Claudio Eva

Stress tensor orientation derived from fault plane solutions in the southwestern Alps

Gephart and Forsyths method has been applied to estimate stress orientations from earthquake fault plane solutions of the southwestern Alps, a region where the tectonic stress regime is known to be fairly complex. Fault plane solutions have been either taken from the literature or computed using data from local and regional networks. Data refer to seismic events of magnitude in the range 2.5–5.3 which occurred in the last decades at depths between 0 and 25 km. Two zones with a different stress orientation have been identified in the studied area (44.0°–45.5°N, 6.5°–8.5°E): the western zone, corresponding to the crest of the alpine belt, where a high-dip maximum compressive stress is found, and the eastern zone (Alps chain to Po Plain transition), characterized by an almost horizontal E-W σ1 and a nearly vertical σ3. Hypocenters of earthquakes used for stress inversion lie in the depth ranges 0–15 km and 5–25 km in the western and eastern zones, respectively. The transition between the two stress domains is very sharp, and this is also indicated by space distribution of earthquake individual misfits to the respective stress models. The findings of the present study are a good match for tectonic models which assume E-W compression derived from the Adria-Europe interaction and producing: (1) major thrusting processes in the eastern side of the chain and (2) secondary tensional effects at very shallow depth beneath the alpine belt crest (western zone of the area studied in this work).

Local and Duration Magnitudes in Northwestern Italy, and Seismic Moment Versus Magnitude Relationships

In the present work, we develop some local magnitude scales for northwestern Italy based on vertical short-period records. This study is motivated by the possibility of applying the computed scales to an instrumental catalog of more than 25,000 local earthquakes, as this region has been continuously monitored by 12 short-period vertical-component (1c) stations since the mid-1980s. Furthermore, a digital network of three-component (3c) broadband or 5 second sensors has monitored northwestern Italy since 1996. Today, a significant number of earthquakes have been simultaneously recorded by both networks, allowing the calibration of the 1c local scale by using magnitudes computed according to a scale derived for the 3c digital network. Moreover, because station Sant’ Anna di Valdieri houses both a 3c (code stv2) and 1c (code stv) sensors, the magnitude scales for the two networks can be developed using the same reference station. The magnitude scale M L = log A + log( R /100) + 0.0054( R − 100) + 3 − S is derived for the 3c digital network with the requirement that the correction S of station stv2 is zero. This scale is based on 10,057 maximum amplitudes (2822 earthquakes) computed from horizontal synthesized Wood-Anderson seismograms, in the hypocentral distance 10 to 310 km and in the range 0 ≤ M L ≤ 5. With respect to an carlier magnitude scale derived for the 3c network constraining the sum of all the station corrections to zero, the magnitudes predicted by the previous equations show an average bias of (−0.2 ± 0.1), which can be ascribed to the different constraint applied to the station corrections. The magnitudes predicted by the scale for the 3c network are used to calibrate magnitude scales based on either total duration or maximum amplitude from synthesized Wood-Anderson seismograms computed for each short-period vertical recording. The magnitude scale obtained considering maximum amplitudes from vertical short-period recordings is M L = log A + log( R /100) + 0.0041 ( R − 100) + 3 − S ′. The reliability of the obtained magnitude scales is assessed using 827 earthquakes different from those we considered in the regression analysis. Finally, the following seismic moment versus local magnitude relations are valid in the western Alps in the range 0 where M L3C is the local magnitude computed starting from the horizontal component of broadband (flat frequency response, from 0.033 to 50 Hz) or semibroadband (flat frequency response, from 0.2 to 40 Hz) sensors and M L1C is the magnitude computed starting from the vertical short-period recordings. [1]: /embed/graphic-1.gif

SEISMIC MULTIPLETS ANALYSIS AND ITS IMPLICATION IN SEISMOTECTONICS

Abstract Doublets analysis techniques suitable to obtain very accurate determination of arrival times of seismic phases on different stations are presented. These methodologies represent a very powerful tool to recognize the seismotectonic structures and to study the propagation characteristics of seismic waves. The signals similarity analysis, performed in the frequency and time domain, has been applied to increase the precision in arrival times definition of seismic phases generated by the same sources and having the same path. Tests on synthetic events have shown how it is possible to recognize the time delay between digital traces with a sensitivity less than the sample rate. In particular, in the time-domain analysis the results obtained using different interpolation functions (parabolic and sinc for the cross-correlation operator) are compared. The analysis slightly suffers from a random noise component added to the signal, allowing the phase picking of highly noised traces. On the contrary, the frequency domain analysis offers the best results in case of high signal-to-noise ratio, while it appears less ‘robust’ with respect to high noise influence. The application of relative location techniques on the obtained arrival times allows to define with very high detail the geometrical shape of seismogenetic structures. As an example, a case of spatial doublets, recorded by the seismic network of Northwestern Italy, that occurred during two days in an area of the Maritime Alps (Italy), has been considered. In spite of the low magnitude of the events, an accurate reconstruction of the spatial distribution of the sources was obtained, allowing to discriminate the fault plane from the auxiliary one in the focal mechanism solution.

Litho—asthenospheric structures of northern Italy as inferred from teleseismic P-wave tomography

Abstract Regional three-dimensional inversions of teleseismic P-wave travel time residuals recorded by high-frequency regional and local seismic networks operating along the Western Alps and surrounding regions were carried out and lithosphere and upper mantle P-wave velocity models down to 300 km were obtained. Residuals of more than 500 teleseismic events, recorded by 98 fixed and temporary seismic stations, have been inverted. The comparison between real residuals and the ones obtained from tomographic model indicates that the method is able to solve the feature of the regional heterogeneities. Where the resolution is good, coherent lithospheric and upper mantle structures are imaged. In the shallower layers, high- and low-velocity anomalies follow the structural behaviour of the Alpine-Apenninic chains showing the existence of very strong velocity contrasts. In the deepest layers, velocity contrast decreases however two deep-seated high-velocity structures are observed. The most extended in depth and approximately trending NE-SW has been interpreted as a wreck of the oldest subduction responsible of the Alpine orogenesis. The second one, connected to the northwestern sector of the Apenninic chain, appears to vanish at depths greater than 180 km and is probably due to still active Apenninic roots. Cross-sections depict the spatial trend of perturbations and in particular outline the sub-vertical character of the Alpine and Apenninic anomalies. Under the Ligurian Sea, the 3-D inversion confirms the uplift of the asthenosphere in agreement with the tectonic evolution of the basin.

The February 23, 1887 tsunami recorded on the Ligurian coast, western Mediterranean

The catastrophic Ligurian Earthquake on February 23, 1887 caused significant damage and numerous casualties in northwestern Italy and southeastern France. The first (main) shock (estimated magnitude of 6.2–6.5) produced tsunami waves observed along 250 km of the Ligurian coast from Genoa to Cannes. Typical tsunami run-up heights were 1–2 m. The first wave was found to be negative, supporting the assumption that the earthquake was produced by release of stress along an offshore normal fault. Computed tsunami waveforms are in qualitative agreement with observed run-ups. Unique 1887 tsunami records in Genoa and Nice harbors were found and analyzed. Numerical computations of eigenoscillations for the old (1887) Genoa harbor were made and found to be in a good agreement with the tsunami observations. The dominant observed period of 22.5 min was shown to be related to the fundamental (Helmholtz) mode of the old port. Long “ringing” and slow decay of tsunami oscillations in Genoa are associated with “pumping” of wave energy from the external basin. Several wave trains are revealed in the record that occur at 2–2.5 h intervals. They are interpreted as edge waves propagating along the Ligurian shelf and reflecting from its borders.

Assessing the Effectiveness of Soil Parameters for Ground Response Characterization and Soil Classification

The average shear wave velocity over the top 30 m of a soil profile (VS,30) represents an usual parameter for soil classification in a modern building code for seismic design. In this work the ground response of about 100 soil profiles in Tuscany and Molise (Italy) is studied through 1-D numerical simulations in order to evaluate the reliability of European and Italian soil classifications based on the VS,30 criterion. The amplification factor, Fa , defined here as the ratio of the pseudo-velocity response spectrum intensity (Housner 1952) at the surface, S Is , to the pseudo-velocity response spectrum intensity at the rock outcrop, S Ir , is related to some soil parameters, such as VS,30, the fundamental frequency of vibration of the soil column, F0, and seismic impedance contrast, Iw . Analyzing the standard deviation of the residual obtained from regression analyses of Fa versus VS,30, F0, and Iw shows that F0 is the most helpful parameter for the prediction of Fa . Hence F0 appears to be more appropriate than VS,30 and Iw for the characterization of the seismic response of a site and, therefore, should not be disregarded in building code soil classifications.

Analysis of Site Amplification Phenomena: An Application in Ripabottoni for the 2002 Molise, Italy, Earthquake

The geophysical working group of the University of Genoa conducted a field experiment to analyze site amplification effects in Ripabottoni, a village in the Molise region of Italy. We used both noise and earthquake recordings, combined with detailed geologic and geotechnical surveys, to define site amplification phenomena. The site effects determination was obtained using the Nakamura technique and the H/V spectral analysis applied to earthquake recordings. The results were validated by applying a one-dimensional simulation code. The computed spectral ratios point out three different typologies of site effects: the southern sector of Ripabottoni is characterized by an absence of local amplification phenomena; the central sector of the village shows a local amplification phenomena with a fundamental frequency of 4–6 Hz; and the northeastern side of the village shows a site response at a fundamental frequency between 2–3 Hz.

Lateral variations of Pn wave velocity in northwestern Italy

The Pn arrival times recorded from seismic networks operating throughout northwestern Italy and surrounding regions were inverted to map the structural variations of the uppermost mantle over the area and to estimate the crustal static delays at each station. By means of careful data selection a quality data set was obtained removing statistical outliers, poorly recorded events, and scantily sampled stations. Moreover, synthetic data were used to evaluate the resolution power of the available data set and the adopted iterative inversion technique. The agreement between synthetic and calculated models is more satisfactory where the path coverage of the rays is quite complete. A low-velocity zone was found beneath the western and northwestern side of the Alps; it may be related to an increase in depth of the Moho, as supported by other geophysical data. The high Pn velocities found in the eastern side of the western Alps indicate the presence of high velocity and density, lower crust rocks of the so-called “Ivrea Body.” The high Pn velocity underlying the Ligurian Sea could be related to a high-velocity structure existing in the upper mantle. The shape of the velocity anomalies matches not only the major tectonic features, but also the Bouguer gravity anomalies of the area.

Regional coda Q variations in the western Alps (northern Italy)

Abstract To study the microseismicity of the central segment of the western Alps, a local temporal seismic network of six stations was installed in addition to the regional centralized network operating in the area. In 3 months about 220 shocks, with magnitude ranging between 1.0 and 3.8, have been analyzed and located. The epicenter distribution shows a possible subdivision into seismogenetic bands. This subdivision is also recognizable from the spectral characteristics of the analyzed seismic signals. A study of the wave attenuation, in a restricted area well covered by the local network, was carried out through a ‘coda’ analysis. The results show strong spatial Qc variations, in agreement with the strong lateral heterogeneities that characterize this area. In particular, the Brianconnais domain, the crystalline massifs and the inner part of the Alpine arc appear to behave as decreasing attenuation bands.

Site-Amplification Effects Based on Teleseismic Wave Analysis: The Case of the Pellice Valley, Piedmont, Italy

The investigation of local amplification phenomena by seismic signal analysis is a fundamental step in carefully defining the seismic response of an area. In this study we investigate the use of teleseismic recordings in assessing seismic- wave amplification in the Pellice Valley (northwestern Alps, Italy). Assuming that teleseismic P waves are sensitive to the deep structure of a basin, we deal with the computation of horizontal-to-vertical spectral ratios (hvsrs) and with the estimate of teleseismic P -wave arrival time delays and P -wave amplifications with respect to a reference site. The reliability of the hvsr results obtained by considering teleseismic signals is confirmed by the agreement with the results coming from both the hvsr of noise and hvsr of S wave of local events methods. Strong correlation between the P -wave arrival time delays and the relative P -wave amplifications with respect to thickness of the low-velocity layers and the geometry of the bedrock is found.