Tsunami of Friday, 11 March 2011

Predicted tsunami amplitudes from the March 11, 2011 earthquake. Image: NOAA Pacific Tsunami Warning Center

The magnitude 9.0 March 11, 2011 earthquake off the east coast of Honshu, Japan generated a tsunami that was recorded throughout the Pacific Ocean. At the Scripps Institution of Oceanography Pier in La Jolla, California, a pressure sensor operated by the Coastal Data Information Program (CDIP) indicated that the tsunami signal arrived at Scripps at approximately 08:50 PST on March 11.

Preliminary analysis shows the tsunami arriving with an amplitude of over 30 cm and a period of 36 minutes. As shown in the following graph, some of the highest peak-to-trough wave amplitudes occured several hours after the initial arrival of the tsunami, and water level changes due to the tsunami persisted for more than three days.

Click here to see official NOAA tsunami record from Scripps Tide Station

Tsunami Arrives at the Scripps Nimitz Marine Facility in San Diego Bay

Scripps Institution of Oceanography Senior Captain Eric Buck observed the impact from the March 11, 2011, tsunami on the Scripps Nimitz Marine Facility (MarFac) on San Diego Bay. In the words of Captain Buck:

From about 1000 to 1025 this morning, we observed abnormal currents just off the MarFac waterfront. We saw strong currents flowing to the north past MarFac into the basin off La Playa. The tide was flooding at this time, but under normal circumstances the current is weak and unobservable.

More dramatic is what we observed from about 1030-1045 as the basin off La Playa had been filled to an unnatural level and now the current had reversed and was flowing (rushing) back out of the bay in a southerly direction. A series of standing waves approximately 10-12 inches high was observed immediately north of MarFac's northern property line (see photos 1 & 2 above).

This outward rush of water created much turbulence and discoloration in the waters off MarFac (see additional photos above). The white caps and choppy waters are from colliding currents - there was no appreciable wind during the day of the tsunami.

Additional inflow/outflow events were observed throughout the day as the water rose and fell with the each tsunami wave, with additional strong events occuring between 4pm and 5pm.

Scripps research vessels in MarFac home port at the time of the tsunami were safely secured alongside the pier in preparation for currents anticipated from this amplitude event. Captain Eric Buck noted that strong currents (6 or 7 knots) persisted at MarFac into the early evening, and required re-securing R/V Sproul to the pier around 1630. Also observed late in the afternoon: a police boat became unmoored and drifted with the strong ebb currents into an eddy just off our quay wharf, and ultimately onto the rocks across the channel.

The tsunami at ebb: strong tsunami currents beneath MarFac pier, and an unmoored boat drifts past MarFac (the height of wet riprap on the shore beyond the boat indicates the change in water level). Photos: Eric Buck

Other observations in San Diego Bay near Nimitz Marine Facility

In San Diego Harbor the amplitude of the tsunami reached 2 feet. These photos show the change in water level at the Southwestern Yacht Club over a 10-minute period on Friday afternoon.
Rise and fall of one tsunami cycle at Southwestern Yacht Club. Photo: Thomas Muschamp

Lessons learned

1) The predicted arrival time of the tsunami represents the start of what can be a very long period of rapidly changing water level and currents, which merits continued attention. During the day of the event, many people assumed the "wave" arrived at 0850 and the whole deal was over by 0900. Not the case.

2) The tsunami was manifested as a repeating series of rapidly changing high- and low-water levels, more like compressed tidal cycles than waves.

3) large peak-to-trough amplitude cycles continued for more than 12 hours after the first arrival; and

4) the effects of the tsunami continued for more than three days.