W Phase Inversions and Seismic Tsunami Warning System in...
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W Phase Inversions and Seismic Tsunami Warning System in Taiwan
for Manila Trench Earthquakes
Po-Fei Chen
Department of Earth Sciences, National Central Universty
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Outline
Motivation
Unit tsunami method
Source inversion of W phase
Conclusions
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Soloviev and Go (1974); 許明光,李起彤 (1996)
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1. Consider model of Earthquake Rupture(GCMT too slow, solution: source inversion of W phase)
2. Compute Static Seafloor Deformation (quickly done by Okada(1985))
3. Interpret the deformation as Initial Condition for Vertical Surface Displacement w ith Zero Initial Velocity
4.Simulate its Propagation(time consuming, solution: unit tsunami method)
Steps for classical tsunami simulation(Okal, 2008):
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Propagation
wavelength >> water depth
The tsunami propagation on open seas is well modelled by the shallow water wave equation.
However, this is the most time consumingstage in tsunami simulation.
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Runup
Linear v.s. Nonlinear Linear approximation breaks downwhen the amplitude is greater than 0.1 water depth.
If we only wish to forecast the amplitudes of the approaching tsunamis. The Runup stage can be excluded and the system is linear.
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Unit tsunami method
For a linear system, the tsunami waves can be expressed as a linear combination of unit tsunamis (Lee et al., 2005).
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0.5°0.5°
1m
unit tsunami event
01_01
14_01
01_10
14_10
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32 tidal stations
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We apply COMCOT (Liu et al., 1998) to simulate the propagation of unit tsunami by solving linear shallow water wave equation in Spherical coordinates.
Grid size : 1 min. Time step : 1 sec Radiation on map boundary
Total reflection on ocean-land boundary
Total time run time : 4hr
Cornell Multigrid Coupled Tsunami Model (COMCOT) is used.
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08_06; y-axis: cm; x-axis: min
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STA/LTA
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Arrival time in minutes
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14_10; y-axis: cm; x-axis: min
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STA/LTA
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Arrival time in minutes
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0.5°0.5°
1m
unit tsunami event
01_01
14_01
01_10
14_10
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Scenario earthquake at 121˚E, 20˚N, 25 kmMw=9.0, strike 10˚, dip 20˚, rake 90˚
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Unit Tsunami method
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Real simulation
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Unit Tsunami method Real simulation
Maximum amplitudes
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Source inversion of W phase(Kanamori and Rivera, 2008)
•A long-period phase, up to 1000 s, arrives before S phase.
•In ray theory, superposition of long-period P, PP, SP and S.
•In normal model theory, superposition of the fundamental mode, first, second and third overtones of spheroidal modes at long period.
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•the long period information•faster speed than the traditional surface waves suitable for rapid and robust determination of seismic source parameters for tsunami warning purposes (Kanamori and Rivera, 2008)
20 min
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(Kanamori and Rivera)
Regional Distance(△ ≦ 12°)
Using F-net data
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Extended BATS
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Broadband Array in Taiwan for Seismology (BATS)
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1996~
Mw > 6.5
16 events
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Data Processing (I)1. The vertical components of BATS data is deconvolved
to displacements with a pass-band of 0.0167 Hz (600 s) to 0.01 Hz (100 s), using a time domain recursive method (Zhu 2003).
2. A time duration from the beginning of P to 15△ s is windowed to contain the most of the W-phase energy.
3. Time windows of the observed waveforms are concatenated for inversion.
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An example of waveform (filtered, windowed, and concatenated)
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Data Processing (II)
4. We compute the theoretical W phase using normal mode summation with pre-computed modes stored in database.
5. The synthetic waveforms are filtered, windowed and concatenated, the same as observed.
6. A linear inversion is performed to obtain the seismic moment tensor using a given hypocenter location and the origin time.
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(Kanamori and Rivera, 2008)
Gm=d
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Red: syntheticsBlack: observations
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Keep stations of good fitting
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Two scenarios
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Centroid Location (I)
1
2
3
4
5
6
7
8
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CentroidLocation(II)
9
10
11
12
13
14
15
16
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1996~
Mw > 6.5
16 events
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PDE Location (I)
1
2
3
4
5
6
7
84
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PDE Location (II)
9
10
11
12
13
14
15
16
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1996~
Mw > 6.5
16 events
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Extended BATS
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Conclusions Database of unit tsunamis is able to predict
arrival times to tidal stations immediately after the occurrence of earthquake.
With earthquake parameters, the database can also quickly predict maximum tsunami amplitudes of tidal stations.
W phase inversion can rapidly provide earthquake source parameters. The construction of extended BATS would enhance its future application.