A shallow earthquake registered by the Korea Meteorological Administration (KMA) as M5.8 hit South Korea at 11:32 UTC (20:32 KST) on September 12, 2016. The USGS is reporting Mb5.4 at a depth of 10 km (6.2 miles).
According to the USGS, the epicenter was located 8.3 km (5.2 miles) S of Gyeongju (population 155 237), 26.9 km (16.7 miles) NNW of Ulsan (population 962 865), 27.2 km (16.9 miles) SSW of Enjitsu, 31.8 km (16.9 miles) SSW of Pohang (population 500 000), and 34.7 km (21.5 miles) SE of Eisen (population 56 006), South Korea.
EMSC estimated there are about 30 million people living in the felt area.
KMA official Yoo Yong-Gyu said: "It was the biggest quake ever to hit the country." Young-Gyu added the tremor was felt all across the country. South Korea started measuring earthquakes in 1978.
Two people have been reported injured, Young-Gyu said, citing data from emergency rescue authorities. One elderly woman was injured by her television that fell from its mounting.
The Yonhap news agency said minor cracks appeared in some buildings, but no major damage was reported.
The previous record in the South Korea was a M5.2 that hit the sea off the southeastern port of Uljin in 2004 and a quake of equal magnitude in the eastern mountainous region of Sokri in 1978.
Today's quake was preceded by a shallow M5.1 (KMA) / M4.9 (USGS) earthquake at 10:44 UTC (19:44 KST). The quakes shook large areas of the country, including the capital, Seoul, where tall buildings swayed.
At least 22 aftershocks with magnitudes of 2 to 3 were detected by 13:00 UTC (22:00 KST).
According to the Daegu Regional Meteorological Administration, the epicenter of the first quake was the Naenam Elementary School in Gyeongju. The epicenter of the second quake was 1.4 km (0.86 miles) away.
The government has ordered people in the quake-hit areas to evacuate to safe areas, the Korea Times reported.
Nuclear power plants near Gyeongju are operating normally, according to Korea Hydro and Nuclear Power. A liquefied natural gas-fueled power station in Ulsan, 26.9 km (16.7 miles) SSW, was shut down in the wake of the quakes.
Subway trains in Busan temporary stopped operation, and KTX bullet trains traveling through the quake-hit area reduced speed in case additional aftershocks damaged rail lines.
Busan International Finance Center said it evacuated most people from its 63-story building as a precautionary measure. As people in the affected areas called their family and friends, some regions had communication disruptions, including on KakaoTalk.
The Korea Times added that the experts speculate the Korean Peninsula is no longer in the "safe zone." "If an earthquake with magnitude 6 or above hits Korea, mass casualties are possible because many buildings are not built to withstand temblors."
USGS issued a green alert for shaking-related fatalities and economic losses. There is a low likelihood of casualties and damage.
Overall, the population in this region resides in structures that are vulnerable to earthquake shaking, though some resistant structures exist.
Recent earthquakes in this area have caused secondary hazards such as landslides that might have contributed to losses.
Estimated population exposure to earthquake shaking
Selected cities exposed
Seismotectonics of Japan and vicinity
The North America plate, Pacific plate, Philippine Sea plate, and Eurasia plate all influence the tectonic setting of Japan, Taiwan, and the surrounding area. Some authors divide the edges of these plates into several microplates that together take up the overall relative motions between the larger tectonic blocks, including the Okhotsk microplate in northern Japan, the Okinawa microplate in southern Japan, the Yangzee microplate in the area of the East China Sea, and the Amur microplate in the area of the Sea of Japan.
The seafloor expression of the boundary between the Pacific and North America plates lies 300 km off the east coasts of Hokkaido and Honshu at the Kuril-Kamchatka and Japan trenches. The subduction of the Pacific plate beneath the North America plate, at rates of 83-90 mm/yr, generates abundant seismicity, predominantly as a result of interplate slip along the interface between the plates. The 1958 M 8.4 Etorofu, 1963 M 8.6 Kuril, 2003 M 8.3 Tokachi-Oki, and the 2011 M 9.0 Tohoku earthquakes all exemplify such megathrust seismicity. The 1933 M 8.4 Sanriku-Oki earthquake and the 1994 M 8.3 Shikotan earthquake are examples of intraplate seismicity, caused by deformation within the lithosphere of the subducting Pacific plate (Sanriku-Oki) and of the overriding North America plate (Shikotan), respectively.
At the southern terminus of the Japan Trench the intersection of the Pacific, North America, and Philippine Sea plates forms the Boso Triple Junction, the only example of a trench-trench-trench intersection in the world. South of the triple junction the Pacific plate subducts beneath the Philippine Sea plate at the Izu-Ogasawara trench, at rates of 45-56 mm/yr. This margin is noteworthy because of the steep dip of the subducting Pacific plate (70° or greater below depths of 50 km depth), and because of its heterogeneous seismicity; few earthquakes above M 7 occur at shallow depths, yet many occur below 400 km. The lack of large shallow megathrust earthquakes may be a result of weak coupling at the plate interface, or simply a reflection of an incomplete earthquake catalog with respect to the length of typical seismic cycles.
The northernmost section of the Philippine Sea plate shares a 350 km boundary with the North America plate that runs approximately east-west from the Boso Triple Junction towards the Izu Peninsula. This short boundary is dominated by the subduction of the Philippine Sea plate beneath Japan along the Sagami Trough, but also includes small sections of transform motion.
The subduction of the Philippine Sea plate under the Eurasia plate begins at the Suruga Trough, immediately southwest of the Izu peninsula. In the northern Tōkai, Tonankai and Nankai sections of this subduction zone, historical data indicate M 8+ earthquake recurrence intervals of 100-150 years. The Tonankai and Nankai sections last ruptured in M 8.1 earthquakes in 1944 and 1946, respectively, while the Tōkai section last broke in 1854. In the 1980's studies began to forecast the imminence of a large earthquake in the Tōkai region, and warned of its potential impact on the cities of Tokyo and Yokohama (the two largest cities in Japan); to date, the expected event has not occurred.
The boundary between the Philippine Sea and Eurasia plates continues south and southwestwards from the Suruga Trough, extending 2000 km along the Nankai and Ryukyu trenches before reaching the island of Taiwan. Along the Ryukyu Trench, the Philippine Sea plate exhibits trench normal subduction at rates increasing from 48 mm/yr in the northeast to 65 mm/yr in the southwest. Convergence and the associated back-arc deformation west of the oceanic trench creates the Ryukyu Islands and the Okinawa Trough. The largest historic event observed along this subduction zone was the M 8.1 Kikai Island earthquake in 1911.
In the vicinity of Taiwan the structure of the Philippine Sea: Eurasia plate boundary and the associated pattern of seismicity becomes more complex. 400 km east of Taiwan a clockwise rotation in the trend of the margin (from NE-SW to E-W), paired with an increase in subduction obliquity creates a section of the plate boundary that exhibits dextral transform and oblique thrusting motions. South of Taiwan the polarity of subduction flips; the Eurasia plate subducts beneath the Philippine Sea plate. Debate surrounds contrasting models of the plate boundary position between the zones of oppositely verging subduction, and the boundary's relation to patterns of seismicity. Many studies propose that crustal thickening causes the majority of regional seismicity, while others attribute seismicity to deformation associated with subduction. Another resolution proposes a tear in the Philippine Sea plate and a complex assortment of subduction, transform, and collisional motion. All the models concede that seismicity around the island of Taiwan is anomalously shallow, with few earthquakes deeper than 70km.
While there are no instances of an earthquake M>8 in the modern record, Taiwan and its surrounding region have experienced eight M>7.5 events between 1900 and 2014. The dominance of shallow M<8 earthquakes suggests fairly weak plate boundary coupling, with most earthquakes caused by internal plate deformation. The 1935 M 7.1 Hsinchu-Taichung earthquake and the 1999 M 7.6 Chi-Chi Earthquake both exemplify the shallow continental crust thrust faulting that dominates regional seismicity across the island. A major tectonic feature of the island is the Longitudinal Valley Fault, which ruptures frequently in small, shallow earthquakes. In 1951, the Longitudinal Valley Fault hosted twelve M≥6 events known as the Hualien-Taitung earthquake sequence.
Large earthquakes in the vicinity of Japan and Taiwan have been both destructive and deadly. The regions high population density makes shallow earthquakes especially dangerous. Since 1900 there have been 13 earthquakes (9 in Japan, 4 in Taiwan) that have each caused over 1000 fatalities, leading to a total of nearly 200,000 earthquake related deaths. In January 1995 an earthquake that ruptured a southern branch of the Japan Median Tectonic Line near the city of Kobe (population 1.5 million) killed over 5000 people. The 1923 Kanto earthquake shook both Yokohama (population 500,000, at that time) and Tokyo (population 2.1 million), killing 142,000 people. The earthquake also started fires that burned down 90% of the buildings in Yokohama and 40% of the buildings in Tokyo. Most recently, the M9.0 Tohoku earthquake, which ruptured a 400 km stretch of the subduction zone plate boundary east of Honshu, and the tsunami it generated caused over 20,000 fatalities. (USGS) More information on regional seismicity and tectonics
Featured image credit: USGS