Velocities of seismic waves propagating through the Bohemian massif from foci in Poland

VELOCITIES OF SEISMIC WAVES PROPAGATING THROUGH THE BOHEMIAN MASSIF FROM FOCI IN POLAND

JAROSLAVA PLOMEROV,~, VLADISLAV BABUSKA, LIBUSE RUPRECHTOVA.

Geophysical Inst i tute, CzechosL Acad. ScL, Prague*)

P e 3 m M e : B cmambe cmpostmca peeuona:ibnbte epe~tena npo6eea eozn I n , Pg, Sn u Sg npuxoOz-

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t[etucKoeo Maccuaa.

1. I N T R O D U C T I O N

This paper is a continuat ion of the study of the propagat ion of seismic waves through the Bohemian Massif from near focal regions. The purpose of the study is to construct regional travel-time curves of the waves that propagate into the Bohemian Massif from the north-east . These regional travel-time curves reflecting the differences in the geological structures of various regions will be used to make the interpretation and location of the foci of near earthquakes more accurate, and they may aid in solving the problems of the structure of the li thosphere in the region of the Bohemian Massif. In the conclusion to the paper, the apparent velocities of waves observed for the Polish loci are compared with the results of previous studies [1, 2], in which the regional travel-time curves for earthquakes with foci in the Friuli and Schwarzwald regions were derived.

2. DATA

At the north-eastern boundary of the Bohemian Massif, on the territory of Poland, there are three focal regions of near shocks. These shocks are associated with deep and surface mining activities [3] in the localities of Lubin (51'5°N 1605°E, ore mine), Upper Silesia (50.4°N 18-9°E, coal mines) and Belchatow (51-3°N 19-4°E, lignite open working). We investigated the events recorded between February 1979 and April 1981, i.e. 38 shocks from the area of Lubin, 18 shocks from Upper Silesia and 4 shocks from Belcl,~atow. The data sample also includes the most intensive shock from the Lubin area recorded on 24 March 1977 with magnitude m b -- 4'8 (ISC). On the whole, 60 earthquakes were studied f rom the focal regions of Lubin, Upper Silesia (the strongest shock with magnitude M z ~ 4"3 (WAR)) and Belchatow (the most intensive shock with M c == = 4-6), recorded by short-period vertical seismographs at the stations of Berggiesshiibel (BRG), Collm (CLL), Ka~persk6 Hory (KHC), Ksi~z (KSP), Moxa (MOX), Prfihonice (PRU) and the mobil station GF1) temporarily recording in the Orlick6 hory (Mts). F rom these three focal regions the seismic waves propagate into the Bohemian Massif along azimuths of 170 ° to 290 ° (Fig. 1).

As in the previous studies [I, 2, 4, 5], we investigated the 4 basic seismic phases, observed and routinely interpreted on seismograms as Pn, Pg, Sn and Sg waves, for the directions corresponding

*) Address: Bo~ni II, 141 31 Praha 4 - Spo~ilov.

56 Studia geoph, et geod. 28 [19841

Velocities o f Seismic Waves . . .

to the foci in Poland. We studied the waves again as far as epicentral distances D ~ 5 °. The shortest distance is between the G F O station and the Lubin focal region (D = 1'16°), which is located in the source-to-station azimuth A Z j = 171 ° (the station-to-source azimuth of G F O is A Z 2 = 352°). We did not take the observations of these shocks at the Ksi~z station into account,

...'7.<. • ." • : :,eLL V ~ . ~ - ~

. . o x o ~ / ~ r < ~ " ' / \ _ X

• i .

. , L : ~ : q... '. . : ." . : . . I.. :: _ : ~ 11o 14o 16 o la*

Fig. 1. Tectonics outline of the Bohemian Massif indicating the loci of seismic shocks and the seismic stations. 1 - sediments of the Alpine-Carpathian foredeep, 2 - Alpine-Carpathian flysch, 3 -- East Alps, 4 - Outer Carpathians, 5 -- Inner Carpathians, 6 - Platform sheet of South Germany, 7 - Polish Palaezoic Platform, 8 - Platform sediments of the Bohemian Massif, 9 -- Upper Silesian Basin, I 0 - Thuringian-Vogtland formation, 11 -- Moldanubicum, 12 - - Moravicum, Moravian part of the Moldanubicum, Brunnia, 13 -- Cesk]7 les and SmrEiny, 1 4 - Tepl~t-Barrandien region, 1 5 - Krugn6 hory (Mts) region, 1 6 - Saxon granulite range, 1 7 - West Sudeten region, 18-- East Sudeten region, 1 9 - Moravian-Silesian region, 2 0 -

Central Bohemian pluton, 21 -- neovolcanites.

because the epicentral distance is less than 1 °. The maximum distance is between MOX had the foci in the Belchatow area (D == 5 °, A Z 1 == 266 °, A Z 2 = 80°). Since mostly shocks of a lower intensity than that of the earthquakes from the Friuli and Schwarzwald areas are involved, not all the types of waves could be observed with equal reliability over the whole interval of epicentral distances.

3. R E G I O N A L TRAVEL-TIME CURVES FOR SHOCKS WITH FOCI IN P O L A N D

I n the c o n s t r u c t i o n o f t r ave l - t ime curves , the a c c u r a c y o f d e t e r m i n i n g the foca l

pa r ame te r s , i.e. the l o c a t i o n o f the focus and the o r ig in t ime o f the shock , h a v e to be

d e t e r m i n e d as a c c u r a t e l y as possible . I f we are dea l ing wi th na tu ra l shocks o f a l o w e r

intensi ty , this r e q u i r e m e n t c a n n o t a lways be fulf i l led. As we p r o v e d in [2] , the foca l

pa ra lne t e r s o b t a i n e d f r o m o b s e r v a t i o n s in loca l se i smic n e t w o r k s are m o r e a c c u r a t e

in l oca t i ng w e a k e r shocks t h a n those g iven by the se i smic cent res ( N E I S , C S E M )

[5] . T h e r e f o r e , in the case o f the Po l i sh shocks we w o r k e d wi th the foca l p a r a m e t e r s

d e t e r m i n e d f r o m the o b s e r v a t i o n s o f the loca l Po l i sh n e t w o r k o f se i smic s t a t ions

Studia geoph, et geod. 28 [1984] 57

.L Plomerov6, V. Babu#ka, L. Ruprechtov6

,,-×o \ " \', \t

~,~ \,, °;io ! ~-×o~ }\ \ ~

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1% ii

~ o

i l - n'~d , , o

7 - XON

o.~ I I I t

q - 77~ Ii,

7 " N~d '

I - ~ I~

o 0

o ~ ,6

" ~ ~ ~ •

m " ~ N =

- ~ ~

5 8 Stud ia geoph, et geod, 28 [1984]


(unpublished communication of Dr. Gibowicz, 1981), in which the location of the hypocentre was mostly compared with the direct effects of the shock in mines.

The regional travel-time curves for the shocks with foci in Poland, recorded by the seismic stations in the Bohemian MassiL were obtained by the linear regression (1) (they are plotted for all observations in Fig. 2):

(1) t , . = (14-19 __ 0'21) D + (4"84 __ 0"66),

tl,o = (19"63 +__

ts, = (26-64 __+

tsg = (33"37 ___

0"19) D - (2.29 _+ 0"61),

1"05) D + (3"17 + 3"51),

0"28) D -- (3'76 _ 0"93).

Since all the shocks for the Lubin area were located in approximately the same point, only the average values of the observations (black symbols) could be shown in Fig. 2. The standard deviations of these average observations are _+ (0.2-0.3)s . All the observations for the loci in Upper Silesia and Belchatow are plotted separately (white symbols in Fig. 2).

The arrival times of all the studied waves, propagating in the given azimuths to epicentral distances < 5 °, are shorter than those that correspond to European travel- time curves for near earthquakes derived by K~irnik [6]. The apparent velocity (Tab. 1) determined from the slopes of the travel-time curves (1) for Pg waves is nearly the same as the velocity corresponding to the European travel-time curve (vpg = 5.62 km/s) and slightly lower for the Sg waves (European travel-time curve: Vs~ = 3-38 km/s). For the loci in Poland, the apparent velocity of the Pn wave is

Table 1. Velocities of seismic waves (km[s).

Type of wave

Poland D ° (WAR)

W. Germany, 1978 D ° (STU)

Friuli, 1976 D ° (CSEM)

K~rnfk, 1954

Jeffreys- Bullen 1940

PH Pa Sn

sa

azimuth

4-6 7-82 ± 0-12 5'0 5'65 ~= 0.05 5"0 4.17 ± 0.16 5.0 3'32 ~ 0'03

170°--290 °

4-2 8"20 ~ 0-24 5"4 5'84 ::k 0"24 3"2 4'68 ~ 0-15 5"3 3'47 :k 0'06

35°--73 °

5,1 8-23 ~: 0-09 5.1 6.01 -_k: 0.11 5-1 4.62 ~ 0.06 5.1 3.57 ~- 0.05

346 ° - 13 °

8'14 5"62 4"53 3"38

180°--360 °

7"83 5'57 4"39 3"36

00--360 °

lower than that derived from the European travel-time curve (vp, = 8.14km/s) and it is the same as the velocity yielded by the standard Jeffreys-Bullen travel-time curve (vp, = 7.83 km/s). The derived travel-time curve of the Sn wave (1)is subjected to large errors because this wave was not observed in the weaker shocks. However, it does indicate a very low value for the apparent velocity Vsn. Our data sample was not large enough for a more detailed analysis.

Studia geoph, et geod. 28 [~984] 59

J. Plomerov6, V. Babu~ka, L. Ruprechtovd

4. AZIMUTAL DEPENDENCE OF APPARENT VELOCITIES

The geometric layout of the seismic stations and of the three studied focal regions is such (Fig. 1) that the range of azimuths of seism;c wave propagation to the stations covers 120 ° and, therefore, does not form a narrow fan with a central direction of the wave propagation into the Bohemian Massif as was the case of the Friuli and also the Schwarzwald earthquakes. For this reason, we shall now discuss the azimuthal dependence of the velocities of propagation of seismic waves to the individual stations for each region separately.

°

Fig. 3a. Apparent velocities of Pn waves (number at marked directions), Pg (numbers at the lines connecting the individual stations with the focus) and Sg (numbers in parentheses) in km/s

derived from observations of shocks with loci in the Lubin area.

For the Lubin focal region, the apparent velocities of the Pn waves {calculated from the slopes of the travel-time curves for selected directions) and of the P9 and $9

waves (calculated directly from the individual observations at each station separately) are shown schematically in Fig. 3a. The apparent velocity of the Pn wave propagating to stations PRU and KHC under the azimuth AZ~ ~ 215 ° (station-to-source azimuth is A Z 2 ~ 35 °) is vp, = 7.9 + 0-1 km/s, which is 0.2 km/s more than the velocity of the waves propagating to stations BRG and MOX (AZ~ ~ 250 °, AZ2 ~ 70°). Also the velocities of waves propagating just through the crust (Pg, Sg) in the SSW direction are higher than those propagating WSW. The highest velocities of the Pg

and $9 waves were observed at station GFI3 which, however, was located close to the focus (D = 1"16 °, A Z 1 = 171°). The lowest velocities were observed for station MOX, which indicates a deceleration in the wave propagation in the region between stations BRG and MOX (Fig. 3a). The standard deviations of the velocities of the Pg and Sg waves are in the interval of +(0"01-0 '06)km/s .

60 Studia geoph, et geod. 28 [1984]


The apparent velocities for the foci in the region of Upper Silesia are given in Fig. 3b. The waves from this focus propagate to the Bohemian Massif in an azimuth fan

PRU V ~"~9~~/ '~

Fig. 3b. Apparent wave velocities derived from shocks in Upper Silesia.

35 ° wide with its axis approximately due west. In this direction the Pn wave propagates at a velocity of 7"8 ± 0-I km/s. Although the station-focus configuration is unfa- vourable for computing the Pn-wave velocities in various directions, it seems that they are higher due WSW than due WNW. All Pg- and S9-wave velocities have standard

Fig. 3c. Apparent wave velocities derived from shocks in the Belchatow area.

deviations of less than ±0-1 km/s and are 0-2-0.3 km/s higher than would correspond to the European travel-time curves, with the exception of the segment between GFI0 and MOX, where the velocities are conspicuously lower: veo = 5"6 ± 0.1 km/s is the velocity determined for this segment by the linear regression from the observations of the stations located in source-to-station aizmuth 270 °. The highest velocities

Studia geoph, et geod. 28 [1984] 61

J. Plomerovd, K Babugka, L. Ruprechtovd

of Pg and Sg waves were observed under the same azimuth in the segment between the focus and the GFI~ ~ station.

The waves from the Belchatow area propagate to the Bohemian Massif in an azimuth fan of the same width as that for Upper Silesia, its axis pointing to WSW (Fig. 3c). A very low Pn velocityis appropriate to this direction, re, = 7.6 __ 0'3 km/s, however, it is subjected to a relatively higher error in comparison with other investigated focal zones. The distribution of the Pg- and Sg-wave velocities is similar to that for the Upper Silesian foci. The Pg wave velocities are again 0.2-0-3 km/s higher than would correspond to the European travel-time curve, an exceptionally high Pg velocity, 6"3 __+ 0"3 km/s, being observed at station KSP. We again observed a marked decrease of velocities, this time along the BRG-MOX segment: veo = 5-5 + + 0.2 km/s. There is a discrepancy between the average of the Pg velocities to the individual stations arriving from different focal regions and the Pg velocity derived from the Pg travel-time curve. The individual Pg velocities (Figs 3 a - c) are calculated, however, directly from the time of propagation and the epicentral distance (it means the time-intercept is supposed to be zero), whereas the Pg velocity given in Tab. 1 is calculated from the slope of the travel-time curves of the Pg wave, which has a negative time intercept (1). Therefore, the individual Pg velocities are higher than the integral velocity in Tab. 1.

5. SEISMIC-WAVE VELOCITIES AND TECTONICS

In the following paragraphs we shall attempt to compare the velocities of seismic waves for all directions of propagation, for which the distribution of the stations in the Bohemian Massif and of the focal regions in its neighbourhood makes it possible. These are apparent velocities derived from regional travel-time curves for northward to north-eastward propagation (waves from the Friuli focal region [1, 4]), for north- eastward propagation (Sclhwarzwald focal region [2, 5J), and for mostly south- westward to westward propagation (Lubin, Upper Silesian and Belchatow focal regions).

The apparent velocities of the Pn waves propagating through the upper mantle in the basement of tectonic formations, in association with which the velocities will be discussed, are shown in Fig. 4. These velocities were determined from the slopes of regional travel-time curves for all the stations at which the Pn wave is observed (the values with the directions marked by the full line) or derived just for selected directions (dashed line) as permitted by the distribution of the stations and foci, and also by the shock intensities. Higher velocities were observed in the central part of the Bohemian Massif, particularly for the NE and NNE propagation (about 8-2 km/s). Also the Saxony-Thuringian zone is characterized by high velocities propagating NE.

Conspicuously lower Pn-wave velocities can be observed in the outer parts of the

62 Studia geoph, et geod, 28 [1984]

Velocities' o f Seismic Waves . . .

Bohemian Massif (an area of intensive Variscan tectonogenesis [8]), particularly in the West and East Sudeten and in the Moravian-Silesian block. The lowest velocities were observed for the Belchatow foci. In this case, the Pn waves propagate due west from the edge of the East European Platform, through the Moravian-Silesian zone on into the Bohemian Massif. Since we are working with apparent Pn-wave velocities, it is probable that the velocities in the NE part of the Bohemian Massif are partly decreased by sedimentary basins, in which the loci are located on the Polish territory. It seems that the azimuthal dependence of the velocities is also evident in the outer

~ BEL

.r.÷ ,'~ ~ // IV [ ~ ~ " " "--" "-~5 . . . . . . . . . . . / KSP 77

d. , =I.?ZI / icHw ~ V ~;'~ J " ~ IKHC:! J / ~ !

l a ° 1 t ° 1 6 ° l a *

Fig. 4. Apparent velocities of Pn waves, in kin/s, propagating into the Bohemian Massif from five focal regions. The numbers that mark the full lines are the velocities derived from observations of all stations, the indicated direction corresponding to the axis of the fan in which the waves propagate from a given focus to the seismic station. The dashed lines mark the directions to selected pairs of stations and the appropriate velocity values are shown. All the velocities were calculated from the slopes of regression lines by which the individual observations were ap-

proximated.

blocks of the Bohemian Massif. Higher Pn-wave velocities (about 7'9 km/s) were found for the S W - N E and the W S W - E N E propagation as compared to the velocities of propagation from the E to the W and from the ESE to the WNW. However, higher apparent Pn-wave velocities were also observed at stations PRU and K H C and it is therefore difficult to decide to what extent the propagation of waves within the Moldanubicum block is responsible for increasing the velocities.

Like the Pn-wave velocities, also the velocities of the crustal phases of P g and S g

are mostly higher in the centre of the Bohemian Massif than within its northern fringe formations (Fig. 5). However, the highest Pg-wave velocities are observed in the Moravian-Silesian region. The lowest velocities, on the other hand, were observed in the Krugn6 hory (Mts) block and in the Vogtland-Thuringian formation east of the M O X station. This finding corresponds to the results of Ber~inek et al. [9], who explained the thick crustal layer of lower velocities in the Krugn6 hory (Mts)

Studia geoph, et geod. 28 [1984l 6 3

J. Plomerovd, V. BabuYka, L. Ruprechtovd

block by a large volume of acidic magmatic rocks, which are also responsible for the negative gravity anomaly in this area. The velocities in the Kru~n6 hory (Mts) block may also be lower due to the higher temperature in the crust as indicated by heat-flow values [10], by thermal springs and neoidic volcanism in the foothill region of the Kru~n6 hory (Mrs). The relatively low velocity of the Pg waves (5.7 km/s) was also observed along the path from the Friuli area to station KHC, where seismic waves propagate across the structure of the basement of the East Alps. This observation agrees with the results of deep seismic sounding [11], which give velocities lower

Fig. 5. Apparent velocities of Pg- and Sg-wave velocities (value in parentheses) in km/s. The arrows indicate the general directions of wave propagation, the hatched places of low (dotted)

and high (dashed) velocities.

than 6 km/s for the upper part of the crust in this region and, moreover, also a conspicuous low-velocity channel.

The azimuth dependence of the Pg-wave velocities is similar to that of Pn waves (Fig. 5). In the region of the Moldanubicum, the velocities of propagation from the NNE to the SSW and from the NE to the SW are 0.2 km/s higher than the velocities of propagation from the ENE to the WSW. This direction and the E - W direction, on the contrary, belong to the fastest in the NE corner of the Bohemian Massif, i.e. in the Moravian-Silesian zone and in the basement of the Upper Silesian Basin. The high velocities may reflect the anticipated turning of the Variscan structures of this region into the E - W direction [12]. In the northern boundary blocks of the Bohemian Massif, the velocities of the Pg waves range from 5"8 to 5"9 km/s without showing any directional dependence. The velocity distribution of the crustal phase $9 (Fig. 5, the values in parentheses) agrees with the Po-wave velocities and thus empha- sizes the observed relations between the crustal-phase velocities and the main features of the tectonics of the Bohemian Massif.

64 Studia geoph, et geod. 28 [1984]

Velocities o f Se&mic Waves . . .

6. CONCLUSION

The travel-time curves derived from the observations of shocks with foci in Poland yield shorter propagation times as far as epicentral distances of 5 ° than corresponds to European travel-time curves [6] and to Jeffreys-Bullen travel,time curves [7]. They will be employed in interpreting and locating shocks from the mining areas of Upper Silesia, Belchatow and particularly from Lubin, where their numbers have recently been increasing. They will also serve to identify the rockbursts in the Ostrava-

Karvin,'i Basin. As in the case of Friuli and Schwarzwald foci, higher Pn- and Pg-waves velocities

were observed in the central part of the Bohemian Massif than in its northern and north-eastern boundary blocks for seismic waves propagating from the NE edge of the Massif. A conspicuous decrease in the velocities of crustal P9 and $9 waves was found for the E-to-W propagation in the Krugn6 hory (Mts).

In the Moldanubicum block, in the Tepl~t-Barrandien block and in the Saxony- Thuringian zone a marked velocity dependence on the direction of propagation can be observed. The highest velocities were observed for the N E - S W and the N N E - SSW propagation (Moldanubicum). It seems that the N E - SW tectonic zones, which play the leading role in the general structure of the Bohemian Massif [13] and are also reflected in the regional gravity anomalies [14], as well as in the enhanced seismo- tectonic mobility of the territory [15], have probably a large depth range and are also manifested in the structure of the upper mantle. On the other hand, high PO- and Sg-wave velocities are observed in the Moravian~Silesian block for the N E - S W and the E - W propagation, but the mantle Pn waves seem to be very slow in those

directions.

Acknowledgement: The authors wish to thank Acad. A. Z~ topek for valuable discussions and Dr. B. T i t t e l for the data of German stations and for critical reading of the manuscript.

Received 30. 11. 1982 Reviewer: A. Zdtopek

References

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[2] J. Plomerov~t, V. Babu]ka , L. Ruprechtov/~: Travel Times of Seismic Waves Propa- gating to the Bohemian Massif from the South-west Direction. Studia geoph, et geod., 25 (1981), 356.

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slovenska, NCSAV, Praha 1961, pp. 15--27. [9] B. Ber/tnek, A. Dudek, M. Zounkovh : Rychlostni modely stavby zemsk6 kfiry v Ceskdm

masivu a Z/tpadnich Karpatech. Sb. geol. v~d, U:~it/t geofyzika, 13 (1975), 7. [10] V. I~.erm/~k: Heat Flow in the Bohemian Massif. J. Geoph., 42 (1977), 455. [11] P. Giese, C. P rodeh l : Main Features of Crustal Structure in the Alps. In "Explosion

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[14] J. I b rma je r : Geological Interpretation on Gravity Maps of Czechoslovakia. In "Geophy- sical Syntheses in Czechoslovakia". Ed. A. Z/ttopek, Veda, Bratislava 1981, 135.

[15] B. Berb.nek, A. Zi~topek: Preliminary Results of Geophysical Syntheses in Czechoslovakia and Central Europe Based on Explosion Seismology until 1980. In "Geophysical Syntheses in Czechoslovakia", Ed. A. ZMopek, Veda, Bratislava i981, 469.

66 Studia geoph, et geod, 28 [19841

Documents

Velocities of seismic waves propagating through the Bohemian massif from foci in Poland