magnitude earthquakes on the San Andreas Fault system in California. We find that the observed recurrence times are mostly longer than the predicted minimum values, and exceptions in the San Andreas system may be explained by elevated slip rates due to larger earthquakes in this region.

Description: Deutsche Forschungsgemeinschaft via MARUM Research Centre/Cluster of Excellence (grants FZT15, EXC309, and IK 107/3‐1) and from the European Research Council (ERC) under the European Union's Horizon 2020 research and innovation program (grant agreement 714430) to M. I.

Description: Published

Description: e2020JB020015

Description: 1T. Struttura della Terra

Description: JCR Journal

Description: A main goal of the European Research Council, Starting Grant, GLASS (InteGrated Laboratories to investigate the mechanics of ASeismic vs Seismic faulting), is to develop a prototype rock deformation biaxial apparatus to examine the physical properties of brittle rocks. Two layers of fault rock are sandwiched between three steel block by a normal load applied using a horizontal oil-dynamics piston. A vertical oil-dynamics piston pushes the internal rock sample of the sandwich in order to slide at constant velocity. With GLASS we are going to build-up a confining pressure around the rock samples under load stress (tri-axial mode) and to measure the fluid flow properties of the rock during the deformation. Working in tri-axial mode with a fluid circulation, the machine is able to measure and to characterize frictional properties of faults on the sample for a wide spectrum of realistic conditions. We have concurrently been working to improve the control and the acquisition system for having a machine very flexible and easy to use for several applications and capable to detect different signals on the rock during frictional sliding in a fluid-rich environment with the goal of comparing these signals to those observed in nature. We began designing the servo controlled machine in October 2010 and have recently installed the apparatus in the HP-HT lab at the INGV in Rome. First tests of this biaxial apparatus confirm the main target of the project.

Description: Unpublished

Description: Harvard University, Boston

Description: 2.3. TTC - Laboratori di chimica e fisica delle rocce

Description: open

Description: USEMS and GLASS are two projects financed by the European Research Council (ERC) as part of the ERC starting grants scheme within the FP7 framework. The rationale behind the funding scheme is to support some of the most promising scientific endeavours in Europe that are being led by young researchers, and to emphasize the excellence of individual ideas rather than specific research areas; in other words, to promote bottom-up frontier research. The general benefits of this rationale are evident in the two ongoing projects that deal with earthquake physics, as these projects are increasingly recognized in their scientific community. We can say that putting excellence at the heart of European Research strongly contributes to the construction of a European knowledge-based society. From a researcher point-of-view one of the most challenging aspects of these projects is to approach and convey the results of the projects to a general public, contributing actually to the construction of knowledge-based society. Luckily, media interest and the availability of a number of new communication tools facilitate the outreach of scientific achievements. The largest earthquakes during the last ten years (e.g. Sumatra 2004 and Japan 2011) have received widespread attention in the media world (TV, W.W.W., Newspaper and so on) for months, and successful research projects such as those above also become media protagonists, gaining their space in the media bullring. The USEMS principal investigator and his team have participated in several dissemination events in the Mass Media, such as interviews wit Italian and French TV national broadcasts (RAI Due TG2, RAI Uno Unomattina, Rai Tre Geo & Geo, FRANCE 2); interviews in scientific journals: SCIENCE (Sept. 2010), newspapers and web (Corriere della Sera, Il Gazzettino, Il Messagero, La Stampa, Libero, Il Mattino, Yahoo, ANSA, AdnKronos and AGI); radio (RadioRai Uno, RadioRai Tre Scienza); documentary “Die Eroberung der Alpen” produced by Tangram Film (Munchen, Germany). The USEMS project started in June 2008, and the GLASS project in October 2010. For both projects we developed a number of web pages through the official web site of the host institution, the National Institute of Geophysics and Volcanology (INGV) in Rome. In these pages we describe the projects, publish pictures and short-movies of the experimental activities and keep the project results up to date. In addition the research teams collaborate within various INGV outreach schemes (school and student visits in the laboratory, writing short news reports for the INGV press office, and interviews with journalists) as well as using WWW channels (Facebook, Youtube) to make the project results available to the general public. Finally, it is notable that the ERC funding agency itself is fully involved in the outreach activities using its own communication channels and its highly skilled resources which promote through brochures, web pages, publications and documentaries the best projects. We are going to improve our effort in this direction up to the end of the projects.

Description: Unpublished

Description: San Francisco

Description: 5.9. Formazione e informazione

Description: open

Description: As part of an interdisciplinary research project, funded by the European Research Council and addressing the mechanics of weak faults, we drilled three 200–250 m-deep boreholes and installed an array of seismometers. The array augments TABOO (The AltotiBerina near fault ObservatOry), a scientific infrastructure managed by the Italian National Institute of Geophysics and Volcanology. The observatory, which consists of a geophysical network equipped with multi-sensor stations, is located in the northern Apennines (Italy) and monitors a large and active low-angle normal fault. The drilling operations started at the end of 2011 and were completed by July 2012. We instrumented the boreholes with three-component short-period (2 Hz) passive instruments at different depths. The seismometers are now fully operational and collecting waveforms characterised by a very high signal to noise ratio that is ideal for studying microearthquakes. The resulting increase in the detection capability of the seismic network will allow for a broader range of transients to be identified.

Description: Published

Description: 31-35

Description: 2T. Sorgente Sismica

Description: JCR Journal

Description: One of the major challenges regarding the disposal of radioactive waste in geological formations is to ensure isolation of radioactive contamination from the environment and the population. Shales are suitable candidates as geological barriers. However, the presence of tectonic faults within clay formations put the long-term safety of geological repositories into question. In this study, we carry out frictional experiments on intact samples of Opalinus Clay, i.e. the host rock for nuclear waste storage in Switzerland. We report experimental evidence suggesting that scaly clays form at low normal stress (≤20MPa), at sub-seismic velocities (≤300μm/s) and is related to pre-existing bedding planes with an ongoing process where frictional sliding is the controlling deformation mechanism. We have found that scaly clays show a velocity-weakening and -strengthening behaviour, low frictional strength, and poor re-strengthening over time, conditions required to allow the potential nucleation and propagation of earthquakes within the scaly clays portion of the formation. The strong similarities between the microstructures of natural and experimental scaly clays suggest important implications for the slip behaviour of shallow faults in shales. If natural and anthropogenic perturbations modify the stress conditions of the fault zone, earthquakes might have the potential to nucleate within zones of scaly clays controlling the seismicity of the clay-rich tectonic system, thus, potentially compromising the long-term safeness of geological repositories situated in shales.

Description: Published

Description: 59-67

Description: 3T. Sorgente sismica

Description: 2IT. Laboratori analitici e sperimentali

Description: JCR Journal