Quake Moved Japan coast by 8 feet: USGS
By the AFP
Japan’s recent massive earthquake, one of the largest ever recorded, appears to have moved the island by about eight feet (2.4 meters), the US Geological Survey said.
“That’s a reasonable number,” USGS seismologist Paul Earle told AFP. “Eight feet, that’s certainly going to be in the ballpark.”
Friday’s 8.9 magnitude quake unleashed a terrifying tsunami that engulfed towns and cities on Japan’s northeastern coast, destroying everything in its path in what Prime Minister Naoto Kan said was an “unprecedented national disaster.”
The quake and its tectonic shift resulted from “thrust faulting” along the boundary of the Pacific and North America plates, according to the USGS.
The Pacific plate pushes under a far western wedge of the North America plate at the rate of about 3.3 inches (83 millimeters) per year, but a colossal earthquake can provide enough of a jolt to dramatically move the plates, with catastrophic consequences.
“With an earthquake this large, you can get these huge ground shifts,” Earle said. “On the actual fault you can get 20 meters (65 feet) of relative movement, on the two sides of the fault.”
He said similar movements would have been seen for Chile and Indonesia.
In December 2004, a 9.1 magnitude quake off Sumatra caused a tsunami that killed an estimated 228,000 people. An 8.8 quake off the coast of Chile in February 2010 killed more than 500.
There was not a similar ground shift in the 7.0 earthquake that devastated Haiti in February 2010, Earle said.
“A magnitude 7.0 is much smaller than the earthquake that just happened in Japan,” he said. “We’ve had aftershocks (in Japan) larger than the Haiti earthquake.”
Kenneth Hudnut, a USGS geophysicist, said experts read data including from global positioning systems to determine the extend of the shift.
“We know that one GPS station moved (eight feet), and we have seen a map from GSI (Geospatial Information Authority) in Japan showing the pattern of shift over a large area is consistent with about that much shift of the land mass,” he told CNN.
Japan Quake May Have Slightly Shortened Earth Days, Moved Axis: NASA
Excerpt from NASA, in Science Daily
This National Oceanic and Atmospheric Administration (NOAA) Global bathymetry map image released on March 11, 2011 shows features of the ocean floor depth (or bathymetry) from a NOAA ETOPO-1 dataset. The image shows the entire Western Pacific basin. Notice how abruptly the Japanese islands rise out of the ocean. Other coastal Asian areas have much more gradual slopes. The islands and mountain ranges throughout the ocean, visible in this imagery, also affect the tsunami travel time and speed. In the open ocean, tsunamis can travel at speeds up to 500 mph (800 kph). This momentum is what creates such a destructive force as the wave moves inland. (NOAA)
The magnitude 9.0 earthquake that struck Japan March 11, 2011 may have slightly shortened the length of each Earth day and shifted its axis.
Using a United States Geological Survey estimate for how the fault responsible for the earthquake slipped, research scientist Richard Gross of NASA’s Jet Propulsion Laboratory, Pasadena, Calif., applied a complex model to perform a preliminary theoretical calculation of how the Japan earthquake, the fifth largest since 1900, affected Earth’s rotation. His calculations indicate that by changing the distribution of Earth’s mass, the Japanese earthquake should have caused Earth to rotate a bit faster, shortening the length of the day by about 1.8 microseconds (a microsecond is one millionth of a second).
The calculations also show the Japan quake should have shifted the position of Earth’s figure axis (the axis about which Earth’s mass is balanced) by about 17 centimeters (6.5 inches), towards 133 degrees east longitude. Earth’s figure axis should not be confused with its north-south axis; they are offset by about 10 meters (about 33 feet). This shift in Earth’s figure axis will cause Earth to wobble a bit differently as it rotates, but it will not cause a shift of Earth’s axis in space, only external forces such as the gravitational attraction of the sun, moon and planets can do that.
Both calculations will likely change as data on the quake are further refined.
In comparison, following last year’s magnitude 8.8 earthquake in Chile, Gross estimated the Chile quake should have shortened the length of day by about 1.26 microseconds and shifted Earth’s figure axis by about 8 centimeters (3 inches). A similar calculation performed after the 2004 magnitude 9.1 Sumatran earthquake revealed it should have shortened the length of day by 6.8 microseconds and shifted Earth’s figure axis by about 7 centimeters, or 2.76 inches. How an individual earthquake affects Earth’s rotation depends on its size (magnitude), location and the details of how the fault slipped.
Gross said that, in theory, anything that redistributes Earth’s mass will change Earth’s rotation.
“Earth’s rotation changes all the time as a result of not only earthquakes, but also the much larger effects of changes in atmospheric winds and oceanic currents,” he said. “Over the course of a year, the length of the day increases and decreases by about a millisecond, or about 550 times larger than the change caused by the Japanese earthquake. The position of Earth’s figure axis also changes all the time, by about 1 meter (3.3 feet) over the course of a year, or about six times more than the change that should have been caused by the Japan quake.”
Gross said that while we can measure the effects of the atmosphere and ocean on Earth’s rotation, the effects of earthquakes, at least up until now, have been too small to measure. The computed change in the length of day caused by earthquakes is much smaller than the accuracy with which scientists can currently measure changes in the length of the day. However, since the position of the figure axis can be measured to an accuracy of about 5 centimeters (2 inches), the estimated 17-centimeter shift in the figure axis from the Japan quake may actually be large enough to observe if scientists can adequately remove the larger effects of the atmosphere and ocean from the Earth rotation measurements. He and other scientists will be investigating this as more data become available.
Gross said the changes in Earth’s rotation and figure axis caused by earthquakes should not have any impacts on our daily lives.
“These changes in Earth’s rotation are perfectly natural and happen all the time,” he said. “People shouldn’t worry about them.”
Workers inspect a caved-in section of a prefectural road in Satte, Saitama Prefecture, after one of the largest earthquakes ever recorded in Japan slammed its eastern coast March 11. (Saitama Shimbun/Associated Press/Kyodo News)