The 1998 debris avalanche at Casita volcano, Nicaragua — investigation of structural deformation as the cause of slope instability using remote sensing

During Hurricane Mitch in 1998, a debris avalanche occurred at Casita volcano, Nicaragua, resulting in a lahar that killed approximately 2500 people. The failure that initiated the avalanche developed at a pre-existing cliff, part of the headwall of a gravitational slide of approximately 1.8 km2 in plan view that cuts the southern flank of the volcano. Structural analysis, primarily based on a high-resolution DEM, has shown that this slide is caused by edifice deformation. Casitaʹs eastern side is spreading radially outwards, forming a convex–concave profile and steepening original slopes. This deformation is possibly facilitated by millennia of persistent hydrothermal alteration of the volcanoʹs core. The gravity slide has some typical features of smaller slumps, such as steep headwalls, an inner flatter area and a pronounced basal bulge fronted by thrusts. The headwall is the source of the 1998 avalanche, as well as several previous mass movements. Edifice deformation has led to extensive fracturing of the hydrothermally altered andesitic source rock, increasing instability further. Field evidence indicates that the gravity slide is still actively deforming, and with steep headscarps remaining, the hazard of future avalanches is increasing. The analysis presented here shows how small but highly damaging landslides can occur during the deformation of a volcanic edifice. We show that identification of instability is possible with remote sensing data and minimal reconnaissance work, implying the possibility of similar efficient and cost-effective analysis at other volcanoes known to host extensive hydrothermal systems. We demonstrate this with a simple structural analysis of two similar stratovolcanoes, Orosı́ (Costa Rica) and Maderas (Nicaragua).