ABSTRACT Prior work suggests that high-frequency seismic attenuation acts as a highly sensitive proxy for crustal permeability and fluid mobility in fractured media. We test the hypothesis that the fault system responsible for the 2016–2017 Amatrice–Visso–Norcia–Capitignano sequence acted as an impermeable seal, compartmentalizing pressurized fluids until dynamic rupture triggered widespread fluid diffusion. By tracking across the sequence the spatiotemporal evolution of the S-wave anelastic attenuation parameter, we identify large, positive low-frequency attenuation anomalies emerging within the hanging wall following the Amatrice mainshock and strictly preceding subsequent large ruptures. Conversely, we observe weaker, negative anomalies in the footwall, anticorrelated in time with those of the hanging wall, revealing a massive asymmetry in fluid redistribution and permeability evolution across the fault system. Furthermore, aftershock migration rates reveal distinct linear alignments in a distance-reduced time space, allowing us to explicitly track and quantify episodes of lateral and upward fluid migration. These physically consistent patterns suggest that stress-driven fluid diffusion directly weakens adjacent fault patches, dictating the spatiotemporal migration of seismicity. We conclude that near-real-time monitoring of seismic attenuation may help detect fluid redistribution in active fault systems and may provide useful information for time-dependent seismic hazard assessment.

The earthquake size distribution is well described by the Gutenberg Richter Law, controlled by the b-value parameter. In recent decades, a great variety of methods for estimating the b-value have been proposed by the scientific community, despite the simplicity of this relationship. All these methods underlie the different views of individual modellers and, therefore, often generate inconsistent results. In this study, we perform a seismological experiment in which we compare different, commonly adopted, methodologies, to estimate the completeness magnitude and the b-value, for seismicity in Central Italy. The intermethod differences are on average equal to 0.4 and 0.3, for Mc and b, respectively, but reach much larger values, especially during more intense seismic activity. This shows that epistemic uncertainty in the b-value plays a more crucial role than intramethod uncertainties, opening new perspectives in the interpretation of discrepant, single studies.

We perform the attenuation local earthquake tomography (P,S, waves) of the complex fault system that ruptured a wide portion of the central Apennines, during the long lasting seismic sequence that started in 2016. Three mainshocks (the 24 August Mw 6.1, the 26 October Mw 5.9, and the 30 October Mw 6.5) hit the towns of Amatrice, Norcia and Visso causing several casualties and diffuse damages. The ruptured faults spread over an 80 km north-northwest-elongated section of a normal-faulting system. A huge amount of seismic data has been collected by permanent and temporary seismic stations since the onset of the sequence.About 230,000 seismic events have been analyzed to retrieve P and S seismic waves arrival times, which allowed us to compute 3-D velocity structure and precise earthquake locations.For all of the earthquakes that meet strict selection criteria based on the signal to noise ratio, we computed the low frequency spectral level and the decay of the amplitude spectra (t*) of both P- and S-waves at about 150 recording sites. In order to avoid source complexity, we selected only events with M

To understand the seismic hazard of a subduction zone, it is necessary to know the geometry, location and mechanical characteristics of the interplate boundary below which an oceanic plate is thrust downward. By considering the azimuthal dependence of converted P-to-S (Ps) amplitudes in receiver functions, we have detected the interplate boundary in the Makran subduction zone, revealing significant seismic anisotropy at the base of the accretionary wedge above the slab before it bends down beneath the Jaz Murian basin. This anisotropic feature aligns with a zone of reduced seismic velocity and a high primary/secondary wave velocity ratio (Vp/Vs), as documented in previous studies. The presence of this low-velocity highly anisotropic layer at the base of the accretionary wedge, likely representing a low-strength shear zone, could possibly explain the unusually wide accretionary wedge in Makran. Additionally, it may impact the location and width of the locked zone along the interplate boundary.

We investigate the presence of the quasi-Love wave (qL) at 51 seismic stations of a temporary seismic network across the western Arabia-Eurasia collision zone. We quantify the intensity of the qL observations from the April 12, 2014 Solomon Islands earthquake by calculating the peak-to-peak amplitude ratios of the qL and Love waves, and compare them with predicted qL intensities from previous shear-wave splitting results. We determine the polarity, timing, and period-dependence of the qL observations within the period range of 50-100 s. Our analysis reveals that the qL observations at stations in the Zagros and Alborz mountain belts exhibit opposite characteristics. In contrast to the Alborz stations, the intensity of qL observations at the Zagros stations exhibits relatively negligible dependence on the period, while their receiver-scatterer distances are considerably period dependent. We approximately locate the anisotropic gradients that generate the qL waves. Our results suggest that a lithospheric gap is responsible for the shallow and abrupt variation in the belt-parallel trend of fast-axis orientations in the westernmost part of the Zagros. Additionally, the period/depth dependence of the anisotropic gradients along the boundary between the central Zagros and central Iran provides insight into the variation in the downward dip of the Arabian lithosphere. The anisotropic gradient located to the north of the Doruneh fault in eastern Iran indicates its role as a major shear zone and lithospheric boundary. Finally, we observe that the spatial distribution of the anisotropic gradient in northeastern Iran matches the higher strain rate areas in the Kopet Dagh Mountains, suggesting coupling between the lithospheric mantle and crust in that region.

ABSTRACT The measurement of earthquake source parameters is affected by large uncertainties, and different approaches lead to large variability in results. One crucial aspect is the trade-off between attenuation (Q) and corner frequency (fc) in spectral fitting: The source corner frequency, inversely proportional to the fault size, can be severely masked by attenuation and site effects. In this article, we describe a method to solve the trade-off based on the fit of displacement spectra to find the source characteristics (corner frequency, fc, and the signal moment, Ω0) and the single-station attenuation operator (t*), in addition to the site response. We follow a parametric approach based on the use of 3D Q seismic tomography and a bootstrap-based method for selecting the best spectra fit. The correction of attenuation with synthetic values derived by 3D attenuation tomography efficiently deals with the trade-off between source and path terms, leading to small uncertainties in the determination of source unknowns (fc and signal moment, Ω0), thus yielding constrained estimates of source parameters for low- to medium-magnitude earthquakes. We show an application to the Emilia 2012 seismic sequence, for which we computed the source parameters for 1240 aftershocks (from an initial dataset of 1748) with local magnitude ranging from 2.0 to 4.7 using the spectral fit from P and S waves. About 80% of stress-drop estimations are characterized by relatively low uncertainties (within 20% of the estimated values), with maximum values of about 40% for the remaining 20%. The attenuation correction is effective to determine source parameters for small-magnitude earthquakes; hence, we obtain reliable estimates of source parameters for the entire aftershock sequence. This approach gives the opportunity to infer the mechanical state of a complete fault system by taking advantage of the larger number of low-magnitude events (with respect to the largest ones) that always follow a major earthquake.

Tomographic images of the lithosphere are the first step to constrain the evolution of mountain belts and their interaction. By inverting new high‐quality P‐ and S‐wave arrivals that sample the entire lithosphere, we determined Vp and Vp / Vs models with reliable resolution in the critical depth range (40–80 km) where plates of the central Mediterranean area interact. This data set yields homogeneous representation of the 3D structure over a critical area at a regional scale. Here, we show that the Alps derive from a laterally continuous underthrusting of the European plate and that the Adria lithosphere was delaminated after the collision. Tomograms resolve the lateral changes of the continental versus oceanic subduction along the Alpine belt and identify original evidence of fluids beneath the orogens able to facilitate the current deformation.

We use seismic waveform data from the AlpArray Seismic Network and three other temporary seismic networks, to perform receiver function (RF) calculations and time-to-depth migration to update the knowledge of the Moho discontinuity beneath the broader European Alps. In particular, we set up a homogeneous processing scheme to compute RFs using the time-domain iterative deconvolution method and apply consistent quality control to yield 112 205 high-quality RFs. We then perform time-to-depth migration in a newly implemented 3D spherical coordinate system using a European-scale reference P and S wave velocity model. This approach, together with the dense data coverage, provide us with a 3D migrated volume, from which we present migrated profiles that reflect the first-order crustal thickness structure. We create a detailed Moho map by manually picking the discontinuity in a set of orthogonal profiles covering the entire area. We make the RF dataset, the software for the entire processing workflow, as well as the Moho map, openly available; these open-access datasets and results will allow other researchers to build on the current study.

Minimum 1D velocity models and station corrections have been computed for the central Mediterranean area using two main data sets. The first one consists of accurate first arrival-time readings from 103 seismic events with magnitude (ML)≥3.5 recorded by the Italian National Seismic Network (RSN) and the AlpArray Seismic Network (AASN) in the period 2014–2021. Earthquakes were selected on the basis of their spatial distribution, epicentral distance to the nearest seismic station, and maximum distance traveled by Pn and Sn phases. This fine selection of high-quality data combined with the spatial density of the AlpArray seismic stations was decisive in obtaining high resolution for upper mantle velocity, especially in the Alpine belt. To obtain a denser coverage of crustal rays, we extended the first data set with P and S arrivals of local earthquakes from Istituto Nazionale di Geofisica e Vulcanologia (INGV) bulletin data (2016–2018). A total of 75,807 seismic phases (47,183 P phases and 28,264 S phases) have been inverted to calculate best-fit 1D velocity models, at regional and local scales. We then test the performance of the optimized velocity models by relocating the last four years of seismicity recorded by INGV (period 2017–2020). The computed velocity models are very effective for routine earthquake location, seismic monitoring, source parameter modeling, and future 3D seismic tomography.

Seismic swarms frequently occur along continental fault systems and their relation with large earthquakes is often contradictory. Such a case is documented in the Pollino mountain range of southern Italy, a decoupling zone where the belt-normal stretching drastically rotates accommodating the differential SE-retreat of the Ionian slab. The paucity of historical large earthquakes has led to hypothesize the presence of a seismic gap. A long-lasting seismic swarm that climaxed with a ML = 5.2 earthquake in October 2012 was therefore thought as a possible signal of an impending large earthquake filling the gap. Seismicity data collected during a 4-years long monitoring are a powerful microscope to look through the seismic swarm. In this study, we present accurate relocations for 2385 earthquakes and high-resolution Vp and Vp/Vs models of the fault system. Seismicity occurred on two separate normal faults that were formerly part of a thrusts and back-thrusts system, originally formed as a pop-up at restraining bends of the Pollino fault, a wrench fault system that inverted the original left lateral sense of slip accommodating a differential motion induced by the southward retreat of the Ionian slab.

In the years between 2011 and 2014, at the edge between the Apennines collapsing chain and the subducting Calabrian arc, intense seismic swarms occurred in the Pollino mountain belt. In this key region, <2.5 mm/yr of NE-trending extension is accommodated on an intricate network of normal faults, having almost the same direction as the mountain belt. The long-lasting seismic release consisted of different swarm episodes, where the strongest event coinciding with a M L 5.0 shock occurred in October 2012. This latter comes after a M L four nucleated in May 2012 and followed by aseismic slip episodes. In this study, we present accurate relocations for ∼6,000 earthquakes and shear-wave splitting analysis for ∼22,600 event-station pairs. The seismicity distribution delineates two main clusters around the major shocks: in the north-western area, where the M L 5.0 occurred, the hypocenters are localized in a ball-shaped volume of seismicity without defining any planar distribution, whilst in the eastern area, where the M L 4.3 nucleates, the hypocenters define several faults of a complex system of thrusts and back-thrusts. This different behavior is also imaged by the anisotropic parameters results: a strong variability of fast directions is observed in the western sector, while stable orientations are visible in the eastern cluster. This tectonic system possibly formed as a positive flower structure but as of today, it accommodates stress on normal faults. The deep structure imaged by refined locations is overall consistent with the complex fault system recently mapped at the surface and with patterns of crustal anisotropy depicting fractures alignment at depth. The possible reactivation of inherited structures supports the important role of the Pollino fault as a composite wrench fault system along which, in the lower Pleistocene, the southward retreat of the ionian slab was accommodated; in this contest, the inversion of the faults kinematics indicates a probable southward shift of the slab edge. This interpretation may help to comprehend the physical mechanisms behind the seismic swarms of the region and defining the seismic hazard of the Pollino range: nowadays a region of high seismic hazard although no strong earthquakes are present in the historical record.

The 2012 Emilia earthquakes sequence is the first debated case in Italy of destructive event possibly induced by anthropic activity. During this sequence, two main earthquakes occurred separated by 9 days on contiguous thrust faults. Scientific commissions engaged by the Italian government reported complementary scenarios on the potential trigger mechanism ascribable to exploitation of a nearby oil field. In this study, we combine a refined geodetic source model constrained by precise aftershock locations and an improved tomographic model of the area to define the geometrical relation between the activated faults and investigate possible triggering mechanisms. An aftershock decay rate that deviates from the classical Omori-like pattern and Vp/Vs changes along the fault system suggests that natural pore pressure pulse drove the space-time evolution of seismicity and the activation of the second main shock. Plain Language Summary Induced seismicity has increasingly become a major topic among Earth scientists. Concerns about a causative role played by the oil-gas extraction and/or the deep injection of disposable water in triggering earthquakes have grown fast in recent years, with many implications for public regulators and stakeholders. We present and discuss the 2012 Emilia (Italy) earthquakes sequence, which is the first debated case in Italy of destructive events possibly induced by anthropic activity. We use coseismic ground deformation measurements and seismic data to define the geometrical relation between the activated faults and investigate possible triggering mechanisms. Our results indicate that fluid pressure at the base of the system was responsible for triggering the two 2012 main earthquakes and exclude any anthropic trigger. Thus, cautious monitoring of seismicity, deformation, and pore pressure can recognize active processes along active faults that can potentially cause earthquakes. Such monitoring should be mandatory to conjugate exploitation activity in seismic active areas with safeness and sustainability.

We compute S velocity profiles of the crust across the Messina Strait (Italy), the tear zone at the southern end of the Ionian subduction zone. Separating Sicily from Calabria, the Messina Strait hosted some of the strongest earthquakes to ever occur in Italy. Here the motion of the Ionian slab with respect to Sicily creates a complex tectonic setting characterized by lithospheric tearing. We show velocity models of the crust, computed from teleseismic receiver function inversion, outlining the differences between Sicily and Calabria. Strong deformation across the Messina Strait between 10-15 and 30 km depth is expressed by strong anisotropy (up to 10%), developed in a ductile shear zone of the crust. The top of these ductile weaker layers could limit the depth extent of future ruptures.

At 01:36:32 UTC on August 24, 2016 an earthquake of magnitude 6.0 occurred in Central Italy, affecting many small towns and municipalities in the Lazio, Umbria, Marche and Abruzzo regions. The event caused severe damages, many victims and 299 fatalities. Only 21 seconds after the beginning of the earthquake, the first automatic location of this earthquake was available and stored in our earthquakes database. The first magnitude estimate followed 68 seconds after the origin time. Few seconds later the INGV seismologists on duty in accordance to the agreed protocols provided the first alert to the Italian Civil Protection Department (Dipartimento di Protezione Civile, DPC) and thereby triggered the seismic emergency protocol. Subsequently, they elaborated the data in order to produce the first manually reviewed hypocenter, which was published on the Institute’s website at 01:53:18 UTC. The sequence following this mainshock generated thousands of earthquakes in the epicentral area, which the INGV automated localization system processed and detected along with the usual seismic activity in the rest of the Italian territory. In this paper we analyze the behavior of the automated system and of the data lifecycle management procedures in such extraordinary conditions. In particular we want to measure the capability of the system to manage the huge data flow, in terms of frequency and size of seismic events and its ability to remain fairly responsive and accurate in accomplishing its duty in the expected time. This will help us to identify potential problems and to suggest necessary improvements to better serve the INGV mission for Civil Protection.