THE 1989-1990 SEISMIC SWARM IN THE ALBAN HILLS VOLCANIC AREA, CENTRAL ITALY

A long seismic swarm has been recorded by a digital microseismic network that the Istituto Nazionale di Geofisica (ING) deployed from April 1989 to March 1990 in the Alban Hills Quaternary volcanic region, about 20 km southeast of Rome. The network consisted of thirteen digital stations equipped with four six-component (weak motion and strong motion sensors), three three-component and six vertical weak motion seismometers, including both broad-band and short-period sensors. More than 3000 earthquakes were detected during about one year of operation. The data recorded during the sequence provided an unprecedented opportunity to resolve details of the seismogenic features of this area that has experienced earthquakes since the Roman age. Although the temporal distribution of seismicity reveals a volcanic-type swarm activity, the recorded seismic signals show medium-high frequency waveforms and clear S-wave arrivals typical of ''tectonic'' earthquakes. About 1100 earthquakes in a magnitude range between M(D) approximately 1.5 and M(D) approximately 4.0 have been located. The seismicity delineates a approximately 6x12km2NW-SE-elongated region that corresponds to the area where the most recent (0.027 Ma) phreato-magmatic activity took place. Almost no earthquakes occurred beneath the central cone and the eastern side of the caldera. The seismicity is shallow and is mostly concentrated in two clusters that are offset laterally and are at slightly different depths (3-5 and 4-6 km). The fault plane solutions of the strongest earthquakes reveal a approximately NE-SW extension, consistent with the regional orientation of the stress field along the Apenninic chain as obtained by other seismological data.