"The last great outer rise earthquakes that occurred were in the 1930s and 1970s," said Charles J. Ammon, associate professor of geoscience, Penn State. "We did not then have the equipment to record the details of those events." The outer rise is the region seaward of the deep-sea trench that marks the top of the plate boundary
In late 2006 and early 2007, two large earthquakes occurred near Japan separated by about 60 days. These earthquakes took place in the area of the Kuril Islands that are located from the westernmost point of the Japanese Island of Hokkaido to the southern tip of the Kamchatka Peninsula. The first event took place on Nov. 15, 2006 when the edge of the Pacific plate thrust under the arc of the Kuril Islands, initiating a magnitude 8.3 event and causing some damage in Japan and a small tsunami that caused minor damage in Crescent City, California. About 60 days later, on Jan. 13, 2007, a magnitude 8.1 earthquake occurred in "the upper portion of the Pacific plate, producing one of the largest recorded shallow extensional earthquakes."
This second earthquake was not at a plate boundary and was not directly caused by subduction -- the moving of one plate beneath the other. Rather, it was a normal faulting event, where the Pacific plate stretched, bent and broke.
While Japan and the Kamchatka Peninsula are active earthquake areas, the region of the Kuril Islands where the large November earthquake occurred, had not had a large earthquake since 1915 and researchers are unsure of the exact nature of that event.
Working with Hiroo Kanamori, the John E. and Hazel S. Smits professor of geophysics, emeritus, California Institute of Technology, and Thorne Lay, professor of Earth & planetary sciences, University of California, Santa Cruz, the Penn State researcher looked at the sequence of seismic activity that link these two earthquakes into a doublet.
"Such large doublet earthquakes, though rare, could be an underestimated hazard," says Ammon. "We are also interested in what these events tell us about how earthquakes interact, how the stresses and interactions allow one earthquake to trigger another."
Looking at the seismic record, the researchers found a series of smaller, foreshock earthquakes beginning about 45 days before Nov. 15. On Nov. 15, there was the magnitude 8.3 earthquake on the plate boundary, the largest event of 2006.
"Within minutes of the Nov. 15 earthquake, seismic activity began on the Pacific plate in the area where the January earthquake would take place," says Ammon. "This large second earthquake generated a larger amplitude of shaking in the frequency range that affects human-made structures than the first earthquake."
Usually, aftershocks from a large earthquake are at least one order of magnitude less than the main event and taper off rapidly. In this case, the events within the Pacific plate east of the plate boundary did not taper off, and the second event that occurred in January was about the same size as the first earthquake.
Earthquakes at plate boundaries in subduction zones occur when the plate that is going under – being subducted – gets temporarily stuck and causes compression in the plate away from the edge. Tension builds and when the plate overcomes the friction holding it, it moves downward, slipping under the top plate and causing an earthquake. According to the researchers, the second earthquake that occurred on the Pacific plate happened because of bending experienced by the pacific plate that occurs before it subducts beneath the upper plate. As the front edge of the plate slipped, the plate east of the November earthquake bent, cracked and broke in January.
Like pie crust, when the Earth's crust bends, small cracks begin to appear – these were the small shocks that began immediately after the first earthquake – but when the bending becomes severe, a larger region of the crust breaks – creating the second, very large event.
In the United States, subduction zones exist only in the Pacific Northwest, Alaska and the area around Puerto Rico. The researchers note, "Triggering of a large outer rise rupture with strong high-frequency shaking constitutes an important potential seismic hazard that needs to be considered in other regions."
Andrea Elyse Messer | EurekAlert!
Massive impact crater from a kilometer-wide iron meteorite discovered in Greenland
15.11.2018 | Faculty of Science - University of Copenhagen
The unintended consequences of dams and reservoirs
14.11.2018 | Uppsala University
Researchers at the University of New Hampshire have captured a difficult-to-view singular event involving "magnetic reconnection"--the process by which sparse particles and energy around Earth collide producing a quick but mighty explosion--in the Earth's magnetotail, the magnetic environment that trails behind the planet.
Magnetic reconnection has remained a bit of a mystery to scientists. They know it exists and have documented the effects that the energy explosions can...
Biochips have been developed at TU Wien (Vienna), on which tissue can be produced and examined. This allows supplying the tissue with different substances in a very controlled way.
Cultivating human cells in the Petri dish is not a big challenge today. Producing artificial tissue, however, permeated by fine blood vessels, is a much more...
Faster and secure data communication: This is the goal of a new joint project involving physicists from the University of Würzburg. The German Federal Ministry of Education and Research funds the project with 14.8 million euro.
In our digital world data security and secure communication are becoming more and more important. Quantum communication is a promising approach to achieve...
On Saturday, 10 November 2018, the research icebreaker Polarstern will leave its homeport of Bremerhaven, bound for Cape Town, South Africa.
When choosing materials to make something, trade-offs need to be made between a host of properties, such as thickness, stiffness and weight. Depending on the application in question, finding just the right balance is the difference between success and failure
Now, a team of Penn Engineers has demonstrated a new material they call "nanocardboard," an ultrathin equivalent of corrugated paper cardboard. A square...
19.11.2018 | Event News
09.11.2018 | Event News
06.11.2018 | Event News
19.11.2018 | Materials Sciences
19.11.2018 | Information Technology
19.11.2018 | Life Sciences