Normal service has been resumed. After a few weeks of unexpected calm, the Earth has returned to a more usual pattern of earthquake activity with quakes in Japan, and the Solomon Islands – and an unusual tremor in Austria.
Although the total number of tremors recorded by the United States Geological Survey isn’t much greater than in previous weeks, the number of earthquakes of a significant size has increased, with 124 of at least magnitude 4 (≥M4.0) compared to 92 the preceding week, 37 of ≥M5.0 (compared to 37) and 9 of ≥6.0 (1).
M6.9, Japan: Biggest Earthquake of the Week
The M6.9 which struck on the island of Hokkaido on 2 February is by no means uncommon: the U.S. list of historic earthquakes in the region records 11 of at least this magnitude in the decade preceding the disastrous Tohoku-oki earthquake of 2011.
The 2013 event was influenced by the same tectonic settings as that of 2011. It occurred at a depth of 103km and this, combined with its onshore epicentre and (relatively) low magnitude explains why no tsunami was generated.
The dominant movement of the western Japan subduction zone is, as its name suggests, convergence: the Pacific plate is moving westwards at a rate of around 3-5cm per year (Yeats) relative to the southern wedge of the Okhotsk microplate.
This subduction zone experiences high stress and has been the location for very many earthquakes, many of them larger than M7.0, as the map of historic seismicity (since 1990) shows.
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