Abstract
The shape and stability of a volcano’s edifice depends on the relationship between eruption rate and the loss or redistribution of material due to erosion, mass-wasting or deformation. This work provides measurements of deformation and shallow mass-wasting at a stratovolcano immediately after an extended period of growth, and demonstrates that high rates of deformation can be associated with shallow edifice processes. We measure displacements and surface property changes on the upper flanks of Arenal, Costa Rica, after a ~40 year period of edifice growth. We present high-resolution satellite radar imagery of the 2011-2013 period that provides evidence of frequent rockfalls and of at least 16 slow-moving, shallow landslides (estimated to be 5-11 m thick, total volume = 2.4×107 m3 DRE). The 2012 Nicoya Earthquake (Mw 7.4) had no measurable impact on the velocities of sliding units at Arenal, but did result in an increase in the area affected by rockfall.
Highlights
The stability of a volcanic edifice depends on the ratio of its exogenous growth to mass loss due to erosion or mass wasting (e.g., Baldi et al, 2008) and mass redistribution due to deformation or compaction (e.g., Borgia et al, 2000; Ebmeier et al, 2012)
The viewing geometry of the images is sensitive to East-West and vertical, but not North-South movements, so line-of-sight displacement appears to be slightly higher to the West of the summit than to the North or South, this may be an artifact of viewing geometry
Consolidation of recent deposits at Arenal is expected to contribute to the net deformation rate, and may be responsible for the similarity in deformation rate around the edifice, it is unlikely to be the primary mechanism for the large horizontal component of motion on the volcano’s upper flanks (Ebmeier et al, 2010)
Summary
The stability of a volcanic edifice depends on the ratio of its exogenous growth to mass loss due to erosion or mass wasting (e.g., Baldi et al, 2008) and mass redistribution due to deformation or compaction (e.g., Borgia et al, 2000; Ebmeier et al, 2012). High growth rates during eruptions lead to oversteepening (e.g., Sparks et al, 1998; Wadge et al, 2006), which, along with the presence of poorly consolidated deposits such as tephras and pyroclastic flows, make mass wasting processes at volcanoes distinct from tectonic mountains. Observations of the rate of flow deposit deformation, and the response of a volcanic edifice to new loads are important for understanding hazard from rockfalls and landslides following an eruption. The post-1968 deposits on Arenal’s western flanks consist primarily of block lavas, though a’a flows were produced early in the eruption and were still present close to the summit in the early 1980s. The thickness of post-1968 material is almost 300 m at Arenal’s summit
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