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