A magnitude 6.7 earthquake rocked Japan today, the 75th aftershock of at least magnitude 6.0 from the devastating magnitude 9.0 quake on March 11, according to the U.S. Geological Survey (USGS). The earthquake data is preliminary and subject to revision.
Japan has been rocked by hundreds of aftershocks since the deadly Tohoku earthquake, the biggest in Japan's recorded history. The aftershocks have been so plentiful that the world's premier earthquake-measurement service has since modified its alert system to filter out the smaller aftershocks. The largest aftershock was a magnitude 7.9 quake that struck less than an hour after the main shock. A magnitude 7.7 also struck that day. A magnitude 7.1 aftershock struck on April 7. [When Will the Aftershocks in Japan End?]
The number of aftershocks seems staggering, but geologists are not surprised, especially for such large mainshock.
The rule of thumb for aftershock strength is that the biggest aftershock will be about one magnitude smaller than the mainshock.
The latest aftershock struck about 54 miles (88 kilometers) east of Honshu. The quake ruptured 20 miles (32 km) below the Earth's surface.
Since the main quake off Japan's northeastern coast, hundreds aftershocks have shaken the island of Honshu, Japan's largest and home to 100 million people. Today's aftershock was caused by thrust faulting near the Japan Trench, the boundary between the Pacific and North America tectonic plates (the huge, moving slabs of the Earth's crust). Thrust faulting happens when one tectonic plate dives under another. In this case, the Pacific plate is diving under the North America plate.
Before the magnitude 9.0 earthquake, only nine magnitude 7.0 or greater earthquakes had ruptured in this subduction zone since 1973.