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We apply the Probabilistic Seismic Hazard Analysis (PSHA) and Deterministic Seismic Hazard Analysis (DSHA) to estimate the input ground motion for three dams in the Campotosto area (Central Italy). The dams, which confine an artificial water reservoir feeding hydroelectric power plants, are located in an active seismic zone between the areas that experienced the 2009 L’Aquila and 2016–2017 Central Italy seismic sequences. The probabilistic disaggregation estimated for a return period of 2475 years, corresponding to the collapse limit state for critical facilities, indicates that the most dangerous fault is associated with a maximum magnitude of 6.75 ± 0.25 within a distance of 10 km. This fault is used in DSHA to emulate the worst-case scenario. To capture the ground motion variability, we input a pseudo-dynamic source model to encompass spatial and temporal variations in the slip, rise time and rupture propagation, heavily affecting the near-source ground motion. Indeed, the ground motion above the rupture volume is mainly influenced by the epistemic uncertainties of rupture nucleation and slip distribution. The computed broadband seismograms are consistent with the near-source shaking recorded during the 2016 MW 6.6 Norcia earthquake and constrain the upper bound of the simulated ground motion at specific sites. Our modelling reinforces the importance of considering vertical ground motion near the source in seismic design. It could reach shaking values comparable to or larger than those of the horizontal components. This approach can be applied in other areas with high seismic hazard to evaluate the seismic safety of existing critical facilities.

The analysis of microseismicity has a fundamental role in understanding earthquakes, giving insights on the long- and short-term driving forces and processes preparing and generating the seismicity occurrence and its evolution in space and time.Recent advances in detection and location algorithms, paired with dense seismic networks, and supported by higher computing capacity, allow dramatic increase in the quality and quantity of low magnitude earthquakes recorded resulting in high resolution earthquakes catalogs in terms of both location attributes and completeness.Such catalogs enable us to analyze small magnitude (M

Seismic data are generally scrutinized for repeating earthquakes (REs) to evaluate slip rates, changes in the mechanical properties of a fault zone, and accelerating nucleation processes in foreshock and aftershock sequences. They are also used to study velocity changes in the medium, earthquake physics and prediction, and for constraining creep rate models at depth. For a robust detection of repeaters, multiple constraints and different parameter configurations related to waveform similarity have been proposed to measure cross-correlation values at a local seismic network and evaluate the location of overlapping sources. In this work, we developed a Python code to identify REs (FINDRES), inspired by previous literature, which combines both seismic waveform similarity and differential S-P travel time measured at each seismic station. A cross-spectral method is applied to evaluate precise differential arrival travel times between earthquake pairs, allowing a subsample precision and increasing the capacity to resolve an overlapping common source radius. FINDRES is versatile and works with and without P- and S-wave phase pickings, and has been validated using synthetic and real data, and provides reliable results. It would contribute to the implementation of open-source Python packages in seismology, supporting the activities of researchers and the reproducibility of scientific results.

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To ensure environmental and public safety, critical facilities require rigorous seismic hazard analysis to define seismic input for their design. We consider the case of the Trans Adriatic Pipeline (TAP), which is a pipeline that transports natural gas from the Caspian Sea to southern Italy, crossing active faults and areas characterized by high seismicity levels. For this pipeline, we develop a Probabilistic Seismic Hazard Assessment (PSHA) for the broader area, and, for the selected critical sites, we perform deterministic seismic hazard assessment (DSHA), by calculating shaking scenarios that account for the physics of the source, propagation, and site effects. This paper presents a DSHA for a compressor station located at Fier, along the Albanian coastal region. Considering the location of the most hazardous faults in the study site, revealed by the PSHA disaggregation, we model the ground motion for two different scenarios to simulate the worst-case scenario for this compressor station. We compute broadband waveforms for receivers on soft soils by applying specific transfer functions estimated from the available geotechnical data for the Fier area. The simulations reproduce the variability observed in the ground motion recorded in the near-earthquake source. The vertical ground motion is strong for receivers placed above the rupture areas and should not be ignored in seismic designs; furthermore, our vertical simulations reproduce the displacement and the static offset of the ground motion highlighted in recent studies. This observation confirms the importance of the DSHA analysis in defining the expected pipeline damage functions and permanent soil deformations.

The design of critical facilities needs a targeted computation of the expected ground motion levels. The Trans Adriatic Pipeline (TAP) is the pipeline that transports natural gas from the Greek-Turkish border, through Greece and Albania, to Italy. We present here the probabilistic seismic hazard analysis (PSHA) that we performed for this facility, and the deaggregation of the results, aiming to identify the dominant seismic sources for a selected site along the Albanian coast, where one of the two main compressor stations is located. PSHA is based on an articulated logic tree of twenty branches, consisting of two models for source, seismicity, estimation of the maximum magnitude, and ground motion. The area with the highest hazard occurs along the Adriatic coast of Albania (PGA between 0.8 and 0.9 g on rock for a return period of 2475 years), while strong ground motions are also expected to the north of Thessaloniki, Kavala, in the southern Alexandroupolis area, as well as at the border between Greece and Turkey. The earthquakes contributing most to the hazard of the test site at high and low frequencies (1 and 5 Hz) and the corresponding design events for the TAP infrastructure have been identified as local quakes with MW 6.6 and 6.0, respectively.

The Antarctic Seismographic Argentinean Italian Network (ASAIN) is a permanent broadband seismic network that has operated since 1995 in the Scotia Sea region, the Antarctic peninsula, and the polar area. It was deployed and is managed in the framework of cooperation between the National Institute of Oceanography and Applied Geophysics and Dirección Nacional del Antártico–Instituto Antártico Argentino and is financially supported by the Programma Nazionale di Ricerca in Antartide. The network consists of seven seismological stations with broadband sensors. ASAIN provides data to Incorporated Research Institutions for Seismology, Observatories and Research Facilities for European Seismology, and GEOFOrschungsNetz. It improves the worldwide seismic networks’ detection capabilities and contributes to refining regional earthquake locations released by the U.S. Geological Survey. The proximity of the seismic stations to the Antarctic continent and their continuous operation in the long term also allows for having a privileged observatory on the ice-related seismicity along the Antarctica peninsula. In this article, we discuss the historical development of ASAIN, its current configuration, and the main characteristics of the seismic stations. Finally, we also provide information on the ASAIN data exchange and the contribution to the scientific research in Antarctica.

A swarm of ~85,000 volcano-tectonic earthquakes started in August 2020 at the Bransfield Strait, between the South Shetland Islands and the Antarctic Peninsula. The Bransfield Basin is a unique back-arc basin, where the past active subduction slowed down dramatically ~4 Ma, leaving a small remnant of the former Phoenix plate incorporated in the Antarctic plate. Today there is no clear evidence for recent normal seafloor spreading. Continental crust is thinning to develop oceanic crust and the current extension is either attributed to the Phoenix Block subduction and rollback or to shear between the Scotia and Antarctic plates. The 2020 seismicity occurred close to the Orca submarine volcano, previously considered inactive. Geodetic data reported a transient deformation with up to ~11 cm northwestward displacement over King George Island. We use a wide variety of geophysical data and methods to reveal the complex migration of seismicity, accompanying the intrusion of 0.26-0.56 km3of magma off the Orca seamount at ~20 km depth. Deeper, clustered strike-slip earthquakes mark the magmatic intrusion at depth, while shallower normal faulting events are induced by the growth of a lateral dike, extending ~20 km NE-SW. Seismicity abruptly decreased after the largest Mw 6.0 earthquake, suggesting the magmatic dike lost pressure with the slipping of a large fault and the opening of upward paths. A seafloor eruption is likely, but not confirmed by sea surface roughness or temperature anomalies. The unrest documents episodic magmatic intrusion in the Bransfield Strait and provides unique insights into active continental rifting.

During the seismic sequence which followed the devastating L’Aquila 2009 earthquake, on 27 May 2009 the OGS (Istituto Nazionale di Oceanografia e di Geofisica Sperimentale) and the GeosisLab (Laboratorio di Geodinamica e Sismogenesi, Chieti-Pescara University) installed a temporary seismometric network around the Sulmona Basin, a high seismic risk area of Central Italy located right at SE of the epicentral one. This area of the central Apennines is generally characterized by low level seismicity organized in low energy clusters, but it experienced destructive earthquakes both in historical and in early instrumental time (Fucino 1915 =XI MCS, Majella 1706 =X-XI MCS, Barrea 1984 =VIII MCS).From the 27 May 2009 to 22 November 2011, the temporary network provided a huge amount of continuous seismic recordings, and a seismic catalogue covering the first seven months of network operation (-1.5≤ML≤3.7, with a completeness magnitude of 1.1) and a spatial area that stretches from the Sulmona Basin to Marsica-Sora area. Aiming to enhance the detection of microearthquakes reported in this catalogue, we applied the matched-filter technique (MFT) to continuous waveforms properly integrated with data from permanent stations belonging to the national seismic network. Specifically, we used the open-source seismological package PyMPA to detect microseismicity from the cross-correlation of continuous data and templates. As templates we used only the best relocated events of the available seismic catalogue. Starting from 366 well located earthquakes we obtain a new seismic catalogue of 6084 new events (-2LLLL≤3.5); the temporal evolution is very articulated with a total duration of one month with different bursts of seismicity and characteristic time extension of approximately one week. C2 instead has a different space-time evolution and consists of different swarm-like seismic sequences more discontinuous in comparison with C1. These swarms are described in greater detail to investigate the influence of overpressurized fluids and their space-time distribution.

The Alto Tiberina normal fault (ATF) in Central Italy is a 50 km long crustal structure that deepens at a low angle (15-20◦). Events on the fault plane are about ten times less than those located i...

Using template matching, we investigate the space-time evolution of seismicity in the period 2008-2018 in Central Apennines. The seismicity appears more persistent at the base of the seismogenic la...

To obtain a 3-D crustal density and shear-wave velocity structure beneath the Po plain, we exploit seismic records gathered from 2006 to 2014 and Bouguer gravity data assembled for the last estimation of the Italian Geoid. 2-D maps for both Love and Rayleigh fundamental mode at periods between 4 and 20 s are obtained applying a tomographic inversion. The defined local dispersion curves are then jointly inverted using a linearized scheme to obtain a 3-D isotropic shear-wave velocity model across the Po plain region. The model, transformed into density through a priori velocity-density relationships, is then the input of the Sequential Integrated Inversion algorithm, which enables us to recover a new 3-D density-shear wave velocity coupling and inferences on the lithology and tectonics. Low and fast S-wave velocities are highlighted for the shallow Pliocene–Quaternary sediments along the foredeep, in front of the Northern Apennines, and for the presence of limestone units in the upper crust, respectively. Whereas sediment trends seem to be consistent with the results obtained, the Mesozoic carbonates, which are inherently characterized by high variability, are less resolved. A major result is the recovery of a high speed (3.3 km/s) - density (2.2 kg/m3) structure in the upper crust (6–10 km) localized beneath the arcuate Po plain thrust front expanding from the external margin of the Ferrara arc toward the Alps and the Adriatic Sea. At the boundaries of this brittle body, we locate earthquakes of the Emilia 2012 seismic sequence and the historical seismicity. Mapping lateral discontinuities in density and shear wave velocity could provide insights in defining strengthening and weakening zones, and in focusing on transition zones often prone to earthquakes.

We developed a hybridization code that merges the full‐wave low‐frequency (LF) signals with stochastic high‐frequency (HF) synthetics to simulate broadband strong ground motion for engineering applications. The approach is applied to reproduce, within the known source and structure constraints, the Kiban–Kyoshin network (KiK‐net) and Kyoshin net (K‐NET) accelerometric data recorded during the 2008 Iwate–Miyagi Nairiku earthquake ( M w 7.0). The results show that our approach is able to simulate satisfactorily the observed waveforms and the related response spectra over the broadband frequency range. We estimated the effects of the hybridization in the peak and spectral parameters through the goodness‐of‐fit criteria: the overall fit improves (∼15%–20%) when, instead of full‐wave calculated seismograms, we use the low‐pass filtered recordings, showing that a good match at LFs is fundamental to obtain reliable and consistent results even in the HF band. Peak ground velocities are less affected than accelerations by the inclusion of the HF contribution and are well reproduced by our method; peak ground accelerations show larger discrepancies due to the intrinsic characteristics of the stochastic model and the small‐scale heterogeneity that affect the seismic radiation and propagation at HFs. Nevertheless, simulated signals reproduce the observed ones fairly well, and an acceptable match is also found in the comparison of the spectral responses. Online Material: Figures showing the COMPEX merging procedure, comparison of recorded and simulated response spectra, and comparison of recorded and synthetic broadband signals; and tables of source parameters, 1D velocity model, and matching frequencies.

A reappraisal of surface wave tomography in the remote Scotia Sea region and surrounding ridges is presented. New group velocity dispersion curves were obtained from local and regional earthquakes recorded at permanent Antarctic stations from 2001 to 2013 and used to update the measurements reported by Vuan et al. (2000). Rayleigh and Love group velocity maps for periods ranging from 15 to 50 s were retrieved using a tomographic inversion. The group velocity anomalies are clearly associated with the major crustal and upper mantle features of the Antarctic, Scotia and South American plates. The updated dataset allows for considerable decrease of the correlation length of the crustal heterogeneities that can be resolved, especially in the west Scotia Sea, central Scotia Sea and Bransfield Basin. Surface wave tomography results were compared with CRUST 1.0 group velocity maps and revealed specific areas where more detailed information is made available by our regional study. In particular, low group velocity anomalies of the Bransfield Strait rifting and continental fragments that are detached from the Antarctic Peninsula and spreading along the South Scotia ridge are not shown by the reference CRUST 1.0 model. A comparison between the average seismic velocities beneath the west and central Scotia Sea shows that both have an oceanic-type structure; however, the crust of the central sea is thicker (12–14 km) and slower than that of the 20 Ma old western sea.

Near-field recordings are very sensitive to the spatiotemporal details of the rupture process while far-field signals show the signature of the overall “point-source” earthquake mechanism. Near- and far-field recording ranges are dependent on the event magnitude and modulate the variability of the ground motion. This study investigates the ground motion and the source-related near-field variability for the 2011 Lorca earthquake, a moderate seismic event (Mw = 5.2) that caused significant localized damage in the Region of Murcia, Spain. The low-frequency content (up to 1 Hz) is simulated by the wavenumber integration method assuming four different source models obtained by inversion of geodetic or seismological data. As a first result, we estimate the variability of the ground motion. We observe that the dispersion in the peak and spectral parameters is larger at LOR, the closest station to the source, and decreases as the source distance increases (more than 50 km far from the source) where the finite-fault effects become negligible. The variability of the pseudo spectral velocity at 2 s is within the ground motion prediction equation ±1 σ, apart from the very near-source station and those stations affected by forward directivity effects. These effects are also found in high-frequency seismograms obtained by the empirical Green’s functions approach., Field work and ambient-noise analysis were partially supported by the Spanish Projects CGL2007-62454 and CGL2010-11831-E.

We explored the detection capability of low-magnitude earthquakes before the 6 April 2009 Mw 6.3 L'Aquila event by using a matched filter technique and 512 foreshocks as templates. We analyzed continuous waveforms from 10 broadband seismic stations in a 60 km radius from the epicenter and for ~3 months before the main shock. More than 3000 new events, mostly located on the main shock fault, were detected to define the spatial-temporal evolution of micro-seismicity. The foreshock sequence was active northwest of the Mw 6.3 hypocenter in January, then migrated toward it at a speed of ~0.5 km/day in middle of February. At that time, in a ~4 km2 patch close to the main shock nucleation point, the cumulative number of earthquakes gradually increased until the Mw 6.3 event. This patch, characterized by a low b-value, played a key role in controlling the preparation stage to the 2009 L'Aquila main rupture.

Strong-motion data consisting of peak ground acceleration and velocity and 5 % damped response spectra are presented for 46 earthquakes of the Emilia seismic sequence which occurred in the Po Plain (northern Italy) in 2012. The data were recorded by the OGS temporary network installed close to the town of Ferrara following the main shock of May 20, 2012. Ground-motion peak parameters and spectral responses are compared with the ground-motion prediction equation (GMPE) of Bindi et al. (Bull Earthq Eng 9:1899–1920, 2011) for soft soils and reverse faults. Peak ground accelerations are in general in good agreement with those predicted by GMPE, while predicted peak ground velocities underestimate the observed data, especially for stronger events at more distant stations. The response spectra follow the trend in peak ground velocities, with observed values higher than predicted values at longer periods. This behavior has been interpreted as a site effect due to the deep soft alluvial cover of the Po Plain, which promotes ground motion characterized by a large low-frequency spectral content that is not yet well modeled by the Italian GMPE. A peculiar behavior was shown by the event occurring on June 6, 04:08:33 UTC, $$\hbox {M}=4.5$$ , located at the eastern edge of the Po Plain, which produced peak ground accelerations exceeding three times the values estimated by attenuation laws. Such a great discrepancy could be related to post-critically reflected S-waves and multiples from the Moho (SmSM).

It has been observed that post-critically reflected S-waves and multiples from the Moho discontinuity could play a relevant role on the ground motion due to medium to strong size earthquakes away from the source. Although some studies investigated the correlation between the Moho reflections amplitudes and the damage in the far field, little attention was given to the frequency content of these specific phases and their scaling with magnitude. The 2012 Emilia seismic sequence in northern Italy, recorded by velocimetric and accelerometric networks, is here exploited to investigate Moho reflections and multiples (SmSM). A single station method for group velocity-period estimation, based on the multiple filter technique, is applied to strong motion data to detect SmSM. Amplitude and frequency scaling with magnitude is defined for earthquakes from \(\hbox {Mw}=3.9\) to \(\hbox {Mw}=5.9\). Finally, the ability of SmSM to affect the ground motion for a maximum credible earthquake within the Po plain is investigated by extrapolating observed engineering parameters. Data analysis shows that high amplitude SmSM can be recognized within the Po plain, and at the boundaries between the Po plain and the Alpine chain, at epicentral distances larger than 80 km, in the period range from 0.25 to 3 s and in the group velocity window from about 2.6 to 3.2 km/s. 5 % damped pseudo-spectral accelerations at different periods (0.3, 1.0 and 2.0 s), and Housner intensities, are obtained from data characterized by large amplitude SmSM. A scaling relationship for both pseudo-spectral accelerations and Housner intensities is found for the earthquakes of the 2012 Emilia seismic sequence. \(\hbox {I}_{\mathrm{MCS}}\) from VII to VIII is estimated, as a result of SmSM amplitude enhancement, at about 100 km for a maximum credible earthquake (\(\hbox {Mw}=6.7\)) in the Po plain, showing that moderate to high damage cloud be caused by these specific phases.

Local response is estimated within the sediment fill of the Tagliamento River Valley (THV), northeastern Italy, using about 90 weak‐to‐medium local and regional earthquakes. Ground motion is recorded at six stations deployed in a linear array across the valley. Site amplification factors are determined using the standard spectral ratio, the generalized inversion technique, and evaluations of receiver functions. Results obtained from earthquake data are then compared with ambient noise analysis. Azimuthal effects in the seismic response are observed locally for waves coming from the east‐southeast, aligned with the longitudinal axis of the valley. The seismic responses evaluated at the mid‐valley and at the valley edge are then convolved with the 1976 M w 6.4 Friuli mainshock recorded at accelerometric station TLM1, located close to the array. The local response at the valley edge, in the town of Cavazzo Carnico, is responsible for an enhancement of the input motion of an average factor of about 4 in the frequency range from 2 to 8 Hz. The elastic demand from the current national regulation for a 475‐year return period and specific for Cavazzo Carnico is overcome by the 5%‐damped pseudoacceleration response spectrum estimated at the valley edge. The underestimation of the Italian code is mostly due to a 2D amplification effect at the valley edge rather than a simple inadequacy of the V S 30 site classification. Online Material: Figures showing locations of historical earthquakes, distribution of fundamental frequency of vibration, waveforms of the M D 5.1 Kobarid earthquake, S ‐wave velocity versus depth, and GIT and RF amplification median values and spectra; tables of seismic station locations and soil type, and of local and regional earthquakes.

SUMMARY The 2009 L’Aquila, Italy, earthquake (Mw 6.3) produced an unprecedented number of seismological records for a normal faulting event, and hence represents an important case study of damaging moderate magnitude earthquakes occurring in densely inhabited areas. We have investigated the source process of the 2009 L’Aquila earthquake in both the low- and broad-band frequency ranges by analysing the teleseismic and strong motion data. A low-frequency (

The plate boundary between the Scotia and Antarctic plates runs along the broadly E-W trending South Scotia Ridge. It is a mainly transcurrent margin that juxtaposes thinned continental and transitional crust elements with restricted oceanic basins and deep troughs. Seismic profiles and regional-scale seismological constraints are used to define the peculiarities of the crustal structures in and around the southern Scotia Sea, and focal solutions from recent earthquakes help to understand the present-day geodynamic setting. The northern edge of the western South Scotia Ridge is marked by a sub-vertical, left-lateral master fault. Locally, a narrow wedge of accreted sediments is present at the base of the slope. This segment represents the boundary between the Scotia plate and the independent South Shetland continental block. Along the northern margin of the South Orkney microcontinent, the largest fragment of the South Scotia Ridge, an accretionary prism is present at the base of the slope, which was possibly created by the eastward drift of the South Orkney microcontinent and the consequent subduction of the transitional crust present to the north. East of the South Orkney microcontinent, the physiography and structure of the plate boundary are less constrained. Here the tectonic regime exhibits mainly strike-slip behavior with some grade of extensional component, and the plate boundary is segmented by a series of NNW-SSE trending release zones which favored the fragmentation and dispersion of the crustal blocks. Seismic data have also identified, along the northwestern edge of the South Scotia Ridge, an elevated region - the Ona Platform - which can be considered, along with the Terror Rise, as the conjugate margin of the Tierra del Fuego, before the Drake Passage opening. We propose here an evolutionary sketch for the plate boundary (from the Late Oligocene to the present) encompassing the segment from the Elephant Island platform to the Herdman Bank. (C) 2012 Elsevier B.V. All rights reserved.

Recent moderate-sized earthquakes ( M L ≤5.2) that occurred in northern Italy demonstrate that the currently available ground-motion prediction equations (GMPEs) largely underestimate shaking in the Po Plain region at hypocentral distances greater than 70 km. In order to investigate this phenomenon, we collected a set of peak ground acceleration (PGA) observations for weak and moderate earthquakes in the area. Nonparametric regression analysis of PGA observations as a function of magnitude and hypocentral distance shows that PGA is systematically enhanced for distances between 70 and 200 km. An indirect estimation through the analysis of the attenuation of macroseismic intensities suggests that the effect also applies to strong earthquakes ( M L >5.5). We performed numerical modeling experiments to investigate the cause of the phenomenon. The characteristics of the computed synthetic seismograms indicate that the enhancement of ground motion is mainly an effect of the reflection of S waves at the Moho ( SmS phase). The analysis of both real and synthetic data shows that the Moho reflection effect is maximized at hypocentral distances between 90 and 150 km, where the PGA increases by a factor larger than 2.4.

Design-elastic displacement spectra for seismic codes and zoning studies of seismic hazard for long-period structures are becoming of utmost importance in modern earthquake engineering. The increase of displacement-based design approaches, together with the availability of good-quality digital strong-motion data, allow the creation of reliable spectral shapes for seismic design of up to 10 s. Nevertheless, to characterize the site amplification effects during large levels of shaking, additional studies are needed, especially in deep sedimentary basins and plains that can be strongly excited by large amplitude long-period waves. Data within deep basins is rare because accelerometric seismic networks are normally deployed on rock or stiff soil, and modeling of waveforms is hence necessary for bridging this gap. Focusing on the numerical modeling of the 3D wavefield in 1D flat layered and 3D structures in the Po and Venetian plains (northern Italy) and on earthquakes with magnitudes M 6.6 and hypocentral distances greater than 50 km, in this study we (1) estimate the displacement response spectra (DRS) from 1 to 10 s in the Po and Venetian plains, (2) compare the DRS from numerical simulations using flat layered and heterogeneous 3D models against the currently available uniform-hazard DRS for Italy, and (3) evaluate the influence of deep sediments on DRS shape and thus on seismic codes. Finally, 1D- and 3D-simulated DRS are compared with DRS obtained from recordings in the Kanto basin where similar long-period displacement is observed.

A compilation of available multichannel seismic profiles acquired along the southern margin of the Scotia Sea east of the South Orkney microcontinent has allowed identifying and mapping the main morphological and structural features of the central segment of the Scotia-Antarctica plate boundary. This margin is composed by several bathymetric highs of variable size and uncertain crustal nature, separated by deep troughs and restricted oceanic basins. Some of these troughs represent pull-apart basins. Three main segments oriented WNW-ESE (the western sector), ENE-WSW (the central sector, here named Bruce Deep), and NE-SW (the eastern sector), have been described. These segments are separated by NNW-SSE-trending release zones, disposed in an en-echelon geometry, which represent mostly strike-slip faults. The western segment corresponds to the northern margin of the South Orkney microcontinent, where a subduction zone seems to be present, even if its present-day activity is unclear. The segment further to the east corresponds to an ENE-oriented basin (Bruce Deep), which separates the Bruce Bank from the eastern promontory of the South Orkney continental platform. To the south of the Bruce Deep, a wide deformation zone with N-verging folds and thrusts (here named Jane Thrust Belt), has been identified from seismic data. The eastern segment of the plate boundary is structurally the less constrained, and may be composed by a series of tectonic lineaments of different lengths. From the Bruce Bank to the east, focal mechanisms maintain a prevalent left-lateral strike-slip motion combined with an extensional component. In this sector, earthquakes are located in a 150. km wide area and on a local scale are difficult to follow unambiguously at the plate boundary. Lithologic analyses on dredged material recovered along a flank of one of the morphological relieves present south of the Discovery Bank to 35°W (here collectively named Irizar Highs), yielded a dominant granitic composition. A similar composition characterizes the rocks collected in the southern flank of the south-easternmost Jane Bank. This suggests a continental crust nature for these bathymetric highs, now dispersed along this sector of the Scotia-Antarctica plate boundary. We propose here a tectonic evolution for this margin, dominated since the Early Miocene by the northward subduction of the Weddell Sea oceanic crust. The development of a dextral, en-echelon transform fault system facilitated the process of fragmentation and dispersion of the crustal blocks, dismembered the subduction zone, and possibly inverted the direction of convergence: Therefore, the Scotia plate would subduct beneath the Antarctic plate, in the western sector, and Weddell Sea would subduct beneath Scotia plate, in the eastern sector. Finally, the activation of left-lateral transtensional strike-slip lineaments generated narrow pull-apart basins in the fore-arc sectors of the convergent zones. Fil: Lodolo, Emanuele. Istituto Nazionale di Oceanografia e di Geofisica Sperimentale; Italia Fil: Civile, D.. Istituto Nazionale di Oceanografia e di Geofisica Sperimentale; Italia Fil: Vuan, A.. Istituto Nazionale di Oceanografia e di Geofisica Sperimentale; Italia Fil: Tassone, Alejandro Alberto. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Departamento de Geología. Instituto de Geofísica "Daniel Valencio"; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Geociencias Básicas, Aplicadas y Ambientales de Buenos Aires. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Geociencias Básicas, Aplicadas y Ambientales de Buenos Aires; Argentina Fil: Geletti, R.. Istituto Nazionale di Oceanografia e di Geofisica Sperimentale; Italia

A spectral analysis of strong-motion data and ambient noise at the Tolmezzo-Ambiesta dam accelerograph (TLM1) site is performed to explain the high horizontal peak acceleration of 0.36 g recorded during the M w 6.4, 6 May 1976 Friuli (northeastern Italy) earthquake. The spectral response of the accelerographic site is estimated from the mainshocks and aftershocks of the 1976 seismic sequence by different techniques. In addition, new seismic data are used to characterize the near- surface S -wave velocity distribution versus depth at TLM1 by means of an analysis of the phase-velocity dispersion of Rayleigh waves. An average, spectral amplification curve is computed for TLM1, whose mean value is at about 1.5. It features two main lobes of amplification with peaks at 2–3.5 Hz and 6–7 Hz that are explained, by the horizontal-to-vertical spectral ratio (hvsr) of noise measurements, as the effects of the vibration of a nearby relief and the dam-reservoir system, respectively. When the site response is evaluated from earthquake recordings, the frequency of resonance due to near-surface S -wave velocity profile at about 5 Hz is strongly attenuated in comparison with the 6–7 Hz peak due to the dam-reservoir system. Finally, by filtering the site effects of the 1976 record, the acceleration peak is reduced from the original value of 349.99 cm/sec 2 to 188.76 cm/sec 2 .

Regional surface wave tomography in the sub-Antarctic Scotia Sea is helpful in revealing the nature of the crust and the S-wave seismic velocity profile beneath the Bransfield Strait. The joint use of our regional network, global seismographic network stations and local temporary arrays provide better lateral resolution than that obtained in our previous studies concerning the Scotia Sea region. Tomographic analysis of data obtained using 10 broad band seismic stations and more than 300 regional events, shows that the Bransfield Basin is characterised by a strong group velocity reduction of 8% with respect to the surrounding areas, in the period range from 15 s to 50 s. The crustal and upper mantle models of the eastern, central and western Bransfield Basin are obtained by joint inversion of Rayleigh and Love local dispersion curves from 15 s to 50 s. In addition our data set is expanded to a broader period interval (1–80 s), in central Bransfield Strait in order to better constrain the upper mantle and shallow crust. The main results can be summarized as follows: (a) the crust thins distinctly from W toward E; the variation is consistent with the type of volcanism, earthquake distribution and bathymetric observations, (b) low upper mantle velocities (soft lid) extend down to depths exceeding 70 km as a consequence of elevated temperatures, (c) the crust beneath the central Bransfield Basin displays continental characteristics with a gradually increasing S-wave velocity distribution versus depth analogous to the East African Rift structure of Kenya, (d) negative velocity gradients are present in the lower crust beneath the eastern Bransfield Basin; these could be interpreted as magmatic bodies originating from decompression melting of the mantle.

The Bruce, Discovery, Herdman and Jane banks, all located along the central-eastern part of the South Scotia Ridge, represent isolated topographic highs, surrounded by young oceanic crust (∼5–23 Ma), whose petrological and structural nature is still the subject of speculation due to the lack of relevant data. Surface wave tomography in and around the Scotia Sea region, performed using eight broadband seismic stations and 300 events, shows that the central-eastern part of the South Scotia Ridge is characterized by negative surface wave group velocity anomalies as large as 6% in the period range from 15 s to 50 s. The spatial resolution of our data set (∼300 km) makes it possible to study a specific area (centred at 61°S and 36°W) that includes Discovery Bank and appears to show dispersion characteristics similar to those found beneath the northern tip of the Antarctic Peninsula and southern South America. Surface wave dispersion curves are inverted to obtain 1-D isotropic shear wave velocity profiles that suggest a continental nature of Discovery Bank. Crustal thickness is in the range 23–28 km with a sub-Moho velocity of 4.1–4.2 km s−1. The boundaries of the negative group velocity anomalies are marked by a high level of seismic activity. The depth of the events and their large seismic moment suggest the presence of continental lithosphere. The continental-type crust of this topographic relief is supported by our interpretation of multichannel seismic reflection profiles acquired across this rise, where the observed seismic structures are interpreted as thinned and faulted continental plateau.

SUMMARY The purpose of this work is to derive a 3-D tomographic image of the shear wave velocity structure of the crust—uppermost mantle in the Aegean area using the group velocities of Rayleigh wave fundamental mode. The database consists of 185 regional earthquakes recorded at broad-band stations that were installed for a period of 6 month in the Aegean area within the framework of a large-scale experiment. In a previous work (Karagianni et al. 2002), an averaged group velocity has been determined using the method of frequency time analysis (FTAN) for each epicentre‐station ray path and the data were used in order to determine the local group velocities for different periods over the area covered by the seismic ray paths. Taking into account additional resolution results obtained for the local group velocities, a grid of 0.5 ◦ was adopted for the Aegean area and a local dispersion curve was defined for each gridpoint. More than 80 local dispersion curves were finally inverted using a non-linear inversion approach, deriving the corresponding 1-D shear velocity models. The interpolation of these models resulted in a 3-D S-wave tomographic image of the crust and uppermost mantle in the broader Aegean area. As expected, as a result of the complex tectonic setting of the Aegean area, strong lateral variations of the S-wave velocities of the crust and uppermost mantle of the studied area are found. In the southern Aegean sea, as well as in a large part of the central Aegean sea a thin crust of approximately 20‐22 km is observed, whereas the remaining Aegean sea area exhibits a crustal thickness less than 28‐30 km. On the contrary, a crustal thickness of 40‐46 km is observed in western Greece along the Hellenides mountain range, whereas in the eastern continental Greece the crust has a typical thickness of approximately 30‐34 km. For shallow depths (

Data from a large-scale experiment which took place in Greece during the period January–July 1997 have been used to investigate the structure of the Aegean area using surface waves. During this experiment, 30 seismic broadband instruments were deployed throughout the whole Greek area. Additional data during the period 1996–2000 from other temporary networks have been included in the dataset. One hundred eighty-five events with magnitudes 4.0VMwV5.5 recorded by these stations have been collected and processed. The individual dispersion curves of the group velocity of Rayleigh waves for each source-station path have been calculated, producing more than 700 paths covering the studied region. These curves have been used to determine Rayleigh group velocity maps using a 2D-tomography method. On the basis of a regionalization of the dispersion measurements, local averaged dispersion curves have been obtained and non-linearly inverted to obtain models of shear-wave velocity versus depth. Since the dispersion curves in the period range 5 sVTV30 s are mostly affected by the crustal structure, the model velocities are estimated down to a depth of approximately 35–45 km. The results from the non-linear Hedhehog inversion as applied to a few local dispersion curves show a crustal thickness of approximately 32 km for the Northern Aegean Sea, and a relatively thin crust of approximately 22–24 km for the Southern Aegean Sea. D 2002 Elsevier Science B.V. All rights reserved.

More than 60 events recorded by four recently deployed seismic broadband stations around Scotia Sea, Antarctica, have been collected and processed to obtain a general overview of the crust and upper mantle seismic velocities. Group velocity of the fundamental mode of Rayleigh waves in the period between 10 s to 30–40 s is used to obtain the S-wave velocity versus depth along ten different paths crossing the Scotia Sea region. Data recorded by two IRIS (Incorporated Research Institutions for Seismology) stations (PMSA, EFI) and the two stations of the OGS-IAA (Osservatorio Geofisico Sperimentale—Instituto Antarctico Argentino) network (ESPZ, USHU) are used. The Frequency-Time Analysis (FTAN) technique is applied to the data set to measure the dispersion properties. A nonlinear inversion procedure, ‘‘Hedgehog,’’ is performed to retrieve the S-wave velocity models consistent with the dispersion data. The average Moho depth variation on a section North to South is consistent with the topography, geological observations and Scotia Sea tectonic models. North Scotia Ridge and South Scotia Ridge models are characterised by similar S-wave velocities ranging between 2.0 km/s at the surface to 3.2 km/s to depths of 8 km/s. In the lower crust the S-wave velocity increases slowly to reach a value of 3.8 km/s. The average Moho depth is estimated between 17 km to 20 km and 16 km to 19 km, respectively, for the North Scotia Ridge and South Scotia Ridge, while the Scotia Sea, bounded by the two ridges, has a faster and thinner crust, with an average Moho depth between 9 km and 12 km. On other paths crossing from east to west the southern part of the Scotia plate and the Antarctic plate south of South Scotia Ridge, we observe an average Moho depth between 14 km and 18 km and a very fast upper crust, compared to that of the ridge. The S-wave velocity ranges between 3.0 and 3.6 km/s in the thin (9–13 km) and fast crust of the Drake Passage channel. In contrast the models for the tip of the Antarctic Peninsula consist of two layers with a large velocity gradient (2.3–3.0 km/s) in the upper crust (6-km thick) and a small velocity gradient (3.0–4.0) in the lower crust (14-km thick).

SUMMARY It has long been observed that damage due to earthquakes depends greatly on local geological conditions. Alpine valleys represent a typical populated environment where large amplifications can take place owing to the presence of surface soils with poor mechanical properties combined to complex topography of the rock basin. In the framework of the EU Interreg IIIB SISMOVALP Project ‘Seismic hazard and alpine valley response analysis’, a stretch of the Tagliamento River Valley (TRV), located in the north-western part of the Friuli Region (Italy) and close to the epicentre of the 1976 Mw= 6.4 earthquake, has been investigated with the aim to define the buried shape of the valley itself. Two non-invasive, low cost, independent geophysical methods were used: (i) detailed gravity survey and (ii) H/V spectral ratio (HVSR) of microtremors. Because of structural geological complexity and active tectonics of the Friuli region, an irregular valley shape was expected in this area. The independent analysis performed by gravity and passive noise, and complemented with refraction seismic velocity profiles, confirms this hypothesis and leads to two models that were consistent, but for some small scale details. The maximum depth estimated is about 400–450 m in the southern part of the valley, while a mean value of 150–180 m is estimated in the northern part. The sediment thickness obtained for this stretch of the TRV is quite large if compared to eastern Alps Plio-Quaternary rates; therefore the valley shape imaged by this study better corresponds to the top of carbonate rocks. Finally, on the basis of the obtained morphology and some direct measurements, we conclude that the TRV features an overall 1-D seismic response (i.e. the resonance is related only to the sediment thickness rather than to the cross-section shape), but in its deepest part some limited 2-D effects could take place.

In the paper ‘‘Reply to comment by Giuliano F. Panza on ‘Rarely observed short-period (5–10 s) suboceanic Rayleigh waves propagating across the Tyrrhenian Sea’’’ by A. Rovelli et al. (Geophysical Research Letters, 33, L10310, doi:10.1029/2006GL025848, 2006), Table 2 is identical to Table 1. The authors apologize to the readers for this error and thank Giuliano F. Panza for outlining the error. The correct Table 2 appears here along with its caption.

We thank Giuliano F. Panza for the interest demonstrated in our study and for having pointed out the error in Figure 3 of Rovelli et al. [2004, hereinafter referred to as R2004GRL]. In his comment, Panza writes that a similar phenomenon was already reported by him in an old paper. This statement is not correct and we feel that probably we did not emphasized enough the innovative character of R2004GRL. His comment gives us the opportunity of better explaining that the phenomenon we observe in the Tyrrhenian Sea is different from those studied by Panza and coauthors.

[1] Shallow earthquakes occurring in the southern part of the Tyrrhenian basin excite a long-duration low-frequency coda that propagates efficiently along paths crossing the Tyrrhenian Sea. Seismograms recorded at regional distances in the Italian peninsula are analysed in the frequency-time domain and compared with synthetics computed in simple vertically varying media. Based on results of our modeling, we interpret the coda as short-period (5–10 s) suboceanic Rayleigh waves. Their large amplitude and long duration, that we emphasize here, are explained as due to the influence of the Tyrrhenian water layer over a fast seafloor with little S-wave attenuation (QS > 300). A satisfactory fit of amplitude and duration of the observed waveforms is obtained assuming a young oceanic model with an average water column of 1 to 2 km and relatively high S-wave velocity (2.5 km/s) in the shallow crust.

The integration of different geophysical techniques is the best way to reduce the ambiguities of any single prospecting method, when characterizing geobodies by their rock properties. Seismic imaging is the main tool for delineating deep targets in 3D, but its quality may increase when the near surface effects are compensated for by gravity or electro-magnetic methods (den Boer et al. 2000, Dell’Aversana 2003, Colombo et al. 2008, 2010, among others). Classical refraction statics, in fact, break down when velocities are not monotonically increasing or the shallow formations are very inhomogeneous. In the last decade, new contributions are emerging from unusual information sources as vibrators’ controllers (Al-Ali et al. 2003, Ley et al. 2006), geological maps and satellite imagery (Vesnaver et al. 2006b, 2009, Laake et al. 2008). Here we review some of these recent results and highlight a few problems that require further analysis. We describe also an ongoing experiment for expanding the band-width of active seismic surveys by integrating them with passive ones.

—More than 150 events, recently recorded by seven seismic broadband stations (OGS-IAA, IRIS, GSETT3-IDC), have been collected and processed to obtain an overview of the crust and upper mantle shear wave velocities.¶Group velocities of the fundamental mode Rayleigh and Love waves, in the period range from 15 s to 50 s, are used to obtain tomographic maps of the Scotia Sea region, the tip of Antarctic Peninsula, and the tip of South America. Errors in the measurements, estimated on clusters, are larger for Love waves than for Rayleigh waves and their averages are 0.060–0.080 km/s and 0.030–0.040 km/s, respectively.¶From the regionalisation of the dispersion measurements, we obtain smoothed local dispersion curves in correspondence with the main geological and tectonic features, and from their nonlinear inversion, the shear wave velocity versus depth profiles.¶The correlation length of the heterogeneity, which can be resolved by Rayleigh waves, varies between 200 and 400 km in most parts of the studied area, but becomes greater near the periphery of the maps. The spatial resolution of Love waves (400–600 km) is poorer than that of Rayleigh waves, due to the deteriorated path coverage and to the larger errors in the group velocity measurements.¶Models of the shear wave velocity in the crust and upper mantle for the tip of South America, the Falkland Plateau, the Scotia Sea, the South Sandwich oceanic spreading ridge, the South Sandwich trench, the South Scotia ridge, the tip of Antarctic Peninsula, the Bransfield Strait and the Drake Passage are presented.¶Our regional models and the existing large-scale models (e.g., CRUST5.1), help to define a 3-D velocity model of the Scotia Sea region to be further investigated by waveform inversion.