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. . 25,000 refugees, with some 15,000 in Fort du France.

10 Government Response

. Paris did send a scientific team, led by Alfred Lacroix, which arrived in late June. . Refugees initially provided with subsistence allowance, but little organization of housing. . Unloading and distribution of international donations of food and supplies very badly organized . In at least one case, custom officials tried to charge duty on donated supplies.

Critical Failures

. Lacroix left in early August, concluding that the worst was over. . Gov’t ordered refugees back to their homes in early August.

Fonds Coré before and after

11 August 30 Eruption

. Little activity between mid-July and mid-August . Mid-August . Nuée down the Riviere Blanche . New dome growth . Large explosive eruptions beginning August 24 . Delegation from Morne Rouge requests evacuation on August 26; request refused by new governor . On August 30, another blast, less powerful but more widely distributed, kills >1000 in Morne Rouge - many of whom had recently returned.

. St. Pierre was eventually resettled and partly Epilogue rebuilt. . By 1910, it had 500 residents. . By 1923, it had a population of 5000 . Residents fled during the eruption of 1929-1932, but St. Pierre was not damaged. . Today, it is a sleepy town of 5000 (Fort du France grew from 22,000 in 1900 to >100,000 in 2000).

12 Lessons

. Full volcanic history needs to be determined - not just one or two previous eruptions . All threats need to be evaluated . Must be done by competent volcanologists . Need for continued monitoring and evaluation of the state of the eruption . Period of quiescence does not mean an eruption is over . Evacuation and emergency response planning needs to be carried out ahead of time . Including evacuation routes . Supply routes . Search and rescue plans . Sheltering and sustenance of refugees

2500 years of Mt. Pelée Eruptions

Pumaceous ash . Modern hazard Cloud deposit Pyro. flow assessment studies (e.g., Westercamp and Traineau, 1983) have delineated the typical kinds of eruptions of Mt. Fine-grained Pelée and their Surge & blast frequency. frequency. fall deposits . These include plinian (isopachs) eruptions, pyroclastic flows, directed blasts, and, in the more distant past, sector collapse.

13 Monitoring Mt. Pelée

. Lacroix established a volcano observatory in 1902. . This functioned until 1925 - after 20 years of quiescence, its was decided to shut down the observatory. . 4 years later (in 1929), Mt Pelée erupted again. . This led to the establishment of the present observatory. . Facilities were upgraded following the eruption of Soufriere of Guadaloupe (neighboring island, also French) in 1976. . Today, the Mt. Pelée Volcanological Observatory (http://volcano.ipgp.jussieu.fr:8080/martinique/stationmar .html) is a modern facility that operates telemetered seismometers, tiltmeters, magnetometers, and cameras spread around the mountain.

Instrumentation on Mt. Pelée

14 Long-term Forecasting and Repose Times . Once the eruptive history of a volcano is known, and provided the volcano has been reasonably active in recent past (historic + time accessible to 14C dating), it is possible make statistical inferences about future activity. . To assess eruption probability, Wickman (1966) proposed a ‘survivor function’, ƒ, defined as: ƒ(t) = N.d(t)/No. . Where N.d(t) is number of observed reposes that lasted longer than time t, and No. is the total number of observed reposes. . ƒ(t) is then the probability that a repose has not ended after time t; i.e., the probability of the volcano not erupting.

Wickman Diagram

. In a Wickman diagram, the log of N.d(t) (number

of observed Log N.d(t) reposes lasting longer than t) is

plotted against Length of repose, t the length of repose (t).

15 Simple Wickman Diagrams . Popocatepetl . Linear relationship indicates repose time is random . Probability of eruption is constant (I.e.., repose times of 1 and 100 yrs equally likely). . Hekla (Iceland) . Compared to Popocatepetl, Hekla shows enhanced probability of repose periods lasting 20-40 years. . This is interpreted as a “loading time” - time required to refill the magma system after an eruption.

Mt. Pelée Wickman Diagram . Mt. Pelée exhibits more complex patterns. This diagram shows all eruptions over the last 5000 yrs . Until 1 century has past, eruption probability is high and constant . After 1 century, probability drops and remains low for 2 centuries. . Indicates a pattern of alternating periods of intermittent eruption over a century or so followed by ~2 centuries of repose.

16 Mt. Pelée -Pyroclastic Flows

. Adjacent figure is a Wickman diagram for only those Mt. Pelée eruptions over the last 5000 years that produced pyroclastic flows . It similar to the preceding diagram, but emphasizes the pattern of eruption periods lasting ~125 years interspersed with ~200 year repose times. . Implication for the future: . Probability of renewed activity on Mt. Pelée remains high through ~2055 . After 2055, ~2 centuries of repose can be expected.

Identifying High Risk Volcanoes

. Some 1500 volcanoes are known to have erupted in past 10,000 yrs. It is too large a task to provide thorough hazard assessment of all. The approach, therefore, is to identify those presenting highest risk and focus efforts on those. . Both Mt. Pelée in Martinique and El Chichon in Mexico are examples of highly hazardous volcanoes that were not recognized as such until after deadly eruptions.

17 High Risk Volcanoes - Yokoyama Rating System . To provide an objective means of determining which volcanoes represented the greatest risk, and were therefore the most appropriate targets of detailed hazard assessment, Yokoyama et al. (1984) developed a rating system. The system was designed to assess both hazard and risk factors. . Hazard and risk factors . Type of past activity . Age of last major explosive activity . Size of area affected during past activity . Occurrence of seismic activity or deformation . Size of population at risk and past fatalities.

Yokoyama Rating System- Hazard Factors Value Hazard Factor 1 High silica content of eruptive product 2 Major explosive activity within 5000 yrs 3 Major explosive activity within 500 yrs 4 Pyroclastic flows within 500 yrs 5 Mudflows within 500 years 6 Destructive tsunami within 500 years 7 Area of destruction within 5000 yrs >10km2 8 Area of destruction within 5000 yrs >100km2 9 Frequent volcano-seismic swarms 10 Significant ground deformation within 50 yrs

18 Yokoyama Rating System- Risk Factors Value Risk Factor 1 Population at risk > 100 2 Population at risk > 1000 3 Population at risk > 10,000 4 Population at risk > 100,000 5 Population at risk > 1,000,000 6 Historic fatalities 7 Historic evacuations

Shortcomings

. Prehistoric activity poorly known . Unprecedented events can occur . Some volcanoes may not have been identified . Long repose time, especially for very explosive volcanoes . Low priority volcanoes may be ignored . Examples: neither Nevado del Ruiz nor Pinatubo made the list

19 USGS NVEWS Assessment

. Hazard Factors: . Series of factors for which the value can be 0 or 1 (except recurrence, which can be 0-4) . Idea is to have many factors, so a missing or wrong one is not critical . Exposure (or Risk) Factors . Mostly also 0 or 1 values . Relative Threat Ranking = Hazard x Risk . Current monitoring levels: score of 0-4 . 0: no ground based monitoring, 1: minimal, 2: limited, 3: basic real time, 4: well monitored

A Well Monitored Volcano . Seismic: 12-20 stations within 20 km of vent. Borehole instruments where practicable. . Deformation: Routine surveys along with sufficient continuous stations (GPS, tiltmeters, and/or borehole dilatometers). . Gas: Frequent airborne or campaign gas measurements. Arrays of continuous sensors. . Hydrologic: Comprehensive database on temperatures and chemistry of springs and fumaroles, real-time monitoring of hill-slope soil moisture, stream discharge, etc., as appropriate. AFM systems for lahar detection where warranted. . Remote sensing: Regular processing and review of satellite images, high-resolution thermal-infrared images and frequent, high resolution, multi-channel visible images. Where practicable, continuous ground-based thermal imaging and Doppler radar coverage.

20 A Minimally Monitored Volcano

. Seismic: . Volcano lies within a regional network; no near-field stations are in place but at least one station is within 50 km of the volcano. Or, a single near-field station is present, but no regional network exists. . Remote Sensing: . Baseline inventory exists of Landsat-class satellite images. Routine scans for eruption clouds are conducted by meteorological agencies.

USGS Hazard Factors Hazard Factors Scoring Ranges Volcano type 0 or 1 Maximum Volcanic Explosivity Index 0 to 3 Explosive activity in past 500 years? 0 or 1 Major explosive activity in past 5000 years? 0 or 1 Eruption recurrence 0 to 4 Holocene pyroclastic flows? 0 or 1 Holocene lahars? 0 or 1 Holocene lava flow? 0 or 1 Hydrothermal explosion potential? 0 or 1 Holocene tsunami? 0 or 1 Sector collapse potential? 0 or 1 Primary lahar source? 0 or 1 Observed seismic activity 0 or 1 Observed ground deformation 0 or 1 Observed fumarolic or magmatic degassing 0 or 1

21 USGS Exposure (Risk) Factors

Log10 of Volcano Population Index (VPI) at 30 km 0 to 5.4

Log10 of approximate population downstream or 0 to 5.1 downslope Historical fatalities? 0 or 1 Historical evacuations? 0 or 1 Local aviation exposure 0 to 2 Regional aviation exposure 0 to 5.15 Power infrastructure 0 or 1 Transportation infrastructure 0 or 1 Major development or sensitive areas 0 or 1 Volcano is a significant part of a populated island 0 or 1

Assessment Results

. 18 very high threat volcanoes, 3 of which are well monitored . 37 high threat volcanoes, 5 of which have no monitoring (4 in Marianas, 1 in Alaska), and 4 of which (including Clear Lake and Mono Craters in CA) have minimal monitoring.

22 Priorities for Increased Monitoring

Aviation- Threat Required Current Score Threat Monitoring Monitoring Monitoring Volcano State Score Level Level Gap Kilauea HI 48 324 4 4 Eruption St. Helens WA 56 267 4 4 Eruption Rainier WA 35 244 4 2 2 Hood OR 28 213 4 2 2 Shasta CA 37 210 4 2 2 South Sister OR 28 194 4 2 2 Lassen CA 31 186 4 2 2 Mauna Loa HI 4 170 4 3 Unrest Redoubt AK 44 164 4 3 1 Crater Lake OR 35 161 4 1 3 Baker WA 14 156 4 2 2 Glacier Peak WA 35 155 4 1 3 Makushin AK 34 152 4 3 1 Akutan AK 42 140 4 3 1 Spurr AK 44 130 4 3 Unrest Long Valley Caldera CA 29 128 4 4 0 Newberry Volcano OR 28 126 4 2 2

Augustine AK 44 123 4 3 1

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