Periodic gravity changes at Poás volcano, Costa Rica

Abstract

The measurement of gravity over active volcanoes is usually aimed either at the development of sub-surface structural models^1–4 or the investigation of temporal changes in gravity associated with magma movements^5–13. The positive gravity anomaly over Poás volcano, Costa Rica⁴ indicates the presence of a cylindrical pipe of denser material extending to several kilometres depth. Although the volcano is active, this feature has remained apparently stable for at least the past four years. However, our programme of high-precision microgravity studies indicates changes with amplitudes of 120 µGal at the crater rim and 50 µGal on the flanks with a periodicty of ∼30 days. These data are interpreted in terms of either relative movements of several metres between the magma body and the stable volcanic edifice or of cyclic density change related to the degree of vesiculation or crystallization of the magma: there is a possibility that these variations are tidally induced. Such studies may have wider relevance in analysing previously unrecognized components of microgravity change and in the monitoring of active volcanoes.