7.3 Earthquake Strikes Off Japan; Little Damage Reported: AIR

December 7, 2012

According to catastrophe modeling firm AIR Worldwide, a magnitude 7.3 earthquake struck off the northeast coast of Japan today, 270 kilometers from the Japanese city of Sendai, at 18:18:24 local time. The event struck in the same plate boundary region impacted by last year’s devastating M 9.0 Tohoku earthquake, although that quake’s epicenter was about 150 kilometers closer to mainland Japan.

According to AIR, given the distance of this event from shore, only minimal non-structural damage is expected, and insured losses are not expected to be significant.

Today’s earthquake shook buildings as far away as Tokyo and prompted the Japan Meteorological Agency (JMA) to issue tsunami warnings for a large part of the northeast, including Miyagi Prefecture, which suffered significant tsunami damage following the Tohoku quake. Thousands of people in at-risk regions evacuated to higher ground. Though tsunami waves as high as one meter did impact the coastal city of Ishinomaki, tsunami warnings were lifted shortly after—indeed, about two hours after the quake struck. (Tsunami warning heights for today’s event had varied between 50 centimeters and two meters, compared to 11 meters for the Tohoku temblor.) Thus far, it is not clear if the waves in Ishinomaki—where vast areas still remain in disarray following the Tohoku event last year—caused any damage.

“Today’s earthquake was also closely followed by at least six aftershocks, the strongest of which was a M6.2 temblor,” said Dr. Tao Lai, principal engineer at AIR Worldwide. “The damage picture from today’s earthquake is still taking shape, but so far there have been no reports of serious damage or injury. During the event, strong shaking was felt in a wide swath of northeastern Japan, extending to Tokyo, where office buildings swayed.”

Although a major concern following the Tohoku event was the crippled Fukushima Daiichi Nuclear Power Station, the plant’s operators, Tokyo Electric Power, reported that the station (located in Fukushima Prefecture, directly south of Miyagi Prefecture) was untouched by today’s event—although workers did move to higher ground at the time of the tsunami warning.

According to AIR, today’s event struck 270 kilometers east of Sendai, Japan, and about 150 kilometers east of the epicenter of the M 9.0 Tohuku earthquake in 2011. It occurred along the Japan Trench where the Pacific Plate subducts under Japan’s Honshu Island, and is associated with the ongoing and rapid (nearly 100mm/year) westward subduction of this plate beneath Japan.

Dr. Lai commented, “The Japan Trench is one of the most seismically active subduction systems on earth. On average, more than one earthquake of magnitude 7 occurs in this 800-km long subduction zone every decade. In the last four decades alone, 12 events of M7 or larger have occurred within 250 km of today’s event. Other large historic earthquakes to have occurred here include a M8.5 event in 1896, a M8.1 event in 1933, and a M7.5 earthquake in 1915.”

“Today’s earthquake—which occurred along the subducting slab of the Japan Trench interface—is likely associated with stresses due to the “bending or unbending” of the crust in this tectonic region and/or evolving stresses due to the earlier Tohoku event. Today’s event is also possibly associated with normal or reverse faults in the lower to middle crust, as the direction of earthquake-related plate movements reveal. The nature of today’s earthquake is rather unusual since so-called “bending or unbending” events are rarely observed in subduction zone areas.”

According to AIR, earthquake insurance penetration in Japan is relatively low (ranging between 14 to 17 percent nationwide). About 70% of all residential construction is estimated to be of wood and about 25% of concrete. Commercial construction consists of more than 50% concrete, about one-third light metal or steel, and less than 10% wood. Residential structures in the region of Japan impacted by today’s quake are generally resistant to earthquake shaking. Some vulnerable structures do exist; they are comprised of non-ductile reinforced concrete frame and heavy wood-frame construction.

Source: AIR Worldwide

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