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Taking the Earth’s Pulse –
Listening to Seismic Noise
Dr Taka’aki Taira
TAKING THE EARTH’S PULSE –
LISTENING TO SEISMIC NOISE
Research Seismologist Dr Taka’aki Taira at the University of California
at Berkeley and his colleagues investigate changes in Earth’s structures
related to earthquake rupture and volcanic eruption by exploring ways to
listen to and interpret ambient seismic noise.
The Earth Not Only Shouts at Us –
It Also Whispers
Earthquakes are scary things. Just the
shaking and rolling of the normally stable
ground in a mild temblor is enough to
frighten people and animals. And the
damage large earthquakes cause – including
the human death toll – seems to make
the news on a regular basis. For example,
recently a violent earthquake struck Gorkha,
Nepal, killing 9,000 people and injuring
22,000. A century ago it was the historic 1906
San Francisco earthquake – one of the largest
natural disasters in the history of the United
States resulting in the destruction of 80%
of the city of San Francisco and the loss of
over 3,000 lives – that resulted in a veritable
eruption of scientific interest in earthquakes,
seismic motions and methods to observe
and forecast these kinds of disasters. For
the last century, scientists the world over
have studied the creaking and cracking
of the Earth’s crust in an effort to warn us
of impending catastrophes. But the big
sounds that accompany such phenomena as
earthquakes and volcanic eruptions are not
all that you can detect with a seismograph.
You can also hear background whispering
of the Earth’s crust – the so-called ‘seismic
noise.’
Seismic noise is the rather nonspecific
term for the fairly persistent, low frequency
vibration of the Earth’s crust from any
number of causes. Also known as ambient
vibrations, it is referred to as ‘noise’
because it is ordinarily an unwanted part
of the signals recorded by seismometers.
Other disciplines besides seismology also
term it ‘noise’ because it is a nuisance for
things that are sensitive to vibrations, like
precision telescopes or the commercial
growing of crystals. On the other hand,
measuring ambient vibrations can be
helpful in engineering, where projects such
as the building of bridges and high-rise
buildings require calculations of the elastic
properties of the soil to determine whether
the structures will be susceptible to shifting
due to earthquakes and other seismic
events. In other words, seismic noise may
indicate whether the ground is firm enough
or not. This is where Dr Taka’aki Taira and his
colleagues concentrate much of their recent
professional attention – studying seismic
noise in relation to actual earthquakes for
determining whether noise is not noise at
all, but a possible indicator of stress in the
tectonic plates near earthquake prone areas.
The Science of Seismic Fault Dynamics is
Itself a Dynamic Science
Since the 1950s and 1960s, scientific
understanding has been that the Earth’s rigid
outer layer – the lithosphere – is broken up
into seven or eight major tectonic plates,
along with a number of smaller plates, that
essentially float on the fluid inner layers of
the Earth. These tectonic plates are always
slowly moving, separating in some places
and colliding in others. But the important
thing is when they collide their edges
grind together causing massive friction.
Where plates collide – called a fault – the
friction of that collision can cause energy to
accumulate over time, which can suddenly
release when the amount of built up energy
overcomes the force of friction between
the two edges. According to this model,
an earthquake results from a sudden slip
on a fault. This slip causes the edges move
against each other, releasing energy in
waves that travel through the Earth’s crust
and causing vibrations and movement
that can lead to damage and destruction.
Simply speaking, the tension builds up over
time until the edges of the fault can’t hold
it anymore and an earthquake results. Thus
scientists have been focusing on observing
the deformation at the Earth’s surface to
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measure the total strain accumulation by this
tectonic plate model. But this is not enough.
We need to know when the Earth can’t
endure the accumulated strain anymore,
i.e., the strength of the crust. Moreover,
scientists recognise the strength might vary
over time. When the crust is weakened,
earthquakes would occur more frequently
even though the accumulated stain is
smaller. This would lead to a more complex
pattern of earthquake cycles. Without
knowing the crustal strength, it is extremely
difficult to forecast earthquakes. Therefore,
observing the temporal variations of the fault
strength has been a long-sought goal of the
earthquake science community over the last
few decades.
Together with Paul Silver of Carnegie
Institution for Science, Dr Taira and his team
developed a new means to monitor fault
strength by analysing seismic waveforms and
microearthquake activities, and published
their findings in Nature. The team found
the first field evidence showing the fault
strength was temporally weakened, and this
temporal weakening was responsible for
clusters of earthquakes in Central California.
More importantly, the reduction of the fault
strength they found in California was induced
by dynamic stress changes from a distant
2004 magnitude 9.1 Sumatra earthquake that
had occurred on the other side of the world.
The implication of their finding is that distant
large earthquakes may increase the risk of
subsequent earthquakes around the globe.
More recently, Dr Taira has concentrated on
recording ambient vibrations – generally
considered nonspecific background noise –
and correlating them with specific volcanic
and earthquake activity.
‘Noise’ Can Be Considered a Heartbeat of
Seismic Activity
In the American Pacific Northwest, Pacific
Oceanic tectonic plates have slid below the
North American Plate for millions of years.
Heat from this tectonic subduction has given
rise to numerous volcanoes from California
all the way up to British Columbia over the
recent geologic past – say, the last 30 million
years. It is also responsible for seismic activity
in the Lassen volcanic area, located at the
southern edge of the Cascade Mountain
Range. Here, at the Lassen Volcanic Center, is
where Dr Taira and his colleagues have their
seismology listening post. The Center sits
above a hydrothermal system that is feared
might be the site of hydrothermal explosions
at some time in the future. Dr Taira monitors
this area, aiming to develop a new way of
forecasting volcanic phenomena by using
seismic noise correlated with actual seismic
and geologic activity.
Dr Taira and his colleague Florent Brenguier,
of the Université Grenoble Alpes, analysed
ambient vibrations at six stations in the
Northern California Seismic Network
around Lassen Peak, a mountain in the
Lassen volcanic area. The data from these
stations was electronically processed to
show changes in the speed of seismic waves
traveling through this area. Variations in
seismic wave speed are indicative of changes
in tectonic stress in the area. Essentially,
they established a quasi-real-time velocity
monitoring system through the use seismic
interferometry with ambient vibrations. Their
monitoring system showed the variability
of seismic velocity over time in response to
stress changes from earthquakes and from
seasonal environmental changes.
Interestingly, dynamic stress changes from
an actual magnitude 5.7 local earthquake
produced a measurable velocity reduction
1 km below the surface. Calculations from
the changes surrounding this earthquake
indicated that the Lassen hydrothermal
system contained highly-pressurised
hydrothermal fluid deep beneath the surface.
Dr Taira’s measurements also show that the
long-term seismic velocity changes closely
follow snow-induced vertical pressure
almost immediately. That is, winter snow
accumulation on the surface actually pushes
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down with enough force to cause changes
in the seismic velocities in the subsurface
hydrothermal fluid. Dr Taira feels that this
is most consistent with a hydrological
load model, where surface loading presses
the hydrothermal fluid out, leading to an
increase in the opening of cracks in the
crust. The weight of snow forces fluid already
below the surface to cause cracks in the
various subsurface rocks and sediment. At
any rate, this effect of the hydrothermal fluid
movement is correlated with reductions of
seismic velocity. This allowed Dr Taira and his
team to deduce that heated hydrothermal
fluid is responsible for the long-term changes
in seismic velocity, and those changes
in velocity can be used to understand –
basically in real time – what is going on with
the subsurface fluids. This is exciting news,
giving hope that monitoring of the ambient
vibrations can tell us what the fluids and
rocks kilometres deep are doing. This in turn
might give an early warning of volcanic or
earthquake activity.
Is This an Isolated Finding?
Dr Taira’s results from the Lassen monitoring
are very exciting, but they aren’t the first
time he’s used ambient vibration monitoring
to listen to earthquakes. Recently, in the
journal Geophysical Research Letters, Dr
Taira and his group published their studies of ambient vibration-based
monitoring they used to look at the temporal variations of crustal
seismic velocities before, during, and after the 2014 magnitude 6.0
earthquake in the South Napa area. This South Napa earthquake is
the largest earthquake in the San Francisco Bay Area, since the 1989
magnitude 6.9 Loma Prieta earthquake. They saw a velocity drop
immediately after the South Napa earthquake. The spatial variability
of the velocity reduction correlated best with the pattern of the peak
ground velocity of the South Napa mainshock. This told them that
fracture damage in rocks induced by the dynamic strain is likely
responsible for this velocity change. About half of the velocity reduction
was recovered at the first 50 days after the South Napa mainshock.
This velocity recovery is a fascinating observation. Dr Taira believes
that these findings after the earthquake may actually indicate a
healing process of damaged rocks. This implies that fault lines can
‘heal’ themselves very rapidly to some extent after they have released
energy in an earthquake. Again, what some folks consider noise – these
ambient vibrations – is giving Dr Taira and his colleagues an important
new technique to monitor seismic activity.
Future Needs and Directions?
ability to understand the underlying mechanisms of earthquakes and
volcanic eruptions. However, their monitoring system is not complete.
Their present system puts out a daily velocity change of the ambient
vibration pattern. If they had more computing power, however, they
could perform massive cross-correlation computations and perhaps
give an hourly update, even streaming it in real-time online. This would
enable researchers around the world to detect changes in the velocity
of ‘noise’ that accompany and perhaps precede a volcanic eruption or
an earthquake.
In addition, Dr Taira tells Scientia that he recently obtained a
grant from National Science Foundation with Rice University and
Lawrence Berkeley National Laboratory to perform an active source
experiment at Parkfield, central California. Recent advancement
of the instrumentation allows the team to generate and detect the
seismic waves in high precision. Previous experiments carried out by
another researcher team found velocity changes that preceded small
earthquakes. This measurement could lead to a sort of ‘stress meter’ at
greater depth to better understand how fault-zone stress is related to
earthquakes. Dr Taira has recently joined this active source experiment
project as a co-Principal Investigator and is planning to go into the field
soon to begin monitoring. This is important research on an important
subject – not noise at all.
What else can we find out about earthquakes and volcanoes by
eavesdropping on them? With the advancements of computer
resources and instrumentation, Dr Taira and his colleagues are
pushing the limits to uncover more of these secrets. Their efforts spent
watching and listening to ‘noise’ will hopefully yield more information
on the genesis of earthquakes and allow us more precision in our
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Meet the researcher
Dr Taka’aki Taira
Berkeley Seismological Laboratory
University of California, Berkeley
Berkeley, USA
Dr Taka’ai Taira did his undergraduate studies in Japan, receiving his
BS in Earth Sciences in 1999 from Yamagata University and his MS in
Geophysics in 2001 from Hokkaido University. He then received his
PhD in Geophysics from the University of Hokkaido. After receiving
his doctorate, Dr Taira did postdoctoral work at the Institute of
Seismology and Volcanology of Hokkaido University, the Department
of Terrestrial Magnetism of the Carnegie Institution for Science and
the Department of Geology and Geophysics of the University of Utah.
He then started a role as an Assistant Research Seismologist at the
Berkeley Seismological Laboratory, where he is now Associate Research
Seismologist.
Dr Taira’s research interests include earthquake and observational
seismology, transient stress changes at seismogenic depth, subsurface
hydrothermal fluid migration, source mechanism of fluid-induced
earthquakes, developing seismic array methodologies, seismic
imaging of crustal structure, seismic wave propagation, and modeling
of conduit flow dynamics. He has authored or co-authored nearly 30
articles published in peer-reviewed journals and other professional
proceedings, as well as multiple oral presentations, media releases
and interviews. Dr Taira is heavily involved in the operation of the
Northern California Earthquake Data Center. Dr Taira’s research has
been recognised by the Young Scientist Award from the Seismological
Society of Japan in 2011 and the Best Young Scientist Poster Award at
the 2013 International Continental Scientific Drilling Program science
conference at Potsdam, Germany. He also received the nation’s top
scientific honour – the Young Scientists’ Prize from the government of
Japan in 2016.
CONTACT
T: (+1) 510 642 8504
E: [email protected]
W: http://earthquakes.berkeley.edu/~taira
FUNDING
National Science Foundation
United States Geological Survey
Southern California Earthquake Centre
France Berkeley Fund
University of California MEXUS (Institute for Mexico and the United
States)
REFERENCES
T Taira and F Brenguier, Response of hydrothermal system to stress
transients at Lassen Volcanic Center, California, inferred from seismic
interferometry with ambient noise, Earth, Planets and Space, 2016, 68,
162.
T Taira, F Brenguier and Q Kong, Ambient noise-based monitoring of
seismic velocity changes associated with the 2014 Mw 6.0 South Napa
earthquake, Geophysical Research Letters, 2015, 42,
6997–7004.
T Taira, DS Dreger and RM Nadeau, Rupture process for microearthquakes inferred from borehole seismic recordings, International
Journal of Earth Sciences, 2015, 104, 1499–1510.
T Taira, R Bürgmann, RM Nadeau and DS Dreger, Variability of fault slip
behavior along the San Andreas Fault in the San Juan Bautista Region,
Journal of Geophysical Research: Solid Earth, 2014, 119, 8827–8844.
T Taira, PG Silver, F Niu and RM Nadeau, Remote triggering of faultstrength changes on the San Andreas fault at Parkfield, Nature, 2009,
461, 636–639.
KEY COLLABORATORS
Paul G. Silver (deceased), Carnegie Institution for Science, USA
Florent Brenguier, Université Grenoble Alpes, France
Fenglin Niu, Rice University, USA
Thomas M. Daley, Lawrence Berkeley National Laboratory, USA
Robert M. Nadeau, University of California, Berkeley, USA
Douglas S. Dreger, University of California, Berkeley, USA
Roland Bürgmann, University of California, Berkeley, USA
Barbara Romanowicz, University of California, Berkeley USA
David R. Shelly, United States Geological Survey, USA
Kiyoshi Yomogida, Hokkaido University, Japan
Robert B. Smith, University of Utah, USA
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