M6.2 Western Japan Earthquake

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Posting Date: October 21, 2016, 11:00:00 AM

Western Japan was struck by an M6.2 earthquake at 05:07 UTC on October 21, 2016, according to the United States Geological Survey (USGS). The USGS recorded the focal point depth at 10 km, making this a very shallow quake that hit about 8 km south of Kurayoshi, Japan, in Tottori Prefecture. No tsunami warning was issued. Power outages and some damage to buildings and roads have been reported.

Figure 1. ShakeMap of the M6.2 earthquake that struck western Japan at 05:07 UTC on October 21, 2016. (Source: USGS)

Other nearby population centers include the Japanese cities of Tsuyama (38.4 km north-northwest of the epicenter), Tottori (42.3 km west-southwest of epicenter), Yonago (43.3 km east-southeast of the epicenter), and Yasugichō (51.3 km east of the epicenter).

Regional Seismicity

About 20% of the world’s M6.0+ earthquakes occur in Japan, one of the world’s most seismically active regions. Four major tectonic plates—the Pacific Plate and the Philippine Plate, and the Amurian Plate and the Okhotsk Plate—come together at Japan. The complex interactions of these plates—where some plates are themselves being subducted even while subducting others—creates an abundance of seismic activity, the effects of which extend under mainland Honshu (Japan's largest island), where the M6.2 quake occurred.

Figure 2. The four tectonic plates that meet at Japan, creating a region prone to seismic activity. (Source: AIR)

The seismicity of the southern part of Japan is dominated by the large subduction interaction between the Philippine and Amurian plates, which produces shallow crustal and deep earthquakes. The Tottori region has previously experienced shallow crustal earthquakes of larger magnitudes:  an M7.4 in 1943 and an M6.6 in 2000, about 50 km and 70 km east and west of the October 21, 2016, event, respectively.  The epicenter of the October 21 M6.2 event is within 20 km of the east-west trending Nagisan and Iwatsubo faults, both of which are considered active in the Japan Headquarters for Earthquake Research Promotion (HERP) seismicity model.  

Reported Impact

Reports of power outages across the affected area have ranged from 32,000–77,000 households; later reports indicate that power has since been restored. The local fire department in the town of Hokuei, Tottori Prefecture, reported that two houses collapsed, a number of others suffered damage (roof tiles were said to be strewn around town), and some roads cracked. A Kurayoshi city government building was also reported damaged, and workers were evacuated.

Exposure at Risk

The cities closest to the October 21 M6.2 quake are home to many: Kurayoshi’s population is 48,347, Tsuyama’s population is 89,717, Tottori’s population is 154,098, Yonago’s population is 141,368, and Yasugichō’s population is 29,368. Buildings in Japan are predominantly wood, steel, or steel-reinforced concrete. Residential exposures are dominated by single-family homes, nearly 90% of which are of wood construction. Apartment complexes in Japan are usually of steel or steel-reinforced concrete construction; only about 24% are of wood construction.

Among commercial buildings, wood is even rarer and is only used for about 12% of the building stock with most of them being steel or steel-reinforced concrete. The small industrial building stock is primarily of steel or light metal with about 20% made of concrete. 

Buildings in Japan range in age greatly, which creates variability in vulnerability to earthquake. Older buildings in Japan are typically quite vulnerable to shake damage, whereas newer buildings constructed to updated design codes will perform better.

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Posting Date: October 21, 2016, 11:00:00 AM