than 8, including the 869 Sanriku earthquake, the 1896 Sanriku earthquake, and the 1933 Sanriku earthquake. Releases of radioactive contamination Radioactive material was released from the containment vessels for several reasons: deliberate venting to reduce gas pressure, deliberate discharge of coolant water into the sea, and uncontrolled events. Concerns about the possibility of a large scale release led to a exclusion zone around the power plant and recommendations that people within the surrounding zone stay indoors. Later, the UK, France, and some other countries told their nationals to consider leaving Tokyo, in response to fears of spreading contamination. In 2015, the tap water contamination was still higher in Tokyo compared to other cities in Japan. Trace amounts of radioactivity, including iodine-131, caesium-134, and caesium-137, were widely observed. The accident released 100–500 petabecquerels (PBq) of iodine-131 and 6–20 PBq of caesium-137 to the atmosphere, according to an estimate by the United Nations Scientific Committee on the Effects of Atomic Radiation. About 80 percent of the atmospheric releases were deposited over the ocean. In addition, 10–20 PBq of iodine-131 and 3–6 PBq of caesium-137 were released directly to the ocean. The Fukushima coast has some of the world's strongest currents and these transported the contaminated waters far into the Pacific Ocean, thus causing great dispersion of the radioactive elements. The results of measurements of both the seawater and the coastal sediments led to the supposition that the consequences of the accident, in terms of radioactivity, would be minor for marine life as of autumn 2011 (weak concentration of radioactivity in the water and limited accumulation in sediments). On the other hand, significant pollution of sea water along the coast near the nuclear plant might persist, due to the continuing arrival of radioactive material transported towards the sea by surface water running over contaminated soil. Organisms that filter water and fish at the top of the food chain are, over time, the most sensitive to caesium pollution. It is thus justified to maintain surveillance of marine life that is fished in the coastal waters off Fukushima. Despite caesium isotopic concentrations in the waters off of Japan being 10 to 1000 times above the normal concentrations prior to the accident, radiation risks are below what is generally considered harmful to marine animals and human consumers. Researchers at the University of Tokyo's Underwater Technology Research Center towed detectors behind boats to map hot spots on