< Back
 |
China Shakes: Mapping earthquake intensity
5/19/2008
|
|
|
USGS ShakeMap: Eastern Sichuan, China earthquake May 12, 2008
|
|
Tibet's desire to edge away from China politically is in ironic contrast to what the two entities are doing geologically. For eons, the High Tibetan Plateau has been inexorably grinding eastward into the edge of the crustal material underlying southeastern China and the Sichuan basin. The two regions sit in the midst of a massive, slow-motion collision between tectonic plates, the India plate crashing into the Eurasia plate at a rate of 50 millimeters per year.
This seemingly modest movement builds up tremendous tectonic stresses. This fact became abundantly and tragically clear on Monday, May 12, 2008 at 2:28 in the afternoon when a band of friction deep beneath Sichuan province suddenly and violently released its stored tectonic energy in the form of a magnitude 7.9 earthquake. The earthquake visited severe damage in the Dujiangyan-Mianzhu-Mianyang area and caused substantial destruction in the provincial capital of Chengdu. At this writing, the Chinese government fears the death toll may reach 50,000.
The USGS ShakeMap above illustrates how the earthquake tremors were experienced in widening ovals around the rupture. This quake was roughly ten times as powerful as those in Northridge, California in 1994 and Kobe, Japan in 1995, which had magnitudes of 6.7 and 6.9, respectively. While a given earthquake is assigned just one level of magnitude, which describes the amount of energy released, ShakeMaps portray the effects of an earthquake at various distances from the epicenter in terms of shaking intensity and potential destruction.
Anyone along the presumed fault line or at the epicenter (star marking) at the time of the temblor would have experienced what is described as violent to extreme shaking, classed as intensity levels of IX to X on the Modified Mercalli Intensity scale and shown in bright red. At these highest levels of intensity, people are thrown off their feet, "most masonry and frame structures (are) destroyed along with their foundations," train rails bend, hillsides collapse in landslides, and canals, rivers, and lakes slosh so severely that they toss their water onto the banks.
Even Beijing,1545 km (960 miles) to the northeast, felt mild tremors from the Chengdu quake. But Chengdu itself, 75 km (50 miles) southeast of the epicenter, sits where an orange-yellow glow blooms on the map around the rupture. At this intensity the shaking feels very strong to severe, people have trouble standing and drivers have difficulty steering, branches break and fall from trees, chimneys tumble down, and towers can twist and fall. According to this scale, "damage is slight to moderate in well-constructed buildings," but as became clear, in Chengdu many buildings were poorly constructed and—like the school that held 900 students—collapsed on their occupants.
At one time intensity maps were constructed over a span of weeks by compiling descriptions of the experience by all postal workers across the affected area. Now underground seismic networks record the acceleration and velocity of ground movement in real time, and a shake map is generated automatically within five minutes of an earthquake.
Sichuan Basin last experienced a massive quake in 1933 when a magnitude 7.5 earthquake killed 9,300 people. Sooner or later all quake-prone areas once again face "the big one." U.S. Geological survey scientists conclude there is a 62 percent probability that the San Francisco Bay area will experience a magnitude 6.7 earthquake, causing widespread destruction, by 2032.
—Jessa Forte Netting
Links
Shakemap us2008ryan
USGS Magnitude 7.9 - EASTERN SICHUAN, CHINA
USGS Natural Hazards - Earthquakes
USGS Earthquake Hazards Program
Quake deaths - Xinhua News Agency Press Release May 16, 2008
For Further Study
Earthquake,
Earthquake engineering,
Seismology,
Plate tectonics,
Designing for and mitigating earthquakes,
ANIMATION: Continental drift,
Q&A: How do earthquakes cause structural damage? How does this relate to the geology of the affected area?,
BIO: Hess, Harry Hammond,
BIO: Vine, Frederick John,
BIO: Wilson, John Tuzo
View all Images of the Week
|
|
|
