A joint study between NASA’s Jet Propulsion Laboratory and the California Institute of Technology has provided the first comprehensive look of the massive amount of data which was gathered the day of the Japanese earthquake.
“This event is the best recorded great earthquake ever,” says Mark Simons, professor of geophysics at Caltech’s Seismological Laboratory and lead author of the study.
The Caltech-led study of the magnitude 9.0 Tohoku-Oki quake was published online in the May 19 issue of Science Express and detailed the first large set of observational data gathered during the megathrust earthquake using a “dense regional monitoring network” which allowed scientists to gather readings from GPS satellite data, along with “globally distributed broadband seismographic networks and open-ocean tsunami data.”
Among the study’s findings:
- The length of fault that experienced significant slip during the quake was about 250 kilometres (155 miles), about half of what would be conventionally expected for an event of this magnitude. The area of greatest slip – 30 meters (98 feet) or more – happened within a 50- to 100-kilometer-long (31- to 62-mile-long) segment.
- High- and low-frequency seismic waves can come from different areas of a fault. The quake’s high-frequency seismic waves were generated much closer to the coast, away from the area of the fault slip, where low-frequency waves were observed.
- The amount of strain associated with the quake’s displacement was five to 10 times larger than normally seen in large megathrust earthquakes. Before now, it was generally believed that the relatively soft material of the seafloor near the Japan Trench could not support such a large amount of stress. Because of this local strengthening of the seafloor, the researchers believe the Pacific and Okhotsk tectonic plates had been pinned together for a long time, perhaps 500 to 1,000 years.
- The area just south of where the fault slipped in March, which is close to Tokyo, should be a focus area for researchers because they do not have data on the area and don’t know yet what it might do in the future.
“This is not something we have documented before,” said Mark Simons, referring to the fact that the area of greatest slip occurred within a 50- to 100-kilometre-long segment. “I’m sure it has happened in the past, but technology has advanced only in the past 10 to 15 years to the point where we can measure these slips much more accurately through GPS and other data.”
“The high-frequency seismic waves in the Tohoku earthquake were generated much closer to the coast, away from the area of the slip where we saw low-frequency waves,” said Jean Paul Ampuero, assistant professor of seismology at Caltech’s Seismological Laboratory who studies earthquake dynamics, who found the fact that high- and low-frequency seismic waves can come from different areas of a fault as the most significant finding of the study.
A longer look at the results and reactions of the scientists studying the quake can be found at Caltech.