Presentation on theme: "Short-term Forecasts of Volcanic Eruptions. Is it possible to forecast exactly when a volcano will erupt? Not yet, but great progress is being made. This."— Presentation transcript:
Is it possible to forecast exactly when a volcano will erupt? Not yet, but great progress is being made. This presentation shows how new models of seismic precursors are providing key limits to forecasting eruptions. Villarrica, Chile, 2004
Before escaping from a volcano, the molten rock that feeds eruptions must break and distort the Earth’s crust as it finds a pathway to the surface. Volcanoes rarely erupt without warning. Most eruptions are thus preceded by small earthquakes and deformation of the ground.
During volcanic crises, earthquakes are the most commonly monitored precursors, because: Emergency seismometers are easy to install. Seismometers can record volcanic earthquakes even if they are not on the volcano itself. Emergency seismometer, Montserrat 1996 (Photo: Bill McGuire)
Months - YearsDays Rate of Seismicity Eruptions are preceded by increasing rates of seismicity. Initial increases may be detected months or years beforehand. The final acceleration tends to occur only days before eruption. Can the final acceleration be used to improve short-term forecasts during a volcanic crisis?
This study focuses on emergencies at volcanoes reawakening after several decades or centuries. Such volcanoes are especially dangerous because: They tend to reawaken with large, explosive eruptions. Few are being monitored permanently; many are not being monitored at all. Local populations are unaware of the threat from the volcano and so no suitable emergency plans have been devised.
Longer intervals between eruptions increase the chances that the next eruption will be explosive. New eruptions normally occur at intervals from years to centuries. Interval between Eruptions Explosivity YEARSCENTURIES
Effusive eruptions can also occur after centuries of tranquillity. Tenerife, Canary Islands (Photo: Carmen Solana) Lanzarote Such eruptions are similar to those seen on Etna, Hawaii and Réunion. If the magma is fluid, such eruptions may feed lava flows, as observed on Tenerife, La Palma and Lanzarote in the Canary Islands.
When magma is more viscous, the eruptions produce lava domes. Since 1995, the growth of a lava dome has been monitored at Soufriere Hills volcano on Montserrat, in the Caribbean.
Lava domes may spread hundreds of metres from the vent. They become much more dangerous if they collapse. When a dome collapses, it disintegrates into a hot landslide of broken lava and volcanic gas, creating a pyroclastic flow. This lethal mixture can travel several kilometres within minutes.
Explosive eruption, October 1997, Montserrat (Photo: Paul Cole) Gas pressure within a dome may also trigger explosive eruptions.
Major explosive (plinian) eruptions expel up to cubic kilometres of magma within 24 hours. They send pulverised magma (ash) 10-35 km or more into the atmosphere. They also feed lethal currents of ash and gas (pyroclastic flows) that race over the ground at hurricane speeds. Such eruptions may devastate hundreds of square kilometres. Photo: Don Swanson, USGS
Examples include: Mount St Helens, USA (main photo) Pinatubo in the Philippines (inset, right) Kliuchevskoi in Kamchatka (inset, left) Photo: NASA Photo: Rick Hoblitt, USGS Photo: Don Swanson, USGS
Forecasting eruptions after long periods of tranquillity
Soufriere Hills volcano, Montserrat, has been erupting since 1995. It is the volcano’s first eruption for nearly 350 years. Note the houses
Most of the world’s major explosive eruptions have occurred at volcanoes reawakening after a century or more of tranquillity. Pinatubo, Philippines in 1991; El Chichón, Mexico, in 1982; Mount St Helens, USA, in 1980; Krakatau, Indonesia, in 1883; Vesuvius, Italy, in 79; Santorini, Greece, in 1650 BC. These include:
How long ahead of time can we reliably forecast eruptions from volcanoes that have been quiet for decades or centuries? Vesuvius, in Southern Italy, has been quiet since 1944. It now supports a population of nearly 600,000 people.
Volcanoes can heal themselves during long periods of tranquillity. Before another eruption, therefore, magma must break open a new path to the surface. As the path is broken open, earthquakes are triggered below the volcano. Magma Location of Earthquake Increasing seismicity thus normally occurs before eruptions.
At first, the number of growing fractures increases with time. Under these conditions, the detected rate of seismicity increases exponentially with time. Most of the fractures remain isolated and so are unlikely to produce a new path for the magma. An exponential increase thus indicates the possible approach to eruption, but does not identify when that eruption might occur. The acceleration in seismicity can be explained by the slow growth of fractures within and below a volcano.
The opening of the main pathway itself is recorded by the peaks in seismic event rate. Under these conditions, the peak rate increases hyperbolically with time. When the number of active fractures exceeds a critical value, these join together to form a new path for the magma. Inverse Event Rate Event Rate Time Conveniently, this means that the minimum inverse rate decreases linearly with time. An eruption is expected when the inverse rate becomes zero.
Soufriere Hills, Montserrat, 1995 Time (Days) Inverse Event Rate (Days) 0.05 10 0 ERUPTION The minimum inverse rate of seismicity decreases as: [Minimum Inverse Event Rate] = (1/ f* ) – * t The gradient, *, measures the critical strain at failure and, for volcanic conditions, is (4.5 3.2) x 10 -3 ; its value for Soufriere Hills is 2.5 x 10 -3. f* is the frequency of strain fluctuations; t is time.
Time (Days) Inverse Event Rate (Days) 0.05 10 0 Soufriere Hills, Montserrat, 1995 The model indicates that the final (hyperbolic) acceleration evolves over ~14 days. However, several days may be needed to recognise the trend. Thus, even under ideal conditions, only a few days might be available to forecast the time of an eruption. Look again at Soufriere Hills. When would you issue an alert? Alarm now? Or now?
Increasing seismicity before eruption has been linked to the opening of a fracture between magma and the surface. How reliable is this acceleration as a precursor? Can it end in a false alarm, without an eruption?
Fractures may also form that do not reach either the magma or the surface. Growth of these fractures may yield the same accelerations in seismicity as pre-eruptive signals. Use of seismicity alone may thus lead to false alarms (blue stars). False Alarms Time Rate of Seismicity
The first signs of unrest may be detected months or years before eruption. The final approach to eruption may trigger an acceleration in seismicity up to 14 days beforehand. Emergency warning times are unlikely to be reliable more than a few days in advance. Episodes of increasing seismicity may also occur without eruption. The public must be made aware of the possibility of false alarms: this is important to maintain public trust during an emergency. Summary At volcanoes reawakening after at least several decades :
Can we distinguish between pre-eruptive and non-eruptive accelerations in seismicity? Can we use precursory signals before the final 14 days to increase warning times? How can we incorporate other types of precursor to improve forecasts? The Future Questions now being addressed include:
The answers will help to avoid a repetition of this:
Herculaneum: victims of the AD 79 eruption of Vesuvius
Project Volcalert Sebastien Chastin Claire Collins Marielle Collombet Torsten Dahm Giuseppe De Natale Jean-Robert Grasso Christopher Kilburn Ian Main Adriana Nave Eleonora Rivalta Giuseppe Rolandi Carmen Solana Claudia Troise Oliver Willetts Photo Credits : Christopher Kilburn unless otherwise stated.
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