Volcanic Hazard Assessment and Long-Term Forecasting EAS 458 Volcanology Introduction . Volcanic hazards generally cannot be eliminated: eruptions cannot be prevented and it is not practical to permanently move large populations out of harms way. Volcanic hazards can be managed and the risk mitigated. Land use planning can limit settlement in the highest risk areas. Building codes can be modified to reduce the risk of structural collapse from ash or volcanic earthquakes. Structures can constructed to divert lahars, and in some cases, small pyroclastic and lava flows. Early warning systems can be implemented that allow limited use of high-risk areas. 1 Role of the Volcanologist . The first steps in this process are identification and assessment of volcanic hazards, and these tasks are primarily the responsibility of the volcanologists. In the subsequent efforts to reduce hazards, for example evacuation route planning, construction of diversion structures, or implementation of improved building codes, volcanologists play only an advisory role. Case History: Mt. Pelee, Martinique – How not to do Hazard Management . Picturesque Mt. Pelée, situated on the northern end of the Caribbean island of Martinique, killed some 28,000 people in 1902. Avoiding this kind of catastrophe is one of the goals of modern volcanology (which really got its start from Lacroix’s study of the volcano after the eruption). What happened in 1902 provides an excellent example of what not to do. 2 Mt. Pelée Volcanic History . 1851-1852: phreatic eruptions . No juvenile material . No casualties, damage restricted to uninhabited region near the vent . 1792 phreatic eruptions . No juvenile material . No casualties, damage restricted to uninhabited region near the vent . ~1600 AD . Pyroclastic flows in the direction of St. Pierre . St. Pierre founded in 1635 . Summit of Pelée unvegetated (hence the name – bald mountain) . ~1300 AD . Dome formation, pyroclastic flows . 23 magmatic eruptions during preceding 5000 years (judged from 14 C dates) Events Preceding the Eruption . 1889 - new fumaroles open in crater Grand Etang just below the summit . April, 1902 - fumarole activity intensified; odor noticed in St. Pierre, 6 km away . Phreatic explosions begin on April 23 . Ash falls on inhabited areas . Ash consisted entirely of lithic fragments, no juvenile material . First small earthquakes . Numerous submarine telegraph cables break, beginning on April 22 . Probably causes by submarine landslides . May 3 . Intensity of explosions increases, 1-5 cm ash falls on St. Pierre; > 5 cm on Le Precheur to the north; small amounts over entire island. Small earthquakes constant enough to occasionally produce constant shaking. Wide fluctuations in flow of Riviere Blanche that are unrelated to rainfall during April & early May. 3 First Casualties . Morning of May 5, large lahar flows down the Riviere Blanche, destroying the Guerin rum distillery and killing 23. Most likely caused by failure of east wall of Grand Etang, releasing the water in the crater lake into Riviere Blanche . 3 successive lahars in 3 minutes . Upon reaching the sea, the first lahar produced a tsunami of about 3 or 4m, pushing water 30 m inland at St. Pierre . No casualties and limited damage from the tsunami Magmatic Phase . Blue glow observed above the summit on May 5 . Probably combustion of magmatic gases . Incandescent blocks of lava observed on the summit on May 6 . Probably marks the emergence of a lava dome . Vulcanian explosions on the evening of May 6 . Dome becomes more apparent on the evening of May 7 . Phreatic explosions continue from southern flank . Produced continuing ash fall at Le Precheur and occasional ash fall at St. Pierre 4 First Pyroclastic Flows . May 7 . Soufriere of St. Vincent (2 islands south) erupts, pyroclastic flows kill 1600; blasts heard in St. Pierre . Evening of May 7 . “At 10:30…we saw a great cloud leave the summit and descend towards Fonds-Coré. But it stopped on mid- slope…then, almost at the same time, the same phenomenon was repeated, always at the same place.” . Morning of May 8 . Several large Vulcanian explosions that blasted ash over and beyond St. Pierre. Societal Background . What were people and the gov’t doing during all this? . Martinique was and is part of metropolitan France (full citizenship, vote in elections, representation in parliament, etc.) . St. Pierre, though not the capital, was the largest city on the island and the cultural and commercial center of the island. 1901 census lists a population of 26,011 . Mt. Pelée was recognized as a volcano (certainly after 1852), but no thorough geologic study had been undertaken and certainly no hazard assessment. Events of 1851-1852 produced a false sense of security since those explosions had done no real harm. People generally expected the same from the 1902 eruption – at least in the beginning. People were concerned about floods, earthquakes, tsunamis, and lava flows and were bothered by the ash flows and stench of sulfur, but the threat of pyroclastic flows and surges was not known or understood. 5 Government Response . Initially, the government did nothing . There were no scientists, much less geologists or volcanologists on the island, but the government did not request any kind of assistance from Paris. The Governor only notified Paris of volcanic activity on May 3. Once ash falls began, the governor made frequent visits to the area, arranged for aid and assisted those wanting to leave the most affected areas - such as Le Precheur. People in mountain and coastal villages were encouraged to take refuge in St. Pierre. 1000-2000 people from neighboring villages likely took refuge in St. Pierre . Roughly 1000 fled St. Pierre on their own initiative, mostly to Fort du France - sulfur stench, ash falls, and lahars finally managed to scare some. Mayor of St. Pierre requested 30 soldiers to keep order and help distribute aid. The governor sent them on the morning of May 8. They never arrived. Investigative Commission . The governor appointed a commission of informed and educated men to assess the situation in early May. The most informed member of this commission was the high school science teacher in St. Pierre. The commission published a report in the local newspaper on May 7 concluding that St. Pierre was safe. The governor, his wife, and all but one member of the commission perished in the eruption. (Another psychological factor was that Fort du France was considered unsafe because of the damage it suffered in an 1839 earthquake). 6 . St. Pierre was destroyed and all The Cataclysm but 2 inhabitants killed at 8:02 AM The Cataclysm (as judged from the loss of telegraph & telephone connection). Witnesses reported seeing an emerging eruption column, a flash of light, followed by the sound of a large explosion (the delay representing the slower travel of sound; blast was heard as far away as Venezuela), and a “glowing cloud” moving toward St. Pierre. The blast was supersonic and ultimately generated a hurricane force return wind. Another major eruption on May 20 completed destruction of the city (though there was essentially no one left to kill). What happened? Column Collapse or Directed Blast? . General, but not complete, consensus that what destroyed St. Pierre is better called a surge and than a pyroclastic flow. Deposit was thin, less than 1 m of material in much of the city (much less, depending on interpretation). Thickness decreases to south, but varies irregularly . Cross bedding abundant . Variable grading . Event was energetic enough to destroy almost all structures, including the cathedral. Victims burned; fires ignited in St. Pierre burned throughout the day . Nevertheless, 2 very different interpretations of what happened have been put forward. Rue Victor Hugo 7 Isopach map of Fisher (1982) Column Collapse Interpretation . Fisher and others (1982, 1983) believe that column collapse occurred within a few seconds of the eruption, the material overtopped the crater, and generated a pyroclastic flow down the Riviere Blanche. A low density, turbulent ash cloud surge then segregated from the high density, laminar pyroclastic flow and moved around and over topography toward St. Pierre. This interpretation is consistent with eyewitness reports of an eruption column preceding the flow and the generally agreed small size of the dome at the time. 8 May 8, 1902 deposits as mapped by Bourdier et al. (1989) . Lacroix (1904), Sparks (1983) and Bourdier et al. (1989) believe that Directed Blast the event was generated by an explosion of the admittedly small Interpretation dome (gas pressure simply exceeded its strength); i.e. a directed blast, Mt. St. Helens style (though much smaller and without a debris avalanche). Sparks (1983) considers it a high velocity pyroclastic flow (i.e., high concentration, laminar flow) . Bourdier et al. (1989) consider it a pyroclastic surge (low concentration, turbulent flow). This interpretation depends on eyewitness reports of a flash and explosion, on a different interpretation of stratigraphy, and on the continuity of stratigraphy from St. Pierre into Riviere Blanche. 9 Survivors . Often said that there were only 1 or 2 (or 3) survivors of the event. This is true only with respect to the actual city of St. Pierre. 151 people were hospitalized in Fort du France, of whom 111 recovered. Some of these were from the surrounding countryside - on the edge of the devastated area; most were rescued from ships in the harbor. Many others survived for a few hours; dozens others were rescued but died en route to Fort du France. Louis-August Sylbaris Aftermath . Though many were rescued from the harbor, there was no organized effort to hunt for survivors in St. Pierre and the surrounding countryside (at least until much later). With St. Pierre destroyed, commerce on the entire island was disrupted. Subsistence economy in the surrounding countryside effectively destroyed as ash, sulfur, and continuing eruptions made farming impossible.