Download Volcanic Earthquake Swarms

DR HELENA BUURMAN
Thunder underground
Dr Helena Buurman, a volcano seismologist, shares the details of her research on earthquake swarms and
eruptions in Alaska, and explains why the summer field season is particularly important in the far north
only place in the world where a 2,000 km
stretch of subduction zone is systematically
monitored in real-time. Most studies of
volcanic activity are focused on only one or
two volcanoes at a time. But thanks to the
monitoring efforts of the Alaska Volcano
Observatory and the Alaska Earthquake
Center, we have data available for almost
the entire length of a subduction zone. The
Aleutian arc is therefore the only place in the
world where we have sufficient data (spanning
over 20 years) to be able to study volcanic
activity on a regional scale. In the study of
volcanic earthquake swarms, I have also been
able to incorporate data from other parts of
the globe, including Iceland, Mexico, the US
Pacific northwest and Kamchatka.
How close is the connection between
earthquake swarms and volcanic activity,
and what importance does this have on the
ability to predict eruptions?
I don’t know of any volcanic eruptions that
have happened without any sort of precursory
earthquake activity. This means that they can
be used to predict eruptions, but the tricky
part for volcano monitoring lies in being
able to tell whether a swarm isn’t leading up
to an eruption, so that we don’t issue false
eruption predictions.
Which regions of the world are you
studying in particular? Who is supporting
your studies?
Most of my work has been in Alaska, studying
volcanoes in the Aleutian arc. This is the
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INTERNATIONAL INNOVATION
There is a wide range of expertise in the staff
at the Alaska Volcano Observatory! When I
was a student I was exposed to a variety of
different disciplines when people needed field
assistants to help collect their data, and these
different experiences definitely helped shape
the way I think about volcanic systems.
I recently co-authored a study of volcanism
in the Aleutian arc that correlated patterns in
volcano seismicity with volcano geochemistry
with Chris Nye, Mike West and Cheryl
Cameron. Chris Nye is an expert in Aleutian
geochemistry and has spent decades thinking
about bigger picture processes that can result
in systematic differences along the Aleutian
arc; it was really exciting for us when we
realised our two very separate datasets (his
geochemical data and my volcano-seismic
data) were closely correlated.
What is next for scientific research
in Alaska?
As our understanding of volcanic systems
increases and we become more confident
in our abilities to monitor and even predict
volcanic activity, the challenge turns to
putting this knowledge into practice and
helping communities that live in the shadows
of volcanoes better understand and prepare
for volcanic activity. There are many small,
extremely remote communities scattered
along the Aleutian arc, many of which are
located near dormant volcanoes. It is crucial
that we as scientists communicate our research
and interpretations of volcanic activity
to those who will be directly affected by
volcanic hazards.
Your research examines tectonic settings;
specifically, whether a volcano is located in
a subduction zone, hot spot or near a fault
zone, etc. How might this contribute to the
characterisation of earthquake swarms?
Could you discuss some of the findings from
your research that were presented at the
American Geophysical Union (AGU)’s 2013
fall meeting?
I compared swarms in different tectonic
settings – subduction zones, hot spots,
spreading centres – to determine how much
of an effect the tectonic setting had on swarm
characteristics. I used statistical comparisons
of the location and magnitudes of earthquakes
in different volcanic swarms, and found that
the tectonic setting had very little influence
on swarm characteristics. This wasn’t a big
DR HELENA BUURMAN
The swarming warning
surprise – scientists have been studying
swarms for decades and would presumably
have come across correlations between
tectonic setting and swarm behaviour long
ago. This was also a little disappointing – while
proving that correlations don’t exist has just
as much scientific value as discovering new
connections between datasets, it is not nearly
as exciting or satisfying!
Do you have any upcoming events that you
are particularly excited about?
Right now we are gearing up for our summer
field season, which is my favourite part of the
year. Alaskan winters are particularly hard on
seismic equipment, and we often need to fix
our seismic stations that have been damaged
by wind, snow, ice or even bears. These remote
sites send continuous seismic data back to
the lab using on-site radios. These seismic
data are used by both the Alaska Volcano
Observatory and the Alaska Earthquake Center
to monitor volcanic and earthquake activity
across the state. For me, feet-on-the-ground
experience is equally valuable as the time
spent behind a computer looking at data, since
it gives me a feel for volcano activity in the
past (you learn a lot about what a volcano can
do by looking at old lava flows and craters)
and therefore how big of an impact it can
have on its surroundings. This also helps me
understand the subtle signals seen on specific
seismic stations – for example, one station
on Augustine Volcano is located close to
the shore, and is able to pick up signals from
cracking sea ice on the beach that none of the
other stations are close enough to record.
Seismological researchers based at the University of Alaska Fairbanks
are investigating the predictive value of earthquake clusters or ‘swarms’
in determining when volcanic eruptions will occur; work which has
already had profound consequences for improved surveillance
VOLCANIC AND SEISMIC disruptions can
be among the most cataclysmic events in
nature. Coupled with this extraordinary power
is a dangerous unpredictability; the forces and
processes that give rise to volcanic eruptions
and major earthquakes are hugely complex and
usually very difficult to observe. Modelling and
predicting these events, therefore, is a difficult
task indeed. Nonetheless, seismologists are
frequently concerned with better understanding
the processes that produce these spectacular
and disastrous events, with the ultimate goal
of identifying early-warning systems for use in
high-risk areas.
One possible predictor of volcanic eruption
comes in the form of the earthquake swarm.
Earthquakes in a swarm share a similar
magnitude, and occur over a short period of
time in a localised region of the Earth’s crust.
When such a swarm takes place beneath an
active volcano, this is usually a telltale sign
that hot gas and magma escaping from the
underlying mantle are being redistributed in the
crust. Most of the time these redistributions are
benign: the superheated materials simply find
their way to new reservoirs, and the pressure
stabilises. Sometimes, however, the pressure is
too much, and there is nowhere left to go but
up. When this is the case, earthquake swarms
precede volcanic eruptions that expel these
materials violently onto the surface.
THE ALASKAN ARC
The problem is that scientists are not sure
whether there are characteristics that can
distinguish pre-eruption swarms from benign
swarms, and a large quantity of data would be
needed to answer this question more definitely.
One very promising area from which to gather
such data is the Aleutian arc, a volcanic arc that
exists on the boundary between the Pacific
and North American plates, which runs along
Alaska’s south coast towards the Kamchatka
Peninsula. Caused by the tectonic subduction
of the Pacific plate beneath the North American
plate, this volcanic arc is particularly active,
with an average of two eruptions occurring
every year.
Using data from the Aleutian arc as well as
other volcanoes around the world, one group
of scientists is aiming to get to the bottom
of earthquake swarms and what they signify
once and for all. Based at University of Alaska
Fairbanks’ Geophysical Institute, postdoctoral
research fellow Dr Helena Buurman and her
collaborators hope to develop a consistent
approach for evaluating the characteristics of
earthquake swarms and their relationship to
volcanic eruptions. By bringing this approach to
swarms in various volcanic and tectonic settings,
they will provide statistically meaningful
information for a number of different geographic
regions, and could ultimately develop methods
for distinguishing between benign swarms and
those predicting eruptions.
THE MISSING METRIC
Although Buurman’s work on the current project
began in June 2013, it was seven years prior to
this that she came up with the novel seismic
metric capable of highlighting eruptive swarms.
While working as part of a group studying the
2006 eruption of Alaska’s Augustine Volcano,
the seismologist found that a good metric to
distinguish between different types of volcanic
earthquake was based on the frequency content
of the seismic signals detected.
Buurman dubbed this value, which was derived
by measuring the ratio of high- to low-frequency
energy in a particular earthquake, the ‘frequency
index metric’. With time, and the reapplication of
this metric across volcanic sites around the world,
it has emerged that certain frequency index types
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INTELLIGENCE
VOLCANIC EARTHQUAKE SWARMS
OBJECTIVES
• To predict volcanic eruptions using
earthquake swarms
• To investigate how volcanic earthquake
swarms relate to the movement of magma
through the Earth’s crust
KEY COLLABORATORS
Dr Michael West, Alaska State Seismologist
and Research Associate Professor, University
of Alaska Fairbanks
FUNDING
National Science Foundation (NSF) – award
number 1044930
CONTACT
Dr Helena Buurman
Principal Investigator
Geophysical Institute
University of Alaska Fairbanks
903 Koyukuk Drive
Fairbanks
Alaska 99775 7320
USA
T +1 907 474 7538
E [email protected]
HELENA BUURMAN earned a BSc (Hons)
in Geophysics from the University of
Edinburgh, UK, in 2006. After a summer
in Iceland working on a variety of research
projects on Icelandic volcanism, she moved
to Fairbanks, Alaska, where she completed
her MS degree in 2009 and PhD degree
in 2013 working with Dr Michael West.
During her time as a student in Alaska,
she worked on projects studying volcanic
activity in Kamchatka, Mexico, Yellowstone
and Alaska, and was heavily involved with
seismic monitoring at the Alaska Volcano
Observatory. In 2013 Buurman began work
on an NSF-funded project on volcanic
swarms. She now also works closely with the
Alaska Earthquake Center, which monitors
earthquakes across the state.
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do appear to have unique associations with
volcanic activity.
In the current project with collaborator Dr
Michael West, Buurman applies this metric to
different types of volcanic swarms in order to
detect signatures for different varieties – and
the results are intriguing. Swarms caused by
the intrusion of magma into the crust without
subsequent eruption, as well as those generated
by the passage of fluid through the crust or
snow and rock avalanches on the slopes of the
volcano, produce a different frequency signature
to eruptive swarms. When an earthquake swarm
precedes a volcanic eruption, it seems, the
range of frequency index values is much higher,
giving them a different seismic signature to their
benign counterparts.
It has emerged that certain
frequency index types do appear
to have unique associations with
volcanic activity
THE FREQUENCY FORECAST
This relationship between frequency index and
volcanic eruption could have huge implications
for the practice of earthquake monitoring, since
it appears to be strongly associated with volcanic
eruption, and is not currently evaluated for every
earthquake. However, further work remains
towards determining the exact nature of this
strong association, and ascertaining whether a
high frequency index is necessarily predictive
of an eruption. For example, it could be that
earthquakes occurring during eruptive activity,
rather than the actual event of eruption, also raise
the frequency index. Buurman remains positive:
“In any case, this apparent relationship between
frequency content and eruptive activity gives us
a tantalising insight into volcanic systems that
until now was not recognised”.
In the process of data collection for this project,
Buurman has gathered datasets from volcanoes
all over the world, including Iceland, the US Pacific
northwest, the Kamchatka Peninsula in the Russian
Far East and Mexico. The next step in the research
programme, however, will be taken by returning,
once again, to the seismic data recorded along the
Aleutian arc. It is in this dynamic environment that
Buurman will wait to test the application of her
metric as a real-time monitoring technique: “And
with two eruptions a year, I hopefully won’t have
to wait too long,” she enthuses.
TALES FROM THE FIELD
Buurman’s research has also focused on
recent volcanic eruptions in the Aleutian
arc. Redoubt Volcano, a mere 170 km from
the city of Anchorage, became the focus
of her research after she became involved
with the 2009 eruption as it unfolded.
From being the first person to recognise an
increase in seismic activity below the crater
in January of 2009, she was closely involved
with data interpretation throughout the
three-month build-up to the eruption. A day
prior to the first of the large ash explosions
that closed down the Ted Stevens
Anchorage International Airport she was
on the mountain, installing seismometers
in several metres of snow and ice, which
she would later use for research into what
caused the eruption.
The data collected by these instruments
proved to be crucial in unravelling the
story of the eruption sequence. Through
close collaboration with scientists from
different disciplines, including geology,
petrology, geodesy and even infrasonics,
Buurman pieced together the chronology
of the eruption, starting with when magma
intruded into the crust and ending with the
cooling and shut-down of the system. These
conclusions, and the seismic data from
which they were drawn, have since provided
the starting point for numerous other
studies on the Redoubt eruption.