<p>The data from USGS is somewhat misleading.</p>

<p>Determining earthquake depths is a difficult problem, particularly if you're using data recorded a long way from the earthquake. Event location methods often become unstable for very shallow earthquakes. With some methods/catalogues it is therefore normal to "fix" the depth of shallow earthquakes, and not attempt to find a more accurate depth. This is why you see so many earthquakes supposedly at 33km; I think the 10km ones may also be fixed. Really, this ought to be made more obvious in the USGS site (I think the information <em>is</em> there if you dig deep enough!).</p>

<p>So, it's impossible to draw any conclusions from these graphs: you're probably just seeing improvements in seismological techniques over time.</p>

CC BY-SA 3.0
2015-11-28T12:04:56.343Z
@avid there was a 7 year drought in Iran and because of the global warming this situation is not going to get better too. Anyway, I don't know the English term but it's said that ground has shifted down for almost 5 meters because of less water underground. Can this possibly be a reason.
user70808
CC BY-SA 3.0
2014-06-09T17:17:46.850Z
to what degree do these methods/catalogues tend to fix(round) the depth? and what is the tolerance interval if there is any? - thank you
user70808
CC BY-SA 3.0
2014-06-09T17:27:47.047Z
I *think* usually the approach is to enforce a *minimum* depth. That is, if the earthquake location procedure wants to place the event shallower than (say) 33km, the depth is then set to 33km and not permitted to change further.
avid
CC BY-SA 3.0
2014-06-09T20:32:08.560Z
I agree that your depth graph shows "fixing" or "snapping" to a specific list of values. Also I remember in my MSc project using modern methods on an old earthquake: I could rule out a number of possible fault orientation scenarios, but could say hardly anything about depth beyond "it was shallow"!
winwaed
CC BY-SA 3.0
2014-06-09T14:40:07.363Z
2014-06-09T14:40:07.363Z
avid
<p>First of all, I am not expert at this so correct me. My hometown is Shiraz and since last year I can sense the earth is moving. I call them  mini earthquakes. But many people around me can not sense it. Some time ago there was an earthquake it felt like a 5 or even 6 richter earthquake but it was a 4 ricther earthquake but then in news they said that the depth was so close to the surface (4 km). I take a look at the USGS archives for Shiraz with in a 90 km diameter (as back in time as it was available) which I think will cover the major and minor fault lines. If you want to take a look its at <a href="http://bit.ly/usgsarchive" rel="noreferrer">a link</a>! usgs archives[a]
. And here is a diagram I made in excel.
<img src="https://i.sstatic.net/rBduV.jpg" alt="as"></p>

<p>This is depth diagram (horizontal axis is the time and starts from 1974). What is interesting for me is that the depth of the earthquakes broke the boundary of 10km and stayed there for a much longer than it used to.And here is the magnitude diagram 
<img src="https://i.sstatic.net/bEKC2.jpg" alt="enter image description here"></p>

<p>This is the map of major fault lines of Iran:
<img src="https://i.sstatic.net/SRNAX.jpg" alt="enter image description here"></p>

<p>This is the same map (only the reverse fault lines):
<img src="https://i.sstatic.net/L0GmS.jpg" alt="enter image description here"></p>

<p>And again the same map (only the lateral fault lines):
<img src="https://i.sstatic.net/R8v49.jpg" alt="enter image description here"></p>

<p>Is this a sign of fault lines building pressure? And why the depth of earthquakes are getting near the surface? Can a geologist make any kind of guess based on the pattern or this is not a pattern at all?</p>

CC BY-SA 3.0
2014-06-09T14:13:01.843Z
Interesting question, thought you might want to know the link to USGS you referenced is not present currently, you might want to added it.
blunders
CC BY-SA 3.0
2014-06-09T14:23:07.107Z
@blunders changed the URL.It had so many parameters I couldn't add it,but now it's there.
user70808
CC BY-SA 3.0
2014-06-09T14:39:17.497Z
Thanks, the data and visualization from USGS was helpful, and posted an answer as a result. Let me know if it is of any use, or if somehow I have misunderstood the intent of your question. Thanks!
blunders
CC BY-SA 3.0
2014-06-11T08:44:03.797Z
Suggestion: To make the first graph clearer, consider putting the y-axis the other way up, so that deeper earthquakes are shown lower on the graph than shallower ones.
Semidiurnal Simon
CC BY-SA 4.0
2021-09-07T11:36:39.367Z
"a 4 ricther earthquake but then in news they said that the depth was so close to the surface (4 km)" you can definitely feel a Mw 4 earthquake so shallow. If close enough (<5 or <10 km distance from the epicenter) you can feel even a magnitude 2 or 3.
EarlGrey
<geophysics><earthquakes>
What does it mean when the depth of earthquakes get near the surface over a period of time?
user70808
# What does it mean when the depth of earthquakes get near the surface over a period of time?

(Asked by: username_0 on 2014-06-09)

First of all, I am not expert at this so correct me. My hometown is Shiraz and since last year I can sense the earth is moving. I call them mini earthquakes. But many people around me can not sense it. Some time ago there was an earthquake it felt like a 5 or even 6 richter earthquake but it was a 4 ricther earthquake but then in news they said that the depth was so close to the surface (4 km). I take a look at the USGS archives for Shiraz with in a 90 km diameter (as back in time as it was available) which I think will cover the major and minor fault lines. If you want to take a look its at [a link](http://bit.ly/usgsarchive)! usgs archives[a]
. And here is a diagram I made in excel.
![as](https://i.sstatic.net/rBduV.jpg)

This is depth diagram (horizontal axis is the time and starts from 1974). What is interesting for me is that the depth of the earthquakes broke the boundary of 10km and stayed there for a much longer than it used to.And here is the magnitude diagram
![enter image description here](https://i.sstatic.net/bEKC2.jpg)

This is the map of major fault lines of Iran:
![enter image description here](https://i.sstatic.net/SRNAX.jpg)

This is the same map (only the reverse fault lines):
![enter image description here](https://i.sstatic.net/L0GmS.jpg)

And again the same map (only the lateral fault lines):
![enter image description here](https://i.sstatic.net/R8v49.jpg)

Is this a sign of fault lines building pressure? And why the depth of earthquakes are getting near the surface? Can a geologist make any kind of guess based on the pattern or this is not a pattern at all?

**Tags:** geophysics, earthquakes

> **Comments:**
> **username_1**: Interesting question, thought you might want to know the link to USGS you referenced is not present currently, you might want to added it.  
> **username_0**: @username_1 changed the URL.It had so many parameters I couldn't add it,but now it's there.  
> **username_1**: Thanks, the data and visualization from USGS was helpful, and posted an answer as a result. Let me know if it is of any use, or if somehow I have misunderstood the intent of your question. Thanks!  
> **username_2**: Suggestion: To make the first graph clearer, consider putting the y-axis the other way up, so that deeper earthquakes are shown lower on the graph than shallower ones.  
> **username_3**: "a 4 ricther earthquake but then in news they said that the depth was so close to the surface (4 km)" you can definitely feel a Mw 4 earthquake so shallow. If close enough (<5 or <10 km distance from the epicenter) you can feel even a magnitude 2 or 3.  

---

## 1 answers

### Answer by username_4 on 2014-06-09. Score: 9

The data from USGS is somewhat misleading.

Determining earthquake depths is a difficult problem, particularly if you're using data recorded a long way from the earthquake. Event location methods often become unstable for very shallow earthquakes. With some methods/catalogues it is therefore normal to "fix" the depth of shallow earthquakes, and not attempt to find a more accurate depth. This is why you see so many earthquakes supposedly at 33km; I think the 10km ones may also be fixed. Really, this ought to be made more obvious in the USGS site (I think the information *is* there if you dig deep enough!).

So, it's impossible to draw any conclusions from these graphs: you're probably just seeing improvements in seismological techniques over time.

> **Comments:**
> **username_0**: to what degree do these methods/catalogues tend to fix(round) the depth? and what is the tolerance interval if there is any? - thank you  
> **username_4**: I *think* usually the approach is to enforce a *minimum* depth. That is, if the earthquake location procedure wants to place the event shallower than (say) 33km, the depth is then set to 33km and not permitted to change further.  
> **username_5**: I agree that your depth graph shows "fixing" or "snapping" to a specific list of values. Also I remember in my MSc project using modern methods on an old earthquake: I could rule out a number of possible fault orientation scenarios, but could say hardly anything about depth beyond "it was shallow"!  
> **username_0**: @username_4 there was a 7 year drought in Iran and because of the global warming this situation is not going to get better too. Anyway, I don't know the English term but it's said that ground has shifted down for almost 5 meters because of less water underground. Can this possibly be a reason.  

---

