A nearfield Tsunami warning system in Taiwan by unit tsunami method
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A nearfield Tsunami warning system in Taiwan by unit tsunami method
Po-Fei Chen1, Yun-Ru Chen2, Bor-Yaw Lin1,3, Wu-Ting Tsai2
1. Institute of Geophysics, National Central University2. Institute of Hydrological and Oceanic Sciences, National Central University3. Central Weather Bureau, Taipei
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Outline Motivation Tsunami simulation Unit tsunami method Database Taiwan Rapid Earthquake Information
Release System (TREIRS) Discussion Conclusions
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(Ho, 1986; Angelier, 1986)
The convergence rates is about 7~8 cm/yr.The convergence rate is 7~8 cm/yr.
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It is necessary to build a nearfield tsunami warning system in Taiwan.
seismicities
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There are three stages for tsunami simulations.
Generation Propagation Runup
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Generation Earthquake parameters(Lon., Lat., depth, magnitude)--------group1 (moment, strike, rake, dip)-----------group2
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Generation
Scaling law (Geller, 1976)
We infer D, L , W from M.
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Generation Input (Lon., Lat., depth) and (L, W, strike, rake, dip) to calculate the
seafloor displacements (Okada, 1985) Elastic Dislocation Theory Project to sea surface elevation for
propagation stage
Different time scale
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Generation
However, (moment, strike, rake, dip) are obtained from fitting teleseismic w
aveforms, a process too slow for nearfield tsunami warning.
Get back to this later!
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Propagation
wavelength >> water depthThe tsunami propagation on open seas is well
modelled by the shallow water wave equation.
However, this is the most time consumingstage in tsunami simulation. In this study, we
focus on calculating this stage in advance and store the results in database.
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Runup Linear v.s. Nonlinear
Linear approximation breaks downwhen amplitude greater than 0.1 water depth.
Fortunately, for this study,we wish to forecast the amplitudes of the approaching tsunamis.Runup stage is not included. The system is linear.
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Linear shallow water wave equation
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Bottom friction=0
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onaccelerati nalgravitatio :
elevation surface free :
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Unit tsunami method
The tsunami waves can be expressed as a linear combination of unit tsunamis (Lee et al., 2005).
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(Lee et al., 2005)
1 meter hight
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(Lee et al., 2005)
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(Lee et al., 2005)
The displacements at the location of unit tsunami determine the coefficient of that unit tsunami.
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We apply COMCOT (Liu et al., 1998) to solve linear shallow water wave equation in Cartesian coordinates.
The propagation of the unit tsunami is simulated. Grid size : 1 min. Time step : 1 sec Radiation on map boundary Total reflection on ocean-land boundary Total time run time : 2hr
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200612261226A TAIWAN REGION Date: 2006/12/26 Centroid Time: 12:26:27.6 GMT Lat= 21.83 Lon= 120.39 Depth= 22.5 Half duration= 7.3 Centroid time minus hypocenter time: -4.4 Moment Tensor: Expo=26 -2.970 0.603 2.370 0.790 -1.220 -1.230 Mw = 6.9 mb = 0.0 Ms = 6.8 Scalar Moment = 3.32e+26 Fault plane: strike=155 dip=32 slip=-86 Fault plane: strike=330 dip=58 slip=-93
Global GMT solution
Pingtung earthquake
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(Okada, 1985)
Standard method
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How about all land and partial land unit source?
Unit tsunamimethod
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f6
f5
f4
f3
f2
f1
I1
I2
I3
I4
I5
I6
1m28km × 28km
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a1
a2
a3
a4
a5
a6
f6
f5
f4
f3
f2
f1
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Quick conclusion
The tsunami waves calculated by the unit tsunami method are consistent with those calculated by standard method.
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(1, 1) (2, 1)
(1, 2)
…
.……
……
……
..…
……
…..
(1, 36)………………
(1, 1)
(1, 36) (32, 36)
(32, 1)
(32, 36)
01
1211 10
09
08070605
04 03
02
20
161514
13
32
3029
28
27
2625
2423
2221
31
1918
17
Tidal Station No.18
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Building database
We have a total of 32 tidal stations.
For each station, we store 32 × 36 waveforms of unit tsunamis.
The total number of traces stored in the database is 32×32×36.
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Generation However, (moment, strike, rake, dip) are obtained from fitting teleseismic wavefor
ms, a process too slow for nearfield tsunami warning.
Now, back to this. We need to incorporate the “Taiwan Rapid
Earthquake Information Release System (TREIRS)” of the CWB to make this warning system work.
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32
地震儀
Station Instrument A900A & S13
Broadband sensor
監測台灣地區地震活動
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TREIRS is done by the accelerograph network.
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For earthquake in this region, TREIRS is capable of reporting its Lon., Lat.,depth and local magnitude within 3~5 mins.
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determinestrike, dip,rake empirically
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Lon., Lat., depth, ML of earthquakes
Lon., Lat., depth, L, W, D, strike, rake, dip
Calculate sea floor displacements
Determine coefficients of each unit tsunami
Linear combination of unit tsunamis from database for each station
Forecast arrival time and maximum amplitude of the approaching tsunamis for each station
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Discussion Advantages of unit tsunami method(1) By calculating the time consuming part of
wave propagation in advance, the warning system is able to do rapid forecasting for nearfield tsunamis.
(2) By determining the coefficients on an event by event basis, the system is flexible to cover all scenario earthquakes with a reasonable size of database.
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Discussion
Size of unit tsunami(1) Small size can resolve the fine
features of sea floor displacements.
(2) Large size can keep the long wave approximation valid.
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Discussion
Tsunami earthquakesTsunami earthquakes are generally defined as those which generate much larger tsunami than expected from their size measured over the seismic frequency band (Kanamori, 2006).(Kanamori, 2006).
In other words, slow earthquakes.In other words, slow earthquakes. E/M deficiency.E/M deficiency.
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Discussion
Energy calculationEnergy calculation Using generalized P wave (P, pP, sP) record
ed at teleseismic broadband stations. Correction for radiation patternradiation pattern, attenuatiattenuati
onon, and geometrical spreadinggeometrical spreading. Definition of Θ=log10(E/M) (Newman and Okal, 1998)(Newman and Okal, 1998)
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Discussion
No tsunami earthquakes for recent offshore earthquakes around Taiwan (Chen et al., 2008).
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Discussion Saturation of body wave magnitude -
potential problem for ML to moment conversion
Potential solution – Development of an Earthquake early
warning system using real-time strong motion signals
(Wu and Kanamori, 2008).
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Conclusions The tsunami waves calculated by the unit
tsunami method are consistent with those calculated by standard method.
The unit tsunami method is flexible to cover scenario earthquakes with a reasonable size of database.
Combined with TREIRS, a nearfield tsunami warning system in Taiwan is feasible.
The method may extend to build a warning system for SCS region.