A strong earthquake struck just off the Japanese island of Honshu yesterday

A strong earthquake struck just off the Japanese island of Honshu yesterday at 13:00 UTC, rattling buildings in downtown Tokyo and briefly cutting power to thousands of residents 48 kilometers to Tokyo's northeast, in Tsukuba City (population 207,000).

The USGS reported a preliminary magnitude of 6.8 for the event and a depth of 35 km. It was one in a series of quakes, including an M6.2 foreshock and multiple M5+ aftershocks—all occurring within about four and a half hours.

“Yesterday’s earthquake occurred in the subduction zone of the Japan Trench where the Pacific plate subducts under Honshu Island at 82 millimeters per year,” said Mehrdad Mahdyiar, director of earthquake hazard at AIR Worldwide.

“According to USGS standards for earthquake fault mechanisms, it was a thrust event; that is, it most likely occurred along the plate interface in the subduction zone. In the past two centuries, large interplate subduction zone events have been frequent along the Japan Trench. In fact, since 1900, there have been four earthquakes exceeding M7 and more than 80 exceeding M6 within 100 km of yesterday’s event.”

Apart from a minor radioactive water leak at a nuclear power plant north of Tokyo and some cracks in older, weak buildings in Ibaraki and Tochigi prefectures (also on Honshu Island) the earthquake caused very little damage.

Though yesterday’s earthquake did not force the closing of any of Japan’s nuclear power plants, a similar-sized earthquake (M6.8) last July shook the center of the country, prompting the close of the world's largest nuclear power facility, owned by Tokyo Electric Power Company.

Because earthquake insurance penetration in Japan is relatively low (ranging between 14%- 17% nationwide) and because this event occurred offshore, AIR does not expect significant insured losses from this event.