focus and epicentre of earthquakes?

Question

Earthquakes occur when two tectonic plates slip past each other. (or under/over each other)
Now, I imagine the contact between these plates as a plane, since both plates are 3D objects, pressed up against each other. 
Then one plate moves, relative to the other one.
Surely the whole plane of contact experieces the jolt. 
If the two plates are to remain the same shape and size, the focus can't be located in one particular spot.
If two carpets are overlapping, and I pull one out from under the other one, there is friction across the entire plane of contact between the two carpets. (all right, I know the analogy is a constructive margin, and earthquakes occur at destructive margins, but surely the principle is the same whichever way the plates are moving?
How come the focus is non dimensional, (a single point) rather than two dimesional, and the epicentre is non dimensional rather than one dimensional?

Kieran Slade · Accepted Answer

In the plates, there is some very slight elasticity as with all objects and this is why you end up with a single point. For example, on a conservative plate boundary there might be one rough patch along it that will cause this build up of pressure, so while the rest of the plate is moving round, this is held onto for slightly longer and gets stuck.
As the rest of the plate continues to move very slightly there will be a point that the rough patch cannot take any more pressure and is suddenly released causing the huge energy release.
Oh and by the way, constructive plate boundaries can have earthquakes too :)

Anonymous · Answer

The focus of the earthquake can be as small as one individual three dimensional crystal  that fractures  and  starts the  earth quake. That crystal has an individual latitude longitude and depth, it's three dimensional  address  in particular coordinate system of the space time continuum. The holocenter is  the focus of the earthquake. The epicenter of the Earthquake is the location on the surface of the earth above where the earthquake  starts. The  slip and propagation of an earthquake along a lateral strike slip fault  is stopped when the stress and strain limits of crystals have not been exceeded so the crystals do not fracture, so there is no more slip or subduction until the stress and strain limits are reached and the next earthquake happens along the same fault.  Look at the USGS earthquake  data web pages. Each earthquake  has a latitude, longitude and sometimes a depth, if known, of the holocenter, the focus of the earthquake is listed. It is not non-dimensional or one dimensional in real life.

Azdo · Answer

The whole plate does push against the other plate across the whole of that side, however when there is an earthquake this tends to have a slip of land in one area and this is the epicentre. After shocks tend to be when other parts of the plate follow but at a less significant force.

? · Answer

Good query! Focus = where it began. Epicenter = The epicenter or epicentre is the point on the planet's surface that is straight above the hypocenter or focal point, the point where an earthquake or underground explosion originates.

focus and epicentre of earthquakes?

4 Answers