Why does China have so many bad earthquakes?

[woodcutter]

Maybe I need a geology forum, but can anyone tell me, why does China suffer so much from Earthquakes? Aren't they supposed to happen (like eruptions) near the edge of tectonic plates? Looking at the map below though, isn't Sichuan pretty far away from such a line? Tangshan too. Korea doesn't get them, for example.

http://en.wikipedia.org/wiki/Tectonic_plates

[SiMaKe]

As for the Sichuan quake, here is what geology.com has to say:

"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."

[imron]

It's not just tectonic plates, it's also fault lines. See here for a map of tectonic plates and fault lines in the Asia Pacific.

[SiMaKe]

And another interesting tidbit from shearsensibility.blogspot.com.

"This isn't a classic plate boundary earthquake, but it's part of the way that continents respond to continent-continent collisions.

In the big picture, India is moving northward and colliding with Asia. That's what created the immense mountains of the Himalayas, and the highlands of the Tibetan Plateau, in the first place. This earthquake, however, was not on the plate boundary itself. It was off on the eastern edge of the Himalayas, on the boundary between the high country and the Sichuan basin."

Apparently Sichuan sits in a "basin" surrounded by three mountain ranges some, or all, of which are still in motion.

Here is a picture which seems to show this basin formation (if you can make it out). The source is the USGS site (neic.usgs.gov/)

[mr.stinky]

http://www.physicalgeography.net/fundamentals/10m.html

this page has a nice chart showing the distribution of 7.0+ magnitude earthquake distribution. from the chart, it seems europe is the most prone to high magnitude quakes.

why does china suffer so much? it's not just the quake magnitude, but also other factors

affect the amount of destruction. probably most important in this case:

population density in the affected area.

poor quality construction.

surface materials: since sichuan is situated in a basin surrounded by mountains, the

buildings are probably built on alluvial (unconsolidated sedimentary) materials

rather than on solid bedrock.

surrounding topography: loose materials on mountainsides and upstream dams can

cause more damage than the earthquake itself.

[self-taught-mba]

Re: http://www.physicalgeography.net/fundamentals/10m.html

Good science; poor history

The greatest loss of life because of an earthquake this century occurred in Tangshan, China in 1976 when an estimated 250,000 people died.

Yep I always learned India is pushing up through Asia.

Large parts of China are prone to quakes.

[woodcutter]

Wow, thanks, I didn't expect such good answers.

It somehow seems a bit of a messy theory. Earthquakes can basically happen all over the place, and the fault line map looks like a cat has scratched madly at certain areas. And then SiMaKe seems to be saying that this Sichuan one didn't happen due to a fault line.

(By the way, Korea has had some biggish earthquakes, historically, but it isn't very prone. However, there is no scratch mark on Korea.......Do the lines change?)

[imron]

However, there is no scratch mark on Korea

Try zooming in on Korea and you'll see the scratch marks.

[SiMaKe]

Woodcutter

To help clear up any misunderstanding, my first post said exactly that - the Sichuan quake was the result of a thrust fault. Sorry it was not clearer.

Mr Stinky makes a good point about Sichuan being on an alluvial plane (I've seen that mentioned elsewhere). This probably contributed to the extensive damage.

And the site http://www.physicalgeography.net/fundamentals/10m.html showing the extent of 7.0+ in Europe (and Eastern US where I am) is interesting. Are these occurring so far beneath the surface (like the ones under Mississippi) that they don't do much damage and thus don't get into the papers? Or have I been sleeping?

[Asiasy]

it`s the first quake i ever experienced.....horrible

[woodcutter]

Pushing my luck further.......what is happening on a fault line? Along the tectonic plates one plate is supposed to be moving in regard to the neighbouring plate (though I have never understood why the result is so localized).

[woodcutter]

If you want to know which places are relatively safe, this map seems quite good....

http://www.seismo.ethz.ch/GSHAP/

This one with more detail is more worrying for S.China folks....

http://www.seismo.ethz.ch/GSHAP/eastasia/asiafin.gif