VAWS/4 WP/03-02
WORLD METEOROLOGICAL ORGANIZATION
=====================================
WORLD METEOROLOGICAL ORGANIZATION (WMO)
IN CLOSE COLLABORATION WITH THE INTERNATIONAL CIVIL AVIATION
ORGANIZATION (ICAO) AND THE CIVIL AVIATION AUTHORITY OF NEW ZEALAND
FOURTH INTERNATIONAL WORKSHOP ON VOLCANIC ASH
Rotorua, New Zealand, 26 - 30 March 2007
Agenda Item 3:
Latent State and Predictions
Title of Paper:
Overview of Volcano Monitoring for Eruption Forecasting and
Alerting
Authors:
Marianne Guffanti, John W. Ewert
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Overview of Volcano Monitoring for Eruption Forecasting
and Alerting
Marianne Guffanti1 and John W. Ewert2
U.S. Geological Survey, Reston, Virginia USA
2
U.S. Geological Survey, Cascades Volcano Observatory, Vancouver, WA USA
1
ABSTRACT
Volcano monitoring is conducted in two general modes: a
forecasting mode before and between eruptions and an
alerting mode when eruptive activity is detected. For the
aviation sector, reliable eruption detection and rapid
alerting are paramount to mitigate risks to en-route aircraft
from encounters with airborne volcanic ash. While similar
methods for monitoring seismicity, deformation, gas flux,
and thermal changes are used for both forecasting and
alerting, there are some differences between the two
modes. Additional techniques used in the alerting mode
include video surveillance, near-field infrasonic pressure
sensors, lightning detectors, airborne infrared cameras,
visual observations, satellite-based multi-spectral sensors,
and weather radar. Ground-based weather radar can
significantly improve the chances of meeting the fiveminute alert benchmark for aviation, particularly at night
and in poor weather conditions, and the combination of
seismic and radar data can provide unambiguous evidence
of an ash-producing eruption. In evaluating monitoring
methods, it is important to recognize that no single
technique works at all volcanoes for the purposes of
forecasting and alerting. The optimal approach is to have
multiple monitoring data streams from different sensor
types received and analyzed together at a Volcano
Observatory.
INTRODUCTION
Monitoring volcanoes is not a routine process. During
unrest (anomalous behavior that may be precursory to
eruptive activity), volcanoes exhibit a wide range in the
behavioral style and duration. Some restless volcanoes
progress to eruption very quickly (days to weeks), others
take months to years, or do not erupt even after exhibiting
heightened unrest.
Eruption styles can vary from
relatively mild events that produce small lava flows or
phreatic emissions to extremely explosive events, and
eruption magnitudes can vary from erupted magma
volumes of 0.001 km3 to (rarely) >100 km3. Generally, an
eruption involves episodes of eruptive activity separated
by non-eruptive intervals of hours to months. The duration
of a single eruptive episode usually ranges from a few
minutes to tens of hours, whereas an entire eruption can
last for a day to decades.
Volcano monitoring is conducted in two general modes,
forecasting and alerting (Ewert and Guffanti, 2006). The
forecasting mode occurs before and between eruptions as
magma moves to the surface, a process that typically
causes earthquakes and tremor, deforms the volcano’s
surface, emits magmatic gases, and changes the thermal
regime of the volcano. Forecasting of expected hazardous
events is especially important when time is needed for
preparedness, such as when people living close to
2
volcanoes may need to evacuate. In the alerting mode, the
objective is detection or confirmation of eruption onset and
timely notification of actual hazards, which is paramount
to mitigate risks to en-route aircraft from encounters with
airborne
volcanic
ash.
Furthermore,
Volcano
Observatories sometimes receive mistaken reports that a
volcano is erupting (for example, normal steaming
becomes more noticeable in certain weather conditions),
and the alerting capability will also provide the important
confirmation that a volcano is not erupting.
Volcano monitoring uses a variety of ground-based,
airborne, and satellite-based techniques. Transmission of
monitoring data occurs via radios, phone lines, Internet,
and/or satellites to scientific facilities – sometimes quite
distant from the monitored volcanoes – for processing and
analysis. Automatic, computer-based data processing
systems make most data available in real to near-real time
for analysis by scientists who may be located in different
facilities. Interpreting monitoring data and forecasting the
future behavior and eruptive potential of a restless
volcano, however, are far from automatic and require
complex analysis by a variety of volcanological experts as
soon as the data are received.
A primer of the various monitoring methods is online at
http://volcanoes.usgs.gov/About/What/Monitor/monitor.ht
ml (last visited 8 March 2007). While similar monitoring
methods are used in both modes, there are some
differences.
MONITORING IN THE FORECASTING MODE
The main monitoring methods for forecasting volcanic
activity are local seismic and geodetic networks on
volcanoes, gas-emission measurements, and multi-spectral
satellite surveillance. Seismic monitoring is the most
widely employed, best understood, and most reliable
monitoring technique in either mode. Seismic networks
are used to locate earthquakes in three dimensions as well
as determine their timing and magnitudes, track any
migration of zones of seismicity, detect tremor associated
with fluid (gas or magma) moving through a constriction,
and characterize other physical processes at the source of
the seismic signal. Digital broadband seismometers, which
can resolve a wide range of ground shaking and usually do
not go off scale or “clip” during intense activity, provide
more data about the physical processes at the source of the
seismic signal than traditional short-period analog
seismometers (Figure 1). However, broadband instruments
are more expensive and require more power and higherbandwidth telemetry. A seismic network with a mix of
broadband and short-period instruments is a good solution
in terms of overall cost, utility, and reliability.
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Measurement of the displacement, or deformation, of the
ground surface of a volcanic edifice is used to model the
geometry and depth of the subsurface magmatic source
and to track the rise of magma toward the surface.
Deformation also is known to precede earthquake activity
at some volcanoes and thus can provide some of the
earliest indication of anomalous activity (see Wicks et al.,
2002). The primary techniques for monitoring volcanic
deformation (e.g., uplift, subsidence, fracturing) are GPS,
interferometric synthetic aperture radar (InSAR), borehole
strainmeters, and electronic tiltmeters.
Continuously telemetered data from permanent GPS arrays
provide good temporal coverage of volcanic deformation,
and such arrays are increasingly being deployed at
volcanoes worldwide.
InSAR is a satellite-based
technique with good spatial coverage of a volcanic edifice;
however, repeat observations are available only weeks or
months apart.
Electronic tiltmeters and borehole
strainmeters are sensitive indicators of short-term volcanic
processes at shallow depths and provide data in real-time.
The optimal approach for deformation monitoring in the
forecasting mode is to use a combination of GPS, InSAR,
and tiltmeters or borehole strainmeters at a volcano.
Dzurisin (2007) provides a comprehensive overview of
modern volcano deformation monitoring.
Magma contains significant quantities of dissolved gases,
which can separate from the magma at depth and be
released into the atmosphere before, during, and after
eruptions. The primary objective of gas monitoring is to
determine changes in the type and rate of released gases,
primarily carbon dioxide and sulfur dioxide. Various
airborne and ground-based methods are used (see
http://volcanoes.usgs.gov/About/What/Monitor/Gas/GasM
onitor.html; last visited 8 March 2007).
A promising new satellite-based tool for monitoring
volcanic degassing in the forecasting mode is the Ozone
Monitoring Instrument (OMI), an ultraviolet sensor on a
polar-orbiting NASA satellite launched in 2004. In
addition to delineating the distinctive sulfur dioxide clouds
that are produced by explosive eruptions, OMI has the
sensitivity and resolution to reveal the subtle degassing of
sulfur dioxide that can occur before eruptions (Carn et al.,
2006). As more volcanoes are studied with OMI,
important new insights about the amount and timing of
magmatic degassing can be expected.
Changes in thermal features often accompany other signs
of unrest. Existing thermal features at a volcano may
increase their thermal output and/or expand in size prior to
eruptive activity. Ground-based, airborne, and satellitebased sensors are employed to monitor and quantify
thermal signals. Thus far, satellite systems have been most
widely employed (Harris et al., 2000). Temporal and
spatial resolutions, as well as meteorological factors, can
limit the utility of these observing systems.
Volcano monitoring is best started well before unrest
begins to escalate at a volcano. Waiting to deploy a proper
monitoring effort until a hazardous volcano awakens and
an unrest crisis is building means that scientists, civil
authorities, businesses, and citizens are caught in a reactive
stance, trying to get instruments and mitigation measures
in place before the situation worsens. Precious data as
well as time are lost in the weeks it can take to deploy a
response to a reawakening volcano. Ewert et al. (2005)
present a methodology for systematically determining
what level of monitoring coverage should be in place at a
volcano before unrest escalates, based on an assessment of
the threat posed by that volcano. In the United States,
many threatening volcanoes lack sufficient monitoring for
useful hazard forecasts.
MONITORING IN THE ALERTING MODE
In the alerting mode, the objective is detection of eruption
onset and timely notification of actual hazards produced by
the eruption. As with eruption forecasting, seismic
monitoring is the primary method in the alerting mode.
Given enough telemetered stations around a volcano,
seismic data alone sometimes can be enough to confirm
that an eruption is in progress. For example, at Augustine
Volcano, Alaska, in 2006, seismic data were the basis for
raising the color-coded alert level to Orange (volcano is
exhibiting heightened unrest with increased likelihood of
eruption) on 10 January 2006 and then to Red 8.5 hr later
on 11 January 2006, which alerted the public and the
aviation sector that an explosive eruption was in progress.
However, at some frequently active volcanoes, seismic
indicators of the onset of an eruptive phase may be
difficult to distinguish from a high level of background
seismicity (e.g., at Redoubt Volcano in 1989; Power et al.,
1994). Having other corroborating observations is critical
for full confidence.
Other techniques useful for detecting and confirming
eruptions include real-time geodetic methods (tiltmeters,
GPS, borehole strainmeters), near-field (<20 km)
infrasonic pressure sensors, lightning detectors, video
surveillance (in fair weather), airborne infrared cameras,
visual observation (e.g., pilot reports), weather radar, and
satellite-based multi-spectral sensors (visible, infrared, and
ultraviolet wavelengths). A recent example of the use of
many of these techniques for eruption alerting is the
response by the Alaska Volcano Observatory to the 2006
eruption of Augustine Volcano (Power et al., 2006).
Although satellite remote-sensing techniques typically do
not have data-download rates rapid enough for real-time
alerting of eruption onset, they are useful for confirming
the occurrence and size of an eruption, particularly at
volcanoes with no ground-based monitoring (for example,
at Anatahan volcano in the Northern Mariana Islands in
2003; Guffanti et al., 2005).
Weather Radar
Of the various methods useful for detecting the onset of
eruptive activity, ground-based weather radar is
particularly effective for confirming ash hazards. Radar
has proven useful in detecting and characterizing volcanicash plumes from numerous eruptions, beginning with the
1980 eruption of Mount St. Helens (Harris et al., 1981).
Radar can be used to estimate plume heights, ash-particle
sizes, and the direction and speed of ash clouds; radar
observations of the eruptive phenomenon are available
instantly and can be made at night and in poor weather
conditions.
A specific example of radar’s utility in providing rapid
warnings of ash hazards to aviation is given by Hoblitt and
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Quaas Weppen (1999). In 1997-1998, Mexico’s National
Center for the Prevention of Disasters (CENAPRED) and
the U.S. Geological Survey deployed an experimental
ground-based Doppler radar in Mexico City to track the
height, direction, and speed of ash plumes from
Popocatepetl Volcano ~60 km distant. Seismic data were
collected and analyzed in conjunction with the radar data.
Simultaneous occurrence of a seismic event and a strong
radar reflector over the volcano provided incontrovertible
evidence that an explosive event had occurred and that ash
was in the air. When the combined real-time seismic and
radar data confirmed an ash-producing eruption had
occurred, CENAPRED would quickly call Mexico City
International Airport to warn air-traffic controllers of
imminent ash hazards.
In response to stated needs of the aviation industry, U.S.
Volcano Observatories strive to notify regional air-traffic
control centers by telephone within five minutes of an
eruption. To significantly improve the chances of meeting
the five-minute alert benchmark, the U.S. Geological
Survey’s Volcano Hazard Program is acquiring a groundbased, portable, Doppler radar (C-band) for deployment
where explosive eruptions pose hazards to aviation. Radar
operated for detecting meteorological phenomena is useful
but not ideal for detecting ash clouds; 360-degreevolumetric scans typical for meteorological purposes can
take up to 10 minutes to complete, and various settings
may not be optimized for ash. By operating its own radar,
the USGS will have full operational control of the unit
with the benefits of virtually no delay between data
acquisition and image availability, full access to archived
data, ability to move the system to different volcanic sites,
and ability to merge radar data with other geophysical data
streams such as seismic data.
CONCLUSION
In evaluating monitoring methods, it is important to
recognize that no single technique works at all volcanoes
for the purposes of forecasting and alerting. Volcanoes are
complex systems, and scientists need to have multiple
sources of data available as quickly as possible on which
to base forecasts and eruption alerts. The optimal
approach is to have multiple monitoring data streams from
different sensor types received and analyzed together by
volcano specialists.
ACKNOWLEDGMENTS
Reference herein to any specific commercial product,
process, or service by trade name, trademark,
manufacturer, or otherwise does not necessarily constitute
or imply its endorsement, recommendation, or favoring by
the United States Government or any agency thereof.
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Monitoring global degassing with OMI: Abstracts
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International Association of Volcanology and
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Chemistry of the Earth’s Interior, 23-27 January 2006,
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Dzurisin, D., 2007: Volcano deformation: Geodetic
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Figure 1. Two seismograms from the same earthquake measured by different instruments. Top panel is a
clipped seismogram from a traditional short-period instrument where the amount of ground shaking has
exceeded the range of the instrument. Bottom panel is from a modern broadband instrument (at a slightly
greater distance from the earthquake) which is responsive to a much larger range of shaking and thus less
likely to clip. The signal in the bottom panel can be better characterized as to whether it results from a
major eruption, small explosion, rock fall, snow avalanche, mudflow, or other common event at a volcano
(from Ewert et al., 2005).
ORANGE issued at 9:10 PM AST on 10Jan2006
8.5 hr later, RED issued at 5:50 AM AST on 11Jan2006
Figure 2. Seismic record from station AUH at Augustine Volcano, Alaska, showing when the Alaska
Volcano Observatory issued color-coded alert levels on 10-11 January 2006. The Observatory had raised the
color code from Green to Yellow on 29 November 2005. AST is Alaska Standard Time.
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