Physics Today: The magnitude 7.9 earthquake that struck near Chengdu May 12 has killed thousands and made millions homeless. It is the largest earthquake to hit China since 1976. According to the US Geological Survey the quake occurred as the result of motion on a northeast striking reverse, or thrust, fault, on the northwestern margin of the Sichuan Basin. The earthquake's epicenter and focal mechanism are consistent with it having occurred as the result of movement on the Longmenshan fault or on a tectonically related fault. The earthquake reflects tectonic stresses resulting from the convergence of crustal material slowly moving from the high Tibetan Plateau, and the strong crust underlying the Sichuan Basin and southeastern China (see maps below). "Earthquakes in this part of China are infrequent but not unexpected" says Harley Benz, Scientist-in-Charge with the USGS's National Earthquake Information Center.

Chengdu has a population of 2 million, with another 9 million in the surrounding urban area, and is about 90 km southeast of the epicenter. Many International companies, such as IBM, Symantec, Microsoft, Intel, Fujitsu, NEC, Motorola, and Nokia, have factories and offices in the region as part of Chengdu's High-Tech Industrial Development Zone. None of those companies are reporting any deaths of employees or major damage to their facilities. The SINA website has photos of the damage done in the city and surrounding towns.

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On a continental scale, the seismicity of central and eastern Asia is a result of northward convergence of the India plate and the Eurasia plate with a velocity of about 50 mm/y. The convergence of the two plates is broadly accommodated by the uplift of the Asian highlands and by the motion of crustal material to the east of the uplifted Tibetan Plateau.

"For this earthquake, because of its size, we should expect to see lots of aftershocks," says Benz. "In the first few hours following the magnitude 7.9, we recorded more than 13 earthquakes, the largest being a magnitude 6. The earthquakes are being located along a northeast trending fault, and they extend over a region about 100 kilometers, or 60 miles, which is consistent with the size of this earthquake.

"In terms of the total number of aftershocks, aftershocks from an earthquake, typically this size, will be occurring weeks and months from now, but typically with time, the number of earthquakes will go down, and the size of the earthquakes will go down, but there are chances of having other large, damaging earthquakes as part of this sequence occurring in the new few days and weeks."

Update 5/27/2008 According to the China Daily, an aftershock on Sunday that was 6.4 on the Richter scale destroyed 71,000 more homes and killed 6 people. The current death toll from the initial earthquake has topped 50,000.