5/6/2023 0 Comments Tohoku earthquake bulletin![]() ![]() Note the number of VIIs and VIIIs near Tokyo, well away from the epicenter the lack of a severe human toll in that metropolitan area is surely a testament to the quality of the nation’s earthquake preparedness. Ground motion also seems to be more intense in coastal and riverine areas, where settlements are built on softer sediments and less bedrock. The ground shaking data is overlaid on a map of population density provided by Oak Ridge National Laboratory.Ī shaking intensity of VI is considered “strong” and can produce “light damage,” while a IX on the scale is described as “violent” and likely to produce “heavy damage.” The pattern of shaking appears to run parallel to the offshore subduction trench, with the intensity decreasing more from east to west, as opposed to north and south. Shades of pale yellow represent the lowest intensity and deep red represents high intensity. Each circle represents an estimate of shaking as recorded by the USGS, in conjunction with regional seismic networks. This map shows the ground motion and shaking intensity from the earthquake at dozens of locations across Japan. Closer to the main shock, coastal regions were devastated by the quake and the resulting tsunami. The event shook buildings and damaged infrastructure hundreds of kilometers away. Initially categorized as magnitude 8.9, the quake was later revised upward to magnitude 9.0 by the Japanese Meteorological Agency (JMA) and the U.S. The global rate of magnitude 9 earthquakes in subduction zones, predicted from statistical analysis of seismicity as well as from moment conservation, is about five per century five actually happened.On March 11, 2011, the largest earthquake in Japan’s modern history struck off the northeast coast, about 130 kilometers (81 miles) east of the mainland region of Tohoku. Another moment conservation method, applied at a point on a major fault or plate boundary, also suggests that magnitude 9 events are required to explain observed displacement rates at least for the Tohoku area. Moreover, moment conservation indicates that variations in estimated corner magnitude among subduction zones are not statistically significant. However, the moment conservation principle yields consistent estimates: for all the subduction zones the corner magnitude is of the order 9.0–9.7. For subduction zones, the seismic or historical record is insufficient to constrain either the maximum or corner magnitude. The moment conservation technique, which we prefer, matches the tectonic deformation rate to that predicted by earthquakes with a truncated or tapered magnitude–frequency distribution. However, for individual zones the statistical method is usually ineffective in estimating the maximum magnitude only the lower limit can be evaluated. There are two quantitative methods for estimating the corner magnitude in any region: a statistical analysis of the available earthquake record and the moment conservation principle. We show that historical magnitudes systematically underestimate this maximum size of future events, but the discrepancy shrinks with time. We consider three questions related to the 2011 Tohoku mega‐earthquake: (1) Why was the event size so grossly underestimated by Japan’s national hazard map? (2) How should we evaluate the chances of giant earthquakes in subduction zones? (3) What is the repeat time for magnitude 9 earthquakes off the Tohoku coast? The maximum earthquake size is often guessed from the available history of earthquakes, a method known for its significant downward bias. ![]()
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