Can we do Earthquake Early
Warning with high-precision
gravity strain meters?
Pablo Ampuero (Caltech Seismolab)
Collaborators: J. Harms (INFN, Italy), M. Barsuglia and E. Chassande-Mottin (CNRS
France), J.-P. Montagner (IPG Paris), S. N. Somala (Caltech), B. F. Whiting (U. Florida)
A multi-disciplinary,
international collaboration:
J. Harms (INFN, Italy)
M. Barsuglia (CNRS France)
E. Chassande-Mottin (CNRS)
J.-P. Montagner (IPG Paris)
S. N. Somala (Caltech)
B. F. Whiting (U. Florida)
Overview
• Earthquake Early Warning Systems: current principles and limitations
• Gravity perturbations induced by earthquakes
• Gravitational Wave detectors: current and future capabilities
• Potential capabilities of an EEWS based on gravity sensors
Mainly based on:
Harms, Ampuero, Barsuglia, Chassande-Mottin, Montagner, Somala and Whiting (2014), Prompt
earthquake detection with high-precision gravity strain meters, manuscript submitted to J. Geophys.
Res., available at http://web.gps.caltech.edu/~ampuero/publications.html
Why do we need Early Warning ?
Expected ground shaking in the Los Angeles basin,
if we had an earthquake of
magnitude M6.5
magnitude M7.0
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Los Angeles
Böse et al., in prep.
Probabilities of events that would cause
at least strong shaking (MMI≥VI)
in the Los Angeles basin
What is Earthquake Early Warning ?
ability to provide a few to tens of seconds of
warning before damaging seismic waves arrive
seismogram
P-wave
• fast
• not damaging
S-wave
• slow
• damaging
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Where is Early Warning used ?
Romania
Taiwan
Turkey
Italy
Japan
Operational systems
Systems under development
Mexico
California
Greece
India
Earthquake Early Warning Demonstration System
How can we use Early Warning ?
1. Public Alert
• warn people to take protective measures (drop-cover-hold on)
• move people to safe positions
• prepare physically and psychologically for the impending shaking
2. Trigger Automatic Responses
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•
•
•
•
slow down/stop trains
control traffic by turning signals red on bridges, freeway entrances
close valves and pipelines
stop elevators
save vital computer information
Limitations:
• chance of false/wrong alerts: need to account for finite rupture size
• no warning in blind zone (~30 km around epicenter)
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Arrays Networked to Track Sources
a network of high-frequency seismic arrays that
will image large earthquakes
with 10-fold better resolution than current
seismic networks
Multiple small-aperture arrays with overlapping fields of view covering a set of faults
Exploit high-frequency waves (10 Hz) to achieve high resolution of rupture processes
ANTS - Pablo Ampuero - Caltech Seismo Lab
The blind zone of an EEWS
Blind zone size in California (Kuyuk and Allen, 2013)
• Blind zone = region close to the
earthquake epicenter where damaging
waves arrive before the warning is
declared
• Size of the blind zone = distance
travelled by S waves at the time the 4th
seismometer detects shaking + signal
processing time + communication
delays
• Can we use geophysical signals that
travel faster than seismic waves?
Static gravity changes induced by earthquakes
• GRACE / GOCE satellite
mission have measure
gravity changes after
vs before large
earthquakes
• Those are STATIC
gravity changes
• Mention Kamioka
superconducting
gravimeter
Matsuo and Heki (2011)
Dynamic gravity changes induced by
earthquakes: theory
Gravity perturbation (acceleration) is related to a potential
𝛿𝑎 = −𝛻𝜓
that satisfies a Poisson’s equation
𝛻 2 𝜓 = −4𝜋𝐺 𝛿𝜌
The density perturbation induced by the seismic deformation is
Assume point-source earthquake in an infinite elastic medium (ignore
free surface effects): use known analytical solutions for the induced
seismic displacement field 𝜉 𝑟, 𝑡
Dynamic gravity changes induced by
earthquakes: theory
We find that the perturbation of the gravity potential is
Distance
 Gravity strain acceleration:
Radiation pattern
Double integral
of seismic moment
Verification: comparison to numerical simulation
We implemented finite kinematic sources and
computation of gravity field in the 3D spectral
element program SPECFEM3D
We find that
errors are smaller
than 5%
http://geodynamics.org/cig/software/specfem3d/
Verification: comparison to numerical simulation
We implemented finite kinematic sources and
computation of gravity field in the 3D spectral
element program SPECFEM3D
http://geodynamics.org/cig/software/specfem3d/
The signal decays as 1/𝑟 5 ,
as predicted,
even for dipping faults
Earthquake spectra compared to gravity sensitivity
Gravity strain acceleration:
Relation to moment rate
function:
𝑑5ℎ
∝ 𝑀0 (𝑡)
𝑑𝑡 5
Epicentral distance = 70 km
Gravitational wave detectors
Devices designed to measure
gravitational waves, minute
distortions of space-time that are
predicted by Einstein's theory of
general relativity
GW: new way to study the universe
Ex: VIRGO, LIGO projects to observe
GW of cosmic origin
(Laser Interferometer GravitationalWave Observatory)
Gravitational wave detectors
Devices designed to measure
gravitational waves, minute
distortions of space-time that
are predicted by Einstein's
theory of general relativity
Ex: TOBA
TOBA concept (torsional bar antenna)
Gravitational wave detectors
Devices designed to measure
gravitational waves, minute
distortions of space-time that
are predicted by Einstein's
theory of general relativity
Ex: TOBA
TOBA concept (torsional bar antenna)
Earthquake spectra compared to gravity sensitivity
Gravity strain acceleration:
Relation to moment rate
function:
𝑑5ℎ
∝ 𝑀0 (𝑡)
𝑑𝑡 5
Epicentral distance = 70 km
Signal to noise ratio
Shortest period resolved
Optimal matched filter detection (with prewhitening)
Conclusions
• Multidisciplinary research, from fundamental to applied, from paperand-pen to high-performance-computing and instrument design
• Next generation GW detector technology can be useful in Earth
science: potential contribution to Earthquake Early Warning Systems
• Advantage over other EEWS approaches: reduces the blind zone
 Earthquake warning sooner and for all
• To do:
• develop signal detection pipeline and demonstrate its capabilities
• Propose an optimal system
• Theory: incorporate free surface effects, etc