Scientists are pursuing earthquakes deep into their subterranean lairs, studying them on land and below the sea. Yet, confronted with the question of when and where the "next big one" will occur, an uncomfortable silence sets in.

Based on history there will be quakes in Japan, also in Tibet, said Leigh Royden of the Massachusetts Institute of Technology.

"Will we ever be able to predict them? I don't know," she said Sunday at a meeting of the American Association for the Advancement of Science.

"There have been a few signals associated with quakes, but only in hindsight," she said. "And those quakes are rare."

Quakes most often occur at the boundaries of the many moving plates that make up the surface of the earth. But some also have been recorded well inside the plates, such as the devastating temblor that shook Wenchuan, China last year or the 1811-1812 quakes at New Madrid, Mo., that briefly caused the Mississippi River to flow backward.

Scientists can tell when a plate is under high stress but don't have a way to determine where a fracture will occur, launching a quake, explained Harold Tobin of the University of Wisconsin.

Nevertheless they continue striving to learn more about quakes and their causes in an effort to find ways to protect lives and property.

James Evans of Utah State University said researchers have drilled nearly 11,500 feet into the San Andreas Fault in California to install instruments in hopes of "really getting into the guts of the fault zone" to record an earthquake.

Tobin is taking part in similar research in the Nankai trough in the western Pacific Ocean, one of the most active seismic areas in the world.

"If we want to understand the physics of how the faults really work, we have to go to those faults in the ocean," he said. "Scientific drilling is the main way we know anything at all about the geology of the two-thirds of the Earth that is submerged."

"The ultimate goal is a series of deep holes in the trough in a few years," he said.

Deep ocean quakes can be especially dangerous if they generate a tsunami, like the giant sea wave that took hundreds of thousands of lives around the Indian Ocean in 2004.

Susan Schwartz, a professor of earth and planetary sciences at the University of California, Santa Cruz, said researchers have uncovered a third type of slip that occurs along plate boundaries.

Some boundaries have been known to creep along steadily without causing quakes while others stick and then release suddenly, shaking the earth.

Now some have been seen to stick and then slip, but only slowly with ground motion taking from weeks to a year and no earthquake occurring.

Her research team found these slow-slip "silent earthquakes" while monitoring the ground beneath the Nicoya Peninsula in Costa Rica.

"At least two slow slip events have occurred beneath the Nicoya Peninsula since 2003," Schwartz said. "When we recorded the first one in 2003, we had only three GPS stations. By 2007, we had 12 GPS stations and over 10 seismic stations, so the event that year was very nicely recorded."

The slow slip phenomenon has also been observed in the Cascadia fault zone off the coast of Washington and British Columbia and Japan's Nankai Trough.

Qiyuan Liu of the Chinese Institute of Geology said his country has installed 297 seismic stations in the region to study the deep structure below the earth in the region around last year's Wenchuan quake. He said the quake occurred in a deep area in a boundary between high- and low-velocity movement of the planet's crust.