The devastating earthquake that hit the heart of China in 2008 caught the region unguarded, and left tens of thousands of people dead.
Because the destruction was so widespread, many people witnessed the ground rip apart, which along with other observations and studies, is helping geologists piece together exactly what happened — and how to prevent future catastrophes.
The area of Wenchuan, China, was crippled by the 7.9-magnitude earthquake that struck on May 12, 2008. The temblor killed more than 70,000 people, injured about 374,000, and left approximately 18,000 missing and presumed dead, according to the U.S. Geological Survey (USGS). More than 45.5 million people across 10 provinces were affected by the quake. Four million people lost their homes. Several cities were almost completely destroyed.
Before the 2008 quake, the region was downgraded to a low seismic risk, so the massive temblor caught many scientists by surprise, which isn't uncommon even in areas known to have frequent earthquakes.
"Unfortunately, it's kind of a truism about the state of our knowledge that we're constantly surprised," said Ken Hudnut, a geologist with the USGS in Pasadena, Calif., who has studied the Wenchuan earthquake.
The most recent comparable earthquake in the area was between 1,000 to 2,000 years ago, suggests a study in the November edition of the journal Bulletin of the Seismological Society of America. The issue of the journal is a special one devoted to studies of the 2008 earthquake.
The eyewitness reports were published in the November/December edition of the journal Seismological Research Letters.
The earthquake ruptured at 6:28 a.m. local time on May 12 along the Longmenshan margin of the eastern edge of the Tibetan Plateau.
The rupture caused a 149-mile-long (240 kilometers) surface crack along one pre-existing fault and an additional 45-mile-long (72-km) surface rupture along another fault.
Field investigations and seismic data show that the earthquake was triggered by active faults of the Longmenshan thrust belt. Here, tectonic stress grew as the Earth's crust slowly moved from the high Tibetan Plateau to the west, against the strong crust underlying the Sichuan Basin and southeastern China.
Once the stress was too much, the earthquake mostly ruptured in what is called a strike-slip fault — these fault systems slide side-to-side when two tectonic plates butt heads. Up to 30 feet (9 meters) of slip may have occurred during the rupture.
Such a big gradient typically raises a red flag about earthquakes and landslides. At least 700 people were buried by a landslide in one area during the Wenchuan quake.
"Take steep topography and shake it hard in an earthquake and stuff's going to come down," Hudnut told OurAmazingPlanet.
Eyewitnesses reported about a mile (1.5 kilometers) of surface faulting in one area. Cracks and fractures were seen on three mountains in the area, and subsidence (the earth shifting to a different level) and street cracks were observed in one city, according to the USGS.
Photographs show buildings split by surface ruptures, and people watched as the ground pulled apart right in their backyards.
Eyewitness accounts of the surface ruptures have been rare, but they are extraordinary, Hudnut said.
Hudnut and colleagues have also interviewed eyewitnesses of the 2010 El Mayor-Cucapah earthquake in Baja California. One person stepped outside right as the ground ripped apart in front of his house and described the rupture as sounding like a race car going by, Hudnut said.
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