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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
magnitude M6.5 magnitude M7.0
Why do we need Early Warning ?
Expected ground shaking in the Los Angeles basin,if we had an earthquake of
Böse et al., in prep.
4
Los Angeles
Probabilities of events that would cause at least strong shaking (MMI≥VI)
in the Los Angeles basin
5
San Andreas Fault
S-P time
P-Wav
e
S-Wav
e
What is Earthquake Early Warning ?
P-wave• fast• not damaging
• slow• damaging
seismogram
S-wave
ability to provide a few to tens of seconds of warning before damaging seismic waves arrive
JapanTaiwan
MexicoTurkey
Romania
Italy
Greece
India
Operational systemsSystems under development
Where is Early Warning used ?
California
Earthquake Early Warning Demonstration System
7
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
How can we use Early Warning ?
2. Trigger Automatic Responses• 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)
ANTS - Pablo Ampuero - Caltech Seismo Lab
Fault
Arrays Networked to Track Sources
Multiple small-aperture arrays with overlapping fields of view covering a set of faultsExploit high-frequency waves (10 Hz) to achieve high resolution of rupture processes
a network of high-frequency seismic arrays that will image large earthquakes
with 10-fold better resolution than current seismic networks
The blind zone of an EEWS
• 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?
Blind zone size in California (Kuyuk and Allen, 2013)
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
that satisfies a Poisson’s equation
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
Dynamic gravity changes induced by earthquakes: theoryWe find that the perturbation of the gravity potential is
Distance Radiation pattern Double integral of seismic moment
Gravity strain acceleration:
Verification: comparison to numerical simulation
http://geodynamics.org/cig/software/specfem3d/
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%
Verification: comparison to numerical simulation
http://geodynamics.org/cig/software/specfem3d/
We implemented finite kinematic sources and computation of gravity field in the 3D spectral element program SPECFEM3D
The signal decays as , as predicted,
even for dipping faults1/𝑟 5
Earthquake spectra compared to gravity sensitivity
𝑑5h𝑑𝑡5 ∝ �̇�
0
(𝑡)
Gravity strain acceleration:
Relation to moment rate function:
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 Gravitational-Wave Observatory)
Gravitational wave detectors
TOBA concept (torsional bar antenna)
Devices designed to measure gravitational waves, minute distortions of space-time that are predicted by Einstein's theory of general relativity
Ex: TOBA
Gravitational wave detectors
TOBA concept (torsional bar antenna)
Devices designed to measure gravitational waves, minute distortions of space-time that are predicted by Einstein's theory of general relativity
Ex: TOBA
Earthquake spectra compared to gravity sensitivity
𝑑5h𝑑𝑡5 ∝ �̇�
0
(𝑡)
Gravity strain acceleration:
Relation to moment rate function:
Epicentral distance = 70 km
Signal to noise ratio
Shortest period resolved
Optimal matched filter detection (with prewhitening)
Preliminary
Conclusions
• Multidisciplinary research, from fundamental to applied, from paper-and-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