Comprehensive Risk Assessment for Natural Hazards - Planat

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Comprehensive risk assessment for natural hazards 4.2.2 Indirect hazards One can distinguish three main types of indirect volcanic hazards. These are lahars, landslides and tsunamis. The first two are often triggered by explosive eruptions and so volcanologists tend to classify them as primary hazards, which is a matter of debate. (a) Lahars They correspond to a rapidly flowing sediment-laden mixture of rock debris and water. One can classify them according to their sediment content. Hyperconcentrated flows contain between 40 and 80 per cent by weight sediment and debris flows more than 80 per cent (Fisher and Smith, 1991). One can categorize these flows as debris flow, mud flow and granular flow (Figure 4.8). (b) Landslides Landslides are downslope movements of rocks which range in size from small movements of loose debris on the surface of a volcano to massive failures of the entire summit or flanks of a volcano. They include slumps, slides, subsidence block falls and debris avalanches. Volcaninc landslides are not always associated with eruptions; heavy rainfall or a large regional earthquake can trigger a landslide on steep slopes (Figure 4.9). (c) Tsunamis Tsunamis may be generated from volcanic activity when huge masses of water are suddenly displaced by an eruption or an associated landslide. The explosion of the Krakatoa volcano in 1883 provoked a tsunami that killed more than 34 000 people. The collapse of Mt Mayuyama in 1792 at the Unzen volcano in Japan generated a major tsunami that killed 15 000 people (Figure 4.10). 37 (d) Others These are other notable indirect hazards such as acid rain and ash in the atmosphere (Tilling, 1989). Their consequences lead to property damage and the destruction of vegetation and pose a threat to airplane traffic. 4.3 TECHNIQUES FOR VOLCANIC HAZARD ASSESSMENT Volcanic hazards may be evaluated through two main complementary approaches, which lead to their prediction (Scarpa and Tilling, 1996): • Medium- to long-term analysis; volcanic hazard mapping and modelling, volcanic hazard zoning. • Short term; human surveillance and instrumental monitoring of the volcano. 4.3.1 Medium- and long-term hazard assessment: zoning In most cases, one is able to characterize the overall activity of a volcano and its potential danger from field observations by mapping the various historical and prehistoric volcanic deposits. These deposits can, in turn, be interpreted in terms of eruptive phenomena, usually by analogy with visually observed eruptions. It is then possible to evaluate characteristic parameters such as explosivity, using the volcanic explosivity index (VEI) listed in Table 4.2 (Newhall and Self,1982),intensity,magnitude and duration.This allows the reconstruction of events and their quantification in terms of, for example, plume elevation, volume of magma emitted and dispersion of the volcanic products. This activity is illustrated within Figure 4.1. Table 4.2 — Volcanic Explosivity Index (modified after Smithsonian Institution/SEAN, 1989) VEI General Volume of Cloud column Qualitative description tephra (m 3 ) height (km) description Classification Historic eruptions up to 1985 0 Non-explosive
38 Each parameter may then be sorted, classified and compared with a scale of values, permitting the zoning of each volcanic hazard. This can be drawn according to: the intensity, for example the thickness or extent of the hazard, such as shown in Figure 4.11; the frequency of occurrence; or their combination. As a general rule, the intensity of volcanic phenomena decreases with the distance from the eruptive centre — crater or fissure. Topographic or meteorological factors may modify the progression of the phenomenon, such as the diversion of lava flow by morphology. Basically the delineation of areas or zones, previously threatened by direct and indirect effects of volcanic eruptions, is the fundamental tool to estimate the potential danger from a future eruption. To fully use zoning maps, it is important to be familiar with the concept of zone boundaries and assessment scales. (a) Zoning boundaries Boundaries are drawn on the volcanic zoning maps using expert judgement based on physical conditions and the estimate of the nature (explosivity, intensity) of the future volcanic activity. They are drawn to one of two scales related to the phenomenon’s intensity and frequency of occurrence, where frequency is estimated from data and geological surveys and anecdotal evidence. (b) Zoning scales In vulcanological practice, each hazard may be zoned according to two assessment scales. The first is the frequency scale and is comprised of four levels. The levels are: • annual frequency → permanent hazard • decennial frequency → very high hazard • centennial frequency → high hazard • millennial frequency → low hazard The second is the intensity scale which would cover aspects such as lava flow extension and thickness of cinders. This scale is also comprised of four levels. They are: • very high intensity → total destruction of population, settlements, and vegetation • high intensity → settlement, buildings partially destroyed → important danger for the population • moderate intensity → partial damage of structures → population partly exposed • low intensity → no real danger for population → damage for agriculture → abrasion, corrosion of machinery, tools,... The establishment of hazard maps is a fundamental requirement but not sufficient for appropriate mitigation action, as their information has to be interpreted in Chapter 4 — Volcanic hazards conjunction with the vulnerability of the exposed environment in a broad sense. 4.3.2 Short-term hazard assessment: monitoring Geophysical and geochemical observations are used to decipher the ongoing dynamics of a volcanic system. The main goal is to develop an understanding sufficient to allow the forecasting of an eruption. To achieve this, a volcano must be monitored in different ways: • In real- or near-real-time through physical or chemical sensors located on the volcano and connected to an observatory by telecommunication lines such as cable telephone, cellular phones and standard UHF or VHF radio via terrestrial or satellite links. • In the field where operators regularly make direct observations and/or measurements. • By laboratory analysis of freshly expelled volcanic material collected in the field. The different tools commonly employed are documented by Ewert and Swanson (1992). They are described below. (a) Seismic activity Volcano-seismic observations allow a major contribution to be made to the prediction of a volcanic eruption. This domain is complex, not only because the seismic sources of the volcanoes imply the dynamics of fluids (liquids and gases) and of solids, but also because the wave propagation evolves in an heterogeneous and anisotropic medium that can be very absorbent. In addition, the topography can be very irregular. Volcano-seismic monitoring records events in an analogue or digital manner. In a first step they are interpreted in terms of source location, of strength (magnitude) and of frequency of occurrence. The location and the magnitude are determined with standard computer programs such as Hypo71 (Lee and Lahr, 1975) or SEISAN (Havskov, 1995). In a complementary step, one tries to infer what had initially generated the vibration, which is termed the source type (tectonic, internal fluid movements, explosions,lava or debris flow,rock avalanches).To facilitate this, one can use a classification based on the shape (amplitude and duration) and frequency content. It is possible to distinguish within the seismic activity, transitory signals, the standard earthquake and/or nearly stationary ones referred to as tremors. The observed events may have frequencies usually ranging from 0.1 to 20 Hz. In the classification of the events based on their frequency content, two broad classes are defined based on the source processes (see for example Chouet, 1996). The first class has earthquakes with high frequencies, and the second has those with low frequencies (often called long period, LP). Those in the first class are produced by a rupture of brittle rocks and are known as tectonic events. If they are directly associated with the dynamics of the volcano, they are defined as volcanotectonic events. The low frequency, volcanic earthquakes and tremors are associated with fluid movement and range in value between 1 to 5 Hz. Observations show that many of
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Comprehensive Risk Assessment for Natural Hazards - Planat

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