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March 11 Sendai Tsunami in San Francisco Bay

The March 11, 2011 magnitude 8.9 Sendai earthquake caused extensive damage in Japan. But most of the damage was caused not by the earthquake itself, but by the earthquake-generated tsunami which struck the Japanese coast only minutes after the earthquake.

The tsunami, which traveled in the ocean at approximately the speed of a modern commercial jet aircraft, struck the California coast approximately 10 hours later. The speed of the wave and the amplitude of the wave are controlled by a number of factors, the primary ones being the orientation of the earthquake rupture, the size of the rupture and the ocean depth. Similar to the large surfing waves at Mavericks, the whole ocean basin acts as a lens to steer the direction of the tsunami. This steering, or refraction, is the reason that the height of the tsunami varied along the California coast 

The Pacific Tsunami Warning Center "tsunami forecast model" (image to below right, courtesy PTWC) predicted the tsunami amplitudes for the Pacific Ocean basin. Due to refraction of the tsunami as it travels across the Pacific northern California received waves of higher amplitude than central California. In San Francisco Bay the tsunami waves spread out after passing through the narrow opening of the Golden Gate and both the wave amplitude and accompanying current speeds were much reduced. Tide gauge records from Ft Point, Tiburon and Vallejo clearly show the arrival of the tsunami in the Bay and the time it took to travel from the Golden Gate to Vallejo. The tsunami amplitude decreased due to the spreading of the wave, and the loss of energy while traveling in shallow water.

The accompanying graph shows the tide gauge records from three locations in San Francisco Bay, Fort Point near the Golden Gate (top/blue curve), the Romberg Tiburon Center for Environmental Studies, SF State on the Tiburon peninsula (middle/red curve) and at the California Maritime Academy at Vallejo (bottom/green curve). Note that the water height readings are relative; they have been offset to have each tidal record clearly visible.

The diagram starts midnight March 11, the day of the Sendai earthquake. During the first eight hours the graphs show the normal sea level changes due to the rise and fall of the tide. At approximately 08:23 March 11, the first signal of the Sendai tsunami registered on the Fort Point tidal gauge. Six minutes later, at 08:29, the tsunami arrival registered on the RTC tidal gauge. There is a small decrease in the tsunami amplitude between Ft. Point and RTC. The tsunami signal was not seen at Cal Maritime until over an hour later at 09:36 and the amplitude was greatly reduced.

The continued high frequency oscillations seen at Ft. Point and RTC are due to a combination of factors: 1) the tsunami waves travelling across the Pacific travel by multiple paths and hence waves on different paths will arrive at different times, 2) aftershocks continue to generate new tsunamis 3) the tsunami can reflect from the ocean boundaries and continue to travel in the ocean, and 4) the tsunami can reflect from the SF Bay boundaries and continue to travel around the bay similar to waves in a bathtub. The records at Ft. Point and RTC show the combination of these effects. At Cal Maritime the secondary effects appear to have dampened out, probably due to the spreading out of the energy in San Pablo Bay and dissipation in the shallow areas of the Bay.

Acknowledgements: The predicted tsunami amplitude figure came from the NOAA Center for Tsunami Research web site: http://nctr.pmel.noaa.gov/honshu20110311/ The Fort Point tidal data are from an instrument maintained by the Bodega Marine Laboratory: http://www-bml.ucdavis.edu/boon/realtime.html . The data from RTC and Cal Maritime Academy are from instruments maintained by RTC and can be accessed at: http://rtc.sfsu.edu/monitoring.htm

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