It allows sudden transition where the crust is too fluid to fracture.
by Simon Redfern Sept 24 2013, 4:00pm +0300
A little more than 90 years ago, British geologist Herbert Hall Turner noticed some data that suggested something unexpected. The only way to make sense of the data was if an earthquake occurred hundreds of kilometers beneath the Earth’s surface.
Since Turner’s observations, deep earthquakes have fascinated seismologists. It is still unclear why they happen, but two studies recently published in Science use different approaches yet reach the same conclusion. These quakes are probably a result of rapid changes in minerals that propagate at up to 14,000 km/hour (nearly 8700 mph).
Such deep earthquakes do not have immediate consequences for humans. But they hold clues about destructive quakes in the Earth’s shallower crust, making it important to understand them.
Not just superficial
Most earthquakes occur in the stiff, brittle outer shell that includes the Earth’s crust. This “seismogenic zone” causes the most devastating and dangerous earthquakes, but it only goes down to about 15km (roughly nine miles) beneath the surface.
As you go deeper, pressure and temperature both increase rapidly, so the nature of earthquakes changes. Rocks move slowly on geological time scales, pushed or pulled by different forces acting on them. At depth, they appear to flow like soft toffee rather than break like peanut brittle.
This is why Turner’s observations of earthquakes more than 600km (372.8 miles) below the surface were puzzling. If the rocks flow slowly, then there shouldn’t really be any sudden shocks that cause an earthquake. Rather, there should be gentle, continuous readjustments to stress.
Suggestions have been floated in the past about what triggers such earthquakes. But Thorne Lay of the University of California at Santa Cruz managed to analyze a deep earthquake that occurred this year on May 24 in the Pacific Ocean beneath the Okhotsk plate. At a magnitude of 8.3, it was four times greater than the 1906 San Francisco earthquake. Indeed, it was the biggest ever recorded at a depth of more than 600km. A near-surface earthquake of the same magnitude could’ve been very destructive, but this was barely noticeable, at least at the surface above.
Recent analysis of an earthquake at Bhuj, India, in 2001 suggests it shared similarities to the Okhotsk event, although it was just 16km deep. At that depth, it caused terrible devastation, including an estimated 20,000 deaths. “There may be things we don’t understand about more shallow earthquakes that we can learn from studying these deep earthquakes,” said Bob Myhill of the University of Bayreuth.
During the Okhotsk event, the Pacific plate was drawn down into the hot mantle that makes up much of the planet’s interior. Lay found that the seismic energy released in the event was so large that it caused fractures as great as 180km (111.8 miles) long near the depth of the earthquake. The rock ruptured at close to the speed of sound, which would be as much as 14,000 km/h under those conditions.