• English Site
• > Under the Scope

Earthquake Research

Earth's Fragile Shell

By Axel Bojanowski

Earthquakes are capable of ripping apart the earth up to 200 kilometers away from their epicenters. Until recently, the actual shift of the earth's crust could only be guessed at. Now, a new technology allows researches to pinpoint the exact effects of violent quakes.

The hectically zigzagging lines scrawled on the paper rolls of a seismogram hardly do earthquakes justice. Even just a one-centimeter needle-tick upwards represents the release of unimaginable amounts of energy as shockwaves roll through the earth's crust shoving incredible volumes of rock to one side or, within seconds, raising the floor of the ocean by many meters.

Yet it is these seemingly infantile scribbles that seismologists rely on to determine how strong a given earthquake was. By comparing seismograms from a number of different sensors, researchers can also determine in which direction the millions of tons of bedrock were shoved. But such measurements are far from precise. A clear picture of how far stone is shifted has eluded seismologists for decades.

That barrier of exactitude, however, appears to have been breached; seismologists Shin 'ichi Miyazaki of the University of Tokyo and Kristine Larson from the University of Colorado may have achieved a breakthrough.

Concentrating on a violent earthquake that shook the north coast of Japan on Sept. 25, 2003, they have been able to create a computer animation that captures the movement of the earth to an incredibly exact degree, with a margin of error of only 4.5 millimeters. And they did it using a satellite navigation system that had been considered unreliable for the real-time measurements necessary for earthquake analysis. Called GPI, the system is the Japanese counterpart to the American Global Positioning System (GPS). And, with 1,200 GPI stations spread out across Japan combined with 3,000 earthquake measuring stations, the film created by the researchers provides the most exact look at an earthquake yet.

The earthquake examined by Miyazaki and Larson began 80 kilometers off the coast of the Hokkaido Peninsula: Suddenly, 25 kilometers below the surface of the earth, the tension between two plates suddenly snapped releasing an amount of energy roughly 80,000 times that of the atomic bomb dropped on Hiroshima. The quake had a magnitude of 8.1 and directly above the epicenter, vast quantities of stone were moved by up to one meter -- at a speed of 25 centimeters per second.

But it is farther away from the epicenter where the earth moves the most, the researchers wrote in the journal Geophysical Research Letters. As their film shows, the most violent shifts occurred 50 kilometers away: millions of tons of the earth's crust moved at a speed of one meter per second and ended up fully nine meters away from their starting point.

Shifting 200 kilometers of stone

Even 100 kilometers north-west of the quake epicenter the surface rock moved two full meters. Indeed the area affected by such a trembler is astonishing: A circle encompassing the entire region that experienced ground displacement had an area roughly the size of the US state of New Jersey. The GPI devices even measured ground shifting 200 kilometers away from the epicenter.

The most surprising result, however, was that aftershocks in the deeply buried bedrock resulted in almost no further shifting of rock on the earth's surface. Following the main quake on the Hokkaido Peninsula, a number of aftershocks, one of them with a magnitude of 7.1, shook the region just south-west of the original epicenter. Surprisingly, however, the earth's crust stayed put. The conclusion of Miyazaki and Larson: Most of the tension present on the surface of the earth must have been released by the first quake.

A comparison between the data from the GPI devices and the seismograms shows that the new technology is much more precise. The seismogram readings had placed the region experiencing the most dramatic shifts 20 kilometers further away from the epicenter than the GPI system shows. "We have demonstrated that a network of navigation devices can accurately measure the shifts of the earth's surface following earthquakes," said Larson.

Until recently, the achievements of the research team had been considered impossible -- GPI and GPS devices weren't exact enough to measure short, sharp movements of the earth caused by earthquakes. Indeed, navigation systems were only used by geologists to measure movements of continental plates over time -- and delivered scientists the proof that the continents move by many centimeters per year.

But now, the satellite technology will be used to measure the immediate movements set off by earthquakes. The National Science Foundation in the United States has taken notice. It is already planning on installing 1,000 GPS stations on the earthquake-prone west coast.