ALBERT

Description: On May 20, 2012, at 02:03 UTC, a Ml 5.9 reverse-fault earthquake occurred in the Emilia-Romagna region, northern Italy, at a hypocentral depth of 6.3 km (http://iside.rm.ingv.it/), close to the cities of Modena and Ferrara in the plain of the Po River. The epicenter was near the village of Finale Emilia where macroseismic intensity was assessed at 7 EMS98 [Tertulliani et al. 2012, this issue], while the closest accelerometric station, MRN, located less than 20 km west-ward at Mirandola (Figure 1) recorded peaks of ground accelerations of about 300 cm/s2 (www.protezionecivile.gov.it/resources/cms/documents/Report_DPC_1_Emilia_EQSd.pdf). The mainshock triggered liquefaction phenomena a few kilometers eastwards of the epicenter, around the village of San Carlo. On the same day, two other shocks of Ml 5.1 followed (02:07, 13:18 GMT; http://iside.rm.ingv.it/). On May 29, 2012, at 07:00 UTC another Ml 5.8 earthquake hit the region (http://iside.rm.ingv.it/), with the epicenter close to the village of Mirandola (Figure 1). Three other strong aftershocks occurred afterwards, of Ml 5.3 (May 29, at 10:55), Ml 5.2 (May 29, at 11:00) and Ml 5.1 (June 3, at 19:20). For a detailed description of the seismic sequence, see Moretti et al. [2012], Scognamiglio et al. [2012], and Massa et al. [2012], in this issue. The Emilia seismic sequence resulted in 25 casualties, several of whom were among the workers in the many factories that collapsed during working hours, and there was extensive damage to monuments, public buildings, industrial sites, and private homes. […]

Description: We carried out a vibration study experiment on a masonry building in the town of Ariano Irpino, southern Italy, using six-channel stations equipped with three-component velocity-transducers and accelerometers and running in continuous modality from January 2006 to December 2007. The analysis of weak motions from several local earthquakes, together with the 3D numerical modelling of the structure, allowed us to identify the first three vibration modes of the target building. Therefore, we checked the validity of ambient noise data to determine the vibration frequencies of buildings. The analysis tools based on earthquake and ambient noise data were conventional, i.e. spectral ratios between homologous components of stations at high floors in the building with respect to a station installed at the basement, and single-station spectral ratios between horizontal and vertical components. The indications derived from earthquakes and ambient noise result in a satisfactory agreement for frequencies between 1 and 20 Hz when using recordings characterized by low levels of amplitude, both for cultural and meteorological noise. In contrast, when the wind speed increases (above 20 km/h, approximately) seismic noise shows an excess of horizontal vibrations at low frequencies (below 2 Hz). These extra-amplitudes are not related to the seismic input vertically incident to the basement, but are probably due to the lateral action of the wind on the building. In contrast anthropic activities do not affect considerably the trend of spectral ratios in the range of frequencies that include the first modes of vibration of the building, even at high noise level.