Researchers find evidence of super-fast deep earthquake

Jul 10, 2014
Red stars depict the location of the 8.3 mainshock and 6.7 aftershock off the Kamchatka Peninsula. Credit: University of California - San Diego

As scientists learn more about earthquakes that rupture at fault zones near the planet's surface—and the mechanisms that trigger them—an even more intriguing earthquake mystery lies deeper in the planet.

Scientists at Scripps Institution of Oceanography at UC San Diego have discovered the first evidence that deep earthquakes, those breaking at more than 400 kilometers (250 miles) below Earth's surface, can rupture much faster than ordinary earthquakes. The finding gives seismologists new clues about the forces behind deep earthquakes as well as fast-breaking earthquakes that strike near the surface.

Seismologists have documented a handful of these events, in which an 's rupture travels faster than the shear waves of seismic energy that it radiates. These "supershear" earthquakes have rupture speeds of four kilometers per second (an astonishing 9,000 miles per hour) or more.

In a National Science Foundation-funded study reported in the June 11, 2014, issue of the journal Science, Scripps geophysicists Zhongwen Zhan and Peter Shearer of Scripps, along with their colleagues at Caltech, discovered the first deep supershear earthquake while examining the aftershocks of a magnitude 8.3 earthquake on May 24, 2013, in the Sea of Okhotsk off the Russian mainland.

Details of a magnitude 6.7 aftershock of the event captured Zhan's attention. Analyzing data from the IRIS (Incorporated Research Institutions for Seismology) consortium, which coordinates a global network of seismological instruments, Zhan noted that most seismometers around the world yielded similar records, all suggesting an anomalously short duration for a magnitude 6.7 earthquake.

The supershear 2013 Sea of Okhotsk earthquake had similar magnitude and fault geometry as the damaging 1994 Northridge earthquake in California, but a much larger depth and faster rupture speed. The high rupture speed (~8 km/s, or 18,000 miles/hour) away from the hypocenter, shown as the red star, concentrates strong shaking on the Mach front. Credit: Zhongwen Zhan, 2014

Data from one seismometer, however, stationed closest to the event in Russia's Kamchatka Peninsula, told a different story with intriguing details.

After closely analyzing the data, Zhan not only found that the aftershock ruptured extremely deeply at 640 kilometers (400 miles) below the earth's surface, but its rupture velocity was extraordinary—about eight kilometers per second (five miles per second), nearly 50 percent faster than the shear wave velocity at that depth.

"For a 6.7 earthquake you would expect a duration of seven to eight seconds, but this one lasted just two seconds," said Shearer, a geophysics professor in the Cecil H. and Ida M. Green Institute of Geophysics and Planetary Physics (IGPP) at Scripps. "This is the first definitive example of supershear rupture for a deep earthquake since previously supershear ruptures have been documented only for shallow earthquakes."

"This finding will help us understand why deep earthquakes happen," said Zhan. "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. So this earthquake provides a new observation for deep earthquakes and high-rupture speeds."

Zhan also believes the new information will be useful in examining ultra-fast earthquakes and their potential for impacting near the earth's surface. Although not of supershear caliber, California's destructive 1994 Northridge earthquake had a comparable size and geometry to that of the 6.7 Sea of Okhotsk aftershock.

"If a shallow earthquake such as Northridge goes supershear, it could cause even more shaking and possibly more damage," said Zhan.

Explore further: Detection of supershear rupture in 2013 Craig, Alaska, earthquake

More information: "Supershear rupture in a Mw 6.7 aftershock of the 2013 Sea of Okhotsk earthquake," by Z. Zhan et al. www.sciencemag.org/lookup/doi/… 1126/science.1252717

add to favorites email to friend print save as pdf

Related Stories

Interpreting the strongest deep earthquake ever observed

Dec 04, 2013

Massive earthquakes that strike deep within the Earth may be more efficient at dissipating pent up energy than similar quakes near the surface, according to new research by Wei et al. The authors analyzed the rupture of the ...

Researchers replicate supershear earthquakes in the lab

Jun 07, 2013

(Phys.org) —A team of geology researchers working in France has for the first time recreated the conditions in a lab that lead to a phenomenon known as a supershear earthquake. In their paper published ...

Sumatra earthquake mysteries examined

May 11, 2012

(Phys.org) -- An earthquake in the Indian Ocean off the coast of Sumatra, Indonesia on 11th April was unusually powerful, at magnitude 8.6, for a “strike-slip” type of quake, and a new analysis of ...

Recommended for you

Better forecasts for sea ice under climate change

Nov 25, 2014

University of Adelaide-led research will help pinpoint the impact of waves on sea ice, which is vulnerable to climate change, particularly in the Arctic where it is rapidly retreating.

User comments : 1

Adjust slider to filter visible comments by rank

Display comments: newest first

betterexists
1 / 5 (6) Jul 10, 2014
Why get Scared, Folks?
gAd got drunk on one fine day & Ate up ALL the Dinosaurs...hIs Bad Teeth prevented hIm from Chewing up Their Bones. They became Fossils.
Then, gAd came up with us, Humans! One day, hE will go berserk Again AND Finish us off all Too!
I am wondering what will be hIs Show after Termination of Humans.
Thank gAd, We did not live along with Dinos. WoW, How Scary! No U.S, No U.K, No U.N.....Nada!
Who Talked of Setting of Sun, B.S

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.