New insight into the factors leading to the 1998 flank collapse and lahar disaster at Casita volcano, Nicaragua

Abstract

During unusually high rainfall associated with Hurricane Mitch in 1998, a flank collapse leading to a disastrous lahar occurred at Casita volcano, Nicaragua. The lack of a similar failure during a rainfall event of comparable magnitude in 1982 (Tropical Storm Alleta) suggests that additional factors influenced the 1998 collapse. We have investigated the potential contribution of erosional flank undercutting, seismic activity and anthropogenic land-cover change at the collapse site. Except for seismic events prior to Mitch, which may have increased flank instability, none of these factors appears significant. Instead, for failure to occur, the following conditions were required: (1) highly fractured slope material allowing deep and rapid infiltration of meteoric water, (2) a less permeable underlying layer to obstruct drainage, (3) strong, continuous antecedent rainfall to build up high pore-water pressure, and (4) episodic, highintensity precipitation during Mitch to generate recurrent pressure waves. Prerequisite (1) was provided by edificewide deformation towards the south-east, seismic activity and proximity to a prominent fault, and local subsidence. Condition (2) was met by clay-rich layers resulting from hydrothermal alteration. More than 1,900 mm of rain fell in the 6 months prior to Mitch without significant interruption, while intense episodic precipitation occurred during the hurricane, satisfying conditions (3) and (4), respectively. The main difference with Alleta was that it occurred at the beginning of the rainy season and, therefore, without sufficient antecedent rainfall. Anthropogenic activity, including land-cover change, did not affect slope stability (i.e. the hazard). However, vulnerability was generated when two towns were established in the lowlands south of Casita, on top of previous lahar deposits. It was greatly increased when approximately 4 km of forest between the collapse site and the towns were cleared, paving the way for a largely unobstructed debris flow. Deforestation also facilitated erosion along the flanks to provide about 78% of the material contained in the lahar when it destroyed the towns, killing more than 2,500 people.

Landslide and tsunami hazard at Yate volcano, Chile as an example of edifice destruction on strike-slip fault zones

“…The immediate causes of major landslides include a wide range of external processes, such as extreme rainfall (Iverson 2000), seismicity (Miller 1960;Davis and Karzulovíc 1963;Evans 1989) or a special combination of factors (Kerle et al 2003), though there may also be no trigger other than the exceedance of a stability threshold. None of the four historic events at Yate correspond to known earthquakes.…”

Influence of substrate tectonic heritage on the evolution of composite volcanoes: Predicting sites of flank eruption, lateral collapse, and erosion

“…Lateral collapses at extinct volcanoes are believed to be triggered by seismic shaking and/or heavy rains with consequent increase of water pore pressure McGuire, 1996;Voight and Elsworth, 1997;Kerle et al, 2003). Since most volcanic regions are located in humid environments (e.g.…”

Changes in spatial point patterns of pioneer woody plants across a large tropical landslide

“…This landslide was triggered by an exceptional rainfall event (500 mm in 24 h) on 30 October 1998, during Hurricane Mitch, and exhibits an altitude range of 150-1350 m a.s.l., an area of 11.21 km 2 , and a complex geomorphology (Kerle et al, 2003). The landslide is surrounded by a highly heterogeneous landscape dominated by remnants of tropical dry forests within an agricultural matrix.…”