Coseismic and aseismic strain offsets recorded by the Gran Sasso strainmeter

An underground low-noise high-sensitivity laser strainmeter, crossing a fau lt in the Gran Sasso massif (central Italy), is discontinuously operating s ince late May 1994, after a few yeats of mechanical stabilization of end mo numents. A swift extensional strain transient reaching a peak of at least 1 mu epsilon occurred in about one month from the beginning of the operating time, followed by a four-month-long coarsely stable period, and a slower d ecay. On June 2nd 1994, in coincidence with two local earthquakes, permanen t extensional offsets of about 10 n epsilon have been recorded. A further a seismic slip of the same order of magnitude and sign has been observed abou t thirty minutes after the last preceding local event, and ten minutes befo re the next following one. At the same time, the arrival of seismic waves f rom a distant earthquake has been recorded by the interferometer. No simila r signal behaviour has been observed any more since then. (C) 1999 Elsevier Science Ltd. All rights reserved.