Had an earthquake of equal magnitude and intensity struck a more developed area, such as Los Angeles — an area prone to similar quakes — the number of casualties would have been exponentially fewer, seismologists suspect.
“Given the same level of shaking, and given the same population exposed to that shaking, in California we’d probably get 100 times or even 1,000 times fewer casualties,” said David Wald, a seismologist with the U.S. Geological Survey.
In China’s Eastern Sichuan province, where earthquake building codes are weak and poorly enforced, buildings are often not designed to withstand quakes of the magnitude that hit the region on Monday. The adobe and clay brick found in many of the region’s structures are brittle and more flimsy than the wood-frame foundations supporting buildings in most California cities, according to structural engineer Kishor Jaiswal, who is working on an earthquake damage assessment project for USGS.
“The structure is brittle and the roof is heavy, which is a primary reason for deaths,” said Jaiswal. “Under serious shaking, these structures completely collapse, and that’s why they kill people.”
Monday’s earthquake was the result of massive shifting of the earth’s crust along the Longmenshan fault system, which stretches 200 kilometers through the Himalayas. A large block of earth along the fault thrust another block upwards, forcing the neighboring Himalayas to the west to rise up, and the valley basin to the east to drop.
Friction from the collision of the plates released energy in the form of sound waves, causing tremendous shaking throughout the Sichuan province. The shaking traveled northeast toward Beijing and southwest toward Hanoi along the fault lines.
It was many such earthquakes over millions of years that enabled the soaring Himalaya Mountains to climb to their current height.
“India is colliding into Asia and won’t stop,” said Lucy Jones, geologist and chief scientist at the USGS Multi-hazards Demonstration Project in Los Angeles. “The Himalayas are the tallest mountains in the world, but they still won’t stop growing.”
Translated as Dragon Gate, the Longmenshan fault is no stranger to destructive quakes. A magnitude 7.5. earthquake in 1933 killed more than 9,300 people. Monday’s quake is roughly the same magnitude as the 1906 San Francisco quake that destroyed more than 500 city blocks.
But damage and injuries are more a result of an earthquake’s intensity than its magnitude. Intensity, marked by roman numerals, measures the impact, or the amount of shaking and damage. This varies by location and distance from the epicenter.
The intensity in Monday’s quake “is some of the highest intensity you can get from an earthquake,” Wald said. “Here you’ve got intensity IX and X in areas with significant population. The numbers [of casualties] are already starting to climb dramatically and will continue to climb.”
With X as the highest intensity, it is believed that more than 15 million people were exposed to intensities from VII to X on the scale, the range considered powerful enough to destroy homes.
“From intensity, you can get a pretty good guess of what the damage will be,” Jones said, adding, “This is going to be one of those situations where the death toll grows dramatically within the next couple of days.”