assessment of site-effects using acceleration time series. application to são sebastião volcanic crater

são sebastião volcanic crater, at terceira island, exhibits stronger ground motion during seismic events than its surrounding areas. inside this crater there is a village that has shown an anomalous seismic behaviour including heavy damages during past earthquakes. both the local geology and the geotechnical characteristics of the soil deposit play a major role in the seismic ground shaking characteristics at the site. recent geophysical and geological surveys executed in 2006 and 2007 added new information that allows a better comprehension of the damage distribution observed during the 1980 earthquake.

during the two seismic crises of 1997 and 1998 it was possible to obtain several records at three different places inside the volcanic crater: escola, junta and misericórdia strong-motion stations. comparison between seismic records allowed the identification and analysis of site effects inside the crater. moreover, two of the most usual site-effect assessment techniques were used, namely, the spectral ratio between the records at the crater and a given reference site (reference-site spectral ratio) and spectral ratios for different components of the same accelerometric station (horizontal-to-vertical spectral ratio). the combined use of these techniques proven to be a very efficient way to characterise the different site effects observed inside the volcanic crater.