We analyze P and S wave spectra from moderate‐ to deep‐focus teleseisms recorded at the Retreating‐Trench, Extension, and Accretion Tectonics (RETREAT) temporary broadband seismic network to assess ...the variations of the Earth mantle attenuation in the northern Apennines region (Italy). For each earthquake, we compute the ratio between the spectrum at each station and the average spectrum, in order to estimate t* residuals (Δt*) from the spectral ratio decay. The number and distribution of the teleseisms useable for the P wave t* calculation allow for a gross azimuthal analysis; although the (Δt*) values at single station display, in most cases, azimuthal‐dependent fluctuations, their overall distribution shows a partition of the study region into two main areas, whose gross features remain almost unchanged over the whole azimuthal range. This partition is confirmed by the S wave t* mean values, computed for each station over the set of useable events. We distinguish a relatively high attenuation area on the western, Tyrrhenian side and a relatively low attenuation area on the eastern, Adriatic side. By correlating our Δt* estimates and the velocity structure derived from the existing tomographic models, we compute the ranges of possible P and S wave Q values in the mantle wedge above the Apennines slab (on the Tyrrhenian side) and in the asthenosphere below the Adriatic region. Furthermore, the determined attenuation properties are used to draw some inferences on the thermal state of the uppermost mantle and on the physical properties of the tectonic elements, which constitute the subduction system in the region.
Within the central Mediterranean geodynamic puzzle, the seismotectonic processes of the northern sector of the Apennines are still under debate. In this framework, we conducted a careful examination ...of seismic catalogues for five years of instrumental seismicity located in the eastern sector of the Tosco-Emiliano Apennines. In our study, we merge two separate seismic bulletins, derived from a small- and a large-aperture seismic network. The joint analysis of the seismic phases of both catalogues allows us to improve event locations and to assess their hypocentral depths. After re-location using a regional velocity model, we found that the spatial distribution of hypocenters follows characteristic patterns at the southwestern and northeastern sides of the chain. Such distribution exhibits a marked axial offset in correspondence of the Livorno-Sillaro Line (LSL), a NE-SW trending lithological discontinuity previously interpreted in terms of a transform zone. Basing on this evidence, and on additional observations related to the morphology of the area, we hypothesise the LSL to represent the shallow manifestation of a discontinuity affecting the whole lithospheric thickness, i.e. a incipient tear fault dislocating the subducting slab.
We present the results of P‐to‐S receiver function analysis to improve the 3D image of the sedimentary layer, the upper crust, and lower crust in the Pannonian Basin area. The Pannonian Basin hosts ...deep sedimentary depocentres superimposed on a complex basement structure and it is surrounded by mountain belts. We processed waveforms from 221 three‐component broadband seismological stations. As a result of the dense station coverage, we were able to achieve so far unprecedented spatial resolution in determining the velocity structure of the crust. We applied a three‐fold quality control process; the first two being applied to the observed waveforms and the third to the calculated radial receiver functions. This work is the first comprehensive receiver function study of the entire region. To prepare the inversions, we performed station‐wise H‐Vp/Vs grid search, as well as Common Conversion Point migration. Our main focus was then the S‐wave velocity structure of the area, which we determined by the Neighborhood Algorithm inversion method at each station, where data were sub‐divided into back‐azimuthal bundles based on similar Ps delay times. The 1D, nonlinear inversions provided the depth of the discontinuities, shear‐wave velocities and Vp/Vs ratios of each layer per bundle, and we calculated uncertainty values for each of these parameters. We then developed a 3D interpolation method based on natural neighbor interpolation to obtain the 3D crustal structure from the local inversion results. We present the sedimentary thickness map, the first Conrad depth map and an improved, detailed Moho map, as well as the first upper and lower crustal thickness maps obtained from receiver function analysis. The velocity jump across the Conrad discontinuity is estimated at less than 0.2 km/s over most of the investigated area. We also compare the new Moho map from our approach to simple grid search results and prior knowledge from other techniques. Our Moho depth map presents local variations in the investigated area: the crust‐mantle boundary is at 20–26 km beneath the sedimentary basins, while it is situated deeper below the Apuseni Mountains, Transdanubian and North Hungarian Ranges (28–33 km), and it is the deepest beneath the Eastern Alps and the Southern Carpathians (40–45 km). These values reflect well the Neogene evolution of the region, such as crustal thinning of the Pannonian Basin and orogenic thickening in the neighboring mountain belts.
Plain Language Summary
I performed the first comprehensive P‐to‐S receiver function analysis in the wider region of the Pannonian Basin using the most recent data set available. My study is based on a relatively long time‐span of seismological dataset (2002–2019) of digital broadband waveforms with uniform automatic waveform processing and thorough quality control procedures. I performed preliminary calculations of the Moho depth and average Vp/Vs ratio of the crust using the H‐Vp/Vs grid search scheme and Common Conversion Point migration method. The main focus was the nonlinear inversion of the receiver functions at each station. In order to take into account possible topography in the major discontinuities, I grouped the receiver functions at the stations according to their back azimuth. The receiver functions obtained from a network of closely spaced stations allowed us to infer a 3D structural and shear‐wave velocity model of the region. I have developed a new interpolation and imaging algorithm. I mapped the thickness of major intracrustal layers and determined their S‐wave velocity conditions and Vp/Vs ratios of these. The Conrad depth, upper crust, and lower crust thickness maps are the first for the Pannonian Basin region. The Moho depth map presents local variations with more finely and better resolved values than previous investigations.
Key Points
First comprehensive receiver function study of the Pannonian Basin and neighboring areas from the hitherto densest network
S‐wave velocity inversion and a novel 3D interpolation scheme
Geodynamic description of the Pannonian Basin in the upper and lower crust
We report the preliminary results from a project (GAPSS-Geothermal Area Passive Seismic Sources), aimed at testing the resolving capabilities of passive exploration methods on a well-known geothermal ...area, namely the Larderello-Travale Geothermal Field (LTGF). Located in the western part of Tuscany (Italy), LTGF is the most ancient geothermal power field of the world. GAPSS consisted of up to 20 seismic stations deployed over an area of about 50 x 50 Km. During the first 12 months of measurements, we located more than 2000 earthquakes, with a peak rate of up to 40 shocks/day. Preliminary results from analysis of these signals include: (i) analysis of Shear-Wave-Splitting from local earthquake data, from which we determined the areal distribution of the most anisotropic regions; (ii) local-earthquake travel-time tomography for both P- and S-wave velocities; (iii) telesismic receiver function aimed at determining the high-resolution (<0.5km) S-velocity structure over the 0-20km depth range, and seismic anisotropy using the decomposition of the angular harmonics of the RF data-set; (iv) S-wave velocity profiling through inversion of the dispersive characteristics of Rayleigh waves from earthquakes recorded at regional distances. After presenting results from these different analyses, we eventually discuss their potential application to the characterisation and exploration of the investigated area.
At 01:36 UTC (03:36 local time) on August 24th 2016, an earthquake Mw 6.0 struck an extensive sector of the central Apennines (coordinates: latitude 42.70° N, longitude 13.23° E, 8.0 km depth). The ...earthquake caused about 300 casualties and severe damage to the historical buildings and economic activity in an area located near the borders of the Umbria, Lazio, Abruzzo and Marche regions. The Istituto Nazionale di Geofisica e Vulcanologia (INGV) located in few minutes the hypocenter near Accumoli, a small town in the province of Rieti. In the hours after the quake, dozens of events were recorded by the National Seismic Network (Rete Sismica Nazionale, RSN) of the INGV, many of which had a ML > 3.0. The density and coverage of the RSN in the epicentral area meant the epicenter and magnitude of the main event and subsequent shocks that followed it in the early hours of the seismic sequence were well constrained. However, in order to better constrain the localizations of the aftershock hypocenters, especially the depths, a denser seismic monitoring network was needed. Just after the mainshock, SISMIKO, the coordinating body of the emergency seismic network at INGV, was activated in order to install a temporary seismic network integrated with the existing permanent network in the epicentral area. From August the 24th to the 30th, SISMIKO deployed eighteen seismic stations, generally six components (equipped with both velocimeter and accelerometer), with thirteen of the seismic station transmitting in real-time to the INGV seismic monitoring room in Rome. The design and geometry of the temporary network was decided in consolation with other groups who were deploying seismic stations in the region, namely EMERSITO (a group studying site-effects), and the emergency Italian strong motion network (RAN) managed by the National Civil Protection Department (DPC). Further 25 BB temporary seismic stations were deployed by colleagues of the British Geological Survey (BGS) and the School of Geosciences, University of Edinburgh in collaboration with INGV. All data acquired from SISMIKO stations, are quickly available at the European Integrated Data Archive (EIDA). The data acquired by the SISMIKO stations were included in the preliminary analysis that was performed by the Bollettino Sismico Italiano (BSI), the Centro Nazionale Terremoti (CNT) staff working in Ancona, and the INGV-MI, described below.
In spite of numerous active and passive seismological investigations, the existence of continuous or interrupted continental subduction below the Western Alps is still open to debate. Many of the ...observations focus on the Moho or the deeper part of the mantle, while reliable information on the Lithosphere‐Asthenosphere Boundary (LAB) below the Alpine region is scarce. Exploiting the data from the dense, broadband AlpArray Seismic Network we present a set of Receiver Function (RF) measurements on the Moho and LAB of a region encompassing the Western Alps, which includes the Ivrea Geophysical Body (IGB), a fragment of mantle placed at a few kilometers depth at the collision margin between Eurasia and Adria plates. We derive seismic velocity profiles of the crust‐uppermost mantle below each station down to about 250 km, through the joint inversion of P and S RF. We constrain the lateral variations of the Moho and LAB topographies across the colliding plates, and quantify the errors related to our measurements. Our results allow us to considerably expand the published data of the Moho depth and to add a unique set of new measurements of the LAB. Our observations show that Eurasia and Adria lithospheres have a comparable thickness (on average 90–100 km), and are colliding below the IGB, and that Eurasia is not presently continuously subducting below Adria. These observations suggest that there is a gap between the superficial (continental) European lithosphere and the deep (oceanic) lithosphere, confirming the discontinuous structure imaged by some seismic tomography models.
Plain Language Summary
The Alps are one of the most studied orogenic systems but a number of questions remain unanswered, such as the existence of continuous or interrupted subduction of the Eurasian plate below the Adria plate in the Western Alps. To answer this question, it is necessary to assess the geometry (such as thickness and position) of the two lithospheric plates that are involved. We use the receiver function technique on seismometer data coming from the dense AlpArray Seismic Network to explore the interfaces that delimit the shallow Earth layers: the Moho and the Lithosphere‐Asthenosphere Boundary. Thanks to these measurements we were able to define the plate geometries below the Western Alps. Our observations show that Eurasia and Adria lithospheres have comparable (on average 90–100 km), that the plates are colliding, and that Eurasia is not presently continuously subducting below Adria. This information helps us understand the current state and recent geodynamic evolution of the Alps.
Key Points
Joint inversion of P and S receiver functions constrains the lithosphere‐asthenosphere structure and Moho and Lithosphere‐Asthenosphere Boundary (LAB) topography below the Western Alps
Across the Ivrea Geophysical Body the colliding Eurasia and Adria lithospheres have a similar average thickness of 90–100 km
Moho and LAB topographies are better reconciled with lithospheric detachment than with a continuous slab below the Western Alps
We derive a three‐dimensional shear‐wave velocity model of the Ligurian‐Provence back‐arc basin (Northwestern Mediterranean Sea) using ocean‐bottom seismometers (AlpArray OBSs) and land stations from ...permanent and temporary seismic networks. The quality of OBS continuous records is enhanced by a specific processing that reduces instrumental and seabed‐induced noises (transients, tilt, compliance). To further improve the resolution of ambient‐noise tomography in the offshore area, we compute the Rayleigh‐wave part of the Green functions for OBS‐OBS pairs by using onshore stations as virtual sources. 2‐D group‐velocity maps and their uncertainties are computed in the 4–150 s period range by a transdimensional inversion of Rayleigh‐wave travel times. The dispersion data and their uncertainties are inverted for a probabilistic 3‐D shear‐wave velocity model that includes probability densities for Vs and for the depth of layer interfaces. The probabilistic model is refined by a linearized inversion that accounts for the water layer in the Ligurian Sea. Our S‐wave velocity and layer boundary probability models correspond well to a recent, high‐resolution P‐wave velocity cross‐section derived from controlled‐source seismic profiling along the Ligurian‐Provence basin axis. A joint interpretation of the P‐ and S‐wave velocity sections along this profile reveals a thin, anomalous oceanic crust of low P‐wave velocities but high S‐wave velocities, intruded by a gabbroic body. The illuminated part of the upper mantle appears to be devoid of serpentinization.
Plain Language Summary
The Ligurian‐Provence basin (Northwestern Mediterranean Sea) is one of the Miocene‐Pliocene back‐arc basins that resulted from the retreat of the Adria subduction in the plate reorganization due to Africa‐Europe convergence. The crustal structure of the basin is still debated, even though it has been probed by active seismic profiling. We compute a high‐resolution shear‐wave velocity model of the Ligurian‐Provence basin and its margins by making optimal use of ambient‐noise recordings of seafloor broadband seismometers. In particular, we improve the usually low quality of surface‐wave signals in noise correlations between seafloor stations by involving correlations with land stations. The joint interpretation of our S‐wave velocity model with a P‐wave velocity section obtained in the basin axis by controlled‐source seismic profiling provides compelling evidence for the presence of a thick sediment pile above a thin, ∼4.5 km‐thick oceanic crust, intruded by gabbroic bodies emplaced at the crust‐mantle transition. These results show the potential of a joint interpretation of P‐ and S‐wave velocity models since they provide reliable answers to a number of debated questions on the petrological nature of the crust and uppermost mantle, in particular in the Ligurian‐Provence basin.
Key Points
Efficient processing scheme to remove transients and reduce tilt and compliance from continuous ambient noise recorded by ocean‐bottom seismometers (OBSs)
Computation of iterative correlations between OBSs based on a virtual reconstruction of the Rayleigh waves
Thin, anomalous oceanic crust with gabbroic intrusions evidenced in the basin axis from a joint interpretation of Vs and Vp models
Within the framework of the European collaborative research initiative AlpArray (http://www.alparray.ethz.ch), the Istituto Nazionale di Geofisica e Vulcanolgia (INGV) deployed overall 20 broad-band ...seismic stations in Northern Italy and on two islands in the Tyrrhenian Sea (Capraia and Montecristo) during Fall-Winter 2015. The temporary deployment (16 stations) will run for two to three years and 4 INGV National Seismic Network accelerometric sites are now equipped with additional permanent broad-band sensors. The 16 temporary stations are equipped with REF TEK 130 digitizers and Nanometrics Trillium Compact 120 s sensors, a couple have Nanometrics Trillium 120P sensors and one a Streckeisen STS2. For each site we describe the settings and discuss the noise levels, the site effects and the preliminary sensitivity analysis.
In this paper, we present the seismological data recorded during the deployment of a dense three‐component seismic network installed a few hours after the 2003 Mw 5.3 Monghidoro earthquake, in ...northern Apennines. The main shock focal solutions derived from polarities distribution and body wave modelling of regional broadband data show a NE–SW striking reverse mechanism. Accurate relative locations of aftershocks and the inversion of focal mechanisms show that earthquakes occurred on a NW‐dipping backthrust within the Adria lithosphere under a NW‐trending horizontal compression. The observed compression is a secondary process possibly explained by differential motion within the Adriatic lithosphere. Fault geometry and kinematics is controlled by pre‐existing structures.
Rapid-response seismic networks are an important element in the response to seismic crises. They temporarily improve the detection performance of permanent monitoring systems during seismic ...sequences. The improvement in earthquake detection and location capabilities can be important for decision makers to assess the current situation, and can provide invaluable data for scientific studies related to hazard, tectonics and earthquake physics. Aftershocks and the clustering of the locations of seismic events help to characterize the dimensions of the causative fault. Knowing the number, size and timing of the aftershocks or the clustering seismic events can help in the foreseeing of the characteristics of future seismic sequences in the same tectonic environment. Instrumental rapid response requires a high degree of preparedness. A mission in response to a magnitude (Ml) 6 event with a rupture length of a few tens of kilometers might involve the deployment within hours to days of 30-50 seismic stations in the middle of a disaster area of some hundreds of square kilometers, and the installation of an operational center to help in the logistics and communications. When an earthquake strikes in a populated area, which is almost always the case in Italy, driving the relevant seismic response is more difficult. …
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