FLANK COLLAPSE AND NEW RELATIVE INSTABILITY ANALYSIS TECHNIQUES APPLIED TO VOLCÁN DE COLIMA AND MT. ST. HELENS

A recently developed technique of analysis applied to stratovolcanoes by Borselli et al. (2011), offers new insights for assessment of degree of instability of flank collapse of volcanic edifices.

The new technique combines three methodologies: 1) the analysis of relative mass/volume deficit in the volcano structure, made using the new VOLCANOFIT 2.0 software (Borselli et al.2011); 2) slope stability by limit equilibrium analysis (LEM) of multiple sectors on the volcano using SSAP 4.0 (Slope Stability Analysis Software, Borselli 2011) which considers fluid internal overpressure and dissipation (Borselli et al. 2011 and rock mass strength criteria (Hoek et al. 2002) ; 3) statistical analysis of major flank debris avalanche ages in the last 10,000 BP, made by stochastic arithmetic methods (Vignes, 1993).

These techniques applied to Volcán de Colima point to the SW quadrant as potentially the most unstable sector of the edifice under a wide range of scenarios. The recurrence interval of major collapse events during the last 10,000 years, calculated here using a stochastic approach, yielding a mean recurrence interval of 2698 yrs, with an uncertainty range of 180 yrs. The analysis point out an increased probability of flank collapse in the interval between -110 yrs and +345 yrs from the present. This generates a series of scenarios ranging from optimistic, considering a collapse within the next 345 years, to pessimistic, derived from the 110-year delay.

The VOLCANOFIT application to Mt. St. Helens pre-eruption1980 DEM shows the distribution of local mass deficit/surplus that is easily correlated with the 1980 incipient flank collapse process.

The proposed new approach may be applied to any stratovolcano with a potential for flank collapse forfuture hazard assessment.