Mayon volcano, Philippines: change of monitoring strategy after microgravity and GPS measurements from 1992 to 1996

Mayon volcano is part of the Bicol volcanic chain on the island Luzon, Phil ippines. During this century there were ten activity periods distributed al most regularly. Because of the density of population (about one million peo ple living in the vicinity of the volcano) three seismological observatorie s are in operation. Measurements of gravity changes started in 1992, just before the eruption o f February/March 1993. Two profiles at the slope were established, connecte d to a regional network around the volcano. In order to enable the determin ation of mass changes between the campaigns the height control was provided by parallel GPS measurements. In all, the network consists of 26 points wh ich were remeasured with three gravimetres at least three times within one campaign. During five campaigns within 4 years the differential GPS gives no signific ant changes of the elevation (within 4 cm). Nevertheless, the gravity incre ased significantly by up to 1500 nm/s(2) (equivalent to 150 mu Gal). As no significant change of elevation is observed (GPS), no extended shallo w magma chamber system below the volcano can be proved. This is in accordan ce with geochemical results indicating a rather undifferentiated magma. The youngest lava which is of interest for the eruption dynamics belongs to th e medium-K basaltic andesite field of the K2O VS SiO2 diagram. A rather qualitative check of groundwater level changes reveals that these cannot be the sources for the observed gravity changes. Thus, the increase of gravity after the eruption of February 1993 can be explained by a mass r edistribution in the volcanic vent from above the level of the gravity poin ts to below. Practical conclusions of these results lead to changes in the monitoring st rategy: Deformation measurements did not reveal any volcanic activities;, a t least for the eruption of 1993 no significant deformation was observed. G ravity could be an indicator for long-term changes. Thus, repeated gravity measurements/GPS, at selected points could be used in parallel to seismic m onitoring to detect slow mass movements prior to changes in seismicity. (C) 2001 Elsevier Science B.V. All rights reserved.