# Why are earthquake considered spot things?

As far as i know earthquake is result of movement of earth crust layers and release of energy at these locations and movements at faults.

So i think its happens somehow along a line or in a wide almost flat layer.

But each earthquake is given a coordination and a depth as if its like a spot explosion.

Why is it like this? Are my assumptions wrong?

• a little information en.wikipedia.org/wiki/Epicenter Dec 27, 2017 at 15:59
• You're right. Earthquakes, are not point sources, i.e., the rupture plane has finite length, width and slip. The reason for point source approximation are historical. For a number of applications, (as long as you're some distance away from the source, it doesn't really matter). Point sources are more convenient to deal with mathematically. People who study strong motion usually use finite fault models, e.g., see earthquake.usgs.gov/earthquakes/eventpage/… Dec 31, 2017 at 16:22

Faults and discontinuities within the Earth's crust are rarely smooth and any smooth parts are short in length.

The degree of roughness along a fault or any discontinuity varies in strength and magnitude. This causes stresses to build up unevenly along faults and discontinuities. When the stresses are too large for one rough patch to withstand there's a break and stress is released causing an earthquake. This is why earthquakes occur at a particular location.

• His question is about the point source approximation of finite fault models. Dec 31, 2017 at 16:21

The co-ordinates and depth refer to the modelled focus (or hypocentre) and the epicentre of an earthquake.

You are correct in that faults can rupture along a plane, but the location and depth information given in an earthquake report is typically based on a point source model. The focus is assumed to be where the rupture begins, with seismic waves propagating outward from there, so the arrival of these waves can be used to calculate its location. The epicentre is directly above the focus on the ground surface and the focal depth is the distance between the two.

The USGS gives some background about the information they (and equivalent agencies in other countries) provide in earthquake reports here: https://earthquake.usgs.gov/earthquakes/eventpage/terms.php

There is a diagram here, and in the link given by a previous commenter illustrating how the focus relates to the fault plane: https://en.wikipedia.org/wiki/Hypocenter.

Here is some information from the USGS about modelling a point source location: https://www.usgs.gov/faqs/how-do-seismologists-locate-earthquake?qt-news_science_products=7#qt-news_science_products

• Actually it's more subtle than that. In the high frequency domain you see a lot of complicated waves related to the rupture propagation along the fault. It's only when you look at the domain where the wavelength is greater than the length of the fault that it appears as a point source.
– bon
Dec 31, 2017 at 9:15
• This is a good point - I've edited to clarify that this the focus is a modelled point source, but please feel free to edit the answer.
– VFT
Dec 31, 2017 at 18:07