Steady deformation at Arenal Volcano, Costa Rica.
Ebmeier, S. K., J. Biggs, T. A. Mather, G. Wadge, and F. Amelung
Satellite measurements of deformation at Costa Rica’s most active volcano, Arenal, show that part of its western flank is moving downslope at a rate of about 7 centimetres every year. We think that this is caused by the weight of lava effused from the volcano over the last 40 years, and speculate that movement is along the plane between material laid down before and after Arenal’s reactivation in 1968. Monitoring the deformation of Arenal’s edifice is important both for understanding the way volcano stability responds to prolonged periods of eruption and for assessing the risk posed by the volcano to the surrounding population.
Arenal has been erupting almost continuously since 1968, when it reawakened after centuries of dormancy. Three days of blast eruptions then covered about 200 square kilometres with ash and destroyed two nearby villages. Today about 7000 people live within approximately 6 kilometres of the volcano, mostly in the town of La Fortuna (Figure 1), where a significant proportion of the population are to some extent dependent on hot spring or volcano related tourism for their livelihood. Over the past 40 years Arenal has erupted almost continously, with the vast majority of new rock added to the western flank of the volcano, making it increasingly asymmetrical and increasing its total volume by about 4%.
Figure 1: Photograph of Arenal Volcano during a relatively quiet period in March 2010, taken from La Fortuna.
Interferometric Synthetic Aperture Radar (InSAR) is a satellite-based measurement technique that allows us to measure the movement of the earth’s surface to centimetric precision, or better. Interferograms are constructed from radar images acquired on different dates and map change in phase over time. After correction, this difference in phase can be used to determine the movement of the earth in the satellite’s line of sight.
Figure 2: a) Map showing the location of Arenal on the Central American Volcanic Arc. b) the average deformation rate measured at Arenal between 2005 and 2009 using RadarSat data. Each complete transistion in colour scale represents 2.6 cm/year of motion.
Over 40 interferograms constructed from two different wavelengths of satellite data were used to measure deformation at Arenal between 2005 and 2009. One of the strengths of InSAR as a technique is its high spatial resolution, which allows us to develop a detailed picture of the shape of the deformation signal (Figure 2). We gained more information about the actual direction of slope movement, as opposed to that in the satellite line of sight, by using two different satellite look directions to resolve the motion into vertical and East-West components (Figure 3). We used this evidence about direction of deformation, along with analysis of the signal shape and variation through time to consider several possible causes for the deformation signal, and concluded that the most likely mechanism is gravity-driven slip.
Figure 3: Profiles though Arenal running East-West, showing a) resolved angle of deformation as arrows with length proportional to magnitude of deformation and b) estimated boundary between pre- and post 1968 eruption deposits.
The rate of deformation at Arenal is unusually high compared to other gravity-driven volcano deformation described in the literature, especially for such a young volcano. This suggests that the plane of failure at Arenal is already well established. Our data suggest that for the period of measurement (2005-2009), slip is constant. However, it is as yet unclear whether this deformation is stabilising or destabilising the slope. Events such as the intrusion of magma into its edifice or a large earthquake could be expected to have a destabilising effect on Arenal’s western slopes and may trigger rapid flank collapse. Detecting any change in the rate of motion on Arenal’s western flanks is therefore important for assessing hazard posed by the volcano.