Difference between Earthquake and explosion signals
Strong underground nuclear explosions produce seismic signals, which are similar to those produced by earthquakes. However, there are some differences between the two signal types, which arise from the nature of the two types of the sources of the energy release. These differences are used to discriminate an underground explosion from an earthquake.
The characteristics of the two types of signals, which distinguish one type of signal from the other are of two kinds. The first kind includes those, which are generally associated with the two types of signals (say in over 80% of the cases studied) , but not always. In the terminology of seismological source discrimination these are called "diagnostic aids". The other types, which are always found to differ in the two cases are called "positive identifies".
Diagnostic Aids
1. Source Depth
Earthquakes originate in faults which are buried at depths in the interior of the earth, several kilometers to several tens of kilometers away from the surface. If the source is found to be at several kilometers depth the source must be explosive. All those signals for which the source depth can be estimated accurately, and is found over, say, 10 kilometers must be from an earthquake source. In that case source depth becomes a positive identifier. If the depth becomes uncertain, the source could be either an earthquake or an explosion. Over 80% of the earthquake signals can be identified on this basis.
2. Source Location
By now areas of earthquake occurrences and potential explosion sites are known reasonably well. If the signal source is located with confidence (i.e. by using data from many seismological observatories, and small standard errors), an earthquake source could be identified on the basis of the source location (this includes depth also). If the source cannot be identified as an earthquake on the basis of source location it could be either an earthquake or an explosion source.
3. Complexity of Seismic signals
Earthquake sources ale much larger in size, and give rise to ground motions of longer periods, which travel to deeper depths and far distances. These signals get reflected and refracted at different depths, and the reflected and refracted signals continue to arrive at a seismograph station for much longer time. Superposition of the signals arriving from different paths and at different times make the record of the signal very complex. While this is generally true for shallow earthquakes and moderately sized explosion sources placed in a region of simple geology, seismic signals from sources at great depths, say 100 km or more, could be very simple, and signals from large nuclear explosions (sometimes several of these fired in succession) could be very complex. Thus complexity of the seismic signal can be used as a diagnostic aid in conjunction with the depth and location of the source.
4. Frequency content in the seismic signal
Explosion sources are comparatively smaller in size, and are located closer to the surface. The energy release from this sources is concentrated in a higher frequency range (higher than a few cycles per second) compared to the earthquake signal (where the vibrations are close to 1 Hz or lower). This comparison is, however, valid for records from the same region (or at least at the same distance and source size, i.e. magnitude).
Positive Identifiers
On the basis of the diagnostic aides most of the seismic signals (over 80%) can be identified as earthquakes. For identifying the remaining ones, more positive identifiers are required. It has been found that for the same body wave magnitude (magnitude estimated from the P-wave, which is called mb) the earthquake source produces stronger surface waves, and a correspondingly higher surface wave magnitude (Ms). A graph of the mb (plotted along the x-axis) and Ms (plotted along the y-axis) enables to discriminate between the two types of sources. The explosion signals show a smaller Ms/mb ratio compared to those from the earthquake sources. If the two types of magnitudes are estimated accurately, the ratio will be well separated from the boundary to enable classification of the signal into one or the other type.
This page was updated on 10-01-11