for National Geographic News
According to Max Wyss, when the Beatles first shook the world with hits like "I Want to Hold Your Hand" and "She Loves You," in the early 1960s, scientists were clueless as to why the Earth literally rattles and trembles on occasion.
"What I'm trying to say is this is a very young, recent advance to know why this planet has earthquakes," said Wyss, who is the director of the World Agency of Planetary Monitoring and Earthquake Risk Reduction in Geneva, Switzerland.
By the late 1960s, as the Fab Four were pulling stunts like rocking a rooftop in Santa Monica, California, scientists began piecing together the theory of plate tectonics.
According to that groundbreaking theory, the Earth's surface is divided into about a dozen thin shells of crust that move about as a result of motions in the Earth's interior.
At mid-ocean ridges, which are known as seafloor spreading centers, new crust erupts from Earth's interior and pushes the plates apart. At so-called convergent boundaries, the plates crash into each other, sometimes forming mountains and volcanoes. At transform fault boundaries plates slide horizontally past each other.
"When the plates push against each other, friction results in stress accumulation, which is released in earthquakes," said Carol Raymond, a geophysicist at NASA's Jet Propulsion Laboratory in Pasadena, California.
In the few decades since the theory of plate tectonics gained widespread acceptance, scientists have been trying to understand the physics of earthquakes. They hope that they can accurately predict when earthquakes may occur and thus save lives and property.
Andy Michael, a researcher with the U.S. Geological Survey's Western Earthquake Hazards Team in Menlo Park, California, said that, unlike the physics that control the weather, the physics that control earthquakes are at this time poorly understood.
"If you know the weather in Kansas today, you can pretty much guess areas a few hundred miles east of Kansas will get that weather tomorrow. Earthquakes happen suddenly, we can't watch the system evolve," he said.
Modern Earthquake Forecasts
Today agencies such as the U.S. Geological Survey issue assessmentsbased on a region's earthquake historythat a given area will experience shaking beyond a certain force. By knowing the location and the size of past earthquakes, scientists can forecast the probability of future earthquakes in the region.
SOURCES AND RELATED WEB SITES