2005 Fukuoka earthquake

The Fukuoka earthquake (福岡県西方沖地震), also known as the Fukuoka Prefecture West Sea Earthquake, struck Fukuoka Prefecture, Japan at 10:53 am JST on March 20, 2005, off the northwest coast of Fukuoka Prefecture, and lasted for approximately 1 minute. The Japan Meteorological Agency (JMA) measured it as peaking at a magnitude of 7.0 and a maximum seismic intesity of less than six, whereas the United States Geological Survey (USGS) reported a magnitude of 6.6. The quake occurred along a previously unknown fault in the Genkai Sea, North of Fukuoka city, and the residents of Genkai Island were forced to evacuate as houses collapsed and multiple landslides occurred in various places. Investigations subsequent to the earthquake determined that the new fault was most likely an extension of the known Kego fault that runs through the centre of the city.

The earthquake caused significant damage in coastal areas such as Genkai Island in Nishi Ward, Fukuoka City, where half of the homes were completely destroyed, as well as in areas including Noko island, Nishiura, Miyanoura, and Shika island. The impact also extended to Fukuoka City and surrounding municipalities such as Shima Town and Maehara City (now Itoshima City). There was one fatality, approximately 1,200 injuries, and around 140 homes completely destroyed. It was the largest earthquake in recorded history near Fukuoka City.

Fukuoka is not as seismically active as many other parts of Japan, and was known prior to the earthquake as one of Japan's safest locations in terms of natural disasters; the previous earthquake, a magnitude 5, had occurred over a hundred years ago and it had been centuries since the city had experienced a serious earthquake.

Earthquake names in Japanese media
The Japan Meteorological Agency referred to the earthquake as the "Earthquake off the western coast of Fukuoka Prefecture" in their press release. While Fukuoka Prefecture and Fukuoka City, as well as major media outlets such as the Asahi Shimbun and NHK, used "Fukuoka Prefecture Western Offshore Earthquake." The Nishinippon Shimbun and Yomiuri Shimbun used "Fukuoka Offshore Earthquake," and the Mainichi Shimbun used "Fukuoka Offshore Genkai Earthquake." Due to slight differences in Japanese grammar and Kanji usage, the names differ slightly in wording and structure among Japanese media, but all refer to the same event and generally have the same meaning.

In English it is most commonly referred to as simply the "Fukuoka earthquake."

City introduction
Fukuoka is located in the northern Kyushu, Japan, and it is in the center of the Fukuoka plain. Fukuoka is the most populous and developed prefecture in Kyushu. Geographically, Fukuoka is close to mainland East Asia, and the nearest megacity to Fukuoka is Seoul, South Korea, rather than a domestic Japanese city. Fukuoka is about as far from Shanghai as Tokyo. As a result, Fukuoka has been the window of east Asian culture flowing into Japan since ancient times, and now there are many direct routes to Korea, mainland China and Taiwan.

Description
The strong earthquake occurred at 10:53 am in the Kyushu region, about 70 km west of Shimonoseki city in Yamaguchi prefecture. The earthquake occurred on Sunday, March 20, 2005, which was also a public holiday known as Vernal Equinox Day, falling in the middle of a three-day weekend.

The depth of the earthquake was extremely shallow, with a presumed Richter scale of 7.0. The Japan meteorological agency said that quake's epicenter was in the sea of Japan, northwest of Fukuoka prefecture, located at approximately 33.443°N latitude and 130.175°E longitude, and the epicenter was about 9 kilometers under the sea. The epicenter was situated about 8 kilometers northwest of Genkai Island, a remote island in the Genkai Sea near the entrance of Hakata Bay, which belongs to Nishi Ward, Fukuoka City. It was also approximately 9 kilometers north-northwest of Nishiura Cape at the northern tip of Itoshima Peninsula.

As of 6 p.m. local time, the earthquake had caused 381 people injured and one dead. Kyodo News Agency, citing Japan's meteorological agency, said it was the strongest earthquake to hit Kyushu since May 1997, and the first to exceed magnitude 6 since 1898. Earthquake experts in Japan said it is rare for a strong earthquake to strike the area. A quake measuring about seven on the Richter scale struck three hundred years ago, but no major quake has struck after that. Fukuoka's most famous major fault, the Kego fault, runs northwest to southeast, roughly parallel to Nishitetsu's Ōmuta train line, and was thought to be 22 km long, terminating at Hakata Bay. It is estimated to be able to produce earthquakes as strong as magnitude 7 at the epicenter approximately once every 15,000 years. When a center is located at a depth of 10 km, it would cause an earthquake of a lower-6 magnitude (similar to the March 20, 2005 earthquake) in downtown Fukuoka. The probability of an earthquake along the known length of the Kego fault occurring within 30 years was estimated at 0.4% prior to the March 20, 2005 earthquake, but this probability has been revised upwards since. According to a National Institute of Advanced Industrial Science and Technology presentation April 12, 2005, supposing the last Kego earthquake had occurred 13,000 years ago, the probability of major activity within 30 years had been revised to 7%, or it were 7,000 years ago, the probability had been revised to 4%. Suppose that an earthquake had occurred along the Kego fault within the last 2000 years, the risk would be unchanged.

This earthquake occurred within the continental plate (Eurasian Plate or Amur Plate), known as an inland crustal earthquake. The focal mechanism involved a strike-slip fault with a pressure axis oriented in an east–west direction. The fault plane was vertical to the ground, and based on the distribution of aftershocks, it is considered to be a left-lateral strike-slip fault extending northwest–southeast.

The fault responsible for this earthquake is provisionally named the "Northwestern Offshore Fault of Fukuoka Prefecture" by the government's Earthquake Research Committee. However, the existence of this fault was not known at the time of the earthquake.

The amount of slip along the fault caused by the earthquake varies according to different sources. According to the Japan Meteorological Agency, it was estimated to be a maximum of approximately 1.7 meters, while according to Nishimura et al., it was about 1.9 meters, and according to Asano et al., it was approximately 3 meters.

Analysis by the Fukuoka District Meteorological Observatory indicated that in Hakata Ward, Fukuoka City, about 30 kilometers away from the epicenter, initial tremors began at around 10:53:47, approximately 7 seconds after the earthquake, followed by the arrival of the main tremors 4 seconds later, lasting for about 20 seconds. Based on this, the duration of the fault rupture that triggered the earthquake was estimated to be several tens of seconds, relatively short for an earthquake of this magnitude.

The aftershock zone extended approximately 30 kilometers northwest from near Shigashima, with slight curvature in the vicinity of the northwest and southeast ends of the fault. Additionally, a small aftershock zone became active near the sea around Midou Peninsula a day after the main earthquake, starting from March 21. This small aftershock zone corresponds to the location of the Ishido-Umino Nakama Fault.

Historical Earthquakes and Geology in the Surrounding Area
The seismic activity in the offshore area northwest of Fukuoka Prefecture has historically been low, with few records of significant earthquake activity. Geological literature published before the earthquake often described northern Kyushu, north of the Beppu-Shimabara Trench, as relatively geologically stable, occasionally experiencing only small earthquakes. Former Fukuoka Governor Aso, on the day of the earthquake, remarked, "Fukuoka has always been said to be safe from major earthquakes, so this is a significant shock."

The largest known earthquakes in Fukuoka City and the Itoshima Peninsula area prior to this event were the Itoshima Earthquake in August 1898 (M6.0, M5.8), followed by earthquakes near Hakata Bay in August 1929 (M5.1) and near Raizan in February 1930 (M5.0). Even including records from ancient documents, there were no precedents for earthquakes of magnitude 7 or higher, making this earthquake the largest in recorded history for Fukuoka City and the Itoshima Peninsula. Additionally, since the establishment of seismic intensity records in 1926, this earthquake marked the first time that seismic shaking with an intensity of 5 or higher was observed in both Fukuoka and Saga Prefectures.

Expanding the scope to include Fukuoka, Saga, and Nagasaki Prefectures, earthquakes such as the 1700 Iki-Tsushima Earthquake (M7) and the 679 Chikushi Earthquake (M6.5 - 7.5) are known. This earthquake occurred approximately 300 years after the last significant earthquake in the region. Despite this, the northwest coast of northern Kyushu, along the Genkai Sea, is relatively inactive in terms of seismic activity compared to other regions in Japan. While earthquakes of around magnitude 7 occasionally occur, contrasting with regions like Bungo Strait where earthquakes of around magnitude 7.5 have been recorded.

No active faults were known in the epicentral area of this earthquake prior to the event. Subsequent seafloor surveys after the earthquake did not reveal any evidence of faulting in the epicentral area. It's believed that slip along strike-slip faults may not produce visible offsets on the seafloor, making them difficult to detect. However, about 10 kilometers northeast of the epicenter (in the direction of the aftershock distribution), there are two segments of seafloor faults several kilometers long, extending in the same northwest–southeast direction as the aftershock distribution. In addition, there are multiple active faults in the northern part of Fukuoka Prefecture, with the Kego Fault, which runs through the center of Fukuoka City, being one of them.

The aftershock area of this earthquake lies almost directly on the extension of the Kego Fault, prompting investigations into their potential relationship after the earthquake. In the 2007 assessment by the Earthquake Research Committee, while the epicentral area of this earthquake was determined not to be directly associated with the Kego Fault itself, it was grouped under the umbrella term "Kego Fault Zone." Although the probability is low, it's acknowledged that there's a possibility of the two faults interacting to generate earthquakes.

Aftershocks
Between the occurrence of the main earthquake and the end of June 2005, there were 375 aftershocks with a seismic intensity of 1 or higher recorded over a period of just over three months. By 9 p.m. Tuesday, 85 aftershocks had been recorded. Within a month of the quake, there were four aftershocks with a magnitude of more than 5, including the largest one of M5.8 on April 20. Within half a year the size of aftershocks continued, a total of several thousand times, but the overall trend gradually reduced. The breakdown by maximum seismic intensity is as follows: one aftershock with a seismic intensity of 5 strong, seven with a seismic intensity of 4, 23 with a seismic intensity of 3, 118 with a seismic intensity of 2, and 226 with a seismic intensity of 1. However, this does not include seismic intensity data from Genkai Island until 6:00 PM on March 21 when the seismometers were installed. Looking at the magnitude (M), during the same period, there were 265 aftershocks with an M of 3 or higher, including 236 with an M of 3, 24 with an M of 4, and 5 with an M of 5.

After the main earthquake, the Japan Meteorological Agency warned that aftershocks with a seismic intensity of around 5 weak might occur in some areas, with the potential for seismic intensity of around 5 strong in certain locations. By the 24th, this potential was revised to seismic intensity around 4.

M5.8 aftershock on April 20
An aftershock hit at 6:11 a.m. April 20 on Japan's southern main island of Kyūshū, the Central Meteorological Agency reported. Although considerable time had passed since the first quake, the aftershock was not unexpected.This aftershock had a strike-slip fault mechanism with a pressure axis oriented in an east–west direction, similar to the main earthquake. The quake, which swayed buildings and shattered some outer walls, was measured to have magnitude of 5.8. 2 and 56 people were severely and slightly injured and treated at a hospital in Fukuoka due to the quake. There were temporary closures of major highways, railway services and Fukuoka's airport. Following reports that the city has only prepared for earthquakes up to a magnitude of 6.5, the aftershock renewed fears that the quakes might cause the Kego fault to become active again beneath Fukuoka, leading to an earthquake as big as, or bigger than, the March 20 quake.

As time passed from the main earthquake, aftershocks tended to decrease, but distinctive activity occurred on the southeast side of the aftershock area. Near Shigashima on the southeast side of the aftershock area, significant aftershock activity occurred after the main earthquake. After a period of relatively few earthquakes in early to mid-April, a series of aftershocks culminated in the largest aftershock on April 20 with an M of 5.8 and seismic intensity of 5 strong. Additionally, around Uminoshima, located approximately 10 kilometers east of the aftershock area, there was significant aftershock activity starting from the day after the main earthquake on March 21, but the activity decreased by mid-April.

In order to more accurately estimate the risk of ongoing or increased seismic activity, teams from Tokyo University, Kōchi University, Hiroshima University and Ōita University surveyed Hakata Bay to determine how far the Kego fault extends. Preliminary results, announced May 1, 2005 indicated that the fault extends nearly as far as Nokonoshima, 2.5 km out into the bay, though no sign of recent activity along the fault was uncovered. The teams also discovered a new fault in the Higashi-ku portion of Hakata Bay. Later findings indicated that the fault responsible for both the March 20 and April 20 quakes was likely an extension of the Kego fault, making its total length approximately 40 km.

Architecture
During this earthquake, more than half of the 225 residential buildings on the Genkai Island in the western part of Fukuoka city, near the epicenter, suffered serious damage. The island was particularly hard hit because it near to the quake and traditional Japanese homes are less vulnerable to earthquakes than "mansions" that are built by engineers. In Kaido, about 120 homes were destroyed and another 55 damaged partially. Traditional Japanese houses, particularly in the areas of Daimyō and Imaizumi, were the most heavily damaged and many were marked for demolition. Insurance payments for damage were estimated at 15.8 billion yen. In Tenjin, many windows were smashed and concrete cracked. Temples and shrines were also damaged a lot. More than half of the 225 residential buildings on the Genkai Island in the western part of Fukuoka city, near the epicenter, suffered serious damage

Transportation
According to public broadcaster NHK, local rail services were suspended after the tremors triggered an automatic safety mechanism. The operation of the San'yō Shinkansen between Shin-Yamaguchi and Hakata was temporarily suspended. The cracks appeared on sidewalks in residential areas.

Other Effects
Officials reported water and gas breaks and power outages. Telephone service in the southern prefecture was jammed after an automatic safety mechanism was triggered by the tremors. The quake also caused 103 gas leaks. The meteorological agency issued a tsunami warning after the strong earthquake, but lifted it at noon. Landslides occurred around Fukuoka, Saga and Nagasaki prefectures.

Impact on Other Countries
Kyushu, which is separated from South Korea by a narrow strait, was felt about 130 miles from the South Korean port city of Busan, where it briefly shook buildings. A Busan police spokesman said no damage was immediately reported.