On May 24th of 2013, a magnitude 8.3 earthquake hit deep beneath the Sea of Okhotsk, between Russia's Kamchatka Peninsula and Japan. The main shock of the earthquake was located at 610 kilometers (379 miles) depth, a rupture in the mantle far below the Earth's crust.
By inverting seismic waves that were observed during the earthquake, researchers have found that this initial shock triggered four subsequent shocks. These four shocks were magnitudes 7.8, 8.0, 7.9, and 7.9. A pressure front from the initial earthquake propagated at a speed of approximately 4.0 kilometers (2.5 miles) per second, setting off three subsequent earthquakes in a line south of the main shock.
The rupture of the second follow-up earthquake sent a secondary rupture front back up north, triggering a third aftershock. By analyzing the results of the secondary ruptures, geologists determined that massive earthquakes that strike deep within the Earth may be more efficient at dissipating pent up energy than similar quakes near the surface.
In total, the entire earthquake sequence took just 30 seconds, and the bulk of the stress was released by the four major shocks. In similar earthquake swarms that occur near the surface, such a release could take hours to days and would likely include a large number of small aftershocks. Based on this, the authors conclude that deep earthquakes are likely more efficient in dissipating stress than shallow earthquakes.
Citation: Shengji Wei, Don Helmberger, Zhongwen Zhan, Robert Graves, ' Rupture Complexity of the Mw 8.3 Sea of Okhotsk Earthquake: Rapid Triggering of Complementary Earthquakes?', Geophysical Research Letters, doi:10.1002/grl.50977