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Institute of Marine Sciences, Physical Oceanography

TSUNAMI RESEARCH GROUP

The Kuril Islands Tsunami of November 2006.

Impact at Crescent City by local enhancement

Non-Linear Shallow Water (NLSW) numerical simulations

Description: Crescent City (CC) is particularly susceptible to tsunami attack. The combination of tsunami signal enhancement by both distant bathymetric features and by local nearshore resonance seems to be responsible for this behavior.
The importance of distant bathymetric features for both tsunami wave amplification and for increased duration of the tsunami arriving at CC were discussed in Kowalik et al. (2007). These distant features were responsible for delivering the late, high energy signal, which was delayed by two hours from the predicted arrival time.
Additional local amplification and increased duration are caused by the shelf bathymetry adjacent to CC. As the initial tsunami arrives at CC, the shelf trapping mechanism tends to excite the gravest of the natural modes. This oscillation is slowly shifted towards the natural mode closest to the period of the incident tsunami wave packet. Short duration wave trains tend to influence the tsunami response at CC harbor in such a way that one hour after the first wave arrival, a much stronger wave will be generated due to the tendency of the shelf to initially ring at the frequency of the gravest mode. Practical implications of distant and local amplification relate to the potential hazard near CC, highlighting the need for awareness of the tsunami signal duration. In this respect two time scales are important for this event: the 2 hours time delay due to distant bathymetric features and the 1 hour delay due to local offshore bathymetry.


References:
Horrillo J., W. Knight and Z. Kowalik (2007), The Kuril Islands tsunami of November 2006. Part II: Impact at Crescent City by local enhancement. Accepted, Journal of Geophysical Research. (Download PDF file, size: 5.24MB)

Kowalik Z., J. Horrillo, W. Knight and Tom Logan (2007), The Kuril Islands tsunami of November 2006. Part I: Impact at Crescent City by distant scattering. Accepted, Journal of Geophysical Research. (Download PDF file, size: 5.28MB)



CRESCENT CITY TIDE RECORD ANALYSIS

Figure 1. Crescent City tide record analysis during Kuril Tsunami of November 15, 2006; a) Blue: recorded sea level; Green: predicted tide; b) tsunami signal (mean sea level and tide removed); c) wavelet analysis using Daubechies wavelet No.3 (normalized values in dB); d) power spectra density estimate using Yule-Walker auto-regressive filter.


Note: The leading edge of the tsunami arrived at 19:45UTC, with the wave amplitude growing to an amplitude of 40 cm one hour later, and culminating about 1-1/4 hours after that (22:00UTC/2:00pm PST) in the highest wave of approximately 1.76 m crest to trough. The higher wave surge arrived at low tide as is indicated in Figure 1a. This low sea level condition enhanced the current in the internal dock (Citizen's Dock) entrance.
Examination of the wavelet analysis indicates that background oscillations (around 20 min) in the Crescent City (CC) shelf/harbor exist before and after the tsunami (Figure 1c).
The largest energy maxima (Figure 1d) are associated with periods of 21, 19, 30 and 16 minutes.
The gauge record (see Figures 1b and 1c) reveals a late arrival of two wave packets starting at approximately 22:45UTC and 25:40UTC respectively. These wave packets have defined envelopes. The growth and decay of wave amplitudes suggest arrival of few forcing waves with periods close to shelf/harbor's mode. This distinct behavior is observed in all tsunami packets. A maximum wave amplitude is attained about 1 hour after the first noticeable wave in the packet. This behavior is observed in some past CC tsunami records as well.



CRESCENT CITY HARBOR GRAVEST MODE OF OSCILLATION

Figure 2. Calculated gravest eigenvalue/eigenperiod for the Crescent City harbor.


Note: The natural mode of oscillation calculated for the Crescent City (CC) harbor is 14.45 min. The most energetic period of the tsunami 21 minutes (see Figure 1d) appears to be too long to be related to a resonance in the harbor. Therefore, this period might be related to a natural period of the shelf adjacent to CC.



OFFSHORE BATHYMETRY

Figure 3. Offshore bathymetry in proximity to Crescent City. Concave and convex curvatures are drawn along shoreline and shelfbreak edge. The red cross indicates focus of energy concentration.


Note: The region within Pt. St. George and Patrick’s Pt. was chosen to investigate tsunami amplification due to this resonance process. Wave rays describe the path of incident wave from the open ocean and reflected by the coastal concave curvature. The refraction, reflection, energy focusing and trapping mechanisms can be explained simply by the spindle-like shape described by the coastal shoreline and shelfbreak edge.



CRESCENT CITY SHELF GRAVEST MODE OF OSCILLATION

Figure 4. Calculated gravest eigenvalue/eigenperiod for the shelf adjacent to Crescent City.


Note: The natural mode of oscillation calculated for the shelf adjacent to Crescent City (CC) is 66.97 min (1 hour + 7 min). Using the wide range of wave periods in a numerical experiment, it was found that the CC shelf initially responds very similarly to different periods as it always tends to energize the gravest mode of the natural oscillations. The trigger of this gravest mode of oscillation can be explained in terms of the refraction/reflection of gravity waves by the peculiar morphology of CC shelf. As the speed of surface wave propagating towards the coast decreases with water depth, wave-rays traveling toward shallow water are refracted against the concave coastline, the wave crest becomes nearly parallel to the shoreline to be uniformly reflected by bottom bathymetry and coast. The resulting reflected wave travels outward and on its way out, is refracted again, concentrating energy close to the center of the convex curvature of the shelfbreak (focus). It is believed that this initial energy trapping starts the gravest mode of oscillation.



THREE INCIDENT WAVE EXPERIMENTS ON CRESCENT CITY SHELF

Figure 5. Computed sea level outside Crescent City harbor. In this experiment only one sinusoidal incident wave with periods of 20, 31 and 40 minutes is used. The sea level is normalized against incident wave amplitude (10 cm).


Note: Figure 5 shows the sea level response of a numerical gauge located just outside Crescent City (CC) harbor. This basin has the tendency to excite the gravest mode of oscillations for all three wave periods, after about 1 hour from the first wave crest the maximum sea level occurs. This tendency is clearly observed in the tide gauge record from Figure 1. The initial tsunami wave arriving around hour 20 is strongly amplified after approximately one hour. The search for natural oscillations inside the CC harbor and at the adjacent shelf revealed that this period was one of the resonant periods defined not by the geometry of CC harbor but by the shoreline and offshore shelf.



VIDEO/ANIMATION

a) Experiment: One sinusoidal incident wave: Period 20 min. Amplitude 10 cm.

Note: The small black window shows sea level recorded by numerical gauges just outside Crescent City harbor.
{RUN the MOVIE]
Notice a higher wave amplitude after 1 hour from the arrival of the first incident wave. As the speed of surface wave propagating towards the coast decreases with water depth, wave-rays traveling toward shallow water are refracted against the concave coastline, the wave crest becomes nearly parallel to the shoreline to be uniformly reflected by bottom bathymetry and coast. The resulting reflected wave travels outward and on its wayout, is refracted again, concentrating energy close to the center of the convex curvature of the shelfbreak (focus, see Figure 3). This initial energy trapping starts the gravest mode of oscillation as it can be seen from the concentration of tsunami energy at the focus. (Download high resolution version. MOV file size: 109MB)



b) Experiment: Three sinusoidal incident waves: Period 20 min. Amplitude 10 cm.

Note: The small black window shows sea level recorded by numerical gauges just outside CC harbor.
[RUN the MOVIE]
In this experiment a wave packet (three waves) are sent into Crescent City (CC) shelf. The forcing waves have periods close to one of the shelf natural modes (20 min). Notice the well define envelopes describing wave amplitudes. The growth and decay of wave amplitudes suggest resonance process. A distinct behavior is observed in all tsunami packet experiments. A higher wave amplitude is attained about 1 hour after the first wave of the packet. This resonance behavior is observed in some past CC tsunami records as well.
The last couple of frames describe maximum wave amplitude and bathymetry. (Download high resolution version. MOV file size: 109MB)



Modified 22 October 2007. Website questions or comments to Juan J. Horrillo.