Open-Coast Tsunami Recording and and Negative Influence of Infragravity Waves A.B. Rabinovich 1,2,...
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Transcript of Open-Coast Tsunami Recording and and Negative Influence of Infragravity Waves A.B. Rabinovich 1,2,...
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.