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The micro-electromechanical system (MEMS)-based seismic network of Trentino (NE Italy) consists of 73 low-cost accelerometers installed close to inhabited areas. These sensors have a suitable sensitivity to detect moderate-to-strong earthquakes but are able to record even weaker seismicity. The densely distributed peak ground acceleration values recorded by MEMS and other types of stations are integrated within the existing seismic monitoring procedure in order to automatically obtain a complete set of strong motion parameters a few minutes after the origin time. The exposure of the resident population and critical buildings is estimated by quantifying the different levels of shaking, which is expressed according to the Mercalli–Cancani–Sieberg intensity scale. These types of results, summarized in synthetic portable document format (PDF), can be useful for civil protection purposes to evaluate the state of emergency after a strong earthquake in a timely manner and to choose how and where to activate first aid measures and targeted structural monitoring.

A site response experiment was performed in the basin of Città di Castello (a small town in Central Italy) in May 2001. This study is part of a project on the evaluation of seismic hazard in seismogenic areas funded by the Gruppo Nazionale Difesa dai Terremoti (GNDT). The experiment consisted of a dense fixed transect configuration with most of the stations recording in continuous mode, and several ambient noise measurements both in single station and in array configuration spread over the investigated area. The dense transect was composed of 26 seismic stations in a crosswise configuration with a maximum inter-station distance of 250 m. The stations were deployed in the southern part of the basin, from the eastern bedrock outcrop to the western edge, across the town. About 70 earthquakes were recorded during 10 days of deployment, generally low magnitude or regional events. We located 23 earthquakes and 17 of them were located using the waveform similarity approach at 4 stations outside the target area. These 4 stations were part of a dense temporary seismic network involved in a previous experiment of the same project, aimed at performing a high-resolution picture of the local seismicity. Delay analysis on the recorded waveforms allowed us to infer the basin geometry at depth and estimate the S-wave velocity of sediments. Moreover, we evaluated relative site response along the E-W transect by performing a standard spectral ratio. Amplification factors up to 9 are found inside the basin; at frequencies above 5 Hz stations closer to the edges show higher amplification, whereas stations located in the middle of the basin, where the alluvial sediments are thicker (CD11-CD14), show higher amplification below 5 Hz. We considered the average amplification in two frequency bands (1-5 Hz and 5-10 Hz), representative of the resonance frequency for 2-3 storey buildings and 1 storey houses,respectively. Our results suggest that the potential hazard for 2-3 storey buildings is higher in the center of the basin (amplification factor up to 6), and for 1 storey houses is higher at the edges (amplification factor up to 5).

Recent seismological studies contribute to better understand the first order characteristics of earthquake occurrence in Italy, identifying the potential sites for moderate to large size earthquakes. Ad hoc passive seismic experiments performed in these areas provide information to focus on the location and geometry of the active faults more closely. This information is relevant for assessing seismic hazard and for accurately constraining possible ground shaking scenarios. The area around the Città di Castello Basin, in the Northern Apennines (Central Italy), is characterized by the absence of instrumental seismicity (M > 2.5), it is adjacent to faults ruptured by recent and historical earthquakes. To better understand the tectonics of the area, we installed a dense network of seismic stations equipped with broadband and short period seismometers collecting data continuously for 8 months (October 2000-May 2001). The processing of ~ 900 Gbyte of data revealed a consistent background seismicity consisting of very low magnitude earthquakes (ML < 3.2). Preliminary locations of about 2200 local earthquakes show that the area can be divided into two regions with different seismic behaviour: an area to the NW, in between Sansepolcro and Città di Castello, where seismicity is not present. An area toward the SE, in between Città di Castello, Umbertide and Gubbio, where we detected a high microseismicity activity. These findings suggest a probable different mechanical behaviour of the two regions. In the latter area, the seismicity is confined between 0 and 8 km of depth revealing a rather well defined east-dipping, low angle fault 35 km wide that cuts through the entire upper crust down to 12-15 km depth. Beside an apparent structural complexity, fault plane solutions of background seismicity reveal a homogeneous pattern of deformation with a clear NE-SW extension.

An inversion of Pn arrival times from regional distance earthquakes (180-800 km), recorded by 94 seismic stations operating in North-Western Italy and surrounding areas, was carried out to image lateral variations of P-wave velocity at the crust-mantle boundary, and to estimate the static delay time at each station. The reliability of the obtained results was assessed using both synthetic tests and the bootstrap Monte Carlo resampling technique. Numerical simulations demonstrated the existence of a trade-off between cell velocities and estimated station delay times along the edge of the model. Bootstrap inversions were carried out to determine the standard deviation of velocities and time terms. Low Pn velocity anomalies are detected beneath the outer side of the Alps (-6%) and the Western Po plain (-4%) in correspondence with two regions of strong crustal thickening and negative Bouguer anomaly. In contrast, high Pn velocities are imaged beneath the inner side of the Alps (+4%) indicating the presence of high velocity and density lower crust-upper mantle. The Ligurian sea shows high Pn velocities close to the Ligurian coastlines (+3%) and low Pn velocities (-1.5%) in the middle of the basin in agreement with the upper mantle velocity structure revealed by seismic refraction profiles.

In August 2000 and July 2001 two seismic sequences, characterized by mainshocks with Ml (local magnitude) respectively 5.1 and 4.8, occurred in the Monferrato region (Italy). The regional seismic network of North-western Italy (RSNI) recorded more than 250 foreshocks and aftershocks. The routine locations, obtained from the Hypoellipse code, show a seismic activity concentrated in a circular area, of a radius of about 15 km, located near Acqui Terme, and randomly distributed in depth. Location errors, due to an uneven azimuthal station distribution of the regional seismic network, prevent recognition of the geometry of the active zone. Waveform analysis revealed the presence of several multiplets. In order to discriminate and successively relocate them, an automatic cross-correlation procedure was applied. Normalized cross-correlation matrixes, for the RSNI stations which recorded almost 90% of considered events, on the basis of a signal to noise ratio analysis, were computed using only S wave time windows. The use of a relocation procedure, based on the double-difference method which incorporates ordinary absolute travel-time measurements and/or cross-correlation differential travel-times, allowed us to overcome the constraints of the uneven distribution of stations giving a quite different frame of seismicity. The improved locations showed that the seismic activity is mainly arranged along a NE-SW oriented volume, at a depth range of 8–20 km, involving the basement crystalline units. This orientation is confirmed by the analysis of the focal mechanisms: most focal solutions show a strike slip component with one of the nodal planes consistent with the main orientation of the seismic events. [ABSTRACT FROM AUTHOR]