March is a bad month for Japan, or so it seems. Three years ago, March 11 saw the largest earthquake in the country’s history and one of the largest ever recorded – but the 2 and 4 of March are also anniversaries of significant tremors of magnitude 8.4 (M8.4) in 1933 and M8.1 in 1952. And yet again this year the earth shook as a smaller, but nevertheless significant, tremor, registered on 2 March 2014.
The truth, of course, is that the Japanese island arc is so earthquake-prone that no month stands out. The USGS list of significant earthquakes from 1891-2011 suggests that September and October are at least as bad as March (although this list includes events according to their significance rather than their size and so is incomplete.)
Nevertheless, the anniversary of these three severe natural disasters, falling so close together, raises the question of Japan’s seismic history.
The March Earthquakes of 1933 and 1952
To say that Japan is vulnerable to earthquakes is an understatement: a quick search of the United States Geological Survey’s earthquake database reveals that since 1900 the Japanese archipelago has suffered 138 seismic events of at least magnitude 7 (≥M7.0) of which 6 were at least M8.0 and one, the greatest of them all in 2011, reached M9.0. By contrast, the San Andreas Fault Zone has experienced just 11 of at least M7.0, none of them larger than M7.7.