Different approaches to estimate the local seismic hazard:on application to the Zelantea Academy Museum in Acireale(Sicily)

Mt. Etna is an active volcano, the slopes of which are densely inhabited, the southeastern one markedly, where the town of Acireale is located. The properly named volcanic hazard is mainly experienced by lava flow invasion and ash downfall. However, the hazard associated with the local and/or regional earthquake activity cannot be neglected. Then, the Zelantea Academy Museum, located in the Acireale downtown, has been chosen as "test site" for different estimates of the seismic hazard on a building of interest for the cultural heritage. A seismic catalogue for the local earthquakes has been compiled, after the collection and revision of the still available catalogues and of coeval and recently published sources. The final catalogue includes 85 local earthquakes and the distribution of their intensity on 64 different localities identified within the Acireale municipality boundaries. A catalogue of the local seismogenic faults (able to generate earthquakes in historical times) has been compiled, too. The coupling of both catalogues allowed us to the following conclusions: i) the most important seismogenic faults affecting the Acireale municipality do not affect the downtown, while the related local earthquakes attenuate their energy (and intensity) in short (few km) distances; ii) the highest seismic intensity (degree X) at the site of Zelantea Academy Museum (in Acireale downtown) was done by the 1693 regional earthquake; iii) over the last 140 years, Acireale downtown has experienced an intensity value of VII only once, while six times the intensity was VI. In the whole, this implies a moderate seismic hazard. The estimate of the seismic hazard at the site has been approached also by the experimental method of the recording seismic noise. Twelve measurements have been performed outside and inside the Zelantea Academy Museum.The spectral ratios HV (horizontal/vertical) show a significant amplification at about 2 Hz (probably due to a 17 m deep discontinuity of the layered shallow subsoil). The same amplification (ca. 2 Hz) has been found inside the building. At frequencies higher than 10 Hz the vertical component is larger then the horizontal one. In conclusion, the site amplification factor is moderate (about 6), while the building does not show evidence of amplification factors due to its shape. A further approach to the estimate of the seismic hazard, based on synthetic seismograms (and spectra) produced by simulating two given earthquake scenarios was done, too. The two scenarios are respectively representative of the largest expected earthquake in the area (the 1693 event), and of a moderate (magnitude ca. 5.5) local event (as the 1818 one). Moderate to strong locally expected accelerations have been evidenced.