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Use of nuclear photoemulsions to investigate the spitak earthquake aftershocks
Nucl. Tracks Radiat. Meas.,
Vol. 21, No. 3, pp. 317-321, 1993
0969-8078/93$6.00+ .00 © 1993PergamonPress Ltd
Printedin Great Britain
USE OF NUCLEAR PHOTOEMULSIONS TO INVESTIGATE THE SPITAK EARTHQUAKE AFTERSHOCKS A. B. AKOPOVA,*A. A. Mom~'~o,* K. I. ~JMAI,~tAN,*S. V. GmGORYAN,tS. G. KAZARYANtand KH. O. SARGS'tA~rt *Yerevan Physics Institute, Alikhanyan Brothers Str. 2, 375036, Yerevan, Armenia; and Tlnstitute of Geological Sciences of the Academy of Science of Armenia, Marshal Bagramyan Str. 24a, 375019, Yerevan, Armenia Abstract--The radon and thoron emanation in soil gases in the epicentral zone of the Spitak earthquake of 7 December 1988 was estimated by the use of nuclear photoemulsions. For measurements of the emanation flow, the stationary observation points of the Laboratory of Seismogeochemistry of the Institute of Geological Scienceswere chosen. The maximum flow was detected in the observation point near the village of Saralandzh. Here, the obtained value exceeded by 10.1 times the average level in the Spitak region (the observation point in the town of Spitak). The flow in the north-west wing of the Spitak break was considerably less and exceeded the average value by only 2.1 and 1.5 times (Gegasar and Nalband villages).A comparative analysis of our data with those obtained in the same observation points by means of the atmogeochemicalmethod shows rather good agreement between the experimental results. The joint application of these two methods allows representation of the geodynamics of the region and prediction of earthquakes.
1. INTRODUCTION
2. EXPERIMENTAL SECTION
FoR THE LAST few years we have been carrying on work in the field with the use of nuclear photoemulsions (NPE) to solve the applied problems of autoradiography. In this work an attempt is made to use NPE for the direct registration of thoron, radon and their daughter products of decay (locating the emulsion layers directly in the soil gas) in order to reveal and trace the dynamics of the fault structures in the seismoactive region.
The layers of NPE were exposed in the soil of the epicentral region of the earthquake of 1988 in Spitak. The experiment was carried out in stationary observation points of the flow of subsoil gases by the Laboratory of Seismogeochemistry of the Institute of Geological Sciences of the Academy of Science of Armenia in the villages Saralandzh, Gegasar, Nalband and in the city of Spitak.
120,.. 100 -
so T, '~ 40 20
0 6
78
150
222
264 Time (hrs)
3(~6
438
510
FIG. 1. The concentrations of Rn ( x ) and Tn (D) in the subsoil gas and their summable content in the atmosphere ( - ) for the observation point in Saralandzh in the period from 29 July to 19 August 1989. The abrupt drop in activity in the interval between 222 and 294 h coincides with the registered earthquake shock of about 2 points (k -- 8). 317
318
A . B . AKOPOVA et al.
rJ
-"
x.A
_
-,t,
,
,%,,,"
FIG. 2. The atmogeochemicalmap of the Spitak earthquake zone from June to September 1989. The lines 1, 2 and 3 coincide with the Spitak, Saralandzh and Alavar sections of the seismogenic fault.
During the field-season of 1989 the members of the Laboratory were determining the element contents of the subsoil gases by the atmogeochemical method (Karapetyan, 1989) particularly of thoron (Tn) and radon (Rn). More detailed measurements were carried out in Saralandzh, where an intensive degassing of Tn and Rn was observed. The values of the measured concentrations of these gases, at intervals of 4 h, from 29 July to 19 August 1989, are given in Fig. 1. The abrupt fall of the Tn and Rn contents, which is clearly visible in the given graph, coincides with two shocks, each of about 2 points (k = 8), which were registered in the morning of 11 August 1989. The atmogeochemical investigations in the zone of the earthquake in Spitak have confirmed on the whole the existing geological faults and discovered
3
i N
2
I,,J)',ll'lllllll Flo. 3. The layout of the passive diffusion dosimeter: (1) nuclear photoemulsion; (2) dielectrictrack detector LR-115; (3) metallic cap; and (4) soil.
new ones. The geochemical map of the mentioned region from July to September 1989 is given in Fig. 2. The numbers put on the isolines correspond to the gas average total values of concentrations of Tn and Rn, in emans, in the subsoil gas and atmosphere. It should be mentioned that the Tn and Rn concentrations on the surface of the Earth are almost equal but as a result of the short half-life of Tn, its concentration drops with altitude (Shimon, 1970). As is obvious from the map, a result of the earthquake was that the Spitak fault was formed with a length of 37 km (Fig. 2, marks 1-3). According to the data of the work carried out in the region, three sections of the fault with different geological characteristics are observed: (1) the Spitak section with a length of 8 km having clearly distinguished seismogene structure (throw-shift with an amplitude of ~ 2 m ) ; (2) the Saralandzh section with a length of 13 km, more passive, on separate parts of which the sections of the fault are visible; and (3) the Aiavar section which is similar to the Saralandzh section in its characteristics. In the region of the Saralandzh section the earthquake did not give rise to great geological dislocations which created a natural reservoir for the accumulation of subsoil gas whose anomalous contents were measured by the atmogeochemical method. The existence of emanation was also confirmed by the registration of the alpha-particles of the decay of the radioactive isotopes of the thorium family and uranium-radium family in the track detector NPE. The NPE layers, dimensions 2 x 2 cm 2, were placed in the stationary observation points in the ground inside an inverted cylindrical cap with a diameter of 8.5cm and height of 9cm. The passive diffusion
INVESTIGATION O F SPITAK E A R T H Q U A K E A F T E R S H O C K S
319
(a)
Fx~. 4. (a) Microphotography of the 2-ray star corresponding to alpha-particlesfrom RaA and RaC decay. (b) Microphotography of the track of a long-range alpha-particle from ThC' decay.
dosimeter (PDD) of this design, with a volume of 0.51 is shown in Fig. 3. The pressure of subsoil gas inside the cap of the P D D was equal to the atmospheric pressure. The measured value of the temperature (t) and the relative humidity (/I) of the gas in the capacity of the P D D before and after exposure were: t = 20°C and h = 40%.
3. DEVELOPMENT AND SURVEY O F DETECTORS After exposure the layers of NPE (thickness 200/~m, type BYa) were subjected to photographic development. The scanning and counting of the number of tracks was carried out under the optical
320
A . B . AKOPOVA et al.
microscope with a magnification of x 300. Measurements of the length of tracks in NPE were made using a × 1350 magnification. Comparison of the characteristics of alpha-tracks in the layers of NPE exposed in the dosimeters and irradiated by calibrated 239pu and 226Ra sources revealed insignificant regression of tracks which were registered during 7 days of exposure in the soil. It was established during the measurements that on the surface of directly adjoining ground the density of registered tracks in all detectors was comparable with the background in the control layers of NPE kept in the refrigerator at t = 5°C. For all detectors the density of alpha-tracks on the surface contiguous with the bulk of the dosimeter exceeded the same quantity in the control layers by a factor of 1.5, 2.3, 3.2, and 15.6 for the stationary points in Spitak, Nalband, Gegasar and Saralandzh, respectively. 4. RESULTS The experiment confirmed the registration of the Tn and Rn emanations which were diffusing from the soil into the air cap of the PDD. Registered in the NPE were alpha-particles from the decay of Tn and Rn and so-called "radioactive fallout" of solid radionuclides with consecutive short life daughter products of thorium and radium: ThA, ThC, ThC', RaA, RaC, RaC'. In the microphotography of Fig. 4, events are reproduced which were discovered in the emulsions. The presence of 2-ray stars (see Fig. 4(a)) formed by alpha-particles with energy E = 5.998 and 7.68 MeV, calculated according to the registered residual ranges, testifies to the decay of the atoms of RaA and RaC' which proves the presence of radon in the subsoil gas (Yagoda, 1949). In the microphotography in Fig. 4(b)
20
Tn RaC' ThA ThC'
-
the track of a long range alpha-particle is shown which has entered the layer with E = 8.78 MeV and is the result of the decay of ThC' that indicates the existence of Tn in the subsoil gas (Yagoda, 1949). For each stationary observation point, alpha-particle energy distributions were obtained. For the point in Saralandzh the distribution is given in Fig. 5. In the energy region of more than 6 MeV, we can see the contribution from alpha-particles of the decay o f Tn deposited on the emulsion surface and the radioactive fallout of ThA, ThC' and RaC' which are the result of the consecutive decay of Tn and Rn. The region E < 6 MeV corresponds to the registration of the alpha-particles that are emitted during the decay of: (1) Rn deposited on the surface of the emulsion from the volume of the PDD; (2) the radioactive fallout ThC, RaA and RaC; and (3) Tn and Rn diffusing in the volume of the PDD. The radionuclides ThC, Rn, RaA and RaC have overlapping energies of alphaparticles from decay: 5.44 < E < 6.09. The alphaparticles emitted during Tn and Rn diffusion in the PDD lose their energy in the interactions with the gas accumulated in the PDD and, therefore, they have energy from 0 to 6 MeV. Therefore, it is difficult to distinguish a certain line in the region E < 6 MeV. 5. SUMMARY The atmogeochemical observations in the area of the earthquake in Spitak show that the anomalous high content of Tn and Rn in the subsoil gas during the June-August period of 1989 was not the same in different parts of the zone. In the observation points near the villages Saralandzh, Gegasar and Nalband the values of the concentrations of Tn and Rn were 4.0, 2.5 and 2.4 times higher than the average level received in Spitak. The measurements carried out in the period from 9 to 16 September 1989, using NPE at the same observation points, have mainly confirmed the data received earlier. So for the same sequence of points an increase in the concentrations of Tn and Rn to 10.1, 2.1 and 1.5 times the average level was obtained. 6. CONCLUSIONS
10
z
3
4
5
6
7
8
9
The experiment showed the possibility of using NPE for the registration of the emanation in the structure of the passive diffusion dosimeter to measure the radioactive gas in soil, building materials, enclosed structures (homes, offices and the like). Comparison of the data received from the measurements of Tn and Rn by the two methods (atmogeochemical and NPE) shows good agreement of the experimental results.
E (MeV)
FXG. 5. The energy distribution of alpha-particles (471 per 10 mm2 of area) registered in photoemulsion at the observation point in Saralandzh.
REFERENCES Karapetyan A. I. (ed.) (1989) A Use of the Geological and Geophysical Methods for the Stud)' of Spitak Earth-
INVESTIGATION OF SPITAK E A R T H Q U A K E AFTERSHOCKS
quake Zones. Report of the Academy of Science of Armenia, Institute of Geological Science Publishing, Yerevan. Shimon A. (1970) The measurement of the natural radioac-
I ~ 21/3---II
321
tivity of the atmosphere in Budapest. Proceedings of the Institute of Experimental Meteorology 17, 33. Yagoda G. (1949) Radioactive Measurements with Nuclear Emulsions. New York.
Vol. 21, No. 3, pp. 317-321, 1993
0969-8078/93$6.00+ .00 © 1993PergamonPress Ltd
Printedin Great Britain
USE OF NUCLEAR PHOTOEMULSIONS TO INVESTIGATE THE SPITAK EARTHQUAKE AFTERSHOCKS A. B. AKOPOVA,*A. A. Mom~'~o,* K. I. ~JMAI,~tAN,*S. V. GmGORYAN,tS. G. KAZARYANtand KH. O. SARGS'tA~rt *Yerevan Physics Institute, Alikhanyan Brothers Str. 2, 375036, Yerevan, Armenia; and Tlnstitute of Geological Sciences of the Academy of Science of Armenia, Marshal Bagramyan Str. 24a, 375019, Yerevan, Armenia Abstract--The radon and thoron emanation in soil gases in the epicentral zone of the Spitak earthquake of 7 December 1988 was estimated by the use of nuclear photoemulsions. For measurements of the emanation flow, the stationary observation points of the Laboratory of Seismogeochemistry of the Institute of Geological Scienceswere chosen. The maximum flow was detected in the observation point near the village of Saralandzh. Here, the obtained value exceeded by 10.1 times the average level in the Spitak region (the observation point in the town of Spitak). The flow in the north-west wing of the Spitak break was considerably less and exceeded the average value by only 2.1 and 1.5 times (Gegasar and Nalband villages).A comparative analysis of our data with those obtained in the same observation points by means of the atmogeochemicalmethod shows rather good agreement between the experimental results. The joint application of these two methods allows representation of the geodynamics of the region and prediction of earthquakes.
1. INTRODUCTION
2. EXPERIMENTAL SECTION
FoR THE LAST few years we have been carrying on work in the field with the use of nuclear photoemulsions (NPE) to solve the applied problems of autoradiography. In this work an attempt is made to use NPE for the direct registration of thoron, radon and their daughter products of decay (locating the emulsion layers directly in the soil gas) in order to reveal and trace the dynamics of the fault structures in the seismoactive region.
The layers of NPE were exposed in the soil of the epicentral region of the earthquake of 1988 in Spitak. The experiment was carried out in stationary observation points of the flow of subsoil gases by the Laboratory of Seismogeochemistry of the Institute of Geological Sciences of the Academy of Science of Armenia in the villages Saralandzh, Gegasar, Nalband and in the city of Spitak.
120,.. 100 -
so T, '~ 40 20
0 6
78
150
222
264 Time (hrs)
3(~6
438
510
FIG. 1. The concentrations of Rn ( x ) and Tn (D) in the subsoil gas and their summable content in the atmosphere ( - ) for the observation point in Saralandzh in the period from 29 July to 19 August 1989. The abrupt drop in activity in the interval between 222 and 294 h coincides with the registered earthquake shock of about 2 points (k -- 8). 317
318
A . B . AKOPOVA et al.
rJ
-"
x.A
_
-,t,
,
,%,,,"
FIG. 2. The atmogeochemicalmap of the Spitak earthquake zone from June to September 1989. The lines 1, 2 and 3 coincide with the Spitak, Saralandzh and Alavar sections of the seismogenic fault.
During the field-season of 1989 the members of the Laboratory were determining the element contents of the subsoil gases by the atmogeochemical method (Karapetyan, 1989) particularly of thoron (Tn) and radon (Rn). More detailed measurements were carried out in Saralandzh, where an intensive degassing of Tn and Rn was observed. The values of the measured concentrations of these gases, at intervals of 4 h, from 29 July to 19 August 1989, are given in Fig. 1. The abrupt fall of the Tn and Rn contents, which is clearly visible in the given graph, coincides with two shocks, each of about 2 points (k = 8), which were registered in the morning of 11 August 1989. The atmogeochemical investigations in the zone of the earthquake in Spitak have confirmed on the whole the existing geological faults and discovered
3
i N
2
I,,J)',ll'lllllll Flo. 3. The layout of the passive diffusion dosimeter: (1) nuclear photoemulsion; (2) dielectrictrack detector LR-115; (3) metallic cap; and (4) soil.
new ones. The geochemical map of the mentioned region from July to September 1989 is given in Fig. 2. The numbers put on the isolines correspond to the gas average total values of concentrations of Tn and Rn, in emans, in the subsoil gas and atmosphere. It should be mentioned that the Tn and Rn concentrations on the surface of the Earth are almost equal but as a result of the short half-life of Tn, its concentration drops with altitude (Shimon, 1970). As is obvious from the map, a result of the earthquake was that the Spitak fault was formed with a length of 37 km (Fig. 2, marks 1-3). According to the data of the work carried out in the region, three sections of the fault with different geological characteristics are observed: (1) the Spitak section with a length of 8 km having clearly distinguished seismogene structure (throw-shift with an amplitude of ~ 2 m ) ; (2) the Saralandzh section with a length of 13 km, more passive, on separate parts of which the sections of the fault are visible; and (3) the Aiavar section which is similar to the Saralandzh section in its characteristics. In the region of the Saralandzh section the earthquake did not give rise to great geological dislocations which created a natural reservoir for the accumulation of subsoil gas whose anomalous contents were measured by the atmogeochemical method. The existence of emanation was also confirmed by the registration of the alpha-particles of the decay of the radioactive isotopes of the thorium family and uranium-radium family in the track detector NPE. The NPE layers, dimensions 2 x 2 cm 2, were placed in the stationary observation points in the ground inside an inverted cylindrical cap with a diameter of 8.5cm and height of 9cm. The passive diffusion
INVESTIGATION O F SPITAK E A R T H Q U A K E A F T E R S H O C K S
319
(a)
Fx~. 4. (a) Microphotography of the 2-ray star corresponding to alpha-particlesfrom RaA and RaC decay. (b) Microphotography of the track of a long-range alpha-particle from ThC' decay.
dosimeter (PDD) of this design, with a volume of 0.51 is shown in Fig. 3. The pressure of subsoil gas inside the cap of the P D D was equal to the atmospheric pressure. The measured value of the temperature (t) and the relative humidity (/I) of the gas in the capacity of the P D D before and after exposure were: t = 20°C and h = 40%.
3. DEVELOPMENT AND SURVEY O F DETECTORS After exposure the layers of NPE (thickness 200/~m, type BYa) were subjected to photographic development. The scanning and counting of the number of tracks was carried out under the optical
320
A . B . AKOPOVA et al.
microscope with a magnification of x 300. Measurements of the length of tracks in NPE were made using a × 1350 magnification. Comparison of the characteristics of alpha-tracks in the layers of NPE exposed in the dosimeters and irradiated by calibrated 239pu and 226Ra sources revealed insignificant regression of tracks which were registered during 7 days of exposure in the soil. It was established during the measurements that on the surface of directly adjoining ground the density of registered tracks in all detectors was comparable with the background in the control layers of NPE kept in the refrigerator at t = 5°C. For all detectors the density of alpha-tracks on the surface contiguous with the bulk of the dosimeter exceeded the same quantity in the control layers by a factor of 1.5, 2.3, 3.2, and 15.6 for the stationary points in Spitak, Nalband, Gegasar and Saralandzh, respectively. 4. RESULTS The experiment confirmed the registration of the Tn and Rn emanations which were diffusing from the soil into the air cap of the PDD. Registered in the NPE were alpha-particles from the decay of Tn and Rn and so-called "radioactive fallout" of solid radionuclides with consecutive short life daughter products of thorium and radium: ThA, ThC, ThC', RaA, RaC, RaC'. In the microphotography of Fig. 4, events are reproduced which were discovered in the emulsions. The presence of 2-ray stars (see Fig. 4(a)) formed by alpha-particles with energy E = 5.998 and 7.68 MeV, calculated according to the registered residual ranges, testifies to the decay of the atoms of RaA and RaC' which proves the presence of radon in the subsoil gas (Yagoda, 1949). In the microphotography in Fig. 4(b)
20
Tn RaC' ThA ThC'
-
the track of a long range alpha-particle is shown which has entered the layer with E = 8.78 MeV and is the result of the decay of ThC' that indicates the existence of Tn in the subsoil gas (Yagoda, 1949). For each stationary observation point, alpha-particle energy distributions were obtained. For the point in Saralandzh the distribution is given in Fig. 5. In the energy region of more than 6 MeV, we can see the contribution from alpha-particles of the decay o f Tn deposited on the emulsion surface and the radioactive fallout of ThA, ThC' and RaC' which are the result of the consecutive decay of Tn and Rn. The region E < 6 MeV corresponds to the registration of the alpha-particles that are emitted during the decay of: (1) Rn deposited on the surface of the emulsion from the volume of the PDD; (2) the radioactive fallout ThC, RaA and RaC; and (3) Tn and Rn diffusing in the volume of the PDD. The radionuclides ThC, Rn, RaA and RaC have overlapping energies of alphaparticles from decay: 5.44 < E < 6.09. The alphaparticles emitted during Tn and Rn diffusion in the PDD lose their energy in the interactions with the gas accumulated in the PDD and, therefore, they have energy from 0 to 6 MeV. Therefore, it is difficult to distinguish a certain line in the region E < 6 MeV. 5. SUMMARY The atmogeochemical observations in the area of the earthquake in Spitak show that the anomalous high content of Tn and Rn in the subsoil gas during the June-August period of 1989 was not the same in different parts of the zone. In the observation points near the villages Saralandzh, Gegasar and Nalband the values of the concentrations of Tn and Rn were 4.0, 2.5 and 2.4 times higher than the average level received in Spitak. The measurements carried out in the period from 9 to 16 September 1989, using NPE at the same observation points, have mainly confirmed the data received earlier. So for the same sequence of points an increase in the concentrations of Tn and Rn to 10.1, 2.1 and 1.5 times the average level was obtained. 6. CONCLUSIONS
10
z
3
4
5
6
7
8
9
The experiment showed the possibility of using NPE for the registration of the emanation in the structure of the passive diffusion dosimeter to measure the radioactive gas in soil, building materials, enclosed structures (homes, offices and the like). Comparison of the data received from the measurements of Tn and Rn by the two methods (atmogeochemical and NPE) shows good agreement of the experimental results.
E (MeV)
FXG. 5. The energy distribution of alpha-particles (471 per 10 mm2 of area) registered in photoemulsion at the observation point in Saralandzh.
REFERENCES Karapetyan A. I. (ed.) (1989) A Use of the Geological and Geophysical Methods for the Stud)' of Spitak Earth-
INVESTIGATION OF SPITAK E A R T H Q U A K E AFTERSHOCKS
quake Zones. Report of the Academy of Science of Armenia, Institute of Geological Science Publishing, Yerevan. Shimon A. (1970) The measurement of the natural radioac-
I ~ 21/3---II
321
tivity of the atmosphere in Budapest. Proceedings of the Institute of Experimental Meteorology 17, 33. Yagoda G. (1949) Radioactive Measurements with Nuclear Emulsions. New York.