At 2:28 pm local time a major M7.9 earthquake struck China near the city of Chengdu in Sichuan province. BBC reports 8500 dead , but USGS analysis shows that 200,000 people were exposed to violent to extreme shaking that could cause heavy damage in well built structures and very heavy damage in vulnerable structures.
Survivor struggles to escape.
Moreover, close to a million people lived where strong shaking could cause heavy damage in vulnerable structures. This USGS analysis suggests that early news reports many be underestimating the extent of this disaster.
The city of Chengdu,with about 11 million inhabitants, 55 miles east of the rupture, was spared from heavy damage because the thrust event concentrated the worst damage near the fault rupture.
The earthquake was a shallow thrust earthquake, similar to the 1971 San Fernando Valley California earthquake in it's fault mechanism and transfer of energy to the surface. However, it was an order of magnitude greater than the San Fernando earthquake, so much more damage over a much larger area can be expected. It was located on a fault in the mountains just northwest of the Sichuan basin.
90 km (55 miles) WNW of Chengdu, Sichuan, China
150 km (90 miles) WSW of Mianyang, Sichuan, China
360 km (225 miles) WNW of Chongqing, Chongqing, China
1545 km (960 miles) SW of BEIJING, Beijing, China
Fortunately for the millions of people living in the basin that strongest shaking was in the mountains along the fault. The aftershock sequence shows that the primary shock began on the southeastern end of the rupture zone and propagated northeast. The heaviest damage can be expected along that rupture path to the northeast of the epicenter because that's where the energy was focused on the surface as the rupture spread northeast.
I'm sitting at my desk on the 19th floor in Shanghai doing work and listening to bad 80's music. I started to feel a little woozy, as if I was dizzy and needed to lay down for a minute. That's when I noticed that the cords on my window shade were swinging back and forth. Earthquake!
The swaying lasted about a minute or two here. I'm sure it was much worse near the epicenter - no word on that yet.
The Sichuan earthquake of May 12, 2008, occurred as the result of motion on a northeast striking reverse fault 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 a tectonically related fault. The earthquake reflects tectonic stresses resulting from the convergence of crustal material slowly moving from the high Tibetan Plateau, to the west, against strong crust underlying the Sichuan Basin and southeastern China.
On a continental scale, the seismicity of central and eastern Asia is a result of northward convergence of the India plate against 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 away from the uplifted Tibetan Plateau.
The northwestern margin of the Sichuan Basin has previously experienced destructive earthquakes. The magnitude 7.5 earthquake of August 25, 1933, killed more than 9,300 people.
P.S. The Richter scale has been replaced by the moment-magnitude scale which directly relates to the energy release of an earthquake.