California Geological Survey - Anniversary of the March 11, 2011 Tohoku-oki Earthquake and Tsunami
Tsunami in Japan
Aerial photo of March 11 tsunami hitting the Sendai Plain in northern Japan (photo credit: unknown).
On March 11, 2011, at 2:46pm (Japanese Standard Time) and March 10, 9:46pm (Pacific Standard Time), a magnitude 9.0 earthquake struck the eastern coast of the Tohuku region, northern Honshu Island in Japan. A large, destructive tsunami was generated locally, with tsunami heights up to 39 meters and flooding that traveled over 10 kilometers inland in places. It is believed that nearly 20,000 people lost their lives in Japan from this event, and that 97% of the fatalities can be attributed to the tsunami alone. A teletsunami (a tsunami that travels a great distance across the ocean) was also generated, propagating out across deep ocean waters, putting the entire Pacific Ocean region at risk. For Japan, tsunami damage to the Fukushima Daiichi Nuclear Plant and land-use planning decisions to rebuild or not rebuild in inundation areas are critical ongoing post-tsunami issues.
TSUNAMI DOCUMENTS AND RELATED INFORMATION:
Tsunami in California
At 12:51am (PST), a little over three hours after the Tohoku earthquake origin time, the West Coast and Alaska Tsunami Warning Center (WC/ATWC) placed the California coast north of Point Conception in a Tsunami Warning, and the coast south of the Point Conception in a Tsunami Advisory. The range of Warning/Advisory forecast tsunami amplitudes varied from 0.3 to 2.5 meters, with the highest surge forecasted for Crescent City. Hourly conference calls were held with the county operational areas, and some communities within the Warning-level area began evacuation procedures.
The tsunami arrived at Crescent City in northern California at 7:30am (PST), on March 11, and moved southward along the coast over the next hour and a half (Table 1). Peak amplitudes at tide gauge locations in the state ranged from a low of 0.15 meters to a high of 2.47 meters at Crescent City. At most locations, the strongest surges were recorded within the first five hours. Because the largest surges arrived at low-tide, only minor inundation outside of harbor and river settings occurred. Several tide gauges, including Crescent City and Santa Barbara, recorded large surges nearly 15 hours after the initial onset when the tide was high. These late surges created hazardous conditions and resulted in additional localized damage. Very strong currents (up to 14 knots in Santa Cruz Harbor) and large water-level fluctuations (over 4.5 meters peak-to-trough in Crescent City Harbor) caused significant damage to two dozen harbors throughout the State. There was one fatality in California (and the West Coast of the U.S.) when tsunami surges knocked a man off some rocks at the mouth of the Klamath River in northern California and he drowned. The state again activated its emergency operation centers to coordinate information and emergency response measures with coastal jurisdictions and the WC/ATWC. Following the tsunami, a State Emergency was proclaimed by Governor Brown, and a Major Federal Disaster was declared by President Obama for three counties (Del Norte, Santa Cruz, and Monterey) because of the significant damage to harbors in Crescent City, Santa Cruz, and Moss Landing. The Federal Disaster damage estimate was about $50M but post-event field assessments and interview with harbor masters put the unofficial estimate closer to $100M; in either case, this was the largest, most damaging tsunami to hit California since 1964.
TSUNAMI DOCUMENTS AND RELATED INFORMATION:
Sunk boats within Crescent City's small boat basin (photo credit: Rick Wilson).
The most severe tsunami effects in the state occurred within the small-boat basin at Crescent City, a harbor with a history of being vulnerable to tsunamis. Maximum tsunami amplitudes of 2.5 meters were forecasted for Crescent City, initiating a full evacuation of the city’s tsunami hazard zone on land and a mass departure of the fishing fleet from the harbor. Within the first two hours of the tsunami activity, the tide gauge recorded a peak amplitude of 2.47 meters, which fortunately occurred at low tide producing minimal inundation of dry land. Strong currents were observed throughout the outer harbor and small-boat basin, with video analysis indicating peak currents of 9 knots at the mouth of the basin. Nearly 15 hours after first arrival, some of the highest tsunami levels were recorded on the tide gauge when large surges occurred at high tide. Large sea-level oscillations caused dangerous strong currents that lasted for over 24 hours. All docks within the small boat basin were heavily damaged or destroyed during the tsunami. Although dozens of boats made it out of Crescent City harbor prior to the tsunami, 16 boats were sunk and 47 were damaged according to the Coast Guard. Dozens of the boats that did evacuate Crescent City harbor were able to find shelter in Humboldt Bay despite large swells that were breaking at the entrance of the Bay. Overall, the harbor sustained over $20M in damages. Dredging of tsunami-related sedimentation within the harbor went on for months following the tsunami, and repair work to docks is ongoing.
Tsunami wave striking the upper Santa Cruz Harbor (photo credit: unknown).
Though no tide gauge exists within Santa Cruz Harbor, peak tsunami amplitudes of 1.6 to 1.9 meters were observed in the harbor constrictions. Approximately three hours after first tsunami arrival, several large, fast-moving tsunami bores were observed moving to the far back of the harbor, causing dramatic buckling of docks where the harbor narrows. Based on preliminary estimates from video analysis, these bores were traveling up to 15 knots in the back part of the harbor. The overall long, rectangular shape of the harbor likely amplified incoming surges causing the strong currents and the bores described. The overall damage to the harbor was more than $28M, with 14 boats sunk and dozens of other boats damaged. Of the harbor’s 29 docks, 23 docks sustained significant damage ranging from severe float cracking to complete dock destruction. Repair work and dredging of tsunami-related sedimentation within the harbor went on for months following the tsunami.
Table 1: Shows forecasted and observed arrival times and tsunami amplitudes as well as a summary of damage for both the February 27, 2010 and March 11, 2011 tsunamis in California. (Source: California Tsunami Clearinghouse.)