GREAT EAST JAPAN EARTHQUAKE AND TSUNAMI OF MARCH 11, 2011
On March 11, 2011, an earthquake struck off the coast of Japan, producing a devastating tsunami that sent walls of water washing over coastal towns, cities and farmland in the northern part of the country and set off warnings as far away the west coast of the United States and South America. Recorded as 9.0 on the Richter scale, it was the most powerful quake ever to hit Japan and the seventh strongest ever recorded globally (tied for forth since 1900). It was also Japan's worst natural disaster since World War II (earlier earthquakes and tsunamis killed more people).
In Japan it became known as the Great East Japan Earthquake and Tsunami or the Tohoku Pacific Offshore Earthquake (Japanese refer to northern Honshu, especially the northeast side of the island as Tohoku). The coastal areas of Iwate, Miyagi and Fukushima prefectures suffered particularly severe damage, due to devastating tsunami more than 10 meters high. Joshua Hammer wrote in the New York Times, “The devastation stretched along hundreds of miles of seacoast; towns washed away by mudslides; trains carried out to sea; thousands of survivors left stranded on roofs, awaiting rescue. The quake also led to... the partial meltdown of at least two nuclear reactors. Only the fact that the epicenter was not near a densely populated area prevented far greater casualties.”
The 9.0- magnitude earthquake was so powerful it shifted the position of the Earth’s figure axis by as much as 15 centimeters and moved Honshu, Japan’s main island, two and half meters eastward. The tsunami generated by the earthquake obliterated towns and damaged hundreds of thousands of buildings, including four reactors at the Fukushima Daiichi nuclear power plant. The cost of disasters worldwide was the highest ever in 2011 ($380 billion), mainly due the earthquake and tsunami in Japan.
The total number of casualties confirmed by Japanese National Police Agency in March 2019 was 18,297 dead, 2,533 missing and 6,157 injured. A total of 196,559 buildings were destroyed or damaged. More than 460,000 were made homeless and sought refuge in shelters. This included 150,000 in Miyagi Prefecture, 47,000 in Iwate Prefecture and 130,000 in Fukushima Prefecture. In the first three days after the disaster Japan’s Self Defense Forces (the Japanese military) rescued 66,000 people, many of them stranded on hilltops and rooftops and among debris. Because reaching them by land was so difficult many had to wait to be retrieved by helicopter, which could carry only a few people at a time. Thousands of others evacuated their homes due to the crisis at the Fukushima nuclear power plant.
The Maximum of 7 on the Japanese seismic scale was recorded in Kurihara, Miyagi Prefecture. Upper 6 was recorded in 34 cities and towns in Miyagi, Fukushima, Ibaraki and Tochigi prefectures. The scale of the disaster took some time to absorb. Headlines in the newspaper the day after the quake read: “At least 20 dead...and some houses were washed away.” By the time these papers were reaching the streets television video was showing massive black waves swallowing up houses and cars and vast empty spaces where towns used to be. Bodies washed ashore at various spots along the coast after having been pulled out to sea by the tsunami’s retreat and then pushed back to the shore. Japanese Prime Minister Naoto Kan told a press conference: “I think that the earthquake, tsunami and the situation at our nuclear reactors makes up the worst crisis in the 65 years since the war. If the nation works together, we will overcome.”
According to the U.S. Geological Survey, there have been only five quakes of magnitude-9.0 or greater since 1900. The strongest was a magnitude-9.5 quake that hit Chile in 1960, and all five were ocean trench earthquakes. The Great East Japan Earthquake is the fourth-strongest on record. The energy of a magnitude-9.0 quake is 32 times greater than that of a magnitude-8.0 quake. [Source: Yomiuri Shimbun, April 30, 2011]
Links to Articles in this Website About the 2011 Tsunami and Earthquake: 2011 EAST JAPAN EARTHQUAKE AND TSUNAMI: DEATH TOLL, GEOLOGY AND THEORIES Factsanddetails.com/Japan ; ACCOUNTS OF THE 2011 EARTHQUAKE Factsanddetails.com/Japan ; DAMAGE FROM 2011 EARTHQUAKE AND TSUNAMI Factsanddetails.com/Japan ; EYEWITNESS ACCOUNTS AND SURVIVOR STORIES Factsanddetails.com/Japan ; TSUNAMI WIPES OUT MINAMISANRIKU Factsanddetails.com/Japan ; SURVIVORS OF THE 2011 TSUNAMI Factsanddetails.com/Japan ; DEAD AND MISSING FROM THE 2011 TSUNAMI Factsanddetails.com/Japan ; ; CRISIS AT THE FUKUSHIMA NUCLEAR POWER PLANT Factsanddetails.com/Japan ;
Death Toll From the Earthquake and Tsunami of 2011
The total number of casualties confirmed by Japanese National Police Agency in March 2019 was 18,297 dead, 2,533 missing and 6,157 injured. A total of 196,559 buildings were destroyed or damaged. More than 460,000 were made homeless and sought refuge in shelters. This included 150,000 in Miyagi Prefecture, 47,000 in Iwate Prefecture and 130,000 in Fukushima Prefecture.
The death toll as of March 2012 was 15,854 in 12 prefectures, including Tokyo and Hokkaido. At that time a total of 3,155 were missing in Aomori, Iwate, Miyagi, Fukushima, Ibaraki and Chiba prefectures. The identities of 15,308 bodies found since the disaster, or 97 percent, had been confirmed at that time. [Source: Japan’s National Police Agency]
As of September 2011, the National Police death toll was 15,774 dead and 4,227 missing. Even at that time the search for bodies was still continuing in the no-go zone around Fukushima nuclear power plant. As of June 11 the death toll reached 15,413, with about 2,000, or 13 percent , of the bodies unidentified. Around 7,700 people were missing. As of May 1, 2011: 14,662 were confirmed dead, 11,019 were missing, and 5,278 were injured. As of April 11, 2011 the official death toll stood at than 13,013 with 4,684 injured and 14,608 people listed as missing. The true death figure is hard to determine because there may be some overlap between the missing and dead and not all residents or people in areas devastated by the tsunami can be accounted for. The final toll is expected to exceed 27,000.
The disaster hit Ishinomaki — a farming and fishing community of 160,000 about 50 kilometers northeast of Sendai — hard. Of 8,000 people still missing across northeastern Japan, 2,770 are from Ishinomaki; it also has the highest confirmed death toll, 3,100. Of 17,000 residents in Minamisanriku, 1,000 were killed.
A large number of victims were elderly and from Miyagi Prefecture . When the death toll topped 10,000 on March 25: 6,097 of the dead were in Miyagi Prefecture, where Sendai is located; 3,056 were in Iwate Prefecture and 855 were in Fukushima Prefecture and 20 and 17 were in Ibaraki and Chiba Prefectures respectively. At that point 2,853 victims had been identified. Of these 23.2 percent were 80 or older; 22.9 percent were in their 70s; 19 percent were in their 60s; 11.6 percent were in their 50s; 6.9 percent were in their 40s; 6 percent were in their 30s; 3.2 percent were in the 20s; 3.2 percent were in their 10s; and 4.1 percent were in 0 to 9.
The conclusion that one draws from this data is that relatively young people were better able to make a dash to safety while the elderly, because they were slower, had difficulty reaching high ground in time.
A forensic examination of 126 victims recovered in the first week after the disaster in Rikuzentakata by Hirotaro Iwase, a professor of forensic medicine at Chiba University, concluded that 90 percent of the town’s fatalities were caused by drowning. Ninety percent of the bodies had bone fractures but those are believed to have occurred primarily after death. The autopsies showed that the victims had been subject to impacts — presumably with cars, lumber and houses — equivalent to a collision with a motor vehicle traveling at 30 to kph. Most of the 126 victims were elderly. Fifty or so had seven or eight layers of clothing on. Many had backpacks with items like family albums, hanko personal seals, health insurance cards, chocolate and other emergency food and the like. [Source: Yomiuri Shimbun]
Before the March 11, 2011 quake the most devastating earthquake was the Great Tokyo Earthquake of September 1, 1923, event, which registered 7.9 on the Richter scale, lasted about five minutes, and flattened Tokyo and the port city of Yokohama, killing 140,000 people. There have been several earthquakes and tsunamis in Japan’s history that have killed between 10,000 and 30,000 people.
First the Earthquake
Kesennuma After On Friday, at 2:46 p.m. Tokyo time, the quake struck. First came the roar and rumble of the temblor, shaking skyscrapers, toppling furniture and buckling highways. Japanese, accustomed to frequent earthquakes, were stunned by this one’s magnitude and the more than 100 aftershocks, many equivalent to major quakes.”I never experienced such a strong earthquake in my life,” Toshiaki Takahashi, 49, an official at Sendai City Hall told the New York Times. “I thought it would stop, but it just kept shaking and shaking, and getting stronger.” In Tokyo it was strong enough to bend the top of Tokyo Tower.
Chico Harlan wrote in the Washington Post, “For those in Tokyo, the earthquake rattled buildings, shook offices. Computers spilled, lights gyrated and workers ducked under desks. But in Sendai, the capital of Miyagi Prefecture, the tsunami turned a city into a confetti of shrapnel and wood — car scraps, engine parts, bricks. Eventually the wall of water that crashed ashore left behind a dark sludge that coated surrounding farm areas. At the Sendai airport, fire soon burned from a small building positioned between the terminals. Cars and propeller airplanes were discarded by the waves, left among splintered wood, and aerial photos made them appear much like toys left out on the floor.” [Source: Chico Harlan, Washington Post, March 12, 2011]
Eight days before the quake a 7.2-magnitude earthquake occurred off the north coast of Honshu not far from epicenter of the main March 11 quake. Tsunami warnings were issued but the tsunamis that came ashore were only about 50 or 60 centimeters in height. There were no reports of injuries or serious damage. The next day a 6.8-magnitude quake rocked the region.
Eric Talmadge and Mari Yamaguchi of AP wrote: “On March 11, Prime Minister Naoto Kan was taking a beating in an Upper House committee meeting over whether he had taken campaign money from a foreign national, which is illegal in Japan. The questioning stopped suddenly when the entire parliament building, a sprawling structure in the center of Tokyo, started to rock. It was 2:46 p.m. All eyes rose to the huge crystal chandeliers above, clinking and shaking violently. "Everyone, please stay in a safe position," committee chairman Yosuke Tsuruho said, grasping the armrests of his upholstered velvet chair. "Please duck under your desk."
Within four minutes, a crisis headquarters was up and running across the street in the prime minister's office. Kan rushed there as soon as the shaking subsided. At 3:37 p.m. he convened a roundtable of his top advisers. Soon after the tsunami hit, Kan's task force was deluged by reports of massive damage up and down the coast, aerial photos and video showing entire villages gone.
Then the Tsunami
The first large tsunami waves struck the coast of northern Japan between 15 minutes and 30 minutes after the earthquake, with coastal areas closest to the quake epicenter being struck first. Waves as high as 12 meters surged onto shore, carrying away cars and boats, setting buildings ablaze and inundating factories, fields and highways. Towering lines of wave uprooted trees, rushed over sea walls trees, ripped down houses and pushed all the debris inland and then pull it back into the sea, leaving behind bits and pieces of this and that and layer of black mud. [Source: New York Times]
Tsunami waves of over 30 meters high were recorded in places where 10-meter waves were pushed higher by narrow inlets and mountains close to the shore. In some of these places houses and even evacuation centers that were supposed to be safe because they were high off the ground were swept away. In other placed the waves swept inland for many kilometers.
The tsunami that struck northern Japan was one of the largest ever recorded and far bigger than scientists thought possible because they did not anticipate such a large earthquake. Researchers at the University of Tokyo estimated that the tsunami wave that struck the Taro District of Miyako in Iwate Prefecture was 37.9 meters in height based on the drift displacement of the port and discovery of lumber from the port on the slope of a mountain 200 meters from the coast. Researchers also found fire engines and fishing boats on the mountain slope. The tsunami waves were so big they destroyed many tidal gauges used to measure wave size. [Source: Kyodo]
Television images showed waves and floodwaters, engorged with floating debris surging inland, pushing aside heavy trucks as if they were toys, depositing buses on four-story buildings ramming fighter jets into houses. Kyodo News reported a gigantic wave sweeping up a ship carrying more than 100 people. “The spectacle was all the more remarkable for being carried live on television,” the New York Times said, “even as the waves engulfed flat farmland that offered no resistance. The tsunami could be seen scooping up every vessel in the ocean off Sendai, and churning everything inland.” [New York Times]
Satellite images revealed how the coastline was changed during the tsunami. Amateur video captured boats being carried down city streets, water rushing through Sendai airport and masses of cars bobbing up and down like rubber ducks in cascades of black water. Shocked observers watched as the water carried away everything in its path.”The tsunami roared over embankments in Sendai city, washing cars, houses and farm equipment inland before reversing directions and carrying them out to sea,” Japanese engineer, Kit Miyamoto, said. “Flames shot from some of the houses, probably because of burst gas pipes.”
The tsunami lasted for two days, as its waves reached as far as Chile before reflecting back towards Japan . By the morning after the tsunami, Japan was filled with scenes of desperation, as stranded survivors called for help and rescuers searched for people buried in the rubble. Several coastal towns were literally wiped off the face of the earth.
Evan Osnos wrote in The New Yorker, “The afternoon of Friday, March 11th, was cool and partly cloudy on the northeast coast of Japan’s main island, a serene stretch once known as the nation’s “back roads.” At 2:46 P.M., as schools were beginning to let out, the ground began to shake. It was violent even by Japan’s standards — the thundering went on for five minutes — and before long Japanese television was warning of a wave charging west across the Pacific Ocean at the speed of a jet. Kicked up from the seabed, the tsunami amplified in size and slowed in speed as it moved into the shallows beside the Japanese coastline, and by the time it touched land it was a wall of water, black and smooth. It was as tall in places as a three-story building, moving at fifty miles per hour. [Source: Evan Osnos, The New Yorker, March 28, 2011]
“Unlike the Indian Ocean tsunami of 2004, the horrific grandeur of this moment unfolded before the unblinking eyes of Japan’s ubiquitous surveillance cameras, mobile phones, and hovering news helicopters, compiling a record of rebuke to the sense of protection once extended by the technology and engineering at the heart of Japanese life.” The tsunami “flicked fishing trawlers over seawalls, crunched them against bridges. It sent fleets of cars and trucks hurtling from parking lots, and turned homes into chips of wood and tile, before heading deeper into Miyagi and Iwate Prefectures across a span of six miles. Rampaging through former farming and fishing villages, and the cosmopolitan city of Sendai, the wave slowed, but remained too fast for most people to outrun on foot.
Tsunamis reached Tokyo but they were small. The first wave to reach Tokyo Bay was about 80 centimeters high. It arrived at 4:40pm about an hour and 45 minutes after the earthquake. The highest was 1.5 meters and came at 7:16 p.m.
Geology of the March 11 Earthquake
The earthquake that struck northern Japan on March 11, 2011 was an ocean trench earthquake. Scientists assume that a quake of this size only occurs once in about 600 years on average. The epicenter was in the ocean about 130 kilometers off the Sanriku region, east-southeast of Miyagi Prefecture’s Oshika Peninsula, according to the Japan Meteorological Agency. The ground moved 5.3 meters eastward and fell by 1.2 meters on Oshika Peninsula.
The 9.0 earthquake lasted for several minutes. It not only violently shook the ground and generated a devastating tsunami, it also moved the coastline and changed the balance of the planet. Global positioning stations closest to the epicenter jumped eastward by up to four meters. Later the Japan Coast Guard found location markers on the seabed near the quake focus — 120 kilometers off the Oshika Peninsula northeast of Sendai, at a depth of 1,700 meters — had shifted 24 meters east-southeast. The closest piece of land to quake epicenter — the Oshika Peninsula northeast of Sendai — moved 5.3 meters and subsided 1.2 meters. Chiba Prefecture near Tokyo shifted 17 centimeters east.
Earthquake Intensity Map
The Geographical Survey Institute said that all 28 benchmarks and triangulation points in the three northeastern Japanese prefectures ravaged by last month's earthquake and tsunami have fallen by a range of 20 to 84 centimeters. The largest fall of 84 cm was observed in Rikuzentakata, Iwate Prefecture, followed by 78 cm in Ishinomaki, Miyagi Prefecture, said the institute that released the findings of its survey which used the global positioning system. A drop of 29 cm was also found in Soma, Fukushima Prefecture, it said. The drops were caused by a sharp movement of plates below the Tohoku region in northeastern Japan due to the magnitude 9.0 earthquake on March 11. These and other findings by the survey can be used to work out measures against spring tides and tsunami, said Toshio Kawahara, an official at the institute. [Source: Kyodo, April 16, 2011]
The 9.0-magnitude earthquake was so powerful that it shifted the island of Honshu eight feet to the east. The energy generated by the quake was equivalent to 336 megatons of TNT, thousands of times stronger than the nuclear explosion at Hiroshima, and 30 times stronger than a 8.0-magnitude earthquake or 800 times more than the Kobe earthquake in 1995. NASA scientists calculated that the redistribution of mass by the earthquake caused the Earth to spin faster, shortening a day on earth by 1.8 microseconds and tilting the Earth slightly. Marcia McNutt, the director of the United States Geological Survey, told CBS, “If we could only harness the energy from this earthquake, it would power a city the size of Los Angeles for an entire year.”
The United States Geological Survey (USGS) said that Friday’s quake was centered off the coast of Honshu, not far from the city of Sendai, at a point about 230 miles northeast of Tokyo and a depth of about 15 miles below the earth’s surface. The USGS said the quake was more than 100 times more powerful than the 8.8-magnitude quake off the coast of Chile a little more than a year before that killed more than 400. It was less powerful than the 9.1-magnitude quake that struck off Northern Sumatra in late 2004. That quake set off a tsunami that killed more than 200,000 people around the Indian Ocean.
The quake occurred in a subduction zone, where the Pacific plate is sliding beneath the North American plate at a rate of about three inches a year. The epicenter was well north of Japan’s primary manufacturing region running southward from Tokyo to Nagoya to Osaka and farther south and west.
On the bottom of the Pacific Ocean from Hokkaido to the Kanto region, the huge Pacific Plate clashes with the plates on which the Japanese islands sit. When the plates can no longer take the strain, they move to adjust the imbalance, causing an earthquake. In January, the Earthquake Research Promotion Headquarters issued a report about the probability and likely size of earthquakes in the sea off the Tohoku region, caused by movement of the plates. [Source: Yomiuri Shimbun]
History of Earthquakes in the Area of the March 11, 2011 Quake
Several Faults Move During the March 11 Earthquake
The Yomiuri Shimbun reported: “The Tohoku Pacific Offshore Earthquake and resulting tsunamis struck across an extraordinarily wide area along the Pacific coast, including the Tohoku and Kanto regions. Experts attributed this to the fact that multiple parts of a fault moved simultaneously... contrary to earlier projections that only one or two sections would move when an earthquake occurred in the sea off the Tohoku region.” [Source: Yomiuri Shimbun, March 13, 2011]
“The Tohoku Pacific Offshore Earthquake started when part of a fault moved in the sea off the Sanriku Coast. The shock caused another part of the fault to move, then another, rapidly increasing the scale of the earthquake. The fault is believed to stretch 400 kilometers north to south, and 200 kilometers east to west. According to the Meteorological Agency, the shock caused parts of the fault located in the areas off the central Sanriku Coast, Miyagi Prefecture and Fukushima Prefecture to move. Parts located in the other two areas also may have shifted, the agency said.” Koshun Yamaoka, a professor of solid-earth geophysics at Nagoya University, said: "I suspect various parts of the fault moved one after another in wide areas ranging from the sea off the Sanriku Coast to Ibaraki Prefecture [to cause the huge temblor]."
“In January, 2011 the Earthquake Research Promotion Headquarters issued a report about the probability and likely size of earthquakes in the sea off the Tohoku region, caused by movement of the plates. The headquarters divided the region into six earthquake focus regions, and predicted that within 30 years, there was a 99 percent chance a massive earthquake would occur off Miyagi Prefecture and a 90 percent or higher chance of one off Ibaraki Prefecture. The probability of a large quake off the southern Sanriku Coast near the Japan Trench was said to be 80 percent to 90 percent, but no more than 7 percent off Fukushima Prefecture. A massive quake was thought unlikely to happen off the central Sanriku Coast. The report showed that the sea off the Tohoku region contains areas where massive earthquakes are more likely to occur than in other regions of Japan.”
Study Pinpoints Trigger of March 11 Earthquake
In October 2011, the Yomiuri Shimbun, the March 2011 earthquake was triggered when plate boundaries that had been stuck tightly together broke apart in an area measuring several dozen square kilometers off Miyagi Prefecture, according to researchers. [Source: Yomiuri Shimbun, October 20, 2011]
The plate boundaries had been firmly wedged against each other, but as one tectonic plate gradually subducted the other, stress built up at the boundaries and caused them to break apart. The removal of this small "seal" that had held the plates in place resulted in them separating along an area stretching about 400 kilometers, according to the researchers. The team of researchers from Shizuoka University and the National Institute for Earth Science and Disaster Prevention reported their finding last week at a Seismological Society of Japan conference in Shizuoka.
The March 2011 earthquake occurred where the Pacific plate subducts another plate beneath the Tohoku region. The quake's hypocenter was off Miyagi Prefecture, and its hypocentral region was about 400 kilometers long and about 200 kilometers wide. Ryoya Ikuta, assistant professor at Shizuoka University, and other experts focused on the strength of the connection between plate boundaries, which can be a cause of seismic activity. They analyzed data on crustal movements taken from the Global Positioning System over the 15 years up until the earthquake.
Their research revealed that plate boundaries in an area covering several dozen square kilometers about 100 kilometers off Oshika Peninsula, Miyagi Prefecture, had been firmly "jammed" together. This area was close to the epicenter of the magnitude-9 quake. The researchers believe the area moved 30 meters when the earthquake occurred, adding that the plate boundaries had possibly been stuck snugly together for several hundred years.
Tsunami Impact Map
Theories About the Geology That Produced the March 11 Earthquake
Kenneth Chang wrote in the New York Times: “The largest earthquakes occur in subduction zones, places where an ocean plate collides with and slides under a continental plate, particularly around the edge of the Pacific Ocean. But some subduction zones seemed to produce more large earthquakes than others. One explanation was offered in 1980, when Hiroo Kanamori of the California Institute of Technology and Larry J. Ruff, now at the University of Michigan, published a paper that said giant earthquakes occurred more often along ocean faults where the subducting ocean plates were geologically young. The younger plates, like those off Alaska and Chile, were warmer, less dense and harder to push down into the Earth’s mantle, their thinking went. Meanwhile, the older, colder and denser ocean plates like those off Java and the Marianas trench in the Pacific would sink more easily and not produce the giant catastrophic quakes.” [Source: Kenneth Chang, New York Times Science Times, March 21, 2011]
“And yet the Pacific plate off Japan is 130 million years old, one of the oldest, and it generated a magnitude-9.0 counterexample. Dr. Scholz of Columbia said the recent quake in Japan fit with a theory that he and Jaime Campos of the University of Chile developed in 1995. By their theory, the colliding tectonic plates off Fukushima were stuck, and should have been producing earthquakes. But the absence of spectacular earthquakes in the near historic record disagreed with their theory, and led Dr. Scholz to believe that something unknown was relieving the stress.”
“Dr. Scholz said that patches of the Pacific plate off Fukushima become stuck as the plate moves under Japan. In the more modest earthquakes of the past 300 years, just one patch would break free. This time, he said, the patches ruptured together, producing a more cataclysmic quake.” “The past 300 years, that hasn’t happened,” Dr. Scholz said. “So if you’re going to use the past history to extrapolate the future, the last 300 years wouldn’t have predicted the recent earthquake.” In Japan’s history, there does seem to have been a precedent for the recent quake, but it took place more than a thousand years ago. See Jogan Tsunami
Tsunami Travel Times
Geology and Oceanography of the March 11 Tsunami
The disastrous events of March 11 began with an ocean trench quake. The Pacific oceanic plate pushed down on the North American plate, which shook with resistance, like a springboard does when pressure is exerted on one end. The focal area was 450 kilometers long and 200 kilometers wide, extending offshore from the Sanriku region to Ibaraki Prefecture. The oceanic plate moved up to 30 meters sideways, and six to eight meters upward. This movement produced a quake with about 1,450 times the energy of Kobe Earthquake, as well as the tsunami that subsequently wrought havoc in coastal areas. [Source: Yomiuri Shimbun, April 30, 2011]
The tsunami resulted from a reverse fault type quake, which in this case occurred when the North American tectonic plate slid over the Pacific Plate, releasing a huge amount of built-up strain. Seawater then was lifted up from the seabed, pushing the sea increasingly higher as it moved closer to land. [Source: Yomiuri Shimbun, March 13, 2001]
The earthquake occurred at a depth of about 20 kilometers, miles, which while relatively shallow by global standards is about normal for quakes in this zone, Emily So, a USGS engineer told the New York Times. When such quakes set off a tsunami, the devastation often comes from a succession of waves, which can cross oceans at 500 miles per hour or more. [Source: New York Times]
The Meteorological Agency and specialists said the unprecedented tsunamis were caused by an extremely shallow focus and a several-hundred-kilometer slip of a huge fault zone stretching from off the Sanriku Coast in Iwate Prefecture as far south as about Ibaraki Prefecture. "The quake's shallow focus and massive scale explain why the tsunami was so huge," Tokyo University Prof. Takashi Furumura said. "[Tsunamis] have accompanied earthquakes off the Sanriku Coast over a 500-year cycle. There was concern [such a massive quake] would occur in the near future," said Active Fault and Earthquake Research Center chief Yukinobu Okamura. [Yomiuri Shimbun, Op. Cit]
The Okhotsk tectonic plate, on which the Tohoku region lies, is gradually being subducted by the Pacific Plate in a westerly direction, building up stress at the plate boundary.This stress accumulates in a relatively short period of time because the plates "jam" strongly and stick because of the way their surfaces are formed. However, this stress is released infrequently. The Japan Trench, where the 2011 earthquake occurred, is deeper than the Nankai Trough (another place where large earthquakes occur), with its subduction progressing at a sharper angle. For this reason, scientists believed the land-side plate would not be pulled too greatly below the sea-side one, and that a massive quake would be unlikely there. They were wrong.
By utilizing data on frequently occurring small earthquakes off the Tohoku region and Global Positioning System data on the speed at which the plates move, Uchida was able to work out the likelihood of the plates in the hypocentral region jamming. He was then able to calculate the timeframe in which enough strain accumulates to cause a magnitude-9 class earthquake based on data obtained through the above process.
According to a study on the frequency of tsunami based on analyses of geological strata, tsunami caused by huge earthquakes are likely to have had hit the Tohoku coastline in intervals of 500 to 1,000 years. "We need to consider there may be cases in which the energy [stresses] accumulates in a period shorter than that," Uchida said.
Cracks have been found in seabed 170 to 200 kilometers off the coast of Miyagi and Iwate Prefectures at sites at depths from 3,200 meters to 5,350 meters that are believed to have created by the earthquake. One of the cracks is 20 centimeters wide and several meters long. The cracks were found and photographed with the Shinkai 6500 manned submarine, which is capable of descending to depths of 6,500 meters below the ocean surface.
The seismic cycle in which another magnitude-9 class offshore earthquake similar to the March 11 Great East Japan Earthquake could hit the Tohoku region may be as short as 260 years, a Tohoku University researcher has found. According to simulations by Naoki Uchida, an assistant professor at the university, the cycle is about 260 years when tectonic plates in the hypocentral region of an earthquake stick strongly to each other. This earthquake potential may extend to 880 years if the plates are less likely to stick. [Source: Yomiuri Shimbun, September 9, 2011]
232 Strong Aftershocks Recorded after March 11
One year after the March 2011 earthquake and tsunami 232 aftershocks measuring 4 or higher on the Japanese seismic scale of 7 had occurred according to the Japan Meteorological Agency. The number is about 24 times the recorded average for the area, which is estimated at about 9.8 earthquakes per year for the decade through 2010. [Source: Yomiuri Shimbun, March 10, 2012]
The aftershock zone covers an area measuring about 600 kilometers north to south from the coast of Iwate Prefecture to Ibaraki Prefecture and about 350 kilometers from east to west. The agency has defined earthquakes with a focus in this region as aftershocks of the March 11 earthquake. It also warned that aftershocks measuring lower 5 or stronger may occur in the area in the future.
In the wake of the disaster, there were 109 aftershocks with an intensity of 4 or higher last March, followed by 50 such aftershocks in April, and six last month. Although the frequency of aftershocks is on the decline, seismic activity is still occurring in the area. An aftershock measuring lower 5 with a focus off Ibaraki Prefecture occurred on March 1. In the one-year period after the March 11 earthquake aftershocks with an intensity of 1 or higher occurred 7,225 times in the area. These aftershocks accounted for about 70 percent of the 10,120 earthquakes measuring 1 or stronger during the same period nationwide. Outside of the aftershock zone, 18 earthquakes measuring lower 5 or stronger were recorded nationwide, with focuses in Shizuoka, Nagano, Akita prefectures and other locations. The agency believes seismic activities have become more frequent across the country.
Reasons for So Many Aftershocks After March 2011 Earthquake and Tsunami
As of April 27 2011, there had been 435 aftershocks with a magnitude of 5 or greater and 11 with a magnitude 7.0 or greater. This is the largest number of aftershocks in Japan’s history. Experts say there are more shocks to come, possibly as strong as magnitude 8. The Japanese Meteorological Agency warned, "Earthquakes have come to occur frequently as the geological dynamics of eastern Japan have changed due to the March 11 earthquake...We have to expect more earthquakes in the magnitude-7.0 class for the time being.”[Source: Yomiuri Shimbun, April 14, 30 2011]
The high frequency of aftershocks can be attributed to the enormous changes in the force exerted in the Earth's surface in the eastern part of Japan after March 11 earthquake, which measured magnitude-9.0 on Richter scale and caused inland plates to move with great force to the east. A magnitude-7.0 aftershock that took place on April 11 in eastern Fukushima Prefecture also had this effect. A fault that previously had not been considered an active threat was the cause of that aftershock.
There are 16 known faults that could cause magnitude-7 class earthquakes in the Tohoku region of northern Honshu. On the eastern ocean side of the focal area of the March 11 quake, another major quake could be triggered by the pulling force on the Pacific plate's shallow section. After the 1896 Meiji-Sanriku Earthquake, the Showa-Sanriku Earthquake took place in the sea to the east of the earlier quake's focal area in 1933, after an interval of 37 years. In a more recent example, a magnitude-7.9 quake took place in the sea east of the Chishima islands in November 2006, and a magnitude-8.2 aftershock occurred 59 days later.
The apparent southern boundary of the aftershock zone from the March 11 earthquake is off the shore of eastern Chiba Prefecture. In that area, the Pacific plate and the Philippine Sea plate slide under the landward plate that includes eastern Japan. Earthquakes off Chiba occurred because faults in the landward plate became active.
Movements of the Philippine Sea plate affect seismic activity in Tokyo Bay. But only a few earthquakes at the magnitude-3 or magnitude-4 levels have been recorded there since March 11. However, Prof. Koshun Yamaoka of Nagoya University pointed out that seismic activity in northern Chiba and southern Ibaraki Prefectures, which is also linked to the Philippine Sea plate, has increased. "An earthquake could hit the Tokyo metropolitan area with its focus directly below the city. We'll have to observe the situation carefully," Yamaoka added.
The professor anticipates the rise in seismic activity in eastern Japan will continue for four or five years. He told the Yomiuri Shimbun magnitude-7 quakes "are likely to occur once or twice in the next month...After that, they can be expected to happen once in two months. The occurrence of such huge earthquakes will gradually decrease to once in a half year. But the possibility remains that a magnitude-8 earthquake could happen at any time.”
Yoshimitsu Okada, president of the National Research Institute for Earth Science and Disaster Prevention, told the Yomiuri Shimbun, "Aftershocks of the March 11 earthquake occurred in shallow places in the Earth's crust and triggered other aftershocks....The frequency of earthquakes has increased by aftershocks caused by aftershocks of the March 11 earthquake.” In March 2011, a government panel of volcano experts said it was unlikely that aftershocks would cause Mt. Fuji or any other volcano in Japan to erupt.
Hot Springs Halt, Erupt from the Ground and Change Color after the Earthquake
The Yomiuri Shimbun reported: Extraordinary events occurred at hot spring sites across the nation in the wake of the Great East Japan Earthquake, including springs spontaneously ceasing to flow, changing color and temperature, and even hot springwater suddenly gushing from the ground in a residential area. [Source: Yomiuri Shimbun, July 19, 2011]
Hot springwater suddenly began pouring from this defunct coal mine ventilation shaft in Iwaki, Fukushima Prefecture, following aftershocks from the Great East Japan Earthquake in April. Kazuhiko Nakamura, 54, proprietor of the long-standing Choseikan hot spring inn in Yahikomura, Niigata Prefecture, said the water stopped flowing from the inn's well just hours after the magnitude-9 earthquake. Expert examinations conducted at Nakamura's request found there was little chance the hot water would start to flow again, leaving him with no choice but to close the inn.
Clear springwater turned milky white after the earthquake at Fukuroda Onsen Sekisho no Yu, a hot spring day spa in Daigomachi, Ibaraki Prefecture. The curious change has become a boon for the facility, which has experienced a marked uptick in visitor numbers. Many guests have remarked on the quality of the springwater and how it makes their skin "silky smooth," according to a staff member. Why the springwater changed color is unknown--one onsen development company suggested the impact of the earthquake may have caused clay to mix into the layer of earth from where the hot spring originates. Day by day, the springwater is gradually returning to its original clarity, to the disappointment of the spa and its customers.
At a defunct coal mine shaft in Iwaki, Fukushima Prefecture, a torrent of hot springwater is gushing from a cylindrical concrete ventilation passage located on a vacant lot that is just 20 to 30 meters from a residential block.The water, which is about 55 C, started pouring out following aftershocks on April 11 and 12 that registered lower six on the Japanese seismic scale of seven. Several thousand tons of springwater has been pouring every day from the passage. The company charged with administering the about-600-meter-deep mine shaft has dug a ditch to channel the water away from nearby residences.
The National Institute of Advanced Industrial Science and Technology, a government-affiliated research body, has been observing 82 natural groundwater reservoirs at 48 locations in the Izu area of Shizuoka Prefecture and the Shikoku region to gauge possible effects of future earthquakes in the Tokai region and elsewhere. According to the institute, water levels--not volume--in 70 percent of the reservoirs fell after the March 11 earthquake, because the ground was "stretched," distributing the water over a wider area. Conversely, in the aftershocks in April, the ground is thought to have been "compressed," causing the reservoirs to narrow and thus force the springwater out, institute researchers said. Usually, groundwater conditions return to normal over time after being disrupted by seismic activity. This time, however, considering the extraordinary power of the Great East Japan Earthquake, some of its effects on hot springs may be permanent.
Tsunami Warning Map
Warning about a Big Tsunami off Northern Japan
A tsunami of that magnitude was supposed to hit elsewhere. Historical data for the region seemed to suggest that earthquakes between 7.5 and 8.0 on the Richter scale occurred every few decades but ones larger than that were unlikely. A 2004 Miyagi prefectural government report on the expected damage from a large-scale quake in northern Japan estimated that a quake's largest magnitude--following simultaneous jolts at neighboring focus areas--would be magnitude 8.0. The report also said the estimated highest water level within 30 minutes after a quake at such indented coastal areas as Kesennuma, Miyagi Prefecture, would be eight-to-10-meters, and two to three meters on plains such as those around Sendai. [Source: Yomiuri Shimbun]
The Washington Post reported, “The consensus among public officials and many scientists in Japan was that the next mega-quake would most likely occur on the Nankai Trough, a tectonic plate boundary southwest of Tokyo. Two sections of that fault had already broken, and since the 1970s the scientific orthodoxy in Japan had been that the easternmost section was primed to break next. Officials had designated the hypothetical event the “Tokai Earthquake...Scientists believed that the section of the Japan Trench off northern Japan around Sendai could experience a magnitude 7 and 8 earthquakes, perhaps as high as 8.4, but not magnitude 9. That’s the difference between a destructive event and a catastrophic one. The earthquake scale is logarithmic: A magnitude 9 is 10 times more powerful than a magnitude 8.” [Source: Andrew Higgins, Brigid Schulte and Joel Achenbach, Washington Post, March 29, 2011]
Yasutaka Ikeda, a University of Tokyo seismologist., however, believed that a catastrophic earthquake in the Japan Trench was indeed possible. “Ikeda’s calculations showed that the magnitude 7 and 8 earthquakes along the Japan Trench were not releasing all the strain that had to be accumulating over time. In 2006, he gave a presentation titled “Long-term and short-term rates of crustal deformation over the northeast Japan arc, and their implications for gigantic earthquakes at the Japan Trench.” His concluding slide stated that most of the strain would be released “in association with a big decoupling event (Mw ~9) on the subduction zone!” He had no idea when this magnitude 9 event might happen. Such is the unfortunate fact of earthquake science.”
Wave heighst from the 2011 tsunami
Historical documents have noted massive tsunami striking the Sendai plains even in ancient times. According to Japan's historical document, "Nihon Sandai Jitsuroku" ("The True History of Three Reigns of Japan"), the Jogan Tsunami occurred on the Sendai coast in 869. That tsunami flooded inland areas more than three kilometers from shore and killed more than 1,000 people, according to the record. According to the research center, the quake that caused the Jogan Tsunami made the fault slip more than seven meters. Analysis of sediments left by the 869 tsunami led to an estimate that the earthquake had a magnitude of 8.3, meaning that tsunami was probably smaller than the one on March 11 2011. [Source: Yomiuri Shimbun, March 13, 2001]
“On the morning of March 11, Ikeda took a flight to China. When he landed, he heard that there had been a magnitude 8.8 earthquake on the Japan Trench. He didn’t believe it at first. There must be some mistake. Ikeda felt sick. He suddenly had no desire to return home. He didn’t want to see what had happened. His theory had been vindicated, but he felt nothing but sorrow and regret. “It’s not a success story at all,” he said later by e-mail. “It’s my regret and many Japanese geologists’ regret that our works had nothing to do with mitigating disaster caused by the M 9.0 earthquake of 11th March. “We did fail.”
Why Wasn’t Such a Large Earthquake Anticipated
The worst-case scenario predicted by the Earthquake Research Promotion Headquarters for the area hit by the March 11 quake was a magnitude-8.0 earthquake. Why was a larger quake not anticipated? The main reason is because the historical data on which their research was based included no reports of earthquakes of such massive scale. [Source: Yomiuri Shimbun, April 30, 2011]
“Records of past seismic activity that scientists use to predict the scale and focal area of future earthquakes only go back about 100 years,” according to an article in the Yomiuri Shimbun. “The details of earthquakes that occurred before that time can only be guessed at, based on examination of sediment brought to land by tsunami, as well as descriptions found in historical documents.”
“For instance, sediment found in coastal areas from Miyagi Prefecture to Fukushima Prefecture is thought to have been carried there by tsunami caused by the Jogan Earthquake, which is believed to have occurred in A.D. 869. Considering the extended area the sediment was spread across, scientists extrapolated the Jogan Earthquake had a magnitude of about 8. The headquarters had planned to expand the scope of its research in April to include predictions of events on the scale of the Jogan Earthquake, but these plans were preempted by the March 11 disaster.”
“As a magnitude-9.0 earthquake was considered beyond the realm of probability, scientists did not contemplate such an event. Previous magnitude-9.0 earthquakes occurred in places where oceanic plates shifted under land plates at a shallow angle. The angle of the Pacific plate, the oceanic plate near Japan, is relatively steep, which led scientists to believe it would exert relatively little pressure on the land plate if it was to shift, and thus be unable to produce enough energy to cause a gigantic earthquake. At a meeting of the headquarters' earthquake research committee on April 11, Katsuyuki Abe, professor emeritus of University of Tokyo and committee president, said: "We worked according to the assumption that no magnitude-9.0 level earthquakes could occur in the area. We have to change our thinking."
Atmospheric Temperature Spiked Before the Earthquake, Could This Have Helped Predict the Quake?
In May 2011, Discover magazine reported that “scientists analyzing the March 11 earthquake in Japan found evidence of a sudden spike in the temperature in the atmosphere above the quake site just before the event. If the spike was related to the quake, and other earthquakes do the same thing, it might help scientists predict such cataclysms in the future. The analysis — which at the time had not yet been published in a journal but rather appeared in the non-peer reviewed arXiv — found that the before the earthquake, the number of electrons in the ionosphere above the epicenter increased dramatically, peaking three days before, and the infrared radiation (that is, the heat) grew as well, peaking just hours before.” [Source: Discover magazine]
“This finding is consistent with the idea that the planet’s atmosphere and geologic activity are linked through something known as the lithosphere-atmosphere-ionosphere coupling mechanism. As the arXiv Blog explains, this idea holds that when a tectonic plate is undergoing great stress before an earthquake, it releases radon gas. The gas ionizes the air, releasing lots of loose electrons, which then go on to cause the condensation of water, a process that releases heat. So far, the connection doesn’t seem like an easily dismissible coincidence: After analyzing more than 100 other earthquakes, the researchers have found that large (less than 5.5 magnitude), shallow (more than 31 miles below the surface) quakes had similar correlations with atmospheric ionization and heat. [References: Dimitar Ouzounov, Sergey Pulinets, Alexey Romanov, Alexander Romanov, Konstantin Tsybulya, Dimitri Davidenko, Menas Kafatos, Patrick Taylor. arxiv.org/abs/1105.2841: Atmosphere-Ionosphere Response to the M9 Tohoku Earthquake Revealed by Joined Satellite and Ground Observations. Preliminary Results]
The editors at Discover pointed out: “It’s easy to jump to conclusions with too little data. While the team hasn’t made any outlandish claims, a number of follow-up analyses are required to see whether this effect is reliably connected to quakes. This spike needs to be seen before more earthquakes, and it needs to be clear that it only happens when an event like this is imminent. If similar jumps in atmospheric ionization and heat can happen in the absence of a quake — that is, if there are lots of false positives — they’re no good as predictors....The paper still needs to be peer-reviewed and published, and that process will put the team’s analysis to the test. Once it’s over that hurdle, the scientists will continue looking for signs that this effect is linked to other large quakes, collaborating with others around the world to collect the necessary data.”
Changes in the Ionosphere Before the March 11 Quake
Some researchers have suggested one possible precursor could have been found about 80 kilometers up in the ionosphere. Atoms in the ionosphere are exposed to energy from the sun and other sources, and divide into ions and electrons. The ionosphere reflects electric waves used for radios and other devices. [Source: Takashi Ito, Yomiuri Shimbun, July 24, 2011]
Prof. Kosuke Heki of Hokkaido University, a researcher of geophysics, checked changes in the density of electrons in the ionosphere using electric waves from Global Positioning System satellites. He found the density over the epicenter rose by up to 10 percent compared with other areas from 40 minutes before the magnitude-9 quake.
A similar phenomenon occurred just before a magnitude-8.8 quake in Chile in 2010 and a magnitude-9.1 quake off Sumatra Island, Indonesia, in 2004. However, the electron density was only marginally higher before a magnitude-8 quake struck off Tokachi area, Hokkaido, in 2003. "Though we don't know why this happens, it could be an effective way to forecast a big earthquake just before it occurs," Heki said.
Masashi Hayakawa, professor emeritus of the University of Electro-Communications, believes the changes in the ionosphere started "about five days before the quake." Hayakawa, who researches the relationship between earthquakes and electromagnetic phenomena, said distortions in the ionosphere were detected by analyzing the transmission of radio waves in the air. He said this was because cracks that occur in the Earth's crust just before an earthquake cause vibrations that pass through the air. According to Hayakawa, the phenomenon occurs from about a week before an inland earthquake of magnitude 6 or larger with a focus up to 40 kilometers underground. Despite this, no clear precursors were detected from movements of the Earth's crust before the March 11 temblor.
Precursor Quakes Before the March 2011 Earthquake
An earthquake occurs when crustal plates that cover the Earth's surface rub against and bump into each other, causing a buildup of pressure that is eventually released as a quake. There are about a dozen such plates. The Tokai Earthquake, which is predicted to strike off Suruga Bay, is expected to be preceded by a "precursor slide" triggered when pressure that has built up along the boundaries of two plates causes one of them to slip under the other. Observation equipment has been set up to monitor any movement of the crusts.
A precursor slide did not occur before the Great East Japan Earthquake, even though it was the same kind of plate boundary quake as the Tokai quake is predicted to be. However, on March 9, two days before the disaster, a magnitude-7.3 quake--which is considered a foreshock of the March 11 quake--occurred close to the latter quake's epicenter.
The epicenters of several aftershocks of the March 9 quake formed a line moving toward that of the March 11 quake. Nagoya University Prof. Koshun Yamaoka, a seismologist, said, "The quake two days before definitely triggered the disastrous quake." But whether an earthquake is a precursor of a bigger one only becomes clear after the more powerful one has occurred.
"Even if we knew of an area where a magnitude-9 quake could occur, forecasting when it might strike would still be difficult," Yamaoka said. The magnitude-7.3 Haicheng Earthquake in China in 1975 was reportedly successfully forecast by observing prior seismic activity, crust movements and unusual animal behavior. Although increasing attention is being paid to phenomena that could be warning signs of an upcoming disaster, only a handful of major earthquakes have been successfully forecast before they unleashed their destructive force.
Did Animals Give a Warning of March 11 Earthquake?
Tsuyoshi Takasawa wrote in the Yomiuri Shimbun, “People in areas affected by the March 11 earthquake have reported witnessing unusual behavior by wild animals shortly before the magnitude-9 temblor hit, stories that lend support to the idea that animals can anticipate natural disasters. [Source: Tsuyoshi Takasawa, Yomiuri Shimbun, July 4, 2011]
"It must've been a warning," said Sachiko Abe, 66, who has lived in disaster-hit Ishinomaki, Miyagi Prefecture, for more than 30 years. For five years, Abe had risen before dawn five days a week to drive her fisherman husband to work. When she opened the front door of her house at around 1:50 a.m. on March 11, she was immediately struck by the cacophony being made by a murder of crows. She had never heard the birds make such a racket before. Peering into the dark sky, she could make out about 50 crows flying around--three times as many as she would usually expect in the area. She remembers her husband being struck by the unusual sound as well. "I've never heard cries like this," he said, his eyes scanning the dark sky.
“Yoshiko Sato, 60, lives in the same area as Abe and also reported birds acting strangely on March 11, Takasawa wrote. “From around 10 a.m. to noon, she heard dozens of black kites--which usually do not make much noise at all--shrieking aggressively, as if fighting. The earthquake struck the area about three hours later.”
“On March 4, a week before the disaster, 54 melon-headed whales between two and three meters long were found beached in Kashima, Ibaraki Prefecture. Masayuki Shimada, chief of the exhibition division at the Ibaraki Prefectural Oarai Aquarium, also known as Aqua World Oarai, believes the whales accidentally wandered into shallow waters and become stranded. Shimada said there is no reason to connect the whales' behavior with the subsequent earthquake. However, a similar phenomenon occurred in New Zealand this year, shortly before a major earthquake hit Christchurch on Feb. 22. According to local media reports, 107 pilot whales became beached on a small island off the country's South Island.” Although there were reports of other strange animal behavior, the connection--if any--between these phenomena and the quake remains unclear.
Although scientists have not established whether there is a relationship between animal behavior and earthquakes, there exists an abundance of anecdotal evidence of animals' ability to predict natural disasters. The Fire and Disaster Management Agency has collected accounts of such incidents and published them on its Web site, under the heading "information on legends related to national disasters." Many of the stories refer to unusual behavior by birds. The maxim, "When the pheasants cry, an earthquake will come," is well known in many prefectures, including Aichi, Chiba, Ibaraki, Iwate and Yamanashi. In Kushima, Miyazaki Prefecture, it is often said, "When crows make a fuss, an earthquake will occur."
Azabu University Prof. Mitsuaki Ota, a veterinary expert, said there are many examples of birds, rats and fish displaying abnormal behavior before an earthquake. "These wild animals likely sense unusual changes in the natural environment," he said. Toshiyasu Nagao, chief of Tokai University's Earthquake Prediction Research Center, said more research should be conducted based on first-hand accounts of odd animal behavior before the March 11 quake.
Team to Bore Into Tectonic Plates Near the Earthquake Epicenter
An international research team will use the deep-sea drilling vessel Chikyu to bore into an area where two tectonic plates meet to study the movements of the plates that caused the Great East Japan Earthquake, according to sources. The vessel will drill about 1,000 meters through the seabed in waters off Miyagi Prefecture in an area of greatest tectonic movement at the time of the March 11 earthquake, and bedrock samples will be examined. The seabed is from 6,000 to 7,000 meters below sea level. [Source: Yomiuri Shimbun , August 14, 2011]
The research, the first of its kind, will be conducted next spring at the earliest. The team hopes the study will be the first step in revealing the mechanism of the magnitude-9 earthquake. The Japan Agency for Marine-Earth Science and Technology, the owner of the Chikyu, will participate in the research from Japan. The research will be one of the projects conducted by the Integrated Ocean Drilling Program, which comprises Japan, the United States and 22 other countries.
According to scientists, the tectonic plates are believed to have moved more than 20 meters near the Japan Trench. However, it is unknown why they moved so much. Some scientists pointed to the possibility that faults branching off from the boundary of the plates also moved, intensifying the tsunami.
The research team believes frictional heat remains at the boundary of the plates and the faults for one or two years after the plates moved. The research team will measure the temperature of the rock and study any deformity caused by heat to learn which faults moved at the time of the earthquake. The team will also study how far the boundary and faults moved, and the speed of the movement. It will also analyze how stable the plates were before the March 11 earthquake.
March 2011 Earthquake and a Future Big Earthquake in the Tokyo Area
In August 2011 the Yomiuri Shimbun reported: “The possibility of a huge plate-boundary earthquake amplified by simultaneous moves in two or more focal areas beneath Tokyo has been increasing since the Great East Japan Earthquake, according to the University of Tokyo's Earthquake Research Institute. [Source: Yomiuri Shimbun, August 2011]
The institute said that since the March 11 disaster, pressure on the tectonic plates beneath the city has changed and two or more focal areas may move simultaneously, resulting in a massive quake. The average number of quakes measured at magnitude 3 or more in the five years preceding the March 11 disaster was about eight a month.The institute discovered that the number of small-scale plate-boundary quakes that are not felt by people has drastically increased following the March 11 earthquake. Also, the preliminary observed number of magnitude-3 or larger plate-boundary quakes between March 11 and Aug. 20 rose about fourfold in an area 60 to 70 kilometers below northern Tokyo Bay. Similarly sized plate-boundary quakes occurring 40 to 55 kilometers below southern Ibaraki Prefecture have increased about 20-fold.
The number of inland quakes has not drastically increased, but the institute said the types of the quakes have clearly changed. There have been huge quakes with shallow focal points below Tokyo and surrounding areas in the past. The 1923 magnitude-8 Great Kanto Earthquake occurred in southern Tokyo Bay.
The government's Central Disaster Management Council has maintained that magnitude-8 quakes will not occur in the near future. But the Great East Japan Earthquake has largely changed conventional thinking about the region's seismology. Naoshi Hirata, a researcher at the institute and a member of the government's Earthquake Research Commitxtee, said, "If there is drastic plate movement, there is no guarantee that the scale of the quake will be in line with the government's prediction of up to magnitude 7.3. "In addition to looking at the intervals of recurrence of Great Kanto Earthquake level quakes, it's necessary to review the predictions," he said.
Image Sources: 1) U.S. Navy; 2) United States Geological Survey USGS); 3) National Oceanic and Atmospheric Administration (NOAA); 4) NASA; 5) U.S. Marines
Text Sources: New York Times, Yomiuri Shimbun, Daily Yomiuri, Washington Post, Los Angeles Times, Kyodo News, National Geographic, The Guardian. Times of London, The New Yorker, Time, Newsweek, Reuters, AP, AFP, and various books and other publications.
Last updated June 2019