Seismologists, study earthquakes including locating the site and source character where they originated. However, one of the challenge they faced is to distinguish the earthquake event from the explosion, as there is a very narrow gap that exists between these and their visual traces look the same in seismic graphs.
And the fact that most seismic stations produce seismograms that are contaminated with explosion events. Whenever received seismic signal ( as recorded by seismograph), it is received after travelling through a medium (earth). This medium modifies its amplitude and frequency and what we receive is the modified signal.
This is why I have decided to write down to you this piece of article so that you can grasp the basic idea on how you can distinguish these two and likely similar seismic events.
Without keeping rounding , the bushes Let's dive down into this talk as described here below.
The following are the factors that can make you aware how earthquakes differ from explosion.
Differences between seismic explosion and earthquake
1. Mode of occurrence
If you observe the seismogram frequently you can notice that most of the explosion events occur in systematic fashion such as at regular period like hourly
While most of earthquake events occur at random fashion, such that there is no definite interval for their occurrences. This form the basis that we can use earthquake statistics especially probability distribution to predict future events occurrences based on past events.
Figure 1: Courtesy: An explosion in S. Norway (M = 1.8, time : 2002 04 11, 13:16)
The explosive charge was 43 T.
Figure 2: Simple earthquake seismic trace.
We will talk in details about this in the upcoming posts on how we can predict future earthquake events by using statistics. Just stay tuned!
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2. Seismic Trace Pattern
When it comes to the trace pattern in most cases of explosion sources are compressional. This means that during seismic propagation only the volume changes without rotation. This can simply mean that Shear (S) - wave generation from most explosions is rare. Hence it is hard to produce a shear source. And this forms the basis of why P-waves are the most important for controlled source seismology
3. Depth of occurrence
In most cases seismic explosions occur in shallow depth compared to earthquakes. For example in seismic reflection surveying that utilize explosion mode of operation at which explosives chemicals are detonated with electrical charges within drill holes (6 - 30m). However this energy may be sufficient to be used in a survey. In earthquakes the shallow focal depth is around less than 70km at which this depth is the most contributing depth for most earthquake occurrences.
4. Source Character
In seismic explosion (exploration), the source character is well known, see figure 1 above. So in this case the received signal is divided by the source signal, to get the multiplicative effect of the medium , while In earthquake studies, the source signal is not known. However the estimate of the medium property is present, when the received signal is divided by the likely effect of the travelling medium, a correct picture of the source signal (at earthquake origin) is obtained.
5. Primary Objective
Despite the fact that, in both cases the division of the received signal either by source signal (seismic exploration) or the medium effect (earthquake studies) is done by a mathematical operator which is called “deconvolution”.The primary objective regarding seismic exploration by using explosion, is that the deconvolved signal is analyzed to get a realistic estimate of the medium property
While in the case of earthquake studies, the deconvolved signal is analyzed to get a realistic estimate of the source character (earthquake origin).
All in All it is very challenging to distinguish these two seismic events through seismogram observation, Although you can utilize the above factors as a basic guide for this purpose. However the best technique is to use explosion filtering in order to block out a certain ground motion within a predefined threshold frequency. However this method is not effective as it may select out various earthquake events that do not exceed that predefined threshold value.
Thank you for your time! And meet you in the next Post.
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