Gravity changes as a precursor to volcanic eruption at Poás volcano, Costa Rica

ABSTRACT

Monitoring of gravity changes at active volcanoes has considerable potential for understanding megma-chamber dynamics and for detecting eruptive presursors. The technique essentially aims to identify changes in sub-surface mass or density corresponding to departures from the free-air gradient (FAG) or Bouguer corrected (terrain-corrected) FAG (BCFAG), respectively. Such gravity changes, of the order of 10-100 ugal, have been reported previously through eruptive periods. Here we present a ten-year data set measured at Poás volcano, Costa Rica, in wich gravity increases in 1987-1989, relative to both the FAG and the BCFAG, preceded the ash-cloud eruptions of april-may 1989. These results constitute the first unambiguous detection of gravity changes as a precursor to eruption, and we interpret them in terms of the intrusion of an ascending, relatively dense magma body(AU)