Subsurface mass redistributions at Mount Etna (Italy) during the 1995-1996 explosive activity detected by microgravity studies

Gravity changes are presented from a series of field microgravity surveys conducted at Mt Etna between August 1994 and November 1996, a period including the 1995-1996 explosive summit activity. Data were collected along a microgravity network of 69 stations at a monthly to annual sampling rate, depending on each subarray of the network. Results show that seasonal changes in water level within the volcano may induce gravity changes of up to 20 mu gal on Etna's southern slope, and indicate that significant magma movement occurred within and below Etna's edifice between 1994 and 1996. In particular, between September 1994 and October 1995, a mass increase of 2 x 10(10) kg occurred 2000 m beneath the summit craters. Between October 1995 and July 1996 this mass was lost, while another 2 x 1010 kg was injected at about 1000 m a.s.l. into the 1989 fracture system. From the gravity data alone, it is not possible to distinguish whether the first shallow intrusion (1994-1995) was then injected laterally into the 1989 fracture, or summit activity was fed by the first shallow intrusion, while new magma entered the 1989 fracture system. While magma was being redistributed within the volcanic edifice, measurements along an E-W-trending profile on the southern slope of the volcano detected some 1.5 x 10(11) kg of magma accumulating 2-3 km below sea level between October 1995 and November 1996.