M 7.9 Quake Devastates Eastern Sichuan, China, Updated

( – promoted by buhdydharma )

At 2:28 pm local time a major M 7.9 earthquake struck China near the city of Chengdu in Sichuan province.   9000 people are feared dead according to Chinese news reports , 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.

Rescuers are searching  for victims.

By FishOutofWater

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, in the Sichuan basin with about 5 million inhabitants, 55 miles east of the rupture, was spared from heavy damage because the earthquake rupture propagated from the southeast to the northwest sparing the cities in the basin. However, towns near the fault rupture well to the northeast of the epicenter were hit hard even though they were as far from the epicenter as Chengdu. In an earthquake like this the distance from the epicenter may be a misleading indicator of potential damage.

NPR reporters were visiting Chengdu when the quake hit

A Horrific Scene at a Middle School in Dujiangyan

We are just leaving the horrific scene at the Juyuan Middle School outside the city of Dujiangyan. Hundreds of parents are still standing in the rain as the army works to find children trapped in the rubble. One parent told us she could hear her son calling. A scene of utter desperation. Back a couple hundred feet was an area where rescuers — peoples armed police — were bringing bodies that had been retrieved. Families were rushing over to see whether the child was theirs. Under tents are families burning incense and candles and paper money next to the shrouded bodies of their loved ones. A terrible, terrible scene.

— Andrea Hsu

For those that may have family or friends in or near Chengdu, from a comment on an NPR blog:

Chengdu:

Please check with Consulate General of American in Chengdu.

Phone: (28) 8558-3992

Fax: (28) 8554-6229

Emergency: 1370-800-1422

Email: [email protected]

Emergencies

The ACS Unit provides emergency assistance to American citizens in distress: when an American is destitute, arrested, separated from minor children, or sick. In an emergency, the Consulate Duty Officer can be reached at any time by calling 1370-800-1422.

Dial 01186 before you dial those numbers if you dial from USA.

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.


By USGS

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

The aftershock sequence lines up along the rupture surface of the main shock. The epicenters of the aftershocks outline the area of maximum destruction on the surface.

By USGS

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.

By USGS

The earthquake was felt in major cities across China including Shanghai as reported here, but those cities will have suffered only minor damage such as objects falling from shelves.

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 cause of the earthquake is the thrusting of the mountain block of the Tibetan plateau over Sichuan basin caused by the tectonic compression related to India’s northward motion.

Tectonic Summary

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.

FYI, The Richter scale has been replaced by the moment-magnitude scale which directly relates to the energy release of an earthquake. I was finishing grad school doing research on an earthquake prediction concept when  Hiroo Kanamori

published the paper that lead to the replacement of the Richter scale.

Update: See Shanghaiist for updates from China.