After the 2009 April 6th Mw 6.3 L'Aquila earthquake (Central Italy) the Italian Civil Defense Department promoted the microzoning study in the ten zones in the epicentral area that suffered major damage. In this paper we present the activities and the results concerning a temporary seismic network installed in the historical L'Aquila city center indicated as "macroarea 1" in the microzoning project. Seismic data were collected to investigate the amplification effects in the city and to support the microzoning activities in verifying both geological profiles and 1D numerical modeling of the seismic response of the city. The conventional spectral approaches using both microtremor and earthquake data allowed to determine the fundamental resonance frequencies and the amplification factors within the city respectively. The spatial variability of these quantities can be related to the geological and geomorphologic characteristics of the investigated area. A comparison between the network data and the data recorded by the two strong motion instruments installed in the city was also made. This allows verifying the relative response of the accelerometric stations that recorded in the city the major events of the sequence.
We present the results of seismological and geophysical investigations performed by the "Istituto Nazionale di Geofisica e Vulcanologia" team operating in Amatrice village (Central Italy), in the emergency phases following the Mw 6.0 event of August 24th 2016, that caused severe damage in downtown and surrounding areas. Data from seven seismic stations equipped with both weak and strong motion sensors are analyzed in terms of standard spectral ratio to empirically define amplification function using a bedrock reference site. Ambient vibration spectral ratios between horizontal and vertical component of motion are also evaluated in a large number of sites, spread out in the investigated area, to recover the resonance frequency of the soft soil outcropping layers and to generalize the results obtained by earthquake data. Ambient noise vibration are also used for applying a 2D array approach based on surface waves techniques in order to define the near-surface velocity model and to verify its lateral variation. The results allows to better understand the amplification factors in the investigated area, showing spatial variation of site effects despite of the homogeneous shallow geological condition indicated by the microzonation studies available at moment of the described field campaign. The analysis reveals a diffuse amplification effect which reaches its maximum values in downtown area with a resonant frequency of about 2 Hz. The obtained results were used to integrate the microzonation studies and they can be used for urban planning and reconstruction activities.
A temporary network of 33 seismic stations was deployed in the area struck by the 6th April 2009, Mw 6.1 (Scognamiglio et al. in Seism Res Lett 6/81, 2010), L'Aquila earthquake (central Italy), with the aim to investigate the site amplification within the Aterno river Valley. The seismograms of 18 earthquakes recorded by 14 of the 33 stations were used to evaluate the average horizontal to vertical spectral ratio (HVSR) for each site and the 123 698 Bull Earthquake Eng (2011) 9:697-715standard horizontal spectral ratio (SSR) between a site and a reference station. The obtained results have been compared to the geological and geophysical information in order to explain the resonance frequencies and the amplification levels with respect to surface geology of the valley. The results indicate that there is no uniform pattern of amplification, because of the complex geologic setting, as the thickness and degree of cementation of the deposits is highly variable.
We investigate the influence of building height on the ability of people to feel earthquakes and observe that, in an urban area, short and tall buildings reach different levels of excitation. We quantify this behavior by analyzing macroseismic reports collected from individuals through the Internet; we focus on elastic regime transitory effects of recent earthquakes in Italy in the local magnitude (M L ) range of 3-5.9. We find a maximum difference of 0.6 intensity units between the top floors of tall (7-10 stories) and short (1-2 stories) buildings at the highest considered magnitudes. As expected, tall buildings experience greater shaking than short buildings during large earthquakes at large source distances. However, we observe the opposite behavior at close distances when the M L is < 3:5. These results can be explained by considering the different spectra radiated by small and large earthquakes and the different fundamental mode resonances of buildings (i.e., shorter buildings have higher resonance frequencies and vice versa). Using idealized building models excited by real acceleration time histories, we compute synthetic accelerograms on the top floors of short and tall buildings and confirm the trend of the observed differences in felt intensities.
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