Volcanic rift-zones and flank instability : an evaluation of ground deformation monitoring techniques
Collapse is part of the growth cycle of volcanoes with active rift-zones. As the volcano develops and grows disequilibrium between the strength of the edifice and the applied stresses, produce instability within the edifice which can potentially lead to catastrophic lateral collapse. The intra-eruptive periods on persistently active volcanoes are often short-lived. The recent intra-eruptive windows at Piton de la Foumaise, Reunion Island (1994-1996), Etna, Sicily (1994-1997) and the less-frequently-active Cumbre Vieja ridge on La Palma (1994-1998), have provided a valuable opportunity in which to compare the background deformation (which can indicate whether the edifice is stable between eruptions). Influences such as gravitational loading, tectonic activity, creep or the intrusion of fresh magma would cause coherent deformation patterns, and even small movements within the estimated errors of the measuring techniques are assessed over time to identify patterns. For each site the background seismicity has remained low and no flank eruptions were recorded during the monitoring program. The results suggest that each site is stable between eruptions. Etna has shown increasing activity since 1995 reflected by a general pattern of inflation. This inflation and a small isolated pocket of inflation on the SW flank are attributed to the intrusion of magma in 1995. A coherent pattern of very minor deformation was observed spanning the 1949 fault of the Cumbre Vieja from 1994 to 1997, however, this was not sustained in 1998. Piton de la Fournaise remained stable during the monitoring program and then erupted at the end of the study in 1998. This inter-eruptive period also provides an optimal time for the transition from one technique to another. On each site the existing EDM (Electronic Distance Measurement) networks were re-occupied using GPS (Global Positioning System), the comparison of individual vectors indicated that the accuracy was approximately 5-12mm, which was less than the expected error between EDM surveys. The networks at all sites have been expanded during the quiescent period and the use of GPS has permitted the optimal positioning of survey stations in order to assess future ground deformation.