The volcanic hazard assessment involves establishing a stratigraphic record of the products of past eruptions and determining the aerial extent of deposits, their origin in the stratigraphic sequence, and the date of the eruptions. To accomplish this, information that exists in the historic record usually must be supplemented with field analysis. Once the stratigraphic sequence is determined, the deposits are classified as to the type of hazard tephra, pyroclastic flow, lava flow, etc. The resulting products are maps and reports which depict the volcanic hazards of an area.
Finally, the volcanic hazard can be graded in terms of severity on a volcanic hazard zonation map. Volcanic Hazard Zonation Map The Sourcebook for Volcanic Hazards Zonation provides an excellent discussion of preparing hazard zonation maps: Volcanic hazards zonation maps have two primary purposes, namely, for long-range planning for uses of land around volcanoes that are thought to be compatible with the hazard from future eruptions, and for determining which areas should be evacuated and avoided during eruptions.
Maps prepared for these two purposes have similarities as well as differences. A hazards-zonation map and an accompanying report designed to guide land-use planning could include estimates of the frequency of events anticipated in the future. Such reports could also include quantitative or other estimates of relative degrees of hazard. In contrast, a zonation map prepared chiefly for the purpose of evacuation could subdivide kinds of hazard, so that people could be removed selectively from different areas according to whether the eruption was expected to produce lava flows, airfall deposits, pyroclastic flows, lahars or combinations of these.
Maps such as these could also be divided into zones based on the anticipated scales of future eruptions, or into sectors determined by which flank of a volcano, or which valley system, might be affected most often by eruptions. The expectable high cost and social disruption caused by evacuation might be reduced by the use of such maps. Both kinds of uses of hazards-zonation maps should be considered during their preparation; both kinds of maps can be prepared from the same basic data, and in some cases one map could be prepared that would serve both purposes Crandall, Examples of volcanic hazard zonation maps used for development planning purposes are shown in Figures and Suggestions for preparing hazard zonation maps for specific volcanic hazards together with numerous examples are available from the UNESCO Sourcebook.
Mitigation of Volcanic Hazards Development-related aspects of volcanic hazard mitigation-reducing the potential loss of life and property damage that can be caused by a volcanic eruption-primarily involve hazard assessments and land-use planning. Other mitigation procedures such as the establishment of monitoring and warning systems, emergency evacuation measures, protective measures, insurance programs, and relief and rehabilitation measures are not treated in this chapter.
Many of these activities are associated with preparedness, which is another phase of hazard management see Chapter 1. Volcanoes which present a short-term hazard and which clearly threaten life or property should be kept under surveillance, and restrictions should be placed on permanent habitation in the areas of greatest hazard. For volcanoes that have long-term periodicity and therefore may or may not pose a hazard during the lifetime of a project, land-use restrictions may not be warranted on purely economic grounds, but development should be planned with a knowledge of the potential consequences of future eruptions.
Obviously, an imminent eruption requires constant monitoring and vigilance and the taking of suitable measures to cope with the impending event. Phase I - Development Diagnosis c.
- Earthquake Hazards.
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Phase II - Development strategy and project formulation. Compared with earthquakes, volcanic hazards are simpler to cope with in development planning because of their point source, the limited extent of the area in which active volcanoes occur, and the limited distance from the source for which volcanic activity poses a serious hazard. The process involves addressing key questions listed in the box above.
Source: Adapted from Crandall, D. Preliminary Mission During the preliminary mission of an integrated development planning study, an initial review is made of the information available. At this time the first two questions shown in the box above can be answered with an acceptable degree of reliability by undertaking an initial volcanic hazard assessment. The procedure, outlined in the box on page 59, uses information from the Preliminary Neotectonic Map of South America and Figure 11 , which is a listing of volcanoes active in the Holocene period, their periodicity, and other summary information on each.
When necessary, local information can supplement this. No specialized expert is needed for this task. Phase I - Development Diagnosis Phase I of a development study requires a diagnosis of a region's development potential. The results of the initial volcanic hazard assessment will lead to different information needs if a volcano in the study area is identified as an imminent, short-term, or long-term threat. If an eruption is determined by geologic evidence to be imminent, mitigation actions must take precedence over all other activities.
The statement appears too self-evident to merit mention, yet surprisingly the principle is not always heeded. For example, the Nevado del Ruiz gave clear signs of the approach of a major eruption in November , one year before the eruption that killed 23, people Tomblin, The moment an eruption appears imminent, full-scale monitoring of the volcano must be initiated if it is not already being done. Warning and evacuation systems must be established.
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Large reservoirs in the path of potential lahars should be either drained or lowered sufficiently to serve as a trap rather than a lubricant to moving mud and water. People occupying the flanks of the volcano should be relocated. Planners can play a role in seeking suitable relocation sites and helping to define relocation mechanisms. Areas adjacent to the volcano that are vulnerable to any specific hazard, particularly lahars and pyroclastic phenomena, should be delineated-at first simply by topographic considerations-and suitable precautions taken.
In short, if an eruption is found to be imminent in an area, the planner's focus abruptly changes from the future to the immediate present. When a short-term volcanic hazard is identified, additional information is needed. Additional information on individual volcanoes can be found in the resources listed in the box on page These sources may be supplemented by more detailed local data such as maps and studies of specific volcanic hazards or historical event and damage assessment studies. Additional information can be inferred from geologic, tectonic, and seismic maps, particularly maps of Holocene or Quaternary geology.
Data on wind prevalent direction and speed are relevant to evaluating tephra hazard. Topography and interpretative soil studies are important for the evaluation of tephra, lava flow, pyroclastic flow, and lahar hazards. The location of reservoirs and other major sources of water that can cause flooding or contribute to the movement of lahars is particularly important data for volcanic hazard mitigation. Information on elements at risk is the same as for earthquake hazards. In some areas with severe volcanic hazards, maps of volcanic hazards, risks, and volcanic hazard land-use zonation are available.
Sources of information include national geologic agencies, national and international volcanic and hazard data centers, national disaster mitigation agencies, universities, and research centers. Volcanoes posing a short-term hazard can be plotted on , to ,scale topographic maps. Local information commonly exists for volcanoes in this category, and some hazard mitigation program may already have been instituted.
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In this case, the task of the planner is to promote land uses and protective measures commensurate with the degree of risk of any area. Sources of information for name of volcano, location, periodicity, location, date of last eruption, effects, and volcanic hazards: Simkin, T. Volcanoes of the World.
Global Volcanism Network. Washington D. Volcanoes with short-term periodicity are presented in capital letters.
Date of last eruption is simplified from Volcanoes of the World using three categories: 1 "Historic" - the actual eruption date is given, sometimes qualified by "? Fatalities caused by one or more eruptions. Destruction of agricultural land or other property damage caused by one or more eruptions. One or more eruptions were explosive. Phreatic explosion was associated with one or more eruptions. Lava flow, lava domes, or spines were associated with one or more eruptions. Destructive mudflows were associated with one or more eruptions. The VEI combines total volume of products, eruptive cloud height, duration of eruption, tropospheric injection, stratospheric injection, and some descriptive terms to yield a index of increasing explosivity as follows: 0 nonexplosive, 1 small, 2 moderate, 3 moderately large, 4 large, 5 very large, cataclysmic.
Preliminary Neotectonic Map of South America. Eustatius, St. Saint Lucia. For all other countries in Latin America and the Caribbean, volcanic hazards are not a significant concern. If "no," volcanic hazards are not a significant concern in the study area. If "yes," go to Step 2. STEP 2: Determine if any part of the study area includes or lies within 30km of any of the volcanoes listed in Figure K "no," volcanic hazards are not a significant concern in the study area.
If "yes," go to step 3. STEP 3 : Using Figure , classify the eruption periodicity of each volcano in the study area as short-term or long-term. Volcanoes with short-term periodicity are shown in capitals. If the volcano is classified as short-term, go to Step 4. STEP 4 : For short-term volcanoes, determine from local authorities if there is any geologic evidence that an eruption is imminent, and if hazard zonation maps have been prepared.
NOTE: The 30km distance is arbitrary, based on the distance from a volcano that lahars, ash, pyroclastic flows, etc. The radius may be shorter or longer depending on such factors as the difference in elevation between the volcano and threatened areas, slope, channel morphology, and prevailing winds. If a hazard zonation map does not exit, one should be prepared as part of the development planning study and should become an integral part of the integrated natural resource inventory.
In this case it will be necessary to obtain the services of a volcanic hazard expert. Having completed the preliminary hazard work during the preliminary mission, the planner will be prepared to draft precise terms of reference for the specialist. With the results of additional studies, the planner can identify potential mitigation measures, comparing costs and potential benefits with all the other elements involved in the development of the study area.
If long-term volcanic hazards are determined to occur in the study area, incorporating hazard considerations into a development study can offer additional benefits. Long-term hazards are often ignored in spite of the fact that surprising eruptions from volcanoes considered dormant or inactive are responsible for tremendous damage. If no local information exists, a difficult decision will be required as to whether the preparation of a volcanic hazard zonation map is justified.
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A volcanic hazard expert can advise on the degree of risk and, commensurately, the effort that should be devoted to additional studies and mitigation measures. Phase II - Development strategy and project formulation In developing areas with short-term volcanic hazards, mitigation measures should be selected if they are not already part of the project identification information.
Land-use restrictions should be instituted for areas having a potential threat of pyroclastic phenomena. In areas where volcanic ash can constitute a hazard, building codes should stipulate appropriate roof construction. In many cases only lahars would merit mitigation measures.
Valley areas in the path of potential lahars could be delineated and land-use restrictions or protective measures in keeping with economic rationality could be instituted. The mitigation measures that can be justified economically for these short-term hazards are limited, since "short-term" is still a lengthy period of time. Awareness of the potential danger may permit a more reasoned development plan. International Association of Volcanology ed. Tsunami Hazards and Their Assessment and Mitigation 2.
Tsunamis and the Development Planning Process. Tsunamis are water waves or seismic sea waves caused by large-scale sudden movement of the sea floor, due usually to earthquakes and on rare occasions to landslides, volcanic eruptions, or man-made explosions. Tsunami Hazards and Their Assessment and Mitigation.