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Some peculiarities of the secular variation of the geomagnetic field in the Carpathians
Physics of the Earth and Planetary Interiors, 77 (1993) 137—141 Elsevier Science Publishers B.V., Amsterdam
137
Some peculiarities of the secular variation of the geomagnetic field in the Carpathians J. Podsklan
v.
a Kuznetsova b and V. Maksimchuk b Observatory, 94701 Hurbanovo, Czech and Slovak Federal Republic Carpathians Branch of the Institute of Geophysics, Academy of Sciences of Ukraine, Lvov, Ukraine a Geomagnetic
b
(Received 28 December 1991; revision accepted 21 April 1992)
ABSTRACT Podsklan, I., Kuznetsova, V. and Maksimchuk, V., 1993. Some peculiarities of the secular variation of the geomagnetic field in the Carpathians. Phys. Earth Planet. Inter., 77: 137—141. The secular variation (SV) of the geomagnetic field is studied in the Western Carpathians and Eastern (Transcarpathian) Carpathians. Possible influences of other geophysical fields on the SV behaviour have been studied. The analysis is based on geomagnetic data from the interval 1979—1984. Comparisons are made with gravitational field anomalies and with recent vertical movements of the Earth’s crust on Deep Seismic Sounding (DSS) profiles No. II. and VI. Connections between recent tectonic processes and the morphology of the SV field are evident in the region studied.
1. Introduction A systematic investigation has been made in the Western Carpathians (Slovakia) and the Eastem (Transcarpathian) Carpathians, not only of the space distribution of the geomagnetic field, but also of its secular variation (SV). Taking into account the fact that observations were all made in the same tectonic zone of the Inner Carpathians, it is interesting to compare the resulting data and their analysis with other geological and geodynamic parameters. Such an analysis is particularly meaningful when it is made along the international DSS profiles, crossing the basic structures of the region and giving a picture of the Earth’s deep crustal structure. The positions of the profiles are shown in Fig. 1. Observations in the Slovakian and Transcarpathian regions were made independently by Slovak and Ukrainian specialists, using approximately the
Correspondence to: J. Podsklan, Geomagnetic Observatory, 94701 Hurbanovo, Czech and Slovak Federal Republic. 0031-9201/93/$06.00 © 1993
—
same observational methods. Very stable proton magnetometers with sensitivities of 0.1 nT were used. The same time interval was used for the comparisons, the years 1979—1984.
_________________________ EAST-EUROPEAN PLATFORM
50 100 150 200 250KM
I
__________
)
I
ç
..~
\...~
KIEV
+
\ •LVOV
...~
~
.
.~ ~.:.
OVO GREA
~“v
\1.
o
~t~_ ..:-_
~
~\__~~
\5:144
/
~/
,—.~
/
4 0
0
.. —“0
~v.:::
Fig. 1. Locations of international DSS profiles No. II and VI.
Elsevier Science Publishers B.V. All rights reserved
138
J. PODSKLAN ET AL.
2. Results of the secular variation study in Sbvakia
points with secular variation data. This is approx2 and imately equivalent to between one pointtwo per points 400 kmof an average distance ap-
A network consisting of 128 stations has been established in Slovakia. The area surrounding each station was checked for homogeneity of the geomagnetic field. The instrument was checked regularly for long-term zero-point point stability by comparing it with the observatory standards at Niemegk and Hurbanovo observatories. When processing the data, observational points were excluded if the measurement was made during a disturbed period and were replaced by new measurements in a few cases. After preliminary processing of the data there remained more than 100
proximately 20 km. The first step in the data processing was the production of a map of the distribution of the secular variation of the total field, F (Fig. 2). The map was computed from the data using the standard procedure of inverse distance weighting. Even after smoothing we see some focus-like regions. They are expected to be caused by crustal composition or other crustal phenomena. One can see a similarity between Fig. 2 and the distribution of earthquakes in Slovakia published by Broucek (Broucek et a!., 1991, fig. 1).
~,j•~____•__((_~-‘
49~—
( 4
____
____
c~
~,
~
-
•___~/~
~ ____
)~iiE~!~) J11~~ / ~
17.0
17.5
IB.0 Geogr. longitude
18.5
19.0
Fig. 2. The secular variation anomalies map for the western part of Slovakia. The solid line in a NW—SE direction shows the position of DSS profile No. VI. The units used in labelling the contour lines are nT yeart.
139
SECULAR VARIATION OF THE GEOMAGNETIC FIELD IN THE CARPATHiANS
For this reason we compared the SV results with other typical features and geophysical fields. Initially we assumed the SV differences to be caused by different magnetizations in the region above the Curie isotherm. We produced profiles from the smoothed data and found four types of behaviour for the anomalous geomagnetic field some profiles) both these phenomena have the and the anomalous SV: (1) in some regions (or on same sign of change and no spatial shift (37%); (2) there exist regions where the changes have opposite signs and no spatial shift (16.2%); (3) as items (1) and (2) but with spatial shift; (4) no evident connection between the phenomena studied (34.1%). All these cases are illustrated in Figs. 3 (a)-(d). This leads to the conclusion, that at least part of the secular variation anomaly studied may be caused by crustal magnetization or is influenced strongly by it. (As we mentioned earlier, all observations at a given point were made at exactly the same point and so it is not possible to account for the effect by, for example, field gradients.) .
nlgl
NW I~
SE
1KJ~OM~k
.
60 20 30
Fig. 4. Cross-section of part of DSS profile VI (Beranek and Zatopek, of SV,of AF; gravitational field, Ag (Ibrmajer, 1981). 1981):change and velocity recent vertical movements, Av (Vanko, 1988). Surface geology: I, Carpathian foredeep; J, Western Flysch Carpathians; K, Vienna basin; L, Little Carpathians; M, Danubian lowlands; hatched area, sedimentary layers; crosses, faults; continuous isolines, velocity of seismic waves; line with hatching, Moho discontinuity.
II ~
________________
.. ~.
We tried other geophysical fields, for the whole .
b
a
___________________
____________________
nT
nT
-
k~
~
~.,• ~,.
6,,,
c
d
Fig. 3. Comparison of the anomalous geomagnetic field (solid line) and the anomalous secular variation of the geomagnetic field (dotted line): (a) inverse behaviour of both phenomena; (b) parallel; (c) spatial shift of phenomena; (d) no evident connection between phenomena. Horizontal scale is distance, vertical scale is amplitude (nT) (nT year ‘ respectively). Vertical scale is only schematic, not proportional to the real amplitude of the phenomena displayed.
territory. Seeking connections with notable gravialthough in some cases we can see a link between tational anomalies gave us no satisfactory answer, the two phenomena. To investigate the character of the SV data with respect to other geophysical information we chose Deep Seismic Sounding (DSS)-IP VI and we made a graphical comparison. Data on the SV change, i.~F,the gravitational field, i~g (Ibrmajer, 1981), the velocity of recent vertical movements, ~v (Vanko, 1988), and the crustal cross-section (Beranek and Zatopek, 1981) are shown in Fig. 4. This comparison shows the existence of connections between the parameters shown. The SV change along the profile correlates well with features of the crustal structure. The low seismic velocity block corresponds, 1), to a zone with of lowest velocity values (6.0 km s
140
J. PODSKLAN ET AL.
negative SV. The velocity drop at depths of 10—15 km reflects the decrease of crustal density at these depths, which, as a rule, is accompanied by an increase of the electrical conductivity. This may be the reason for the SV change minimum at this part of the profile. lt seems to be clear that higher amplitudes of the SV anomalies are connected with faults in the crust.
two points is in the range 3—5 km. Systematic studies of the SV have been made since 1976, together with other geophysical and geodynamic investigations. The main goal is to study the connection between SV anomalies, deep structure and regional geodynamics. The connection between SV features and the block structure of the crust has been established (Verbitsky et al., 1988). The observational methods were approximately the same as in the Slovakian region. The character of SV changes studied along the international DSS-II profile (Naukova Dumka, 1987), together with the gravitational field, ~g, and data on the velocity of recent crustal movements, ~v, are given in Fig. 5. The crust has a more complicated structure in the Transcarpa-
3. The results of the study in the Transcarpathians A network of 100 points has been established in the Transcarpathians. The distance between
SE
NW
____
20
~ ~
+
++
N
+
______
Vv~8V\v
V
____
~AE~A 80
80
—
1~
2~
—
3~
—
~
~:.
6~
—
—
—
7J1 ~
Fig. 5. Comparison of the geomagnetic field secular variation, AF; gravitational field, Ag; velocity of recent vertical movements, Av; and the deep structure of the crust along profile DSS-II. 1, young basement surface; 2, protofundament surface; 3, crust—mantle surface; 4, Moho discontinuity; 5, sedimentary layers; 6, Baikal epoch rock basement; 7, rocks of consolidated crust, with acid composition granitic complex (v = 6.2—6.4 km ~_1); 8, rocks of consolidated crust of basic composition basaltic complex; 9, effusive rocks; 10, rocks of crust mantle ‘mixture’ (v = 7.4—7.6 km ~_1); 11, asthenosphere surface as determined from 3). I, Prepanonian deep fault; II, Transcarpathian deep magnetotelluric data; 12, deep faults from DSS; 13, density values (g cm fault.
SECULAR VARIATION OF THE GEOMAGNETIC FIELD IN THE CARPATHIANS
thian foredeep than in the Danubian lowlands. There are volcanic structures within the sedimentary cover of the fold, but in the zone of conjuction of the Inner and Outer Carpathians, where the Transcarpathian deep fault passes, there is a large regional conductivity anomaly. This fault separates two blocks, which differ in both their composition and their thickness, the shift of the Moho being 25 km. The Transcarpathian deep fault is also the basic seismotectonic line of the region, and the epicentres of the strongest local earthquakes (Dragovo on 14 September 1937, intensity 7 degrees of the MSK scale; Dolgoye on 26 December 1872, intensity 7 degrees; Svalyava on 5 February 1908, intensity 7 degrees and other cases) occur along it. After repeat levelling, which was done during the time interval studied, a sinking of the surface in the near-fault zone has been established, which indicates the presence of extensional forces in this region. In the gravity field also, along the profile DSS-VI, we see a decrease in the direction of the Flysch Carpathians, which is connected with a thickening of the sedimentary cover and the crust in this direction. There is a minimum in the SV curve, complicated by local gradient zones, which is spatially connected to the above-described zone of the Transcarpathian fault and with the associated conductivity anomaly. It is evident that tectonic processes, presently active in this zone, and the associated structural features are reflected in the morphology of the SV field.
4. Conclusions The investigations which have been performed in the territories of Slovakia and the Transcarpathians show that features of the crustal structure and recent geodynamics are reflected in the mor-
141
phology of the anomalous SV field. This indicates the possibilities that the study of local SV anomalies has for the refinement of the deep structure of the lithosphere and the study of recent tectonic processes. The authors hope that improvements to the station network through further cooperation will enable the acquisition of new, interesting data, which are necessary for explaining the above-described anomalies in the secular variation of the geomagnetic field.
Acknowledgement The authors acknowledge the help of Dr. M. Bielik of the Geophysical Institute, Bratislava who assisted us with the gravity data for Slovakia.
References Beranek, B. and Zatopek, A., 1981. Earth’s crust structure in Czechoslovakia and in Central Europe by methods of explosion seismology. In: A. Zatopek (Editor-in-Chief), Geophysical Syntheses in Czechoslovakia. Veda, Slovakian Academy of Science, Bratislava, pp. 243—264. Broucek, I., Bune, V.1., Medvedeva, N.S., Polyakova, T.P. and Szeidovitz, Seismic activity of Geophys. the Western Carpathians G., and 1991. adjacent regions. Contrib. Inst. Slov. Acad. Sci., 21: 105—124 (in Russian with English abstract). Ibrmajer, J., 1981. Geological interpretation of gravity maps of Czechoslovakia. In: A. Zatopek (Editor-in-Chief), Geophysical Synntheses in Czechoslovakia. Veda, Slovakian Academy of Science, Bratislava, pp. 135—148. Litosfera Tsentralnoy i Vostochnoy Evropy, Geotraversy I, II, V, 1987. Naukova Dumka, Kijev, 167 pp. Vanko, J., 1988. Map of recent vertical movements of the Western Carpathians in Slovakia in the period 1952—1977. Geodet. Kartogr. Obzor, 34: 216—222. Verbitzky, T.Z., Kuznetsova, V.G. and Somov, V.1., 1988. The results of complex investigations on the Carpathian geodynamic polygon. J. Geodyn., 9: 177—186.
137
Some peculiarities of the secular variation of the geomagnetic field in the Carpathians J. Podsklan
v.
a Kuznetsova b and V. Maksimchuk b Observatory, 94701 Hurbanovo, Czech and Slovak Federal Republic Carpathians Branch of the Institute of Geophysics, Academy of Sciences of Ukraine, Lvov, Ukraine a Geomagnetic
b
(Received 28 December 1991; revision accepted 21 April 1992)
ABSTRACT Podsklan, I., Kuznetsova, V. and Maksimchuk, V., 1993. Some peculiarities of the secular variation of the geomagnetic field in the Carpathians. Phys. Earth Planet. Inter., 77: 137—141. The secular variation (SV) of the geomagnetic field is studied in the Western Carpathians and Eastern (Transcarpathian) Carpathians. Possible influences of other geophysical fields on the SV behaviour have been studied. The analysis is based on geomagnetic data from the interval 1979—1984. Comparisons are made with gravitational field anomalies and with recent vertical movements of the Earth’s crust on Deep Seismic Sounding (DSS) profiles No. II. and VI. Connections between recent tectonic processes and the morphology of the SV field are evident in the region studied.
1. Introduction A systematic investigation has been made in the Western Carpathians (Slovakia) and the Eastem (Transcarpathian) Carpathians, not only of the space distribution of the geomagnetic field, but also of its secular variation (SV). Taking into account the fact that observations were all made in the same tectonic zone of the Inner Carpathians, it is interesting to compare the resulting data and their analysis with other geological and geodynamic parameters. Such an analysis is particularly meaningful when it is made along the international DSS profiles, crossing the basic structures of the region and giving a picture of the Earth’s deep crustal structure. The positions of the profiles are shown in Fig. 1. Observations in the Slovakian and Transcarpathian regions were made independently by Slovak and Ukrainian specialists, using approximately the
Correspondence to: J. Podsklan, Geomagnetic Observatory, 94701 Hurbanovo, Czech and Slovak Federal Republic. 0031-9201/93/$06.00 © 1993
—
same observational methods. Very stable proton magnetometers with sensitivities of 0.1 nT were used. The same time interval was used for the comparisons, the years 1979—1984.
_________________________ EAST-EUROPEAN PLATFORM
50 100 150 200 250KM
I
__________
)
I
ç
..~
\...~
KIEV
+
\ •LVOV
...~
~
.
.~ ~.:.
OVO GREA
~“v
\1.
o
~t~_ ..:-_
~
~\__~~
\5:144
/
~/
,—.~
/
4 0
0
.. —“0
~v.:::
Fig. 1. Locations of international DSS profiles No. II and VI.
Elsevier Science Publishers B.V. All rights reserved
138
J. PODSKLAN ET AL.
2. Results of the secular variation study in Sbvakia
points with secular variation data. This is approx2 and imately equivalent to between one pointtwo per points 400 kmof an average distance ap-
A network consisting of 128 stations has been established in Slovakia. The area surrounding each station was checked for homogeneity of the geomagnetic field. The instrument was checked regularly for long-term zero-point point stability by comparing it with the observatory standards at Niemegk and Hurbanovo observatories. When processing the data, observational points were excluded if the measurement was made during a disturbed period and were replaced by new measurements in a few cases. After preliminary processing of the data there remained more than 100
proximately 20 km. The first step in the data processing was the production of a map of the distribution of the secular variation of the total field, F (Fig. 2). The map was computed from the data using the standard procedure of inverse distance weighting. Even after smoothing we see some focus-like regions. They are expected to be caused by crustal composition or other crustal phenomena. One can see a similarity between Fig. 2 and the distribution of earthquakes in Slovakia published by Broucek (Broucek et a!., 1991, fig. 1).
~,j•~____•__((_~-‘
49~—
( 4
____
____
c~
~,
~
-
•___~/~
~ ____
)~iiE~!~) J11~~ / ~
17.0
17.5
IB.0 Geogr. longitude
18.5
19.0
Fig. 2. The secular variation anomalies map for the western part of Slovakia. The solid line in a NW—SE direction shows the position of DSS profile No. VI. The units used in labelling the contour lines are nT yeart.
139
SECULAR VARIATION OF THE GEOMAGNETIC FIELD IN THE CARPATHiANS
For this reason we compared the SV results with other typical features and geophysical fields. Initially we assumed the SV differences to be caused by different magnetizations in the region above the Curie isotherm. We produced profiles from the smoothed data and found four types of behaviour for the anomalous geomagnetic field some profiles) both these phenomena have the and the anomalous SV: (1) in some regions (or on same sign of change and no spatial shift (37%); (2) there exist regions where the changes have opposite signs and no spatial shift (16.2%); (3) as items (1) and (2) but with spatial shift; (4) no evident connection between the phenomena studied (34.1%). All these cases are illustrated in Figs. 3 (a)-(d). This leads to the conclusion, that at least part of the secular variation anomaly studied may be caused by crustal magnetization or is influenced strongly by it. (As we mentioned earlier, all observations at a given point were made at exactly the same point and so it is not possible to account for the effect by, for example, field gradients.) .
nlgl
NW I~
SE
1KJ~OM~k
.
60 20 30
Fig. 4. Cross-section of part of DSS profile VI (Beranek and Zatopek, of SV,of AF; gravitational field, Ag (Ibrmajer, 1981). 1981):change and velocity recent vertical movements, Av (Vanko, 1988). Surface geology: I, Carpathian foredeep; J, Western Flysch Carpathians; K, Vienna basin; L, Little Carpathians; M, Danubian lowlands; hatched area, sedimentary layers; crosses, faults; continuous isolines, velocity of seismic waves; line with hatching, Moho discontinuity.
II ~
________________
.. ~.
We tried other geophysical fields, for the whole .
b
a
___________________
____________________
nT
nT
-
k~
~
~.,• ~,.
6,,,
c
d
Fig. 3. Comparison of the anomalous geomagnetic field (solid line) and the anomalous secular variation of the geomagnetic field (dotted line): (a) inverse behaviour of both phenomena; (b) parallel; (c) spatial shift of phenomena; (d) no evident connection between phenomena. Horizontal scale is distance, vertical scale is amplitude (nT) (nT year ‘ respectively). Vertical scale is only schematic, not proportional to the real amplitude of the phenomena displayed.
territory. Seeking connections with notable gravialthough in some cases we can see a link between tational anomalies gave us no satisfactory answer, the two phenomena. To investigate the character of the SV data with respect to other geophysical information we chose Deep Seismic Sounding (DSS)-IP VI and we made a graphical comparison. Data on the SV change, i.~F,the gravitational field, i~g (Ibrmajer, 1981), the velocity of recent vertical movements, ~v (Vanko, 1988), and the crustal cross-section (Beranek and Zatopek, 1981) are shown in Fig. 4. This comparison shows the existence of connections between the parameters shown. The SV change along the profile correlates well with features of the crustal structure. The low seismic velocity block corresponds, 1), to a zone with of lowest velocity values (6.0 km s
140
J. PODSKLAN ET AL.
negative SV. The velocity drop at depths of 10—15 km reflects the decrease of crustal density at these depths, which, as a rule, is accompanied by an increase of the electrical conductivity. This may be the reason for the SV change minimum at this part of the profile. lt seems to be clear that higher amplitudes of the SV anomalies are connected with faults in the crust.
two points is in the range 3—5 km. Systematic studies of the SV have been made since 1976, together with other geophysical and geodynamic investigations. The main goal is to study the connection between SV anomalies, deep structure and regional geodynamics. The connection between SV features and the block structure of the crust has been established (Verbitsky et al., 1988). The observational methods were approximately the same as in the Slovakian region. The character of SV changes studied along the international DSS-II profile (Naukova Dumka, 1987), together with the gravitational field, ~g, and data on the velocity of recent crustal movements, ~v, are given in Fig. 5. The crust has a more complicated structure in the Transcarpa-
3. The results of the study in the Transcarpathians A network of 100 points has been established in the Transcarpathians. The distance between
SE
NW
____
20
~ ~
+
++
N
+
______
Vv~8V\v
V
____
~AE~A 80
80
—
1~
2~
—
3~
—
~
~:.
6~
—
—
—
7J1 ~
Fig. 5. Comparison of the geomagnetic field secular variation, AF; gravitational field, Ag; velocity of recent vertical movements, Av; and the deep structure of the crust along profile DSS-II. 1, young basement surface; 2, protofundament surface; 3, crust—mantle surface; 4, Moho discontinuity; 5, sedimentary layers; 6, Baikal epoch rock basement; 7, rocks of consolidated crust, with acid composition granitic complex (v = 6.2—6.4 km ~_1); 8, rocks of consolidated crust of basic composition basaltic complex; 9, effusive rocks; 10, rocks of crust mantle ‘mixture’ (v = 7.4—7.6 km ~_1); 11, asthenosphere surface as determined from 3). I, Prepanonian deep fault; II, Transcarpathian deep magnetotelluric data; 12, deep faults from DSS; 13, density values (g cm fault.
SECULAR VARIATION OF THE GEOMAGNETIC FIELD IN THE CARPATHIANS
thian foredeep than in the Danubian lowlands. There are volcanic structures within the sedimentary cover of the fold, but in the zone of conjuction of the Inner and Outer Carpathians, where the Transcarpathian deep fault passes, there is a large regional conductivity anomaly. This fault separates two blocks, which differ in both their composition and their thickness, the shift of the Moho being 25 km. The Transcarpathian deep fault is also the basic seismotectonic line of the region, and the epicentres of the strongest local earthquakes (Dragovo on 14 September 1937, intensity 7 degrees of the MSK scale; Dolgoye on 26 December 1872, intensity 7 degrees; Svalyava on 5 February 1908, intensity 7 degrees and other cases) occur along it. After repeat levelling, which was done during the time interval studied, a sinking of the surface in the near-fault zone has been established, which indicates the presence of extensional forces in this region. In the gravity field also, along the profile DSS-VI, we see a decrease in the direction of the Flysch Carpathians, which is connected with a thickening of the sedimentary cover and the crust in this direction. There is a minimum in the SV curve, complicated by local gradient zones, which is spatially connected to the above-described zone of the Transcarpathian fault and with the associated conductivity anomaly. It is evident that tectonic processes, presently active in this zone, and the associated structural features are reflected in the morphology of the SV field.
4. Conclusions The investigations which have been performed in the territories of Slovakia and the Transcarpathians show that features of the crustal structure and recent geodynamics are reflected in the mor-
141
phology of the anomalous SV field. This indicates the possibilities that the study of local SV anomalies has for the refinement of the deep structure of the lithosphere and the study of recent tectonic processes. The authors hope that improvements to the station network through further cooperation will enable the acquisition of new, interesting data, which are necessary for explaining the above-described anomalies in the secular variation of the geomagnetic field.
Acknowledgement The authors acknowledge the help of Dr. M. Bielik of the Geophysical Institute, Bratislava who assisted us with the gravity data for Slovakia.
References Beranek, B. and Zatopek, A., 1981. Earth’s crust structure in Czechoslovakia and in Central Europe by methods of explosion seismology. In: A. Zatopek (Editor-in-Chief), Geophysical Syntheses in Czechoslovakia. Veda, Slovakian Academy of Science, Bratislava, pp. 243—264. Broucek, I., Bune, V.1., Medvedeva, N.S., Polyakova, T.P. and Szeidovitz, Seismic activity of Geophys. the Western Carpathians G., and 1991. adjacent regions. Contrib. Inst. Slov. Acad. Sci., 21: 105—124 (in Russian with English abstract). Ibrmajer, J., 1981. Geological interpretation of gravity maps of Czechoslovakia. In: A. Zatopek (Editor-in-Chief), Geophysical Synntheses in Czechoslovakia. Veda, Slovakian Academy of Science, Bratislava, pp. 135—148. Litosfera Tsentralnoy i Vostochnoy Evropy, Geotraversy I, II, V, 1987. Naukova Dumka, Kijev, 167 pp. Vanko, J., 1988. Map of recent vertical movements of the Western Carpathians in Slovakia in the period 1952—1977. Geodet. Kartogr. Obzor, 34: 216—222. Verbitzky, T.Z., Kuznetsova, V.G. and Somov, V.1., 1988. The results of complex investigations on the Carpathian geodynamic polygon. J. Geodyn., 9: 177—186.