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

Outline Motivation Tsunami simulation Unit tsunami method Database Taiwan Rapid Earthquake Information

Release System (TREIRS) Discussion Conclusions

(Ho, 1986; Angelier, 1986)

The convergence rates is about 7~8 cm/yr.The convergence rate is 7~8 cm/yr.

It is necessary to build a nearfield tsunami warning system in Taiwan.

seismicities

There are three stages for tsunami simulations.

Generation Propagation Runup

Generation Earthquake parameters(Lon., Lat., depth, magnitude)--------group1 (moment, strike, rake, dip)-----------group2

Generation

Scaling law (Geller, 1976)

We infer D, L , W from M.

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

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!

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.

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.

Linear shallow water wave equation

0

0

0

Hy

ςgH

t

Q

Hx

ςgH

t

P

y

Q

x

P

t

ς

y

x

Bottom friction=0

stressesshear bottom :,

),(flux volume: Q,

onaccelerati nalgravitatio :

elevation surface free :

x y

hvQhuPP

g

Unit tsunami method

The tsunami waves can be expressed as a linear combination of unit tsunamis (Lee et al., 2005).

(Lee et al., 2005)

1 meter hight

(Lee et al., 2005)

(Lee et al., 2005)

The displacements at the location of unit tsunami determine the coefficient of that unit tsunami.

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

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

(Okada, 1985)

Standard method

How about all land and partial land unit source?

Unit tsunamimethod

f6

f5

f4

f3

f2

f1

I1

I2

I3

I4

I5

I6

1m28km × 28km

a1

a2

a3

a4

a5

a6

f6

f5

f4

f3

f2

f1

Quick conclusion

The tsunami waves calculated by the unit tsunami method are consistent with those calculated by standard method.

(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

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.

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.

32

地震儀

Station Instrument A900A & S13

Broadband sensor

監測台灣地區地震活動

TREIRS is done by the accelerograph network.

For earthquake in this region, TREIRS is capable of reporting its Lon., Lat.,depth and local magnitude within 3~5 mins.

determinestrike, dip,rake empirically

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

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.

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.

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.

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)

Discussion

No tsunami earthquakes for recent offshore earthquakes around Taiwan (Chen et al., 2008).

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).

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.