US research reports evidence of super-fast deep earthquake

Researchers said Thursday they have discovered the first evidence that deep earthquakes breaking at more than 300 kilometers underground can rupture much faster than previously thought.
In a study published in the U.S. journal Science, researchers at the Scripps Institution of Oceanography discovered the first super-fast earthquake, in which its rupture speed exceeds that of the seismic waves called shear waves it radiates, while examining the aftershocks of a 8.3-magnitude earthquake on May 24, 2013, in the Sea of Okhotsk off the Russian mainland.
"Prior to our study, such supershear ruptures have been reported in only six or seven earthquakes, but they all occurred in shallow earthquake events," lead author Zhongwen Zhan of Scripps told Xinhua. "In this case, what we have observed is one that occurred at a depth of over 600 kilometers."
After analyzing the data of the 2013 Okhotsk earthquake, Zhan and colleagues found the rupture speed of a 6.7-magnitude aftershock, which happened 640 kilometers below ground, was extraordinary -- about eight kilometers per second. That is nearly 50 percent faster than the aftershock's shear wave velocity.
"For a 6.7 earthquake you would expect a duration of seven to eight seconds, but this one lasted just two seconds," co-author Peter Shearer, a geophysics professor at Scripps, said in a statement. "This is the first definitive example of supershear rupture for a deep earthquake since previously supershear ruptures have been documented only for shallow earthquakes."
The researchers said the finding may offer new clues about the mechanism of deep earthquakes, which has been a puzzle since their discovery in the 1920s.
"One quarter of earthquakes occur at large depths, and some of these can be pretty big, but we still don't understand why they happen," Zhan said. "So this earthquake provides a new observation for deep earthquakes and high-rupture speeds."