Image of the estimated fault slip due to the 9.0 Tohoku-Oki earthquake. The fault responsible for the quake dips under Japan, starting at the Japan Trench (indicated by the barbed line), the point of contact between the subducting Pacific Plate and the overriding Okhotsk Plate. The magnitude of fault slip is indicated both by the color and the contours, which are at 26-foot (8-meter) intervals. The question mark represents the researchers’ current lack of information about the seismic potential of the region south of the most recent event. Image credit: Caltech
The study of the magnitude 9.0 Tohoku-Oki quake, led by researchers at the California Institute of Technology in Pasadena, and published online in the May 19 issue of Science Express, details the first large set of observational data from this rare megathrust earthquake event. The researchers used observations from a dense regional monitoring network that allows measurements of Earth movement to be gathered from GPS satellite data, along with globally distributed broadband seismographic networks and open-ocean tsunami data, to begin to construct models that describe how Earth moved due to the quake. JPL researchers Susan Owen, Angelyn Moore and Frank Webb provided the GPS observations on which the study’s fault slip model was based. They analyzed the GPS data from Japan’s network and found the large horizontal and vertical movements that were used to determine where the earthquake ruptured the subduction zone fault.
Among the study’s findings…
- The length of fault that experienced significant slip during the quake was about 155 miles (250 kilometers), about half of what would be conventionally expected for an event of this magnitude. The area of greatest slip — 98 feet (30 meters) or more — happened within a 31- to 62-mile-long (50- to 100-kilometer-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.
