Elena Spagnuolo

• PhD Geophysics
• Istituto Nazionale di Geofisica e Vulcanologia

Strain accumulation and release in shallow sections of subduction zones results in a broad variety of fault slip behaviors, from slow aseismic creep to abrupt coseismic slip. The variety of slip styles in megathrust environments remains a matter of debate although the role of pressurized fluids is increasingly substantiated. Yet, understanding the...

Faults exhibit dynamic weakening during large displacements (>1 m) at seismic slip velocities (>0.1 m s−1), but the role of this weakening in small-displacement induced earthquakes (M 3–4), such as those in the Groningen Gas Field (the Netherlands), remains unclear. We conducted seismic slip-pulse experiments on Slochteren sandstone gouges (SSG) us...

Earthquakes are frictional instabilities caused by the shear stress decrease, that is, dynamic weakening, of faults with slip and slip rate. During dynamic weakening, shear stress depends on slip, slip rate, and temperature, according to constitutive laws governing the earthquake rupture process. In the laboratory, technical limitations in measurin...

Natural fault zones are complex, spatially heterogeneous systems. Rock deformation experimental studies simplify the complexity of natural fault zones either as a surface discontinuity between intact rocks (bare‐rock surfaces) or as a few mm‐thick gouge layer. However, depending on the simplified fault type and its slip history, the response to app...

The trench-parallel Atacama Fault System (AFS) evidences a long history of deformation since the Cretaceous in the Coastal Cordillera of northern Chile. Conspicuous fault scarps are expressed in the topography of this cordillera attesting to the most recent fault activity, characterized by normal faulting. Particularly, between 23°-25°S, metric fau...

Numerous field and laboratory studies have been conducted to investigate the relationship between radon variation and seismic events, as well as the complex link between radon emission and rock deformation mechanisms. However, a clear understanding of this correspondence and systematic observations of these phenomena are still lacking, and recent e...

Typical rocks at shallow depths of seismogenic faults are fluid-rich gouges. During earthquakes, on-fault frictional heating may trigger thermal pressurization and dynamic fault weakening. We show that frictional melting, rather than thermal pressurization, occurred at shallow depths during the 2008 MW 7.9 Wenchuan earthquake, China. One year after...

Large seismogenic faults consist of approximately meter-thick fault cores surrounded by hundreds of meter-thick damage zones. Earthquakes are generated by rupture propagation and slip within fault cores and dissipate the stored elastic strain energy in fracture and frictional processes in the fault zone and in radiated seismic waves. Understanding...

While sliding at seismic slip‐rates of ∼1 m/s, natural faults undergo an abrupt decrease of shear stress called dynamic weakening. Asperity‐scale (<<mm) processes related to flash heating and weakening and, meso‐scale (mm‐cm) processes involving shear across the bulk slip‐zone, related to frictional melting or viscous flow of minerals, have been in...

The 2008 Mw 7.9 Wenchuan earthquake generated ∼270 and ∼80 km long surface ruptures along the Longmenshan fault belt, namely the Yingxiu‐Beichuan fault (YBF) and the Guanxian‐Anxian faults (GAF), respectively. So far, most of the frictional investigations were performed on the YBF gouge materials. Here, we present the results of rotary shear fricti...

Theoretical studies predict that during earthquake rupture faults slide at non‐constant slip velocity, however it is not clear which source time functions are compatible with the high velocity rheology of earthquake faults. Here we present results from high velocity friction experiments with non‐constant velocity history, employing a well‐known sei...

Tectonic pseudotachylytes are thought to be unique to certain water–deficient seismogenic environments and their presence is considered to be rare in the geological record. Here, we present field and experimental evidence that frictional melting can occur in hydrothermal fluid–rich faults hosted in the continental crust. Pseudotachylytes were found...

We performed a suite of experiments aimed at examining the frictional properties of unaltered basalts at conditions considered to be representative of slip at shallow depths in volcano-tectonic environments and in-situ geo-energy basaltic sites. Scientific drilling and field studies on exhumed subsurface faults and fractures analogues suggest that,...

Experiments that systematically explore rock friction under crustal earthquake conditions reveal that faults undergo abrupt dynamic weakening. Processes related to heating and weakening of fault surfaces have been invoked to explain pronounced velocity weakening. Both contact asperity temperature Ta and background temperature T of the slip zone evo...

The understanding of earthquake physics is hindered by the poor knowledge of fault strength and temperature evolution during seismic slip. Experiments reproducing seismic velocity (∼1 m/s) allow us to measure both the evolution of fault strength and the associated temperature increase due to frictional heating. However, temperature measurements wer...

In subduction zones, seismic slip at shallow crustal depths can lead to the generation of tsunamis. Large slip displacements during tsunamogenic earthquakes are attributed to the low coseismic shear strength of the fluid-saturated and non-lithified clay-rich fault rocks. However, because of experimental challenges in confining these materials, the...

Calcite and dolomite are the two most common minerals in carbonate-bearing faults and shear zones. Motivated by observations of exhumed seismogenic faults in the Italian Central Apennines, we used a rotary-shear apparatus to investigate the frictional and microstructural evolution of ca. 3 mm thick gouge layers consisting of 50 wt % calcite and 50...

On-fault processes during earthquakes contribute to seismic rupture propagation and slip. Here we investigate clast fragmentation in an experimental pseudotachylyte (solidified seismic melt) produced with a rotary shear machine. We slid for 0.44 m (corresponding to Mw ≥ 6 earthquakes), at slip rates > 1 m/s, pre-cut samples of quartz + phyllosilica...

In subduction zones, seismic slip at shallow crustal depths can lead to the generation of tsunamis. Large slip displacements during tsunamogenic earthquakes are attributed to the low coseismic shear strength of the fluid-saturated and non-lithified clay-rich fault rocks. However, because of experimental challenges in confining these materials, the...

Plain Language Summary Tectonic pseudotachylytes are solidified melts produced by rapid sliding of faults during earthquakes. A long‐lasting unsolved dispute in earthquake physics regards the abundance of pseudotachylytes in nature and the relevance of frictional melting as a seismic‐related process. Although experimental and theoretical arguments...

Calcite and dolomite are the two most common minerals in carbonate-bearing faults and shear zones. Motivated by field examples from exhumed seismogenic faults in the Italian Central Apennines, we investigated the frictional and microstructural evolution of gouge mixtures consisting of 50 wt % calcite and 50 wt % dolomite using a rotary-shear appara...

High‐viscosity fluids are often used during hydraulic fracking operations in georeservoirs. Here we performed dedicated experiments to study the influence of fluid viscosity on fault reactivation and associated induced earthquakes. Experiments were conducted in the rotary‐shear machine Slow to HIgh Velocity Apparatus on experimental fault of Wester...

Pseudotachylytes (i.e., solidified frictional melts) remain the only unambiguous signature of seismic slip in the rock record so far. However, pseudotachylytes are considered to be rare in the geological record and found discontinuously along faults. Here we describe, to our knowledge, the first pseudotachylytes ever found in South America. The ps...

During seismic slip, the elastic strain energy released by the wall rocks drives grain fragmentation and flash heating in the slipping zone, resulting in formation of (nano)powders and melt droplets, which lower the fault resistance. With progressive seismic slip, the frictional melt covers the slip surface and behaves as a lubricant reducing the c...

Smectite clays occur in subduction zone fault cores at shallow depth (approximately 1 km; e.g., Japan Trench) and landslide décollements (e.g., Vajont, Italy, 1963). The availability of pore fluids affects the likelihood that seismic slip propagates from deeper to shallow fault depths or that a landslide accelerates to its final collapse. To invest...

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 accelerat...

Grain size reduction due to cataclasis is a key process controlling fault frictional properties during the seismic cycle. We investigated the role of cleavage planes on fracturing and microstructural evolution during cataclasis in wet and dry carbonate fault gouges (50 wt% calcite, 50 wt% dolomite)deformed in a rotary-shear apparatus over a wide ra...

Fluids are pervasive in fault zones cutting the Earth's crust; however, the effect of fluid viscosity on fault mechanics is mainly conjectured by theoretical models. We present friction experiments performed on both dry and fluid-permeated silicate and carbonate bearing-rocks, at normal effective stresses up to 20 MPa, with a slip-rate ranging betw...

Evidence of temperature rise is useful for identifying where within a fault zone earthquake slip has occurred, as well as some attributes of the earthquake such as energy output. Biomarker thermal maturity can elucidate structures within a fault zone that were hotter than surrounding rock, and therefore likely to have hosted earthquakes. We perform...

Supporting Information S1

The safe application of geological carbon storage depends also on the seismic hazard associated with fluid injection. In this regard, we performed friction experiments using a rotary shear apparatus on precut basalts with variable degree of hydrothermal alteration by injecting distilled H2O, pure CO2, and H2O + CO2 fluid mixtures under temperature,...

The ideal treatment for intracranial aneurysms has been highly controversial in the last few decades. It is particularly difficult to decide between clipping vs. coiling when it comes to an aneurysm that has already been treated. The authors performed a review of the literature published in the last ten years amongst the main neurosurgical publicat...

Faults accommodate deformation by aseismic (slip rates << 0.1 mm/s) and seismic (average slip rate ~1 m/s) slip. The frictional dissipation during seismic slip, especially for moderate to large in magnitude (M > 6) earthquakes, should result in bulk temperature increase of the fault slipping zone. One moderate to large magnitude earthquake (M>6) nu...

Faults accommodate deformation by aseismic (slip rates << 0.1 mm/s) and seismic (average slip rate ~1 m/s) slip. The frictional dissipation during seismic slip, especially for moderate to large in magnitude (M > 6) earthquakes, should result in bulk temperature increase of the fault slipping zone. One moderate to large magnitude earthquake (M>6) nu...

Seismological, tsunami and geodetic observations have shown that subduction zones are complex systems where the properties of earthquake rupture vary with depth as a result of different pre-stress and frictional conditions. A wealth of earthquakes of different sizes and different source features (e.g. rupture duration) can be generated in subductio...

The 2011 Tōhoku-Oki earthquake revealed that co-seismic displacement along the plate boundary megathrust can propagate to the trench. Co-seismic slip to the trench amplifies hazards at subduction zones, so its historical occurrence should also be investigated globally. Here we combine structural and experimental analyses of core samples taken offsh...

Ultramafic pseudotachylytes have been regarded as earthquake fossils formed at mantle depths (i.e., >30 km). Here we show that pseudotachylytes hosted by ultramafic rocks from three localities have distinct magnetic properties. Fresh host-peridotites contain only small amounts of coarse-grained magnetite. In contrast, the ultramafic pseudotachylyte...

One moderate- to large-magnitude earthquake (M > 6) nucleates in Earth's crust every three days n average, but the geological record of ancient fault slip at meters-per-second seismic velocities (as opposed to subseismic slow-slip creep) remains debated because of the lack of established fault-zone evidence of seismic slip. Here we show that the ir...

Abstract: In this multi-methodological study, microstructural observations of fault rocks are combined with micromechanical property analyses (Contact Resonance Atomic Force Microscopy, CR-AFM) and with rotary friction experiments (SHIVA apparatus) to find evidence of seismic to aseismic slip and understand the nanoscale rheology of clay-bearing, c...

Carbonates are a recurring lithology in most of active seismic areas worldwide, such as the Apennines (Italy). Here, typical fault products are gouges and cataclasites made of mixtures of carbonate minerals (i.e. calcite and dolomite) that occasionally exhibit a foliation. Natural fault gouges often contain minerals with strong anisotropies, such a...

Many earthquakes propagate up to the Earth’s surface producing surface ruptures. Seismic slip propagation is facilitated by along-fault low dynamic frictional resistance, which is controlled by a number of physico-chemical lubrication mechanisms. In particular, rotary shear experiments conducted at seismic slip rates (1 ms−1) show that phyllosilica...

Nanoparticles and amorphous materials are common constituents of the shallow sections of active faults. Understanding the conditions at which nanoparticles are produced and their effects on friction can further improve our understanding of fault mechanics and earthquake energy budgets. Here we present the results of 59 rotary shear experiments cond...

In this study we investigate directivity effects associated to the August 24, 2016 Mw 6, central Italy earth-quake taking into account the source rupture heterogeneities. We use the directivity predictor proposed by Spudich et al. (2004) which is derived from the isochrones theory. The directivity is computed using a source to site geometry and a f...

On August 24, 2016 a Ml=6.0 earthquake occurred in Central Apennines, Italy, between the towns of Norcia and Amatrice, causing severe destruction and casualties in a wide area around the epicenter. We present a preliminary analysis of continuous radon concentration data collected from the second half of 2012 to the day after the earthquake by a lon...

In the present study, we improve the probabilistic seismic-hazard assessment (PSHA), taking into account fault rupture-related parameters that sensibly affect the azimuthal variability of the ground motion. The study area is the Marmara region (Turkey), characterized by one of the highest levels of seismic risk in Europe and the Mediterranean regio...

Recent estimates of fracture energy G ′ in earthquakes show a power-law dependence with slip u which can be summarized as G ′ ∝ u a where a is a positive real slightly larger than one. For cracks with sliding friction, fracture energy can be equated to G f : the post-failure integral of the dynamic weakening curve. If the dominant dissipative proce...

We present a series of high-velocity friction tests conducted on Westerly granite, using the Slow to HIgh Velocity Apparatus (SHIVA) installed at INGV Roma with acoustic emissions (AEs) monitored at high frequency (4MHz). Both atmospheric humidity and pore fluid (water) pressure conditions were tested, under effective normal stress in the range 5-2...

Earthquakes are the result of slip along faults and are due to the decrease of rock frictional strength (dynamic weakening) with increasing slip and slip rate. Friction experiments simulating the abrupt accelerations (>>10 m/s2), slip rates (~1 m/s), and normal stresses (>>10 MPa) expected at the passage of the earthquake rupture along the front of...

Empirically based rate-and-state friction laws (RSFLs) have been proposed to model the dependence of friction forces with slip and time. The relevance of the RSFL for earthquake mechanics is that few constitutive parameters define critical conditions for fault stability (i.e., critical stiffness and frictional fault behavior). However, the RSFLs we...

Laboratory experiments reproducing seismic slip conditions show extreme frictional weakening due to the activation of lubrication processes (DiToro et al., 2011). Due to a substantial variability in the details of the weakening transient, generalization of experimental results and comparison to seismic observations has not been possible so far. Her...

Rupture fronts can cause fault displacement, reaching speeds up to several ms−1 within a few milliseconds, at any distance away from the earthquake nucleation area. In the case of silicatebearing rocks the abrupt slip acceleration results in melting at asperity contacts causing a large reduction in fault frictional strength (i.e., flash weakening)....

Earthquake fault dynamics: Insights from laboratory experiments

[1] We performed 31 friction experiments on glassy basalts (GB) and glass-free basalts (GFB) at slip rates up to 6.5 ms-1 and normal stress up to 40 MPa (seismic conditions). Frictional weakening was associated to bulk frictional melting and lubrication. The weakening distance (Dw) was about three times shorter in GB than in GFB but the steady-stat...

We performed ~40 friction experiments on glassy basalts (GB) and glass-free basalts (GFB) at slip rates up to 6.5 m/s and normal stress up to 40 MPa (seismic deformation conditions). Once slip was accelerated, friction decayed abruptly with a shorter weakening distance (Dw) in GB than in GFB but with a steady-state friction higher in GB than in GFB...

We investigated the rupture history of the 2009 April 6th (Mw 6.1) L’Aquila earthquake by using a non-linear inversion of strong motion, GPS and DInSAR data. The joint inversion solution reveals a heterogeneous slip distribution characterized by two main asperities, located up-dip from the hypocentre and south-east wards along strike direction resp...

Recent experiments systematically explore rock friction under crustal earthquake conditions (fast slip rate 1<V<6 m/s and intermediate normal stress 5<σ<50 MPa) revealing that faults undergo abrupt dynamic weakening and lubrication associated to thermally triggered physico-chemical processes. We use such experimental data to test various thermal we...

While it is widely recognized that fluids control earthquakes nucleation and evolution, their effects on coseismic sliding friction is only conjectured. To shed light on these effects, more than 100 high velocity friction experiments were conducted on carbonate- (Carrara marble, porosity

Real-time seismology has made significant improvements in recent years, with source parameters now available within a few tens of minutes after an earthquake. It is likely that this time will be further reduced, in the near future, by means of increased efficiency in real-time transmission, increasing data coverage and improvement of the methodolog...

Experiments performed on rocks at deformation conditions typical of seismic slip, show an extremely low friction coefficient, the activation of lubrication processes and a power-law strength decay from a peak value to a residual, steady-state value. The weakening curve has an initially very abrupt decay which can be approximated by a power-law. The...

We propose a method to introduce a refined representation of the ground motion in the framework of the Probabilistic Seismic Hazard Analysis (PSHA). This study is especially oriented to the incorporation of a priori information about source parameters, by focusing on the directivity effect and its influence on seismic hazard maps. Two strategies ha...

In several earthquake prone areas of Europe (e.g. Italy, Greece, Turkey) destructive mainshocks, as well as associated foreshocks and aftershocks, nucleate within and propagate through thick sequences of carbonate. To understand the frictional behavior of carbonate-bearing faults during co-seismic slip we performed an experimental study on 3mm-thic...

Fluids play a fundamental physical (fluid pressure, temperature buffering, etc.) and chemical (dissolution, hydrolytic weakening, etc.) role in controlling fault strength and earthquake nucleation, propagation and arrest. However, due to technical challenges, the influence of water at deformation conditions typical of earthquakes (i.e., slip rates...

The accurate evaluation of a ground motion intensity measure for future earthquakes is necessary to inform earthquake-engineering decision. One widely adopted strategy is the probabilistic approach that uses the ground motion predictive equations to assess a ground motion intensity measure. Nevertheless at present the available empirical models are...

The 1999 M7.6 Chi-Chi earthquake on the Chelungpu fault in Taiwan is especially well characterized. Therefore we have investigated the frictional behavior of the Chelungpu fault during seismic slip, using high-velocity friction tests conducted on clay-rich fault gouges from cores obtained from the Taiwan Chelungpu fault Drilling Project. Rotary fri...

The Gole Larghe Fault Zone (GLFZ) in the Italian Southern Alps is characterized by the occurrence of cataclasites and pseudotachylytes (solidified frictional melts) formed along pre-existing magmatic cooling joints over a fault zone width of ca. 500 m, under ambient conditions of 9-11 km depth and 250-300°C (the "base" of the seismogenic zone in th...

While it is widely recognized that fluids influence fault strength and earthquake nucleation, propagation and arrest, their effects on co-seismic sliding friction are only conjectured. To shed light on these effects, 55 high velocity (>1ms-1) friction experiments were conducted at room temperature on Carrara marble samples in the presence of pore f...

Rotary shear machines are becoming a popular tool for rock friction experiments. Such machines are of particular interest to study conditions approaching those of natural earthquakes (slip rates of one meter per second or more, normal stress of several tens of MPa, total slip up to several meters). One key issue for a successful rotary shear experi...

Dynamic rock friction is the basic parameter that controls fault slip during an earthquake. However, at seismic slip rates little is known about how the dynamic friction coefficient (μ) varies with rock composition, normal stress, displacement, ambient temperature and fluid conditions. Important insights arise from experiments performed with dedica...

Fault surface roughness is a principal factor influencing fault and earthquake mechanics. However, little is known on roughness of fault surfaces at seismogenic depths, and particularly on how it evolves with accumulating slip. We have studied seismogenic fault surfaces of the Gole Larghe Fault Zone, which exploit precursor cooling joints of the Ad...

In recent years several experimental studies have been performed using rotary-shear apparatus to investigate the frictional behavior of gouge materials at seismic slip rates. However, because of technical difficulties confining gouge layers, a majority of these experiments were conducted at normal stresses <2-3MPa, making extrapolation to natural c...

A challenging goal in experimental rock deformation is to reproduce the extreme deformation conditions typical of coseismic slip in crustal earthquakes: large slip (up to 50 m), slip rates (0.1-10 m/s), accelerations (> 10 m/s2) and normal stress (> 50 MPa). Moreover, fault zones usually contain non-cohesive rocks (gouges) and fluids. The integrati...

Earthquake nucleation on pre-existing surfaces is governed by frictional instabilities which can be described by state parameters. These parameters may evolve with cumulative slip and progressive acceleration eventually driving the system to catastrophic failure under a given far-field stress. Studying the transition towards catastrophic failure re...

Physical and chemical fluid/rock interactions control seismic rupture nucleation, propagation, arrest and recurrence. Several experimental studies explored the effects of pore fluid pressure (Pp) on the sliding behavior of faults. Most of them were performed with bi and tri-axial apparatus at high temperature and high confining pressure. However, d...