A magnitude 4.7 quake struck off the southeast coast of the prefecture at 4.08am on Monday, just 16 minutes after a 6.3 tremor nearby. In a little over six hours from 5.37pm on Saturday, there had been no fewer than five tremors ranging from 4.3 to a magnitude of 6.1 in the same area.
The previous day, a 5.4-magnitude quake struck a little further north, but still close to the Chishima Trench. Running parallel to the Japanese coast, the 2,200km (1,400-mile) fault is where the Pacific tectonic plate subducts under the North American plate, causing a great deal of seismic activity.
The Japan Meteorological Agency (JMA) said that none of the quakes were severe enough to trigger additional seismic activity and that they were being treated as isolated incidents. But the agency did warn that the Chishima Trench had been the source of megaquakes in the past.
