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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! usgs archives[a] . And here is a diagram I made in excel. as

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

This is the map of major fault lines of Iran: enter image description here

This is the same map (only the reverse fault lines): enter image description here

And again the same map (only the lateral fault lines): enter image description here

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?

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  • $\begingroup$ Interesting question, thought you might want to know the link to USGS you referenced is not present currently, you might want to added it. $\endgroup$
    – blunders
    Jun 9, 2014 at 14:13
  • $\begingroup$ @blunders changed the URL.It had so many parameters I couldn't add it,but now it's there. $\endgroup$
    – user70808
    Jun 9, 2014 at 14:23
  • $\begingroup$ 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! $\endgroup$
    – blunders
    Jun 9, 2014 at 14:39
  • $\begingroup$ 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. $\endgroup$
    – Semidiurnal Simon
    Jun 11, 2014 at 8:44
  • $\begingroup$ "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. $\endgroup$
    – EarlGrey
    Sep 7, 2021 at 11:36

1 Answer 1

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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.

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  • $\begingroup$ 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 $\endgroup$
    – user70808
    Jun 9, 2014 at 17:17
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    $\begingroup$ 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. $\endgroup$
    – avid
    Jun 9, 2014 at 17:27
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    $\begingroup$ 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"! $\endgroup$
    – winwaed
    Jun 9, 2014 at 20:32
  • $\begingroup$ @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. $\endgroup$
    – user70808
    Nov 28, 2015 at 12:04

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