On 21 August 2017, an M D 4.0 earthquake struck Ischia island in the Tyrrhenian Sea off the coast of Naples, Italy. In spite of its modest magnitude, the earthquake caused two deaths and severe building damage on the northern side of the island. Initial hypocenter locations based on arrival times were highly uncertain and several proposed moment tensor solutions were inconsistent. These contradictory observations prompted a new calculation of the earthquake parameters using alternative methods. Our new approach, based on the determination of P-wave particle motion, evaluation of rotated spectra, and accurate calculation of S-minus-P travel time, yields a hypocentral depth of 2 km and a location in the same area as the devastating seismic event that struck Ischia in 1883. We invert the moment tensor for a best-fitting double couple (DC), obtaining an M w 3.9 with a normal mechanism and an optimal depth of 8 km. Calculation of the full moment tensor results in (1) 36% negative isotropic component and 26% negative compensated linear vector dipole (CLVD) components, (2) a better fit at a shallower source depth than for the corresponding DC, and (3) a magnitude estimate of M w 4.1. Modeling of the waveform and the first motion recorded in Ischia's station IOCA predicts, however, a negative polarity, which is in disagreement with the observation. We therefore suggest a complex rupture process, with an initial shallow normal-faulting event that triggered a subsequent shallow underground collapse.

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.

The Fucino basin (Central Italy) is one of the largest intramountain alluvial plain in the Apennines range. It has a tectonic origin related to the presence of important systems of faults located in its northern and eastern edges. Some of these faults are still active and capable of generating strong seismic events. Site effects related to the soft soils filling the basin can be very important. In this paper we show the preliminary results of a seismic network installed in the Fucino area in order to collect information about site amplification effects and geometry of the basin. We analyze ambient seismic vibrations and recordings of about 150 local earthquakes mainly related to the seismic sequence of the April 6th 2009 Mw 6.3 L'Aquila event. Moreover the strongest events of L'Aquila sequence were analyzed at the three permanent strong-motion stations operating in the area. Using standard spectral techniques we investigate the variation of resonance frequencies within the basin. The ground motion recorded in the Fucino plain is mainly characterized by strong energy at low-frequencies ( f < 1 Hz) affecting both horizontal and vertical components. This is particularly evident for stations deployed in correspondence of very thick deposits of sedimentary filling, where a significant increase of ground-motion amplitude and duration is likely caused by locally generated surface waves. The amplification at low-frequencies (<1 Hz) on the horizontal components can reach up a factor of 10 in comparison to nearby stiff sites. However, we found evidences of seismic amplification phenomena also for stiff sites surrounding the basin, including stations of the Italian strong motion network. The independent geological information and the shallow shear-velocity profiles available for the basin can be combined with resonance frequencies 123 1962 Bull Earthquake Eng (2011Eng ( ) 9:1961Eng ( -1985 for deriving representative geological sections to be used as base for future numerical 2D-3D modeling of the basin.

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