Importance of Broadband Seismic and Barometric Observations at Magnetic Observatories Toshihiko Iyemori 1 , Kent Taira 1 , Mitsuru Matsumura 1 , and Desheng Han 2 1 Graduate School of Science, Kyoto University, Kyoto, Japan 2 Polar Research Institute of China, Shanghai, China
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Importance of Broadband Seismic and Barometric Observations at Magnetic Observatories Toshihiko Iyemori 1, Kent Taira 1, Mitsuru Matsumura 1, and Desheng.
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Importance of Broadband Seismic
and Barometric Observations at Magnetic Observatories
Toshihiko Iyemori1, Kent Taira1, Mitsuru Matsumura1,
and Desheng Han2
1Graduate School of Science, Kyoto University, Kyoto, Japan2Polar Research Institute of China, Shanghai, China
NAK - H 2009.07.17 16:00 – 17:00UT
1-sec averaged data fromDMI Fluxgate magnetometer (FGE-91)(Spec.: 0.1nT resolution)
(NAK)
Nakanoshima
100km
Noise or Signal ??
KNY - H 2009.07.17 16:00 – 17:00UT NAK - H 2009.07.17 16:00 – 17:00UT
★Coincidence of the magnetic variations at Tonghai(THJ) and that of the seismic wave arrival at Kunming(KMI) → magnetic variations caused by the seismic waves
THJ magnetic field
KMI displace-ment
IRIS broad-band seismometer (20Hz1sec averaged )
Fluxgate magnetometer
● PHI
Importance of colocated Broadband seismic and magnetic measurements
1nT
0.4m
epicenter
Characteristics of the magnetic and seismic variations
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・ Ground displacement: N-S component precedes vertical by 90 degrees → Rayleigh wave (E-W comp. → Love wave)
・ Period (frequency) shift 30 sec →20 sec
Power spectral density ( MEM ) Red : Mag. H-comp. Blue : Vertical displacement
V
Displacement at Kunming
N precedes V by 90 degrees Reyleigh wave
Time
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Inclination of the sensors cannot reproduce the observed magnetic oscillation because; 1. amplitude is twice of the effect of the sensor inclination 2. magnetic oscillation is polarized in the H-Z plane
Observed H and Z
Effect of sensor inclination
Observed displace-ment
Crustal dynamo
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J=σ(V×B+E)
V
V
Crustal dynamo mechanism is simple compared with Piezo magnetic effect or fluid effect
GGGrownd motion Induced mag.
Simulation of magnetic variation by the crustal dynamo mechanism
Assumption・ Displacement decreases to zero at the depth of 1.5x(wave length)・ Effective resistivity is 50Ωm including the upper mantle ・ The phase velocity of the Reileigh wave with period of 25 second is 3.83km/s (Aki and Richards, 2002)
Depth dependence of amplitude of the Rayleigh wave (Uzu,2001)
Observed displacement is used as the surface displavement and the underground displacement is estimated by the model below.
Magnetic effect from sensor inclination
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Estimation of the sensor inclination
Removing the sensor inclination effect
・ Phase velocity of the Rayleigh wave: 3.83km/s Correction of the distance between KMI and THJ Magnetic data are shifted 31 seconds behind ・ Coordinate transformation (rotation) to separate the Rayleigh wave from the Love wave The epicenter is in the direction of wave propagation
Displacement in 2 sec
Nenative H
Negative Z
Correction based on the velocity of the seismic wave necessary
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Observation
Effect of sensor inclination
Observed – Effect of inclination For H and Z component.
Estimated crustal dynamo effect
Ground displacement
Comparison of observed magnetic variation with the estimation by a crustal dynamo simulation
SUMMARY
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1. Magnetic variations caused by the vertical acoustic resonance between the ground and ionosphere via the ionospheric dynamo have been detected at various occations.
2. To identify the cause of small amplitude magnetic oscillations, micro-barometric observation is important. It could also be useful for meteorological research.
3. Magnetic variation with amplitude of about 0.1-0.2 nT was observed during the passage of the Rayleigh wave just after the Great Sumatra earthquake.
4. The amplitude and phase relation of H and Z component agree well with an estimation by a crustal dynamo simulation with observed seismic wave (Rayleigh wave) data.
5. The effective resistivity necessary to explain the observed amplitude is about 50(Ωm).
6. For quantitative analysis, it is desirable to put a broad-band seismometer at the same location with high-
resolution magnetometer.
The 1st ICSU World Data System Conference – Global Data for Global Science
September 3 – 6, 2011 in Kyoto, Japan
Thank you for your attention !
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Chishima Islands Oki Earthquake (2007.01.13) Mag. at MMB & Displacement at KNPMemambetsu (MMB) Geomagnetic fields
Kunnep (KNP) Broadband seismometer
D
Another example
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Love wave dominates in this event Direction of VxB vector is parallel to the wave propagation Electric current does not flow(?) V
B
VxB
Wave propagation
(J)
Crustal dynamo origin
Acoustic wave origin
Internal Gravity wave origin
Pi2, Pi1
SC
Substorm, Storm
Sq
( ~ upper limit)
Period (second)
Frequency (Hz)
nT
Amplitude Spectra of Rapid Magnetic Variations
Almost all of the short period magnetic variations have been assumed to be of ionospheric or magnetospheric origin. Events of Crustal and/or lower atmospheric origin