Volcano Hazards Mitigation -- A Complicated Business

Faced with the daunting task of trying to unravel the tremendous variety and complexity of volcanic processes, especially during a rapidly evolving volcano crisis with lives and property at risk, today’s volcanologist can ill afford to rely on any one specialty to furnish the necessary tools and information. Instead, diverse techniques from geology, hydrology, seismology, geodesy, geochemistry, and related fields must be brought to bear and, if necessary, tailored to the situation at hand.

Experience worldwide has shown that no single technique can provide unambiguous information about the cause and likely outcome of volcanic unrest. For example, earthquakes near a volcano might indicate fracturing of brittle rock around a pressurizing magma body, intrusion of magma or other fluid into brittle host rock, or release of tectonic strain not directly related to magmatic processes. Even the occurrence of long-period (LP) earthquakes that are commonly associated with magmatic or hydrothermal processes is inherently ambiguous. Such earthquakes are generally thought to indicate the presence of a two-phase fluid in a resonating crack (Chouet, 1988, 1996a, 2003). However, this fluid could be gas exsolved from fresh magma that is rising and decompressing, hypersaline brine derived from stagnant magma that is cooling and crystallizing, or gas-charged magma itself. Without complementary data from other monitoring techniques to help resolve this ambiguity, the precise significance of LP earthquakes for volcano hazards is difficult to discern, especially during a crisis. Furthermore, some models for LP earthquakes require a relatively narrow range of conditions for resonance to occur, suggesting that some (most?) fluid transport occurs without generating LP earthquakes. Even when they do occur, swarms of shallow LP earthquakes could be caused by pressurization of a magma body as a result of intrusion, boiling within a hydrothermal system in response to magmatic or tectonic changes, or migration of fluids released from a stagnant, cooling magma body. The hazards implications vary widely and can be explored further only by combining seismic information with that from other disciplines.

By itself, any single volcano-monitoring technique provides information that is ambiguous or subject to alternative explanations. For example, an increase in the flux of sulfur dioxide gas might indicate the recent arrival at shallow depth of fresh, gas-charged magma, but the absence of such an increase does not preclude the same interpretation. This is because SO₂ is very soluble in water and can be effectively ‘scrubbed’ by groundwater (Doukas and Gerlach, 1995). Carbon dioxide is less soluble in water than SO₂ , so an increase in CO₂ followed by an increase in SO₂ might be attributed to newly emplaced magma that released both CO₂ and SO₂ and eventually dried out a conduit to the surface, allowing SO₂ to escape. For confirmation, though, the volcanologist is likely to reach for additional tools from seismology and geodesy.

No amount of monitoring is likely to provide definitive information about the explosivity of newly emplaced magma, and thus about the extent or degree of hazards posed by an impending eruption. For this, the volcanologist relies on the tools of field geology and stratigraphy, under the assumption that the volcano will likely behave in a manner similar to the ways it has behaved in the past. Past performances are chronicled in the stratigraphic record of past eruptive products, but the record is always incomplete and impossible to read unequivocally. In some cases, past eruptive behavior can be highly variable and therefore is a poor guide to the next event. Moreover, there is always the possibility of an unusual or unprecedented event, such as the catastrophic landslide and lateral blast at Mount St. Helens in 1980. In hindsight, similar events had occurred before but the evidence lay buried until the landslide and subsequent erosion cut deeply into the volcano’s north flank (Hausback and Swanson, 1990). Only by combining information from a variety of sources can scientists hope to decipher the cause of volcanic unrest, and thus provide useful information to public officials charged with the difficult task of reducing risk while minimizing social and economic disruption.

source: Volcano Deformation by D. Dzurisin