Open-Coast Tsunami RecordingOpen-Coast Tsunami Recording and and

Negative Influence of Infragravity WavesNegative Influence of Infragravity Waves

A.B. RabinovichA.B. Rabinovich1,21,2, F.E. Stephenson, F.E. Stephenson22, and D.G. Goring, and D.G. Goring33, ,

11 P.P. Shirshov Institute of Oceanology, Moscow, Russia P.P. Shirshov Institute of Oceanology, Moscow, Russia

22 Institute of Ocean Sciences, Sidney, B.C. , Canada Institute of Ocean Sciences, Sidney, B.C. , Canada

33 Mulgor Consulting Ltd., Christchurch, New ZealandMulgor Consulting Ltd., Christchurch, New Zealand

Present scheme of tsunami monitoring

on the coast of British Columbia based on tsunami

warning and PWLN stations

Upgrated Tsunami Warning SystemUpgrated Tsunami Warning System(Canadian Hydrographic Service)(Canadian Hydrographic Service)

Location of tide gauges on the coast of British Columbia

3 Tsunami Stations10 PWLN Stations

Sampling: 1 min

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

Date Region M Stations2001-07-23 Peru 8.4 Langara, Bella Bella, Queen

Charlotte, Port Hardy, Winter Harbour, Tofino, Bamfield, Victoria

2001-10-12 Q. Charlotte (Canada, BC)

6.3 Port Hardy, Winter Harbour, Tofino, Bamfield

2003-01-22 Colima (Mexico)

7.4 Winter Harbour, Tofino, Bamfield

2004-11-02 Vancouver I. (Canada, BC)

6.7 Tofino, Bamfield

2004-12-26 Sumatra (Indian Ocean)

9.3 Bella Bella, Port Hardy, Winter Harbour, Tofino, Bamfield, Victoria

2005-06-15 California 7.4 Tofino, Bamfield

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.

BC spectra and transfer functions

An attempt to identify the Hokkaido Tsunami of September 25, 2003 in the BC records. This plot clearly demonstrates the

problem at Langara station (very high background noise).

BC tide gauge records

Location of the New Zealand tide gauges

Sampling: 1-minute

Storm of August 17-21, 2003

New Zealand

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.

“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]

Generation of long background waves(From Rabinovich [1993])

Formation of IG-waves in the ocean

From Rabinovich [1993]

Map of Langara Island with positions of permanent tide gauge station Langara and temporary station Henslung

Langara

Stormy Langara…

…and quiet Henslung

Henslung and Langara recordsHenslung and Langara records

Simultaneous longwave records at the Henslung and

Langara stations

Zoomed segments

Location of the Sumner Head and Lyttelton tide gauges

New ZealandNew Zealand

Lyttelton and Sumner Head recordsLyttelton and Sumner Head records

Simultaneous longwave records at the Sumner

Head and Lyttelton stations

Zoomed segments

Spectra or background noise in Sumner Head during “normal” (windy) and very calm weather

Spectral ratio of “windy” and “calm” longwave spectra

New Zealand

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

British Columbia (a)

British ColumbiaNew Zealand (b)

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

(a) British Columbia (b) New Zealand

Tsunami source functionTsunami source function2001 Peru Tsunami (2001 Peru Tsunami (MMww= 8.4)= 8.4)

0.1)()(ˆ

)()(

)(S

)()( 21

0

0

0

background

observed

EA

S

SESR

So

urc

e f

un

ctio

nFrequency (cpm)

Three consecutive days

Six stations

British ColumbiaNew Zealand

October storm October storm (27.10.2002)(27.10.2002)

IGIGwaveswaves

IGIGwaveswaves

British Columbia

Source functions reconstructed for four New Zealand stations during a storm passage

New Zealand: August storm (17-21.08.2003)New Zealand: August storm (17-21.08.2003)

Main results:Main results:

(1)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.

(2)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.

(3)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.