Figure 1– Epicenters distribution for nuclear explosions and earthquakes for which focal mechanisms solutions were obtained, and its stereograms

A Supplement to the Comprehensive Test-Ban Treaty recommends using, among other parameters, first motion

of P-wave and focal mechanism for standard filtration of phenomena. The basis of this criteria application for seismic

events nature discrimination is in known character of compression and waves distribution from different

sources. In most cases compression waves and waves of earthquakes have quadrant distribution, force

source is double couple force. Ordinary rock failure is a source of waves, in this case negative displacement

are recorded in first motions on seismograms in all azimuths from a source. Explosion is a source of all-round expansion,

positive displacement should be observed in first motions of longitudinal waves in all directions from an epicenter.

However, our practical observations showed that it is not always possible to discriminate an explosion using these

criteria. In some cases on the seismograms of the same explosion obtained in different azimuths from an epicenter both

positive and negative displacement were observed in first motions. For some explosions focal mechanisms were

determined using standard technique that uses double dipole as a force source. Parameters of focal mechanisms

received for explosions, as rule, were typical for earthquakes focal mechanisms of the region. The present work is

aimed at revealing explosions which can be considered as exclusions according to their first motions in longitudinal

waves and focal mechanism parameters. We have focused on different explosions types: nuclear, calibration, industrial.

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Nuclear explosions

information about focal mechanisms for 19 nuclear explosions conducted at the Test Sites of Caspian depression for

period 1968-1983. The explosions focal mechanisms were constructed using standard technique that uses double-couple

as a force model of a source. Figure 1 shows the nuclear explosions epicenters map and stereograms for which focal

mechanisms solutions were obtained.

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In all cases the focal mechanisms are reverse and oblique reverse faults of east-west or north-west direction dipping

angularly 45 degrees that contradicts the expected mechanisms of explosions being the sources of all-round expansion.

It was interesting to compare the parameters of obtained solutions of nuclear explosions and focal mechanisms of

earthquakes for the region. As the region has low seismicity, focal mechanisms were determined for three earthquakes only.

All three solutions are similar being normal faults or oblique-slip faults realized under conditions of horizontal extension.

The results of the comparison showed that the dislocations character and orientation of the main stress axes in earthquake

sources at near Caspian region differ significantly from the same parameters obtained for nuclear explosions.

The earthquakes focal mechanisms catalogue of Mostryukov A.O., Petrov V.A. (Moscow, 1994) contains

Calibration explosions

In 1998 - 2000 calibration experiments Omega were conducted at Degelen site of the Semipalatinsk Test Site (STS).

The charge mass in all cases was 100 tons, explosions Omega-2 and Omega-3 were conducted in the same tunnel.

The explosions were recorded at the distance range 0 – 1000 km: Omega-1 was recorded by 43 stations, Omega-2 -

by 29 stations, Omega-3 - by 20 stations. The seismograms of the stations located at different azimuths from the

explosion epicenters show compression waves (+) and waves (-) in first motion of P-wave in all three cases,

although, in theory, explosions should have signs (+) in all azimuths. Figure 2 shows an example of seismogram that

clearly shows a sign (-) in first motions.

Distribution of positive and negative signs of first motions of P-wave for all three explosions was close to quadrant

that allowed to construct focal mechanisms using standard technique. As the explosions Omega-2 and Omega-3

conducted in the same tunnel had the same signs at the same stations, the focal mechanism was constructed using

grouping method.

Figure 3 shows the results of focal mechanism determination for Omega -1 explosion and generalized focal

mechanism for the explosions Omega-2 and Omega-3. The parameters of focal mechanisms of these explosions:

orientation of nodal planes and main stress axes are the same, differences in strike azimuth values did not exceed the

construction accuracy.

The focal mechanism is a combination of steep shears of north-west and north-east direction with minor

components on the plane dip that as seen from the Figure 4a, coincides with dislocations in earthquake source

occurring at the region, and corresponds to strike and kinematics of the main fault system of the region. As for main

stresses in earthquake sources, its direction differs significantly from those obtained from explosion (Figure 4b).

It is possible to suppose that explosions influence conducted in tunnels located in a fault body cause local redistribution

of stresses released as a result of shear along the fault plane according to its extension and kinematics. Body waves

caused by these shear, apparently, were recorded in first motions at some observation stations.

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Figure 2 – The record of first of P-wave. Omega -1 explosion recorded by ZRN station and temporary station S7

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Figure 3 – Stereograms of explosions focal mechanisms. Omega-1 (left), Omega-2 and Omega -3 (right)

Figure 4 – Stereograms of focal mechanisms (a), and direction of compression stress axes (b) on a faults scheme of the STS

- ; Stereograms of earthquakes

- Stereograms of explosions.

Direction of compression stress axes- ; in earthquake sources

- in explosions sources

About 5000 explosions are conducted on the territory of Kazakhstan and recorded by the IGR NNC RK seismic

stations network (Figure 5) each year. As rule, first motions of P-waves in all azimuths from explosions epicenters have

(+) signs that corresponds to theoretically expected compression waves in all directions.

However, there were several cases, when first motions of P-waves, in different azimuths from explosion epicenters, had

compression waves (+) and wave (-). These “abnormal explosions” make up about 0,2% of total amount of

explosions recorded per year. In 2008 – 2010, 25 “abnormal explosions” within energy range K=6.4 – 8.0 were recorded;

its epicenters are located at quarries of Chu-Ili mountains, Karatayu, Dzhungariya.

Distribution of signs allowed to construct focal mechanisms of “abnormal explosions” using standard technique, note

that the best focal mechanisms solution corresponded to the sources depth (equivalent double couple) within 0 -10 km.

Figure 6a shows stereograms of quarry blasts focal mechanisms on the epicenters map and comparison with earthquakes

focal mechanisms in corresponding structures (Figure 6b).

Focal mechanisms of “abnormal explosions” and earthquakes are oblique reverse faults along the planes of north-west or

north-east extension corresponding to direction of main fault system direction: Karatayu and Northern Tien Shan.

Orientation of compression stress axes by data of focal mechanisms in “abnormal explosions” sources (Figure 6)

and earthquakes with K=6-7 is not stable, however, in both cases compression stress close to meridian direction prevails.

Thus, the dislocations character obtained for “abnormal explosions” can be related to geological structure and strain

stress of corresponding regions. Probably, these abnormal cases can appear as a result of strain stress of geological

structures in which industrial blasts are conducted.

CONCLUSION

Different types of explosions (nuclear and chemical) conducted on the territory of Kazakhstan revealed deviation from

expected distribution of signs in first motions of seismic waves. Several stations record clearly waves.

Focal mechanisms can be constructed for different types of explosions using standard technique.

Focal mechanism type of explosions conducted at the regions of tectonic faults very often consistent with that for

earthquakes occurred at the same region.

Thus, it is sown that discrimination criteria between earthquakes and explosions basing on first motions of P-waves

and focal mechanism is not universal.

Industrial blasts

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Figure 5 – The map of explosions epicenters from July 1 to December 31, 2010. Number of events - 2651

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Figure 6 – Stereograms and direction of compression stress axes from industrial blasts (a) and earthquakes (b) on the epicenters map

стереограммы фокальных механизмов: - землетрясений;

- ядерных взрывов

EARTHQUAKES FOCAL MECHANISMS AS CRITERION FOR EXPLOSIONS DISCRIMINATION

(T2-P20)

Poleshko N.N., Mikhailova N.N. Institute of Geophysical Research, Kurchatov, Kazakhstan