Coseismic and aseismic strain offsets recorded by the Gran Sasso strainmeter.

An underground low-noise high-sensitivity laser strainmeter, crossing a fault in the Gran Sass0 massif (central Italy), is discontinuously operating since late May 1994, after a few years of mechanical stabilization of end monuments. A swift extensional strain transient reaching a peak of at least 1 ccc occurred in about one month from the beginning of the operating time, followed by a four-month-long coarsely stable period, and a slower decay. On June 2nd 1994, in coincidence with two local earthquakes, permanent extensional offsets of about 10 nc have been recorded. A further aseismic slip of the same order of magnitude and sign has been observed about thirty minutes after the last preceding local event, and ten minutes before the next following one. At the same time, the arrival of seismic waves from a distant earthquake has been recorded by the interferometer. No similar signal behaviour has been observed any more since then.