The fast individuation and modeling of faults responsible for large earthquakes are fundamental for understanding the evolution of potentially destructive seismic sequences. This is even more challenging in case of buried thrusts located in offshore areas, like those hosting the 9 November 2022 Ml 5.7 (M w 5.5) and M L 5.2 earthquakes that nucleated along the Apennines compressional front, offshore the northern Adriatic Sea. Available on- and offshore (from hydrocarbon platforms) geodetic observations and seismological data provide robust constraints on the rupture of a 15 km long, ca. 24° SSW-dipping fault patch, consistent with seismic reflection data. Stress increase along unruptured portion of the activated thrust front suggests the potential activation of longer portions of the thrust with higher magnitude earthquake and larger surface faulting. This unpleasant scenario needs to be further investigated, also considering their tsunamigenic potential and possible impact on onshore and offshore human communities and infrastructures.

We calculate seismic velocity structure of the north Zagros suture zone, west Iran, to resolve the crustal features at the boundary of Arabian-Central Iranian collision. We compute teleseismic receiver functions (RFs) for 46 stations along a transect crossing the suture. Through harmonic analysis and inversion of the RF data, we obtain information on the characteristics of the suture zone at depth. The RFs and their harmonics show a low angle NE dipping boundary between the overriding layer and a midcrustal low velocity zone, which corresponds to the suture zone. The overriding high velocity feature (Vs similar to 3.8 km/s) is interpreted as an intermediate depth crustal complex exhumed close to the surface through imbricate thrust faulting and enhanced by crustal buoyancy due to continental underthrusting. Significant anisotropy is found above and below the suture zone: We interpret it in terms of slow-symmetry-axis anisotropy and derive clues on the ongoing deformation processes.

Persistent seismic swarms originate along the normal faulting system of central Apennines (Italy). In this study, we analyze the space‐time‐energy distribution of one of the longer and more intense of these swarms, active since August 2013 in the high seismic risk area of the Gubbio basin. Our aim is to verify if information relevant to constraint short‐term earthquake occurrence scenarios is hidden in seismic swarms. During the swarm, the seismic moment release first accelerated, with a rapid migration of seismicity along the fault system, and suddenly dropped. We observe a decrease of the b ‐value, along the portion of the fault system where large magnitude events concentrated, possibly indicating that a fault patch was dynamically stressed. This finding suggests that the onset of seismic swarms might help the formation of critically stressed patches.

•Quantitative estimates of scattered quasi-Love waves in Italy.•Large-scale SKS fast-axis pattern is consistent with scattered qL.•Results argue for laterally-variable horizontal anisotropic symmetry axis.•qL originate at slab edges due to gradients in asthenospheric flow.

Since the late 1960s - early 1970s, seismologists started studying the elastic properties of the Earth crust looking for signals from the Earth interior indicating that a large earthquake is coming. To be useful for prediction a signal needs to: 1) occur before most large earthquakes and 2) occur only before large earthquakes. Up to now, no one has ever found such a signal, but since the beginning of the search, seismologists developed theories that included variations of the elastic property of the Earth crust prior to the occurrence of a large earthquake. The most popular is the theory of the dilatancy: when a rock is subject to stress, the rock grains are shifted generating micro-cracks, thus the rock itself increases its volume. Inside the fractured rock, fluid saturation and pore pressure play an important role in earthquake nucleation, by modulating the effective stress. Thus, measuring the variations of wave speed and of anisotropic parameter in time can be highly informative on how the stress leading to a major fault failure builds up. In 1980s and 1990s such kind of research on earthquake precursors slowed down and the priority was given to seismic hazard and ground motions studies, which are very important since these are the basis for the building codes in many countries. Today, we have dense and sophisticated seismic networks to measure wave-field characteristics: we archive continuous waveform data recorded at three components broad-band seismometers, we almost routinely obtain high-resolution earthquake locations. Therefore, we are ready to start to systematically look at seismic-wave propagation properties to possibly reveal short-term variations in the elastic properties of the Earth crust. One seismological quantity which, since the beginning, is recognized to be diagnostic of the level of fracturation and/or of the pore pressure in the rock, hence of its state of stress, is the ratio between the compressional (P-wave) and the shear (S-wave) seismic velocities: V-p/V-s. Variations of this ratio have been recently observed and measured during the preparatory phase of a major earthquake. In active fault areas and volcanoes, tectonic stress variation influences fracture field orientation and fluid migration processes, whose evolution with time can be monitored through the measurement of the anisotropic parameters. Through the study of S-waves anisotropy it is therefore potentially possible to measure the presence, migration and state of the fluid in the rock traveled by seismic waves, thus providing a valuable route to understand the seismogenic phenomena and their precursors. On the other hand, only in the very recent times with the availability of the continuous seismic records, many authors have shown how it is possible to estimate the relative variations in the wave speed through the analysis of the cross-correlation of the ambient seismic noise. In this paper we first analyze in detail these two seismological methods: shear wave splitting and seismic noise cross-correlation, presenting a short historical review, their theoretical bases, the problems, learning, limitations and perspectives. We, then, compare the main results in terms of temporal trends of the observables retrieved applying both methods to the Pollino area (southern Apennines, Italy) case study.

In this study we map the distribution of the b-value of the Gutenberg-Richter law-as well as complementary seismicity parameters-along the fault responsible for the 2009 M-W 6.1 L'Aquila earthquake. We perform the calculations for two independent aftershock sub-catalogs, before and after a stable magnitude of completeness is reached. We find a substantial spatial variability of the b-values, which range from 0.6 to 1.3 over the fault plane. The comparison between the spatial distribution of the b-values and the main-shock slip pattern shows that the largest slip occurs in normal-to-high b-values portion of the fault plane, while low b-value is observed close to the main-shock nucleation. No substantial differences are found in the b-value computed before and after the main-shock struck in the small region of the fault plane populated by foreshocks.Citation: De Gori, P., F. P. Lucente, A. M. Lombardi, C. Chiarabba, and C. Montuori (2012), Heterogeneities along the 2009 L'Aquila normal fault inferred by the b- value distribution, Geophys. Res. Lett., 39, L15304, doi: 10.1029/ 2012GL052822.

The largest dataset ever recorded during a normal fault seismic sequence was acquired during the 2009 seismic emergency triggered by the damaging earthquake in L'Aquila (Italy). This was possible through the coordination of different rapid-response seismic networks in Italy, France and Germany. A seismic network of more than 60 stations recorded up to 70,000 earthquakes. Here, we describe the different open-data archives where it is possible to find this unique set of data for studies related to hazard, seismotectonics and earthquake physics. Moreover, we briefly describe some immediate and direct applications of emergency seismic networks. At the same time, we note the absence of communication platforms between the different European networks. Rapid-response networks need to agree on common strategies for network operations. Hopefully, over the next few years, the European Rapid-Response Seismic Network will became a reality.

A M W 6.3 earthquake struck on April 6, 2009 the Abruzzi region (central Italy) producing vast damage in the L'Aquila town and surroundings. In this paper we present the location and geometry of the fault system as obtained by the analysis of main shock and aftershocks recorded by permanent and temporary networks. The distribution of aftershocks, 712 selected events with M L ≥ 2.3 and 20 with M L ≥ 4.0, defines a complex, 40 km long, NW trending extensional structure. The main shock fault segment extends for 15–18 km and dips at 45° to the SW, between 10 and 2 km depth. The extent of aftershocks coincides with the surface trace of the Paganica fault, a poorly known normal fault that, after the event, has been quoted to accommodate the extension of the area. We observe a migration of seismicity to the north on an echelon fault that can rupture in future large earthquakes.

Teleseismic receiver functions from a seismic experiment in a small area of the northern Apennines, Italy, reveal strong crustal structure variations across the mountain chain. Receiver functions imaging and full waveform inversion technique are used to constrain the S‐wave velocity profile in the crust and to reconstruct the geometry of the main seismic discontinuities at depth. We highlight the presence of the main mode‐converting discontinuities in the study area. Most importantly, we identify the crust‐mantle transition which is represented, almost everywhere in the study area, by a sharp S‐wave velocity increase (over 4 km/s) at depth between 35 and 40 km. However, farther west, the S‐wave velocity reaches values typical for the sub‐crustal mantle at about 54 km depth, possibly marking the locus where the subducting Adriatic plate starts to dip into the mantle. Here the presence of a shallower discontinuity at about 36 km depth, with S‐velocity values around 4 km/s, can be interpreted as the Moho signature of the overriding Tyrrhenian plate.

Surface waves from the great Sumatra‐Andaman earthquakes of 2004 and 2005 that cross Italy south of ∼44°N display Love‐to‐Rayleigh scattered waves (quasi‐Love phases) diagnostic of sharp lateral gradients in the anisotropic properties of Earth's upper mantle. Surface waves that traverse Italy further north lack this distinctive phase, documenting a change in the upper mantle fabric that is corroborated by a shift in the fast polarization of shear wave birefringence. These observations suggest that orogen‐parallel asthenospheric extension behind the retreating Apennines slab has limited geographical expression. We hypothesize that subduction rollback currently terminates at 44°N, while the upper mantle flow pattern further to the north has been recently rearranged.

In the south-eastern corner of the Tyrrhenian basin, in the central Mediterranean Sea, a tight alignment of earthquakes along a well-defined Benioff zone marks one of the narrowest active trenches worldwide, where one of the last fragment of the former Tethys ocean is consumed. Seismic tomography furnishes snapshot images of the present-day position of this slab, and seismic anisotropy allows to reconstruct the past kinematics of the subduction process. Using seismic anisotropy fast directions as a proxy for the present and past mantle flow, we look backward for the seismic traces of the slab motion through the western-central Mediterranean mantle, from the starting locus of subduction toward its present day position. The result of combining independent data sets provides a coherent pattern of anisotropy that illustrates an example of slab rollback from its initiation point to its present-day position.

Unusual features in teleseismic receiver function, computed at a single three‐component seismic station, reveal the presence of a dipping interface in the shallow crust with a pronounced seismic velocity contrast in the Campania‐Lucania region, southern Apennines (Italy). The locus of this finding is the Pergola‐Melandro basin, situated between the maximum intensity areas of two of the most destructive earthquakes reported in the Italian seismic catalogue: theM∼ 7.0 Val d'Agri earthquake (1857) and theMs= 6.9 Irpinia earthquake (1980). This area is not associated with known historical events and for this reason is currently object of investigation as a potential seismic gap, where the probability of future ruptures is higher than in surrounding regions, also for the static stress increase caused by the two above mentioned earthquakes. Through an inversion procedure applied to the computed receiver functions, we are able to satisfactorily constrain the strike, dip, depth, and velocity contrast of this crustal discontinuity, whose geometric characteristics closely match those proposed by some authors for the sources of the two neighboring earthquakes. We argue that the detected interface is the seismogenic source which fills the gap. The findings of this study could open new perspectives both in the study of seismic gap areas and in the identification of historical earthquake sources, offering a new and effective tool to locate, investigate, and constrain buried or unknown faults.