Alto Tiberina Research Articles

A strainmeter array as the fulcrum of novel observatory sites along the Alto Tiberina Near Fault Observatory

Abstract. Fault slip is a complex natural phenomenon involving multiple spatiotemporal scales from seconds to days to weeks. To understand the physical and chemical processes responsible for the full fault slip spectrum, a multidisciplinary approach is highly recommended. The Near Fault Observatories (NFOs) aim at providing high-precision and spatiotemporally dense multidisciplinary near-fault data, enabling the generation of new original observations and innovative scientific products. The Alto Tiberina Near Fault Observatory is a permanent monitoring infrastructure established around the Alto Tiberina fault (ATF), a 60 km long low-angle normal fault (mean dip 20°), located along a sector of the Northern Apennines (central Italy) undergoing an extension at a rate of about 3 mm yr−1. The presence of repeating earthquakes on the ATF and a steep gradient in crustal velocities measured across the ATF by GNSS stations suggest large and deep (5–12 km) portions of the ATF undergoing aseismic creep. Both laboratory and theoretical studies indicate that any given patch of a fault can creep, nucleate slow earthquakes, and host large earthquakes, as also documented in nature for certain ruptures (e.g., Iquique in 2014, Tōhoku in 2011, and Parkfield in 2004). Nonetheless, how a fault patch switches from one mode of slip to another, as well as the interaction between creep, slow slip, and regular earthquakes, is still poorly documented by near-field observation. With the strainmeter array along the Alto Tiberina fault system (STAR) project, we build a series of six geophysical observatory sites consisting of 80–160 m deep vertical boreholes instrumented with strainmeters and seismometers as well as meteorological and GNSS antennas and additional seismometers at the surface. By covering the portions of the ATF that exhibits repeated earthquakes at shallow depth (above 4 km) with these new observatory sites, we aim to collect unique open-access data to answer fundamental questions about the relationship between creep, slow slip, dynamic earthquake rupture, and tectonic faulting.

Statistically significant difference between earthquake size distributions of independent and triggered seismicity

The Alto Tiberina Fault system, located in Central Italy, is an active structure about 60 km long composed of a principal low-angle normal fault and several minor synthetic and antithetic splays. The system is monitored by a dense seismic network, giving us the opportunity to construct high-definition seismic catalogs with a low completeness magnitude. We analyze the clustering properties of the 2010-2015 seismicity by using a 3D stochastic declustering algorithm that also includes the earthquakes’ depth. We demonstrate that the earthquake size distribution is strongly correlated with the clustering of seismic events and their depth; in particular, the principal fault and secondary faults show an opposite behavior both in terms of earthquake size distribution and clustering properties.

Non‐Linear Seismic Velocity Variations Observed During a Seismic Swarm in the Alto Tiberina Low Angle Normal Fault From Ambient Noise Correlation Measurements

AbstractFrom seismic interferometry, we investigate the strain sensitivity to seismic velocity variations related to a seismic swarm activity that occurred in 2013 along the Alto Tiberina low angle normal fault. We compute daily auto‐correlation functions of ambient noise recorded at seismic stations located in the vicinity of the fault over the course of 10 years. Using the stretching technique, we compute daily velocity variations smoothed over a period of 100 days with a time lapse approach. Through the application of an optimization procedure based on synthetic modeling, we separate the non‐tectonic, thermoelastic and rain induced velocity variations, from the tectonic components. Consequently, we unravel a significant velocity drop of 0.033% coinciding with the swarm occurring at seismogenic depth (3–5 km). Additionally, the time evolution of the velocity changes shows a direct relationship with the strain rate rather than the strain indicating a non‐linear behavior of the crust induced by aseismic slip. The deduced strain sensitivity, exhibiting an order of magnitude comparable to that observed within volcanic settings, confirms this non‐linear behavior and suggests the presence of pressurized fluids at depth.

Unraveling Earthquake Clusters Composing the 2014 Alto Tiberina Earthquake Swarm via Unsupervised Learning

AbstractEarthquake swarms represent a particular mode of seismicity, not directly related to the occurrence of large earthquakes (e.g., aftershocks) but rather driven by external forcing such as aseismic deformation or fluid migration in fault systems. Sometimes their occurrence overlaps with observable geodetic signals in space and time, indicating a direct link. However, the low resolution of geodetic observations tends to obscure the small scale spatial and temporal dynamics of swarms. In this work, we automatically extract clusters of seismicity related to the 2014 Alto Tiberina swarm sequence (Italy) using an unsupervised clustering approach that exploits space and time information of the seismicity. The quantitative characterization of each cluster indicates that the overall swarm is composed of spatially and temporally confined (sub) swarms each of which could potentially be driven by small‐scale aseismic deformation process. This observation aligns with similar findings during slow slip events in subduction zones.

A novel infrastructure for the continuous monitoring of soil CO2 emissions: a case study at the alto Tiberina near fault observatory in Italy

Static and dynamic stress, along with earthquakes, can trigger the emission and migration of crustal fluids, as frequently observed on the surface and within the upper crust of tectonically active areas such as the northern Apennines of Italy. To investigate the origin of these fluids and their interconnection with the seismogenic process, we complemented The Alto Tiberina Near Fault Observatory (TABOO-NFO), a multidisciplinary monitoring infrastructure composed of a dense array of seismic, geodetic, strain, and radon sensors, with a proper geochemical network grounded on four soil CO2 flux monitoring stations and weather sensors, placed near the main vents of the superficial manifestations. The TABOO-NFO is a state-of-the-art monitoring infrastructure, which allows for studying various geophysical parameters connected to the deformation processes active along a crustal fault system dominated by the Alto Tiberina fault (ATF), which is a 60 km long normal fault dipping at a low angle (<15°–20°). The region is favourable for conducting geochemical studies, as it is characterised by the presence of over-pressurised fluids trapped at certain depths and superficial manifestations associated with the emission of large quantities of fluids. After describing the theoretical framework and the technological aspects based on which we developed the geochemical monitoring network, we described the data recorded in the first months. Over the studied period, the results showed that soil CO2 flux was primarily influenced by environmental parameters, and that the selected sites received a regular supply of deep-origin CO2.

Spatiotemporal Evolution of the Seismicity in the Alto Tiberina Fault System Revealed by a High‐Resolution Template Matching Catalog

AbstractThe Alto Tiberina Fault system, located in the Northern Apennines (Italy), consists of a low angle normal fault (LANF) which radiates micro‐seismicity that can be explained by continuous creep. On top of the LANF, a network of syn‐ and antithetic high angle faults frequently hosts seismic swarms, one of which has been associated with a transient aseismic deformation signal. To study in detail the seismicity and its relationships with aseismic deformation processes occurring in this fault system, we apply template matching on seismic data recorded at an array of borehole stations, to derive a high‐resolution earthquake catalog. Thanks to the additionally detected events, we are able to reveal time periods of increased spatial‐ and temporal clustering during an aseismic deformation event. This reflects the complex evolution of aseismic slip together with the complexity of the shallow fault system. Along the LANF, we observe a bimodal type of seismicity, with diffuse seismicity active continuously, and short‐lived bursts of seismicity that could indicate rapid fluid releases. We additionally identify repeating earthquakes. These events not necessarily match a simple creep model and therefore open the possibility for new models to explain the seismicity along the LANF.

Seismicity Rate Changes and Geodetic Transients in Central Apennines

AbstractUsing template matching and GPS data, we investigate the evolution of seismicity and observable deformation in Central Apennines. Seismicity appears more persistent at the base of the seismogenic layer than in the shallower crust. Diffuse activity is reported on segments at depth, alternating along strike with apparent quiescence on segments that experienced one or more Mw6+ earthquakes in 1997, 2009, and 2016. Central Apennines are likely underlain by a sizeable shear zone with areas of diffuse seismicity bounding shallow normal faults where Mw6+ earthquakes occurred. The deformation observed at the surface seems to follow the seismicity variations at the base of seismogenic layer along the Apenninic chain. Principal and independent component analyses of GPS data exhibit a transient when the 2016 foreshock sequence starts. This transient propagated northward from the Campotosto fault up to the Alto Tiberina fault system and has likely loaded the Mw6+ 2016 earthquake sequence.

Intermittent Slip Along the Alto Tiberina Low‐Angle Normal Fault in Central Italy

AbstractThe Alto Tiberina normal fault (ATF) in central Italy is a 50‐km‐long crustal structure that dips at a low angle (15–20°). Events on the fault plane are about 10 times less frequent than those located in its shallower syn‐ and antithetic hanging‐wall splays. To enhance ATF catalog and achieve a better understanding of the degree of coupling in the fault system, we apply a template matching technique in the 2010–2014 time window. We augment by a factor 5 the detections and decrease the completeness magnitude to negative values. Contrary to what previously observed on ATF, we highlight intermittent seismic activity and long‐lasting clusters interacting with sequences on the shallower splays. One of these episodes of prolonged seismic activity, detected at the end of 2013 on a 30‐km‐long ATF segment, suggest the ATF active role during an aseismic transient unraveled by geodetic data.

Partially Locked Low‐Angle Normal Faults in Cohesive Upper Crust

AbstractIt is well established that slip on a frictionally weak low‐angle normal fault (LANF) can be more favorable than breaking a steep fault in strong crust. Very few studies, however, have considered the specific effect of crust and fault cohesion on LANF viability. We do so using Limit Analysis, a methodology for predicting the optimal orientation of faults with varying strength subjected to a specific set of boundary conditions. Accounting for crustal cohesion in our models reduces the lowest admissible LANF dip and even allows slip on high‐friction LANFs if the contrast between crust and fault cohesion is large. Fault cohesion, however, increases the lowest admissible LANF dip and introduces a locking depth above which LANF slip is not mechanically feasible. This is consistent with observations of steep splay faults rooting onto LANFs in a variety of settings. We further demonstrate that locking depth can help constrain LANF cohesion, friction, and fluid pressure on the Alto Tiberina (Italy) and western Corinth (Greece) LANFs. Specifically, assuming a measured fault friction of 0.2–0.3, we find that the shallow locking depth of the Alto Tiberina fault requires either (1) moderate fluid overpressure (57% of lithostatic) with cohesion of 8–12 MPa or (2) strong overpressure (77% of lithostatic) with cohesion of 13–20 MPa along the fault. By contrast, the larger locking depth characterizing the western Corinth LANF can reflect greater fault cohesion.

Did the Deadly 1917 Monterchi Earthquake Occur on the Low‐Angle Alto Tiberina (Central Italy) Normal Fault?

Did the Deadly 1917 Monterchi Earthquake Occur on the Low‐Angle Alto Tiberina (Central Italy) Normal Fault?

Change-point analysis ofVP/VSratio time-series using a trans-dimensional McMC algorithm: applied to the Alto Tiberina Near Fault Observatory seismic network (Northern Apennines, Italy)

Change-point analysis of<i>VP</i>/<i>VS</i>ratio time-series using a trans-dimensional McMC algorithm: applied to the Alto Tiberina Near Fault Observatory seismic network (Northern Apennines, Italy)

Effects of lateral variations of crustal rheology on the occurrence of post-orogenic normal faults: The Alto Tiberina Fault (Northern Apennines, Central Italy)

The Northern Apennines (NA) are characterized by formerly compressive structures partly overprinted by subsequent extensional structures. The area of extensional tectonics migrated eastward since the Miocene. The youngest and easternmost major expression of extension is the Alto Tiberina Fault (ATF). We estimate 2D rheological profiles across the NA, and conclude that lateral rheological crustal variations have played an important role in the formation of the ATF and similar previously active faults to the west. Lithospheric delamination and mantle degassing resulted in an easterly-migrating extension-compression boundary, coinciding at present with the ATF, where (i) the thickness of the upper crust brittle layer reaches a maximum; (ii) the critical stress difference required to initiate faulting at the base of the brittle layer is at a minimum; and (iii) the total strengths of both the brittle layer and the whole lithosphere are at a minimum.Although the location of the fault is correlated with lithospheric rheological properties, the rheology by itself does not account for the low dip (~20°) of the ATF. Two hypotheses are considered: (a) the low dip of the ATF is related to a rotation of the stress tensor at the time of initiation of the fault, caused by a basal shear stress (~100MPa) possibly related to corner flow associated with delamination; or (b) the low dip is associated to low values of the friction coefficient (≤0.5) coupled with high pore pressures related to mantle degassing.Our results establishing the correlation between crustal rheology and the location of the ATF are relatively robust, as we have examined various possible compositions and rheological parameters. They also provide possible general indications on the mechanisms of localized extension in post-orogenic extensional setting. The hypotheses to account for the low dip of the ATF, on the other hand, are intended simply to suggest possible solutions worthy of further study.

Seismic swarms and diffuse fracturing within Triassic evaporites fed by deep degassing along the low‐angle Alto Tiberina normal fault (central Apennines, Italy)

AbstractWe present high‐resolution elastic models and relocated seismicity of a very active segment of the Apennines normal faulting system, computed via transdimensional local earthquake tomography (trans‐D LET). Trans‐D LET, a fully nonlinear approach to seismic tomography, robustly constrains high‐velocity anomalies and inversions of P wave velocity, i.e., decreases of VP with depth, without introducing bias due to, e.g., a starting model, and giving the possibility to investigate the relation between fault structure, seismicity, and fluids. Changes in seismicity rate and recurring seismic swarms are frequent in the Apennines extensional belt. Deep fluids, upwelling from the delaminating continental lithosphere, are thought to be responsible for seismicity clustering in the upper crust and lubrication of normal faults during swarms and large earthquakes. We focus on the tectonic role played by the Alto Tiberina low‐angle normal fault (ATF), finding displacements across the fault consistent with long‐term accommodation of deformation. Our results show that recent seismic swarms affecting the area occur within a 3 km thick, high VP/VS, densely cracked, and overpressurized evaporitic layer, composed of dolostones and anhydrites. A persistent low VP, low VP/VS volume, present on top of and along the ATF low‐angle detachment, traces the location of mantle‐derived CO2, the upward flux of which contributes to cracking within the evaporitic layer.

On the mechanical behaviour of a low-angle normal fault: the Alto Tiberina fault (Northern Apennines, Italy) system case study

Abstract. Geological and seismological observations have been used to parameterize 2-D numerical elastic models to simulate the interseismic deformation of a complex extensional fault system located in the Northern Apennines (Italy). The geological system is dominated by the presence of the Alto Tiberina fault (ATF), a large (60 km along strike) low-angle normal fault dipping 20° in the brittle crust (0–15 km). The ATF is currently characterized by a high and constant rate of microseismic activity, and no moderate-to-large magnitude earthquakes have been associated with this fault in the past 1000 years. Modelling results have been compared with GPS data in order to understand the mechanical behaviour of this fault and a suite of minor syn- and antithetic normal fault segments located in the main fault hanging wall. The results of the simulations demonstrate the active role played by the Alto Tiberina fault in accommodating the ongoing tectonic extension in this sector of the chain. The GPS velocity profile constructed through the fault system cannot be explained without including the ATF's contribution to deformation, indicating that this fault, although misoriented, has to be considered tectonically active and with a creeping behaviour below 5 km depth. The low-angle normal fault also shows a high degree of tectonic coupling with its main antithetic fault (the Gubbio fault), suggesting that creeping along the ATF may control the observed strain localization and the pattern of microseismic activity.

Assessment of earthquake locations in 3‐D deterministic velocity models: A case study from the Altotiberina Near Fault Observatory (Italy)

AbstractThe accuracy of earthquake locations and their correspondence with subsurface geology depends strongly on the accuracy of the available seismic velocity model. Most modern methods to construct a velocity model for earthquake location are based on the inversion of passive source seismological data. Another approach is the integration of high‐resolution geological and geophysical data to construct deterministic velocity models in which earthquake locations can be directly correlated to the geological structures. Such models have to be kinematically consistent with independent seismological data in order to provide precise hypocenter solutions. We present the Altotiberina (AT) seismic model, a three‐dimensional velocity model for the Upper Tiber Valley region (Northern Apennines, Italy), constructed by combining 300 km of seismic reflection profiles, six deep boreholes (down to 5 km depth), detailed data from geological surveys and direct measurements ofPandSwave velocities performed in situ and in laboratory. We assess the robustness of the AT seismic model by locating 11,713 earthquakes with a nonlinear, global‐search inversion method and comparing the probabilistic hypocenter solutions to those calculated in three previously published velocity models, constructed by inverting passive seismological data only. Our results demonstrate that the AT seismic model is able to provide higher‐quality hypocenter locations than the previous velocity models. Earthquake locations are consistent with the subsurface geological structures and show a high degree of spatial correlation with specific lithostratigraphic units, suggesting a lithological control on the seismic activity evolution.

On the Relationship betweenMwandMLfor Small Earthquakes

Abstract Estimating the moment magnitudes ( M w ) of a small earthquake is a challenging task. One viable option to measure its size is to calculate its local magnitude ( M L ) and convert it to the physically based M w . Unfortunately, to correctly perform such a conversion is not easy; moreover, even though many studies demonstrate that the equivalence between M L and M w is incorrect for small events, these two parameters are sometimes thought to be strictly equivalent, regardless of the earthquake’s size. Using random vibration theory, we show that, below M w ∼4, the M L of a small earthquake is proportional to the logarithm of its seismic moment, and the following relationship holds: We test our findings on a high‐quality data set in the Upper Tiber Valley (northern Apennines, Italy), composed of events in the range of 0≤ M L ≤3.8, for which we compute accurate estimates of M L and M w . Online Material: Details of the processing procedure, figures of the empirical regional attenuation functional, and source terms of 1191 events from the Alto Tiberina fault (ATF) data set and earthquake catalog.

Crustal fracturing and presence of fluid as revealed by seismic anisotropy: case histories from seismogenic areas in the Apennines (Italy)

An automatic analysis code, Anisomat+, was developed, tested and improved to calculate anisotropic parameters: fast polarization direction and delay time. Anisomat+ was applied on data coming from three zones of the Apennines in Italy. For each area, anisotropic parameters have been interpreted to determine the fracture and stress field taking into account the geological and structural settings. It was recognized that the averages of fast directions are NW-SE–oriented at all sites, in agreement with the orientation of maximum horizontal stress as well as with the strike of the main fault structures. The mean values of normalized delay time range from 0.005 s/km to 0.007 s/km and to 0.009 s/km, respectively for L'Aquila region, Alto Tiberina Fault area and Val d'Agri basin, suggesting a 3-4% of crustal anisotropy. Moreover, for each area, the spatial distribution of anisotropic parameters is examined, and for L’Aquila 2009 seismic sequence also temporal distribution is discussed.

An automatic modular procedure to generate high-resolution earthquake catalogues: application to the Alto Tiberina Near Fault Observatory (TABOO), Italy.

An automatic modular procedure to generate high-resolution earthquake catalogues: application to the Alto Tiberina Near Fault Observatory (TABOO), Italy.

Interaction between regional and local tectonic forcing along a complex Quaternary extensional basin: Upper Tiber Valley, Northern Apennines, Italy

In extending areas undergoing regional tectonic uplift, the persistence of subsidence at a normal-fault hanging-wall depends on the competition between regional and local tectonic effects. When regional uplift exceeds the subsidence of the hanging-wall block, denudation prevails at both the hanging-wall and the foot-wall. When local tectonic subsidence exceeds regional uplift, sedimentation occurs over the hanging-wall block, supplied by foot-wall erosion. We analyzed a Pliocene–Quaternary continental basin, currently crossed by the Tiber River in Italy. The tectono-sedimentary evolution of the basin developed at the hanging-wall of a regional low-angle extensional detachment, the Alto Tiberina Fault, in the axial region of the Northern Apennines of Italy. This area is affected by regional uplift on the order of 0.5–1.0 mm/yr. The present-day activity of the fault is revealed by both microseismicity and geodetic (GPS) data. We investigated the mid- (10–100 ka) and long-term (0.5–3.0 Ma) evolution of the three depocenters by studying the continental Pleistocene succession infilling the basin as well as fluvial terraces and higher paleosurfaces carved into the Pleistocene deposits.By using surficial geologic data and an interpretation of a set of seismic reflection profiles, we show that the three depocenters experienced a fairly similar evolution during the Pliocene–Early Pleistocene, when a 1000-m-thick continental succession was deposited. On the contrary, geomorphological observations indicate that, at the beginning of the Middle Pleistocene, a switch occurred in the evolution of the three depocenters. In the northernmost Sansepolcro sub-basin, bounding normal faults are active and hanging-wall subsidence outpaces regional uplift. Concurrently, in the Umbertide and Ponte Pattoli sub-basins uplift dominates over the hanging-wall subsidence, promoting river incision and exhumation of the Pleistocene deposits. For these two depocenters, by means of terrace-river correlations, we estimate that the incision rate is ∼0.3–0.35 mm/yr, suggesting a maximum tectonic subsidence of 0.2 mm/yr.The identification of a heterogeneous uplift pattern along the hanging-wall of the Alto Tiberina Fault, driven by different displacement rates of its fault splays, allowed us to characterize fault segments with different activities and, possibly, different seismic behaviors.

The Alto Tiberina Near Fault Observatory (northern Apennines, Italy)

&lt;p&gt;The availability of multidisciplinary and high-resolution data is a fundamental requirement to understand the physics of earthquakes and faulting. We present the Alto Tiberina Near Fault Observatory (TABOO), a research infrastructure devoted to studying preparatory processes, slow and fast deformation along a fault system located in the upper Tiber Valley (northern Apennines), dominated by a 60 km long low-angle normal fault (Alto Tiberina, ATF) active since the Quaternary. TABOO consists of 50 permanent seismic stations covering an area of 120 × 120 km&lt;span&gt;&lt;sup&gt;2&lt;/sup&gt;&lt;/span&gt;. The surface seismic stations are equipped with 3-components seismometers, one third of them hosting accelerometers. We instrumented three shallow (250 m) boreholes with seismometers, creating a 3-dimensional antenna for studying micro-earthquakes sources (detection threshold is M&lt;span&gt;&lt;sub&gt;L&lt;/sub&gt;&lt;/span&gt; 0.5) and detecting transient signals. 24 of these sites are equipped with continuous geodetic GPS, forming two transects across the fault system. Geochemical and electromagnetic stations have been also deployed in the study area. In 36 months TABOO recorded 19,422 events with M&lt;span&gt;&lt;sub&gt;L&lt;/sub&gt;&lt;/span&gt; ≤ 3.8 corresponding to 23.36e-04 events per day per squared kilometres; one of the highest seismicity rate value observed in Italy. Seismicity distribution images the geometry of the ATF and its antithetic/synthetic structures located in the hanging-wall. TABOO can allow us to understand the seismogenic potential of the ATF and therefore contribute to the seismic hazard assessment of the area. The collected information on the geometry and deformation style of the fault will be used to elaborate ground shaking scenarios adopting diverse slip distributions and rupture directivity models.&lt;/p&gt;