Open-Coast Tsunami Recording and Negative Influence of Infragravity Waves

1. Open-Coast Tsunami Recording and Negative Influence of Infragravity Waves A.B. Rabinovich1,2, F.E. Stephenson2, and D.G. Goring3, 1 P.P. Shirshov Institute of Oceanology, Moscow, Russia 2 Institute of Ocean Sciences, Sidney, B.C. , Canada 3Mulgor Consulting Ltd., Christchurch, New Zealand

2. Upgrated Tsunami Warning System(Canadian Hydrographic Service) Present scheme of tsunami monitoring onthe coast of British Columbia based ontsunami warning and PWLN stations

3. Location of tide gauges on the coast of British Columbia 3 Tsunami Stations 10 PWLN Stations Sampling: 1 min

4. Tsunamis recorded on the BC coast: 1999-2005

5. BC spectra and transfer functions In contrast to the spectra of other stations, spectra of Langara at frequencies >0.02 cpm (periods <50 min) are“white” or “blue ”. The corresponding topographic transfer function at these frequencies is going up.

6. BC tide gauge records An attempt to identify the Hokkaido Tsunami of September 25, 2003 in the BC records. Thisplot clearly demonstrates the problem at Langara station (very high background noise).

7. Location of the New Zealand tide gauges Sampling: 1-minute

8. New Zealand Storm of August 17-21, 2003 Three different instruments at the same site gave almost identical results. That means that high background noise is related to physical factors rather than to instrumental problems.

9. Generation of long background waves (From Rabinovich [1993]) “Long waves” here refers to periods between swell and tides, that is, from about ½ minute to 12 hours… The most conspicuous thing about waves in this frequency range is their absence. Munk [1962]

11. Formation of IG-waves in the ocean From Rabinovich [1993]

12. Map of Langara Island with positions of permanent tidegauge station Langara and temporary station Henslung

13. Langara

14. Stormy Langara…

15. …and quiet Henslung

16. Henslung and Langara records Simultaneous longwave records at theHenslung andLangara stations Zoomed segments

17. New Zealand Location of the Sumner Head and Lyttelton tide gauges

18. Lyttelton and Sumner Head records Simultaneous longwave records at the Sumner Headand Lyttelton stations Zoomed segments

19. New Zealand Spectral ratio of “windy” and “calm”longwave spectra Spectra or background noise in Sumner Headduring “normal” (windy) and very calm weather

20. British Columbia (a) British Columbia New Zealand (b) Spectra of simultaneous records at (a) Henslung and Langara BC, and(b) Lyttelton and Sumner Head, New Zealand.

21. (a) British Columbia (b) New Zealand Spectral ratio and coherence between (a) Langara and Henslung, British Columbia;and (b) Sumner Head and Lyttelton, New Zealand.

22. Three consecutive days Tsunami source function 2001 Peru Tsunami (Mw= 8.4) British Columbia New Zealand Six stations

23. October storm (27.10.2002) British Columbia IG waves IG waves

24. New Zealand: August storm (17-21.08.2003) Source functions reconstructed for four New Zealand stations duringa storm passage

25. Main results: • High-frequency background noise associated with IG-waves at open coast stations (such as Langara and Sumner Head) complicate the recording and detection of tsunamis and, consequently, causes troubles in using these stations for tsunami warning. • Despite common opinion, tide gauges installed in bays and harbours are more appropriate for detection of small tsunamis, even though harbours are affected by resonant seiche oscillations.

26. Based on present analysis, the decision was made to relocate the Langara tsunami warning station from its present location to Henslung Cove. This decision was validated when the recent 2004 Sumatra tsunami was clearly recorded at six harbour-located BC stations, but was not identified at the Langara station.