Earthquakes have shaken apart cities and towns for thousands of years, subjecting human and animal communities to sudden, unpredictable and potentially life-altering jolts.
Though scientists have learned much about the mechanics of temblors in recent decades, there are still gaps in their understanding of the nature of these earth-rending events. One particular area of interest is in pinpointing the parts of faults that produce different types of seismic activity.
With the help of some new tools, a few groups of scientists are getting a more in-depth (literally, in some cases) view of earthquake faults and the motions they produce.
Two teams of seismologists are studying different fault systems half a world apart, with one drilling down below the ocean floor off the coast of Japan, and the other monitoring a fault near Costa Rica for the slow slip that produces "silent earthquakes."
Some of the early findings of the projects were recently presented at the annual meeting of the American Association for the Advancement of Science, in Chicago. Studies like these could aid in earthquake-preparedness and help prevent some of the damage quakes can cause.
Deep-sea drilling
Japan is one of the most earthquake-prone regions in the world, sitting nearly on top of two subduction zones, the areas where one of Earth's tectonic plates is shoved underneath another.
One such subduction zone, the Nankai Trough, sits offshore to the south of the island of Honshu and has a history of the powerful temblors dubbed mega-thrust earthquakes.
An international team of scientists is using a new deep-sea drilling ship to explore the fault zone to help understand how it generates earthquakes and the tsunamis that sometimes accompany them. The project is called NanTroSEIZE (for Nankai Trough Seismogenic Zone Experiment) and is the first geologic study of underwater subduction zone faults.
"If we want to understand the physics of how the faults really work, we have to go to those faults in the ocean," said Harold Tobin of the University of Wisconsin-Madison, the project's co-chief scientist.
Subduction zones angle upward as one plate slides underneath the other; friction builds between the plates as they try to slide past one another, until the system gives and ruptures, causing an earthquake.
Of course, not all parts of the fault are equally as complicit in causing the quake: While both the shallow and deep parts of the fault slip, only the deeper parts cause the temblor because the stresses on the upper portions of the fault are much weaker, Tobin explained.
The part of the fault where the earth-shaking movement originates is called the seismogenic zone. "It's where the stress overcomes that friction" between the plates, Tobin told LiveScience. This zone extends from around 1.2 miles (2 kilometers) to 19-25 miles (30-40 kilometers) below the Earth's surface, he added.
During the first stage of the decade-long project, Tobin and his team drilled down into the shallow part of the fault zone to see if they could find distinct, localized signs of faulting. And they did.
The cores (long cylinders of rock) drilled out from the fault showed a narrow band of finely ground "rock flour" revealing a fault zone between the upper and lower plates that is only about 2 millimeters thick — roughly the thickness of a quarter.
The rock was "shattered and broken literally," Tobin said. "These cores, poor things, have probably had thousands of earthquakes."
The next stage of the project, set to begin in May, will drill down deeper into the fault zone and place instruments in the holes to monitor the stresses and motions of the plates.
Silent slips
In Costa Rica, Susan Schwartz of the University of California, Santa Cruz, and her colleagues are examining an entirely different, and much quieter, fault phenomenon.
While earthquakes are generated by the sudden, high-energy jolt of two plates slipping past one another in a matter of seconds, another type of motion, discovered only within the last decade, goes by unnoticed by everything but the networks of GPS instruments strung along a fault.
This motion, known as a slow slip event, also happens when the two plates slide past each other, but it occurs so slowly that the ground doesn't shake. However, GPS instruments can measure the displacement of the ground that can happen over the course of days or weeks.
The phenomenon was originally discovered at the Nankai Trough, and has since been found to occur in the fault zone off the coast of the Pacific Northwest, as well as beneath the Nicoya Peninsula of Costa Rica.
"At least two slow slip events have occurred beneath the Nicoya Peninsula since 2003," Schwartz said.
Since then Schwartz and her team have expanded their GPS network with funding from the National Science Foundation and recorded another event in 2007.
Schwartz and her team will be monitoring for more events and trying to relate them to the fault motions that produce earthquakes.
The slow slip events don't seem to occur in the same places that are highly stressed and tend to produce quakes, so "a big question is whether [the slow slip] is loading the locked area, making it more likely to break, or relieving stress on the fault," Schwartz said.
Even if slow slip events, also called "silent earthquakes," do increase the stress on the loaded portions of the fault, it's unlikely that they would increase the likelihood of a major earthquake, Schwartz said.
"It's a very, very, very small change in stress," she told LiveScience.
Knowing where the slow slip is taking place along the fault could help bracket the area likely to break in a big quake though, Schwartz noted.
More research must of course be done to further understand earthquakes in general and these two phenomena in particular.
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