The 1998 Casita volcano flank collapse and lahar: An overview

Category Tectonic & Seismotectonic
Group GSI.IR
Location International Geological Congress,oslo 2008
Author Devoli, Graziella۱; Cepeda, José۱; Kerle, Norman۲
Holding Date 03 September 2008

A catastrophic landslide occurred at the southern slope of the Casita volcano (Nicaragua, Central America) on 30 October 1998, triggered by the rainfall associated with Hurricane Mitch. The event started as a flank collapse which rapidly evolved into a debris avalanche and transformed into a lahar, that completely buried two small towns 6 km downstream and killed about 2500 people. Landslide and geological investigations at this volcano have been neglected in the past, but since the occurrence of this lahar, several studies were undertaken aimed at its assessment in different aspects.
Ten years after its occurrence, this study summarizes the knowledge acquired in previous investigations, presents an improved geologic and tectonic setting of the volcano summit and reconstructs the history of instabilities at this volcano. Then, the research attempts to improve our understanding of the initial failure processes and the factors instigating the instability of 1998 based on previous knowledge with additional unpublished field data and new observations. New insights are provided into the geology, tectonics, pre- and post-failure geometry and stratigraphy of the scarp area, the failure initiation and propagation, the lithologies and the geotechnical characteristics that governed the failure mechanisms, the sequence of failures and the types of failure mechanisms. Analyses indicate that the flank collapse took place in three stages of failure, which have occurred along an almost planar sliding surface at the interface between the volcanic breccia and the lower impermeable units of clay-rich pyroclastic deposits and altered lavas with low shear strength. The failures then propagated into the overlying loose materials. The study established the most likely failure mechanism of the first stage and identified the most likely mechanism of failure of the second stage. The mechanism of the third stage of failure is still unclear. Slope stability back-analyses of the second stage of failure were done, and the stability of the remaining slope was assessed using limit-equilibrium methods. The slope stability back-analyses allowed to obtain a range of undrained shear strengths at the base of the breccia (470 to 640 kPa) and to investigate the sensitivity of the factor of safety to hydrostatic pressures and the thickness of the breccia and the colluvium. The results of the analyses performed for undrained conditions, which are considered the most likely to occur in these settings, indicated that the remaining southern slope is unstable if it becomes saturated during intense and/or prolonged rainfall, i.e. the same conditions as those produced during Hurricane Mitch. For the perched water table scenarios a factor of safety lower than 1 is obtained if the colluvium berm is removed from the toe, but in "dry" conditions the slope is stable as long as the colluvium at the toe is not removed.