October 25, 2013 Earthquake: E Coast, Honshu, Japan

Felt (IV) at Komae, Misawa and Morioka; (III) at Ayase, Kawasaki, Tokyo, Yamoto and Yokosuka; (II) at Yokohama. Felt widely in the Greater Tokyo Area and along the eastern coast of Honshu from Miura to Misawa. Recorded (4 JMA) in Fukushima, Ibaraki, Miyagi and Tochigi.

Tectonic Summary

The October 25, 2013 M 7.1 earthquake offshore of Honshu, Japan occurred as the result of normal faulting in the shallow oceanic crust of the Pacific plate. The earthquake occurred outboard (east) of the Japan Trench, which marks the seafloor expression of the subduction zone plate boundary between the Pacific and North America plates, and is immediately up-dip of the source region of the March 2011 M 9.0 Tohoku earthquake. At the latitude of this earthquake, the Pacific plate moves westwards with respect to the North America plate at a rate of 83 mm/yr before subducting beneath the island of Honshu. Note that some authors divide this region into several microplates that together define the relative motions between the larger Pacific, North America and Eurasia plates; these include the Okhotsk and Amur microplates that are respectively part of North America and Eurasia.

The location, depth, and focal mechanism of the October 25 2013 event are consistent with normal faulting rupture near the outer-arc high of the Japan Trench. In this region, normal faulting is encouraged by both the bending of the Pacific plate as it enters the subduction zone, and by stresses transferred from the locked subduction thrust interface to the west. Since the March 2011 Tohoku earthquake, two large events of M 7.7 and M 7.3 have occurred in the vicinity of the October 25, 2013 earthquake. The M 7.7 event, on March 11, 2011, was also a normal faulting event near the outer-arc high and occurred 95 km north of the October 25 event. The M 7.3 event, on December 7, 2012, was a more complex earthquake resulting from thrust motion near the trench 100 km to the northwest of the October 25 earthquake. Since March 2011, 10 additional events, ranging in magnitude from M 6.1-6.4, have occurred in this region east of the Japan Trench.

(above from reference #13456)

TOKYO (AP) - A magnitude 7.3 earthquake struck offshore of Japan's crippled nuclear power plant early Saturday, triggering small tsunamis but causing no damage... There was no damage and only one minor injury was reported, the national Fire and Disaster Management Agency said. Tsunamis of up to 40 centimeters (15 inches) hit four areas along the coast, but a tsunami advisory was lifted less than two hours after the quake. The epicenter of the 2:10 a.m. Tokyo time (1710 GMT) quake was about 290 kilometers (170 miles) off Fukushima, and it was felt in Tokyo, 300 miles (480 kilometers) away. "It was fairly big, and rattled quite a bit, but nothing fell to the floor or broke. We've had quakes of this magnitude before," Satoshi Mizuno, an official with the Fukushima prefectural government's disaster management department, told The Associated Press by phone. "Luckily, the quake's center was very far off the coast." Mizuno said the operator of the troubled Fukushima plant, Tokyo Electric Power Co., said no damage or abnormalities had been found. Japan's meteorological agency issued a 1-meter (3-foot) tsunami advisory for a long stretch of Japan's northeastern coast. It put the quake's magnitude at 7.1, while the U.S. Geological Survey recorded a magnitude of 7.3. The U.S. Pacific Tsunami Warning Center did not post warnings for the rest of the Pacific. The meteorological agency reported tsunamis of 40 centimeters in Kuji city in Iwate prefecture and Soma city in Fukushima, as well as a 20-centimeter tsunami at Ofunato city in Iwate and a 30-centimeter tsunami at Ishinomaki in Miyagi prefecture. (reference #10786)