[KathyKeegan]

(05-26-2015, 11:24 PM)Skywise Wrote:

(05-26-2015, 10:04 PM)KathyKeegan Wrote:

(05-18-2015, 06:48 AM)KathyKeegan Wrote: Is that close enough to India yet? It just had a 4.9 southwest of India.

I just was thinking that if there was an increase in quakes around the plate that is shoving Nepal into China it would have to have repercussions both on the ridges and boundaries of plates elsewhere. There is no action without reaction sort of thing.

There is something known as the elastic rebound theory which explains how stresses build in the Earth's crust along a fault line and are then released. Here's a couple of videos:







As you can see the ground deforms as the stress builds up, and springs back during the earthquake. Just like a pebble in a pond, when the ground snaps back (the earthquake), ripples spread out. These are the earthquake waves, the ground motions that we feel.

The area of deformation is local to the fault zone. So is the area of rebound after the quake. This deformation can be measured using something called InSAR (Interferometric Synthetic Aperture Radar), usually done with satellites. The resulting images show how the ground shifted during the quake. Here's an InSAR image of the Nepal quake:


from: http://www.travelinggeologist.com/2015/0...ke-searle/

The tight rings represent the area of ground displacement resulting from the quake. As you can see, it's concentrated in the local region where the fault slipped. There is some slight change at a distance, but some of this can be due to causes other than the quake. InSAR is amazingly sensitive. But looking at the map, the maximum displacement is near Kathmandu and by the time you move south to Hetauda, things are still normal. That's only a distance of 27 miles.

On the other hand, there is the idea of faults being affected by the passing earthquake waves and that has been studied. But that's an effect that occurs within minutes or hours after the quake.

Now, that isn't to say that the stress field doesn't have an effect. For example, here is the resulting cumulative stress field from the Landers (M7.3) and Hector Mine (M7.1) earthquakes in the southern California deserts. I then plotted on the map the locations of larger quakes in the years afterward. The red areas represent locations where ground stress has increased; blue is lower stress. As you can see, nearly all the quakes occurred in areas of increased stress.


from: http://www.skywise711.com/quakes/index.html#8

I made that image more than five years ago so it doesn't show more recent quakes.

It is widely conjectured that the Hector Mine quake was triggered by the Landers quake seven years earlier. The epicenter of Hector Mine is smack in the middle of a high stress lobe from the Landers quake. But again, this is within tens of miles.

Basically, the 'stress field' from a quake doesn't go far enough to affect faults thousands or even hundreds of miles away. The change in the stress field is just too small. The Earth's crust just isn't that solid. At these scales, rock is almost "spongy". Imagine pressing your finger into a sponge on one side. The other side doesn't deform.

Brian

Ok, thanks for taking the time to explain it.