![]() * Originally, seismologists measured the magnitude of short-period seismic waves to indicate earthquake magnitude, and in the 1960s it became possible to measure longer-period seismic waves, which more accurately indicate the size of large earthquakes. This describes the surface intensity at particular places, rather than the magnitude of the earthquake itself. Japan has a seismic intensity scale in shindo units 0 to 7, with weak/strong divisions at levels 5 and 6, hence ten levels. The logarithmic Richter magnitude scale (or more precisely the Moment Magnitude Scale more generally used today*) measures the overall energy released in an earthquake, and there is not always a good correlation between that and intensity (ground motion) in a particular place. PGA has long been considered an unsatisfactory indicator of damage to structures, and some seismologists are proposing to replace it with cumulative average velocity (CAV) as a more useful metric than ground acceleration since it brings in displacement and duration and "operators are able to determine the absence of potential damages with high confidence" according to the IAEA. Peak ground acceleration (PGA) or design basis earthquake ground motion (DBGM) is measured in Galileo units – Gal (cm/sec 2) or g – the force of gravity, one g being 980 Gal. Various systems are used in planning, including Probabilistic Seismic Hazard Assessment (PSHA), which is recommended by IAEA and widely accepted. The International Atomic Energy Agency (IAEA) has a Safety Guide on Seismic Risks for Nuclear Power Plants. It is estimated that, worldwide, 20% of nuclear reactors are operating in areas of significant seismic activity. In France for instance, nuclear plants are designed to withstand an earthquake twice as strong as the 1000-year event calculated for each site. Nuclear facilities are designed so that earthquakes and other external events will not jeopardise the safety of the plant. Three of these at the Fukushima Daiichi plant subsequently caused an INES Level 7 accident due to the 15 m tsunami causing loss of power leading to loss of cooling and subsequent radioactive releases. In March 2011, eleven operating nuclear power units shut down automatically during the Great East Japan Earthquake.In 1999, three nuclear reactors shut down automatically during the devastating Taiwan earthquake, and were restarted two days later.In 1995, the closest nuclear power plants, some 110 km north of Kobe, were unaffected by the severe Kobe-Osaka earthquake, but in 2004, 2005, 2007, 20 Japanese reactors shut down automatically due to ground acceleration exceeding their trip settings. ![]() Japanese, and most other, nuclear plants are designed to withstand earthquakes, and in the event of major earth movement, to shut down safely. ![]()
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