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

<p>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.</p>

CC BY-SA 3.0
2017-12-31T16:21:15.717Z
His question is about the point source approximation of finite fault models.
stali
CC BY-SA 3.0
2017-12-27T10:58:19.903Z
2017-12-27T10:58:19.903Z
Fred
<p>The co-ordinates and depth refer to the modelled focus (or hypocentre) and the epicentre of an earthquake.</p>

<p>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. </p>

<p>The USGS gives some background about the information they (and equivalent agencies in other countries) provide in earthquake reports here: <a href="https://earthquake.usgs.gov/earthquakes/eventpage/terms.php" rel="nofollow noreferrer">https://earthquake.usgs.gov/earthquakes/eventpage/terms.php</a></p>

<p>There is a diagram here, and in the link given by a previous commenter illustrating how the focus relates to the fault plane: <a href="https://en.wikipedia.org/wiki/Hypocenter" rel="nofollow noreferrer">https://en.wikipedia.org/wiki/Hypocenter</a>. </p>

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

CC BY-SA 3.0
2017-12-31T09:15:51.677Z
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
CC BY-SA 3.0
2017-12-31T18:07:31.957Z
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
CC BY-SA 3.0
2017-12-31T08:01:55.863Z
2017-12-31T18:06:00.357Z
2017-12-31T18:06:00.357Z
VFT
<p>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.</p>

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

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

<p>Why is it like this? Are my assumptions wrong?</p>

CC BY-SA 3.0
2017-12-27T15:59:15.677Z
a little information https://en.wikipedia.org/wiki/Epicenter
trond hansen
CC BY-SA 3.0
2017-12-31T16:22:14.273Z
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 https://earthquake.usgs.gov/earthquakes/eventpage/us2000bmcg#finite-fault
stali
<earthquakes>
Why are earthquake considered spot things?
Nemexia
# Why are earthquake considered spot things?

(Asked by: username_0 on 2017-12-27)

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?

**Tags:** earthquakes

> **Comments:**
> **username_1**: a little information https://en.wikipedia.org/wiki/Epicenter  
> **username_2**: 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 https://earthquake.usgs.gov/earthquakes/eventpage/us2000bmcg#finite-fault  

---

## 2 answers

### Answer by username_3 on 2017-12-27. Score: 3

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.

> **Comments:**
> **username_2**: His question is about the point source approximation of finite fault models.  

---

### Answer by username_4 on 2017-12-31. Score: 2

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>

> **Comments:**
> **username_5**: 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.  
> **username_4**: 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.  

---

