Volcano monitoring around the world is woefully incomplete despite technological advances.
Volcanoes' ascent of magma is typically accompanied by:
Numerous small earthquakes.
The release of magmatic gases.
Surface deformation.
Systematic volcano monitoring to detect these phenomena began in 1845 with the completion of the Osservatorio Vesuviano.
The Hawaiian Volcano Observatory followed and celebrated its 100th anniversary this year.
Today, the World Organization of Volcano Observatories has 80 members.
Detection systems have increased in range and sophistication.
Advanced models of volcanic processes are helping to interpret monitoring data.
Key problems remain with:
Distinguishing volcanoes that will erupt from those that will not.
Global data coverage.
Seismic Signals
Seismic signals remain a key aspect of volcano monitoring.
As magma moves toward the Earth's surface:
Stress changes in the volcanic edifice.
Magma rupture and stick-slip motion of the magma body.
Lead to highly regular seismic patterns, often referred to as volcanic tremor.
The signals are typically very weak and may be missed by regional networks, requiring a dedicated network of seismometers near the volcanic edifice.
Seismic monitoring is therefore at the heart of every volcano observatory.
Early attempts to interpret seismic signals on volcanoes used methods adopted from earthquake seismology.
Simple event counts or amplitude estimates were used as crude indicators for the level of volcanic activity.
In the past 20 years, broadband seismic sensors have enabled detection of seismic signals from volcanic earthquakes in a wide frequency range, allowing volcano seismologists to distinguish between different types of volcanic events and to attribute different signals to different volcanic processes.
Conceptual models help to detect and quantify magma or fluid movements, or to identify stress changes in the volcanic edifice.
Short-term forecasting can be achieved by interpreting systematic changes in seismic energy release as changes in magma ascent rates and changes in seismic patterns and spectral characteristics as indicators of critical changes in magma properties.
Compared with global seismology, where data exchange is routine, volcano observatories are more independent and less willing to share data.
Particularly during a crisis, raw seismic data are often confidential, such that only the local observatory can give advice.
Some observatories have established links to research institutions; advice to authorities should be channeled through the observatories or official scientific advisory committees.
Maverick interpretations from outside groups can be a problem.
Ground Deformation
Most volcanic eruptions are preceded and accompanied by ground deformation.
Methods to measure surface movements include:
High-precision leveling
Electronic distance measurement with lasers
Ground tiltmeters
Global Positioning System (GPS)
These methods are typically used in combination.
Strain meters in boreholes, one of the world's most sensitive geophysical instruments, are used at very few volcanoes.
Deformation data were long interpreted with a simple point source model, the Mogi model, but today's numerical models incorporate representations of crustal rheology and of differently shaped pressure sources.
With these new models, the shape and depth of the magma chamber can be determined more precisely.
Satellite-Based Methods
Satellite-based methods provide a route to detecting unrest on currently unmonitored volcanoes.
Interferometric synthetic aperture radar (InSAR) uses the phase component of radar images to determine the position of the Earth's surface.
Simultaneously recorded images from different radars produce digital elevation models, vital for predicting the path and run-out of pyroclastic flows and lahars, and time-separated images measure deformation.
The radar beam can pass through clouds, which is particularly useful in the tropics, where cloud cover frequently obscures visual observations.
The satellite view provides a global perspective, mapping tectonic strain across continents and allowing the exploration of volcanoes in remote, underfunded, or inaccessible environments.
As a result, the list of volcanoes previously thought to be dormant but now known to be showing signs of unrest is growing rapidly.
These observations enable targeting of resources for more detailed, ground-based monitoring.
Despite its enormous potential, InSAR is still a young endeavor.
Technical issues, such as repeat times and mission longevity, are in the hands of the space agencies.
The European Space Agency's (ESA) Sentinel satellites, due for launch in 2013, are expected to acquire data over all land masses every 6 days for the next 20 years.
Attaching radar systems to unmanned aerial vehicles (UAVs) promises greater flexibility and bespoke acquisition plans, although flight paths are more complex than those of satellites, complicating the data interpretation.
The challenges will be to distinguish between magmatic, hydrothermal, and even atmospheric errors and to determine which processes will culminate in eruption.
Volcanic Gas Composition
Measurements of volcanic gas composition and fluxes, as well as temperature, have long been stalwarts of monitoring.
Until recently, these measurements were made on fumaroles and hot springs, sometimes placing scientists at high risk.
In situ data remain very valuable, but there has been prodigious progress in remote measurements from ground-based instruments and satellites.
Analytical petrological estimates, notably from melt inclusion studies, have advanced understanding of gas inventories.
Networks of ultraviolet spectrometers and imaging cameras provide detailed sulfur dioxide (SO2) time series that can be combined with seismic and deformation data to give a much more complete and informative picture of volcano behavior.
Likewise, (SO2) and thermal data are now measured from satellites, although it is proving hard to get agreement between the ground-based and satellite measurements.
Furthermore, (SO2) data are often difficult to interpret:
A decrease in (SO2) may mean that the threat of eruption is diminishing because of decreased magma supply.
The gas is being trapped at depth and the threat is increasing.
Other novel monitoring techniques include infrasound and portable ground radar, which are already being deployed to document explosive eruptions and ash clouds.
Muon tomography holds promise for imaging the interior of lavas, and UAVs may provide new monitoring capability, but both methods will need considerable development to become widely used.
Early Warning Challenges
Despite these technological advances and the increasing integration of different data types, early warning of eruptions still faces major challenges.
The most important issue is how to tell whether a period of volcanic unrest will lead to eruption.
There are more cases of unrest that do not lead to eruption than those that do.
False alarms are one of the most problematic issues for observatories.
Evacuations that are called but then nothing happens can undermine public trust, whereas evacuations that are called too late or not at all can lead to tragedy.
Volcanic systems are likely to fail suddenly.
Sometimes, very minor differences in system properties determine whether failure and eruption occur or not.
The exact timing of eruptions may be difficult to predict, and it is even likely that some volcanic systems are inherently unpredictable.
Monitoring can provide insights into patterns and consequences of activity that can help draw evacuation plans.
Most volcanoes around the world are not monitored effectively or at all.
A study of 441 active volcanoes in 16 developing countries reveals that 384 have rudimentary or no monitoring, including 65 volcanoes identified as posing a high risk to large populations.
Satellite systems such as InSAR can provide regional, or even global, data but are rarely applied in real time.
This unsatisfactory situation is to some extent ameliorated by rapid response teams at the request of countries during volcanic emergencies, notably the Volcanic Disaster Assistance Program of the U.S. Geological Survey.
Even well-funded observatories suffer from a wide gap between research developments and their implementation as forecasting tools on an operational level.
Developing capacity and closing these gaps is a priority for the volcanological community.