Combined GPS, EDM and triangulation surveys of the rapid down-slope motion of the western flank of Arenal Volcano, Costa Rica

Here we present initial conclusion of a one year-long combined geodesy campaign on Arenal Volcano, in Costa Rica (2008-2009). Our data shows evidence for an active local tectonic surrounding and a 7cm/yr displacement field on the western flank of the volcano edifice that is in general agreement with recent INSAR studies. Moreover, the rigorous combination of simultaneous GPS, EDM and triangulation measurements provide a time-cost effective approach to volcanic monitoring. Arenal volcano, in Costa Rica, is an active basaltic andesite stratovolcano located at the southern end of the Guanacaste volcanic range. Arenal's current long-lived magmatic activity may be coming to an end but downslope motion of the western flank suggests a risk of sector collapse. In order to study this, during 2008 and 2009 we deployed a joint geodetic network, comprising GPS, EDM (Electronic Distance Measurements) and triangulation measurements on a total of 12 benchmarks. Nine benchmarks monitor movements of the western flank of Arenal while three external benchmarks, located away from the volcanic system, monitor local tectonic movements. A joint adjustment of the combined network using the software suite GAMIT, FONDA and GLOBK allowed us to maintain a high repeatability along the time despite short field campaigns. In agreement with a quantitative comparison with existing INSAR, our results show a downward displacement on the western flank. However, in our full 3D survey, we observe that these displacements shift to a northern and even northeastern direction at the foot of the volcano. This suggests that if a sector collapse is underway it must have a northwestern direction. Our favoured preliminary explanation is, in agreement with local workers, that the deformation is caused by the weight of the recent lava field. The differential displacements between the three external points agree with neotectonic studies of the presence of active faults in the north of the Arenal graben.