low-frequency rotational seismology: observations with ... · conclusions! ring laser technology is...
TRANSCRIPT
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Low-Frequency Rotational Seismology:Observations with Ring Laser Technology
H. Igel1, A. Cochard1, A. Fichtner1, A. Flaws1,2, U. Schreiber3 , B.Schuberth1, W. Suryanto1, D.N. Pham1, A. Velikoseltsev3
, F. Vernon4, J.
Wassermann1, M. Bernauer1, F. Bernauer1, D. Kurrle1
1Department of Earth and Environmental Sciences, LMU Munich2Department of Physics and Astronomy, Christchurch, New Zealand3Fundamentalstation Wettzell, Kötzting4IGPP, La Jolla, USA
! What is rotation in seismology? (Why bother?)
! The ring laser instrument
! Observations: What to do with rotations?! Waveform comparison with translations! Tomography without travel times! Free oscillations of the Earth!!!
! Array-derived vs. directly measured rotations
! Conclusions and Outlook
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Rotation is the curl of the wavefield… it separates P- and S-waves in isotropic media
!!!
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Ground velocity
Seismometer
Rotation rate
Rotation sensor
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Rotation from seismic arrays?... by finite differencing ...
xyyxzvv !"!#$
vy
vx
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vy
vx
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Rotational motion
estimated from
seismometer recordings
seismometers
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Rotations – why bother?
! Standard seismological observations are
contaminated by rotations (e.g. OBS, strong motions,
free oscillations)
! Tiltmeters (rotation around horizontal axes) are
contaminated by translations
! 6C sensor may allow integration to displacements
! Rotations may contribute to co-seismic structural
damage
! Rotational measurements provides additional
wavefield information (phase velocities, structure,
propagation direction, anisotropy, etc)
! ... and may allow putting further constraints on
rupture processes ...
Instruments
Earthquake engineering
Tomographyand sources
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THEORY
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Radiation froma point source
Near fieldterm containsthe staticdisplacement
Intermediateterms
Far fieldterms: themain ingredientfor sourceinversion, raytheory, etc.
Ground displacement
Aki and Richards (2002)
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The rotational part
• Rotations are zero before S arrival
• Far-field P-rotation is not zero! Only the sum ofall contributions cancel!
Cochard et al. (2006)
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Basic seismograms, full space
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Rotation rate and transverse accelerationplane-wave propagation
Plane transversely polarized wave propagating in x-direction with phase velocity c
kctkxftxuy /)(),( !! ="=
)(),(),( 2 tkxftxutxa yy !! "##== !!Acceleration
ctxtxa 2),(/),( !="!
Rotation rate and acceleration should be in phase and the amplitudesscaled by two times the horizontal phase velocity
Rotation rate [ ] !"
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2
1,0,00,,0
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1),( tkxfkutx y ,,!!
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Instrumentation
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The ring laserat Wettzell
ring laser
Data accessible at www.rotational-seismology.org
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How can we observe rotations?-> ring laser
Ring laser technology developed by thegroups at the Technical University Munichand the University of Christchurch, NZ
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Ring laser – the principle
PfSagnac
!
AÙ "=#4
A surface of the ring laser (vector)" imposed rotation rate (Earth‘s rotation +
earthquake +...)# laser wavelength (e.g. 633 nm)P perimeter (e.g. 4-16m)$f Sagnac frequency (e.g. 348,6 Hz sampled at
1000Hz)
Resolution down to O(10-11) rad/s
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The Sagnac Frequency(schematically)
Tiny changes in the Sagnacfrequencies are
extracted to obtainthe time series with
rotation rate$f -> %
Sagnac frequency sampled with 1000Hz (Reftek)-> instantaneous freq. using Hilbert transform
Rotation rate sampled with 20Hz
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The PFO sensor… built for seismology
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The PFO sensormode hopping
... mode hops can be avoided by cleaning through getterfunction (reducing outgassing)
Sag
nac
sign
al
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Cross-axis sensitivity:tilt-Ringlaser coupling
… negligible for far-field observations …
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Horizontal axes?
! First application inVIRGO project(gravitational wavedetection, Pisa, Italy)
! 3C planned for PFO
! Options to get around„lock-in“ problem:! 45 degree
arrangement,projections
! Running RL with„dithering“ out of lock-in
RL1RL2
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Data, Data, Data
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Mw = 8.3 Tokachi-oki 25.09.2003transverse acceleration – rotation rate
From Igel et al., GRL, 2005
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PFO Observations
Kamtchatka M7.6
California M3.9
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Compatibility with MS(surface wave magnitude)T=30s, c=4300m/s
3.3log66.1log 1010 ++= DT
AM
S
3.9log66.12
2
2
10102),(2!!
=="DM
Sz
S
cTDMA
cT
##
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Instrument correction!transverse acceleration – rotation rate
Time (s)
Before correction
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Instrument correction!transverse acceleration – rotation rate
After correction
Time (s)
... an independent confirmation of the quality ofthe instrument correction ...
Time (s)
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Love wave dispersiontransverse acceleration – rotation rate
From Igel et al. (GJI, 2007)
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… dispersion …M7.4 Kuril islands, 15/1/09
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Love wave dispersion from 28 events
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… why the large variations?
… overtones alone can do it …(Kurrle et al., in preparation)
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Sumatra M8.3 12.9.2007
P
P Coda
Modelling the ratio of Energy in P-Rotations and Translationsallows putting constraints on crustal scattering (Pham et al.,
BSSA, 2009)
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Adjoint method and rotationssensitivity kernels
Fichtner and Igel, 2009, BSSA.
Sensitivity of travel times (or rotationamplitudes alone)
Sensitivity of acceleration/rotation(apparent shear wave speed)
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… tomography without travel times!
… the observables are frequency dependent amplitude ratios ofrotations and translations …
Bernauer et al, Geophysics, in print
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EARTHQUAKE on 29/09/2009 at 17:48(UTC) SAMOA ISLANDS REGION 164 km SSiumu MAGNITUDE: Mw 7.9
Widmer et al., BSSA, 2009
Ring laser record, Wettzell, Germany
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… long periods ….
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First observations ofeigenmodes with ring laser!
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Synthetics ….
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Rotations using Seismic Arrays
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Array measurementsDec 2003-Mar 2004
xyyxzvv !"!#$
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First comparison of array-derived rotations (black)and direct ring laser measurements (red)
From Suryanto et al (2006, BSSA)
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Array vs. direct
Wassermann et al., 2009, BSSA.
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Step table tests with low-resolution sensors
See poster byWassermann et al!
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Conclusions
! Ring laser technology is the most promising technologyfor broadband far-field observations (Hz – mHz range,10-11-10-6 rad/s)
! Measuring tilts (horizontal component of rotation) inbroadband seismology is a fundamental researchquestion!
! Collocated observations of rotations and translations areproviding additional structural information!
! Our current state of knowledge:! Broadband, far field: ring lasers
! Seismological applications, portable: fiber-optic gyros
! Strong motion: fluid-based sensors (e.g., R1)
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Future
! A 6C permanent observatory (with surrounding BBarray) at PFO
! Cooperation with Gravitational Wave Experiments (e.g.,LIGO, VIRGO)
! Applications in earthquake engineering, sourceproblems, reservoir seismics, strong motion seismology,ocean bottom seismology, planetology
! Tilts, tilts, tilts!
-> www.rotational-seismology.org (IWGoRS)
-> IWGoRS Meeting in Europe, provisional schedule for2nd Meeting in Prague 12-15 October, 2010
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Using Surface Wave Ray Theory
Ferreira and Igel, 2009, BSSA.
l >> 1
Love wave dispersion from point measurement!
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Max. cross-corr. coefficient in sliding time windowtransverse acceleration – rotation rate
Small tele-seismic event
P-onset
S-waveLove waves Aftershock
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M8.3 Tokachi-oki, 25 September 2003phase velocities ( + observations, o theory)
From Igel et al. (GRL, 2005)
Horizontal phase velocity in sliding time window
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Real vs. Synthetics : Papua event
Cochard et al., 2006
Observations
Synthetics
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Collocated measurements of translations and rotationsGrenoble Basin (synthetics) and the LA basin
Apparent shear velocity from ratio acceleration/rotation
Basin model
Stupazzini et al., 2009, BSSA, and Wang et al. BSSA, 2009
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Rotational signals in the P-coda?frequency dependence
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… observable for all events!
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Rotational signals in the P-coda?azimuth dependence
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Love waves – direction of energy… from rotations and translations measured at one point …
correlations in Love wave window
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P-Coda energy direction… comes from all directions …
correlations in P-coda window
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P-SH scatteringsimulations with ADER-DG
translations
rotations
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P-SH scatteringsimulations with ADER-DG
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Ratio of Rotational energy vs.Translational energy
Observedrange of
energy ratios
Pham et al., 2008, BSSA, subm.
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Love wave wave rotationsJWKB synthetics
3D
PREM
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Love wave dispersion-> S-wave tomography
Ferreira and Igel., 2009, BSSA.
Acceleration
Spectral ratio
Rotation
Oceanic Crust Continental Crust