Large quantities of seawater remained trapped inland Saturday, hampering rescue operations, after coastal land areas subsided following Friday's killer earthquake.
Large quantities of seawater remained trapped inland Saturday, hampering rescue operations, after coastal land areas subsided following Friday's killer earthquake.
The Geospatial Information Authority of Japan (GSI) confirmed with a global positioning system survey that the ground sank about 70 centimeters in Ishinomaki, Miyagi Prefecture, after the magnitude-8.8 quake.
The rate of subsidence was about 66 cm in Kesennuma and 30 to 40 cm in Higashi-Matsushima, both in Miyagi Prefecture, according to the GSI.
Researchers say the ground, which was raised by the push of the Pacific plate, fell back down as the upward force was released by the earthquake.
"Some of the inland areas are now below sea level, preventing seawater (driven in by the tsunami) from flowing back to the sea," said Junichi Miyamura, an associate professor at Hokkaido University's Institute of Seismology and Volcanology.
"The water is left like an irrigation pond," he said.
Takeshi Sagiya, a professor of seismology at Nagoya University, also says the inundation has occurred as a result of land subsidence due to crustal change from the earthquake.
"In some places, the coastline may have receded as seawater that poured over embankments remained there or waves carried away coastal sand," he said.
Another seismologist, Yuji Yagi, an associate professor at the University of Tsukuba, said a more accurate analysis showed the destruction of a fault occurred in an area extending 500 kilometers north to south off Aomori Prefecture to Ibaraki Prefecture, and 200 km wide.
The fault showed significant movement at two points: an area from Iwate Prefecture to off Miyagi Prefecture, and one off Ibaraki Prefecture.
The destruction started first at the northern point and then traveled south to Ibaraki in about 70 seconds, he said. The fault shifted up to 20 meters, according to Yagi.
The moment magnitude (Mw), which measures the size of an earthquake in terms of the shift in a fault, came to an estimated 9.0, larger than his first analysis, Yagi said.
It was almost comparable to the 2004 jolt off Sumatra, Indonesia, which caused a massive tsunami and had a moment magnitude of 9.1.
Meanwhile, two other seismologists said Friday's shock was by far greater in scale than was expected for the area.
"It was an ocean-trench type earthquake of the largest level possible for waters near Japan," said Yoshiaki Kawata, executive director for the Disaster Reduction and Human Renovation Institution.
He said the quake's focus was located in an area where no earthquakes had occurred since the Meiji Era (1868-1912), leading to a long accumulation of energy.
Kawata said it is necessary to examine the possibility of jolts in such areas in compiling an anti-disaster plan for the future.
Yasuhiro Umeda, a professor emeritus at Kyoto University, also said the magnitude, at 8.8, was surprising to seismologists because a quake of that scale was never expected in that area.
Given the location of the epicenter, it should have been a plate-boundary type that occurred close to an ocean trench, he said.
He called for caution, saying major aftershocks with a magnitude of 7.6 to 8 could occur for at least the next six months.
In Friday's quake, Umeda said a fault 600 km by 150 km collapsed, far larger than the 1896 quake that caused tsunami off the Sanriku coast.
The major destruction of the fault pushed up seawater from the bottom at a depth of 3,000 meters, sending it crashing over a wide land area.
Umeda said it is unlikely the temblor will trigger a serial Tokai, Tonankai and Nankai earthquake in central to western Japan.
"But should the three hit together, it would be a jolt as large as the latest one," he said. "Preparatory steps must be reviewed by learning from the damage this time around."