A nearfield Tsunami warning system in Taiwan by unit tsunami method

1. A nearfield Tsunami warning system in Taiwan by unit tsunamimethod Po-Fei Chen1, Yun-Ru Chen2, Bor-Yaw Lin1,3, Wu-Ting Tsai2 1. Institute of Geophysics, National Central University 2. Institute of Hydrological and Oceanic Sciences, National Central University 3. Central Weather Bureau, Taipei

2. Outline • Motivation • Tsunami simulation • Unit tsunami method • Database • Taiwan Rapid Earthquake Information Release System (TREIRS) • Discussion • Conclusions

3. (Ho, 1986; Angelier, 1986) The convergence rate is 7~8 cm/yr. The convergence rates is about 7~8 cm/yr.

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

5. There are three stages for tsunami simulations. • Generation • Propagation • Runup

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

7. Generation Scaling law (Geller, 1976) We infer D, L , W from M.

8. 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

9. Generation • However, (moment, strike, rake, dip) are obtained from fitting teleseismic waveforms, a process too slow for nearfield tsunami warning. Get back to this later!

10. 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 consuming stage in tsunami simulation. In this study, we focus on calculating this stage in advance and store the results in database.

11. Runup • Linear v.s. Nonlinear Linear approximation breaks down when 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.

12. Bottom friction=0 Linear shallow water wave equation

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

14. 1 meter hight (Lee et al., 2005)

15. (Lee et al., 2005)

16. The displacements at the location of unit tsunami determine the coefficient of that unit tsunami. (Lee et al., 2005)

18. 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

19. Pingtung earthquake 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

20. (Okada, 1985) Standard method

21. Unit tsunami method How about all land and partial land unit source?

22. f6 f5 f4 I6 f3 I5 I4 I3 f2 I2 I1 f1 1m 28km × 28km

23. f6 f5 f4 a6 f3 a5 a4 a3 f2 a2 a1 f1

27. Quick conclusion • The tsunami waves calculated by the unit tsunami method are consistent with those calculated by standard method.

28. (1, 36) (32, 36) Tidal Station No.18 01 ……………… (1, 36) (32, 36) 02 32 .………………..………….. 04 31 03 05 30 06 29 07 08 28 09 10 11 27 12 13 26 14 25 15 24 17 16 23 18 19 20 22 21 (1, 2) … (1, 1) (2, 1) (1, 1) (32, 1)

29. 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.

30. Generation • However, (moment, strike, rake, dip) are obtained from fitting teleseismic waveforms, 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 監測台灣地區地震活動 Broadband sensor 32

33. TREIRS is done by the accelerograph network.

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

36. determine strike, dip, rake empirically

38. 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

39. Discussion • Advantages of unit tsunami method • By calculating the time consuming part of wave propagation in advance, the warning system is able to do rapid forecasting for nearfield tsunamis. • 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.

40. Discussion • Size of unit tsunami • Small size can resolve the fine features of sea floor displacements. • Large size can keep the long wave approximation valid.

41. Discussion • Tsunami earthquakes are generally defined as those which generate much larger tsunami than expected from their size measured over the seismic frequency band (Kanamori, 2006). • In other words, slow earthquakes. • E/M deficiency.

42. Discussion Energy calculation • Using generalized P wave (P, pP, sP) recorded at teleseismic broadband stations. • Correction for radiation pattern, attenuation, and geometrical spreading. • Definition of Θ=log10(E/M) (Newman and Okal, 1998)

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

44. 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).

46. 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.