Geologists create volcanic hazard maps using scientific data to portray potential future geological events; the end users are principally public safety officials. Typical maps use a few simple polygons to outline areas of potential inundation or cover by a few categories of flows based on past frequency and size. Uncertainties in data regarding flow characteristics complicate the construction of accurate hazard maps. Generally, there are inadequate exposures of good sections, poorly known extents of units, and imprecise volumes for deposits. Crisis conditions limit the time available for field and laboratory work. Computer models can simulate possible scenarios, but the volumes, styles of emplacement, and source starting locations are poorly known in many cases. The large uncertainty in initial conditions is seldom taken into account in the construction of hazard maps, and these uncertainties are rarely passed on to the end users of the maps. TITAN2D is a computational model for volcanic block-and-ash flows and rock avalanches of various types and scales, and it forms the core of the TITAN toolkit for volcanic hazard analysis, which can integrate high-performance computing, database management, and visualization to a very sophisticated level. TITAN provides a solution to mapping problems by providing a probabilistic calculation of inundation depth that takes into account many of the critical uncertainties.

Studies of the type, extent, and volume of Holocene pyroclastic and lahar deposits have concluded that future eruptions of El Misti volcano, even if moderate in magnitude, will pose a serious threat to the city of Arequipa, Peru. After describing the most probable volcanic scenarios at El Misti, this paper concentrates on lahar and flood risk assessment. Scenarios were derived with the help of the simulation codes LAHARZ and TITAN2D. The lahar risk assessment varies significantly depending on the method selected. LAHARZ simulations indicate that a considerable part of the urban areas and infrastructure could be severely affected. Losses due to impacts inflicted by lahars in three selected parts of the urban area are estimated to be in the order of 40–100 million U.S. dollars. In the case of TITAN2D, the resulting lahar-affected area only includes infrastructure assets mainly located along the Río Chili. Results indicate that although simulation codes could be useful tools in the analysis of lahar hazard scenarios, it is still premature to regard them as accurate sources of information for actual decision making related to risk mitigation at the local level. More research is required to further adjust simulation codes and refine risk scenarios. The first priority for the mitigation of the volcanic hazard faced by the city of Arequipa should be improvement of the risk map (a hazard map has already been drawn and is under scrutiny) and the preparation of contingency plans.

A new numerical code for simulating flows of granular material, TITAN2D, is used to model the Merapi-type block and ash flows resulting from the 1991 eruption of Colima Volcano, México. The 1991 block and ash flows reached distances of up to 4 km from the vent with a total estimated volume of 8 × 10 5 m 3 . The block and ash flows were modeled using a digital elevation model (DEM) of the region and compared to field data using a quantitative center-line comparison method. Input parameter values, which dictate flow dynamics, were varied to demonstrate the effect these parameters have on the program output. Analysis showed that the TITAN2D model performs best in replicating the center lines of the 1991 Colima block and ash flows when using an initial volume value representative of a single flow event rather than a total deposit volume.

Abstract The relationship between valley morphology and ash-cloud surge development for 12 pyroclastic density currents (PDCs) at Soufrière Hills Volcano (SHV), Montserrat is investigated. Channel slope, sinuosity and cross-sectional area were measured from high-resolution digital elevation models (DEMs) using geographical information system (GIS) software; and were compared to geometric parameters of the deposits. The data illustrate three surge-generation regimes: a proximal area of rapid expansion; a medial deflation zone; and a steadier distal surge ‘fringe’. The extent to which these regimes develop varies with flow volume. For larger flows, within the proximal and medial regimes, a strong inverse correlation exists between surge detachment and valley cross-sectional area. Surge detachment is also correlated with observed and modelled flow velocities. Areas of topography-induced increases in velocity are interpreted to result in more pervasive fragmentation and fluidization, and thus enhanced surge generation. Distally, surge deposits appear as fringes with decaying extents, indicative of more passive expansion and decreasing velocity. The results indicate that surge mobility and detachment are a complex product of flow mass flux and topography, and that future efforts to model dense–dilute coupled flows will need to account for and integrate several mechanisms acting on different parts of the flow.

Abstract Volcanic hazard and risk at Soufrière Hills Volcano (SHV), Montserrat has been assessed in a consistent and quantitative manner for 14 years (1997–2011) during highly variable eruptive activity involving andesitic lava-dome growth, which has placed serious constraints on Montserratian society. This work has been carried out by the Scientific Advisory Committee (and predecessors) in collaboration with the Montserrat Volcano Observatory. We describe the organizational context of these assessments, the types of hazards and the methods used to analyse them. Knowledge elicitation using hazard scenarios and analysis by the Classical Model method were employed to formulate probabilistic forecasts of future hazardous events over the next year, and to quantify risks to individuals and Montserrat society generally. We devised a scheme for assessing the likelihood that the volcanic system had stopped receiving basalt magma, considered to be the main driver of the eruption. The accuracy of forecasts was tested using Brier Skill Scores: 83% of forecasts for events that were critical to life had positive skill, as measured by this method. We also discuss how government responded to our assessments. The continuous series of quantitative volcanic hazard and risk assessments described here is the only one of its kind.

Abstract The probability of inundation of vulnerable areas by pyroclastic flows and surges generated by lava dome collapse at Soufriere Hills Volcano, Montserrat, is assessed. The runouts of the flows are simulated by two coupled one-dimensional models, the first representing the valley-following block-and-ash flows (avalanches) and the second the surges orthogonal to them. An ensemble of simulations is developed that honours the observed runout–frequency relationship of the flows and the likelihood of flows originating from a particular part of the dome. The former relationship is based on the observed eruption history from 1995 to 1998 and the second from 1995 to 2003. The avalanche component of the models is parameterized using a random selection from a library of triads of friction coefficients that produce simulation fits to field-observed flow deposits at 1 km runout increments.

Abstract The 1995–present eruption of Soufrière Hills Volcano on Montserrat has produced over a cubic kilometre of andesitic magma, creating a series of lava domes that were successively destroyed, with much of their mass deposited in the sea. There have been five phases of lava extrusion to form these lava domes: November 1995–March 1998; November 1999–July 2003; August 2005–April 2007; July 2008–January 2009; and October 2009–February 2010. It has been one of the most intensively studied volcanoes in the world during this time, and there are long instrumental and observational datasets. From these have sprung major new insights concerning: the cyclicity of magma transport; low-frequency earthquakes associated with conduit magma flow; the dynamics of lateral blasts and Vulcanian explosions; the role that basalt–andesite magma mingling in the mid-crust has in powering the eruption; identification using seismic tomography of the uppermost magma reservoir at a depth of 5.5 > 7.5 km; and many others. Parallel to the research effort, there has been a consistent programme of quantitative risk assessment since 1997 that has both pioneered new methods and provided a solid evidential source for the civil authority to use in mitigating the risks to the people of Montserrat.