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Focal depth distribution of Swedish earthquakes
T29 Tect5~5physi~s, 63 (1979) TZS-T31 Q Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands Letter
Section
c_
Focal depth distributionof Swedishearthquakes
MARKUS R,AtrH Seismological Institute, Box 51 Y, S-r51 20 Uppmla {Sweden) (Submitted August 23,197S;
accepted for publication November X7,1978)
ABSTRACT I%th, M., 1979. Focal depth distribution T29Jr3 1.
of Swedish earthquakes. Tectonophysics,
53:
Instrumental and macroseismic focal depths of Swedish earthquakes show sta~~tic~ly a high degree of correlation. The mejority of the earthquakes is located in the lower granite, probably associated with a low-velocity layer, whereas crustal discontinuities appear to be relatively less seismic.
Focal depth is a significant seismic parameter for information on structural and tectonophysical properties and for discrimination between different hinds of events. Unfortunately, difficulties are often connected with the dedication of focaI depth to such an accuracy as requestedfrom various apph~tio~s. In a recent cataiogue of earthquakesin Sweden for the interval 1951-1976 (B&h, 1978a), focd depths are cakulated both instrumentally and macroseismically in ah possible cases. The purpose of the present paper is partly to examine the correlation between focal depths, determined in the two ways mentioned, partly to examine possible reIations between the focaI depths and the averagecrustalstructure.This investigation of the depth distribution is ~ppl~en~ to earlierinvestigationsof magnitude distribution (B&h, 1978a), of time distribution (B&h, 197%) and of epicenter distribution (B&h, 1978d) of Swedish earthquakes. FOCAL DEPTH
Instrumental focaf depths (h) are calculated from differences of arrivaI times of crustal waves, i.e. from kinematic properties, whiIe macroseismic focaI depths (h,) are cakulated from the intensity-distancedependence, i.e. from dynamic properties. Detailsof the two calculation methods are given in B&h (1978a). Fig. 1 shows the frequency distribution of the two focaf
T30
Fig. I. Frequency distribution (n) cf focal depths of Swedish earthquakes from instrumental (h) and macr~~mie (h,) ~cu~ati~~ (modes marked). For h,, two curves are given: “actual” refers to the directly &served number per km, “percent” to the percentage distribution (% per km). For h, the two corresponding curves coincide. The suggested P-wave velocity distribution in the average Swedish crust is shown to the right in the figure.
depths for Sweden for the interval 1951-1976, ~cl~d~g 99 &values and 70 h,-values. In Fig. 1, the frequencies are ~rnoo~~ con~ut~vely by the rule fa + 2b + c)/4. The depth range 5-34 km is covered. The co~elatiun coefficient betwam the twu frequency curves (30 pairs of observations) is iO.77 f 0.07, which is highly significant. This high correlation refers to the statitical presentation of our depth data, whereas individual values may show large errors and often poor correlation. The high statistical correiation between h and hn is interesting, especially as the calculation of the two depth values depends on entirely different methods and data. The mean depths are h = 17.0 rt 6.8 km and h, = 14.8 f 6.0 km, The modes of the frequency ~t~b~tio~ curves are h = 14 km and h, = 15-16 curve has a cfear km. Besides the primary mode h = 14 km, the i~t~rnent~ ~terest~g because of the secondary mode h = 22 km. This is p~ic~~ly almost perfect ~eement with the modes of macrose~mi~ depths earlier for Fe~o~~d~a for the intervaI 189X---1930 ~~~~, 1954, detain fig. 4). VELOCITY
STRUCTURE
Figure 1 shows a suggested average crustal structure
for Sweden, for
T31
whose details reference is made to B&h (1978a, b). The main features are summarized as follows, referring only to the P-wave: (1) The superficial granitic layer O-l.4 km with P-wave velocity V =: 5.82 kmlsec (Pg2; B&h et al., 1976). This is the site of near-surface phenomena, like rockbursts, not included in our depth calculations (B&h and WahlstrSm, 1976; Bath, 1977). (2) The main granitic layer 1.4-19 km, V = 6.22 km/set (Pgl). (3) The low-velocity layer in the lower granite, tentatively in the depth range 12-16 km, V = 6.14 km/see. Most of our earthquakes are associated with this low-velocity layer. (4) Conrad discontinuity at a depth of 19 km. It should be noted that this level is associated with minima in the dep~-~equency curves. (5) The upper basaltic layer in the range 19-31 km, V = 6.64 kmlsec (P*). (6) A tentative low-velocity layer at 21-23 km, corresponding to the secondary mode of h. Contrary to the granitic low-velocity layer, the basaltic low-velocity layer is not substantiated by any reliable phase readings. (7) The lower basaltic layer at 31-38 km, V = 7.03 km/set (P*), well documented, especially by corresponding S-waves. (8) Moho discontinuity at a depth of 38 km. Moho is also not associated with any maximum in the depth-frequency curves. (9) The upper mantle P-wave velocity just below 38 km is 7.84 km/set (Pn). CONCLUSIONS
Our results can be summarized in the following points: (1) Statistically there is a high degree of correlation between instrumental and macroseismic focal depths of Swedish earthquakes. (2) Most Swedish earthquakes are probably associated with low-velocity layers, i.e. weakness layers. This is particularly true for the lower granite, where maxima of depth-frequency curves coincide with well documented Pwaves ascribed to a low-velocity layer. (3) Crustal d~cont~uities correspond to minima in the depth-frequency curves, suggest~g that the layers are in welded contact at these d~~ont~uities.
REFERENCES B&h, M., 1964. Seismicity of Fennoscandia and related problems. Gerlands Beitr. Geophys., 63: 173-208. B&h, M., 1977. A rockburst sequence at the Grilngesberg iron ore mines in central Sweden - Part II. Seismol. Inst., Uppsaia, Rep. No. 5-77, 21 pp. B&h, M., 1978a. Earthquakes in Sweden in 1951 to 1976. Swedish Geol. Survey, in press. B&h, M., 1978b. A seismicity map of Sweden. Tectonophysics, 49: Tl-TS.
T32 B&h, M., 1978c. Suspected periodicities of earthquakes in Sweden. Tectonophysics, 51: T55--T62. B&h, M., 1978d. Deep-seated fracture zones in the Swedish crust. Tectonophysics, 51: T47-T54. B&h, M. and Wahlstrijm, R., 1976. A rockburst sequence at the GrZingesberg iron ore mines in central Sweden. Seismol. Inst., Uppsala, Rep. No. 6-76, 30 pp. B%th, M., Kulh&rek, O., Leong, L.S., LindstrSm, D., Meyer, K., Rubio, M., Van Eck, T. and Wahlstriim, R., 1976. A seismic refraction investigation of superficial granitic layering. Seismol. Inst., Uppsala, Rep. No. 7-76, 21 pp.
Section
c_
Focal depth distributionof Swedishearthquakes
MARKUS R,AtrH Seismological Institute, Box 51 Y, S-r51 20 Uppmla {Sweden) (Submitted August 23,197S;
accepted for publication November X7,1978)
ABSTRACT I%th, M., 1979. Focal depth distribution T29Jr3 1.
of Swedish earthquakes. Tectonophysics,
53:
Instrumental and macroseismic focal depths of Swedish earthquakes show sta~~tic~ly a high degree of correlation. The mejority of the earthquakes is located in the lower granite, probably associated with a low-velocity layer, whereas crustal discontinuities appear to be relatively less seismic.
Focal depth is a significant seismic parameter for information on structural and tectonophysical properties and for discrimination between different hinds of events. Unfortunately, difficulties are often connected with the dedication of focaI depth to such an accuracy as requestedfrom various apph~tio~s. In a recent cataiogue of earthquakesin Sweden for the interval 1951-1976 (B&h, 1978a), focd depths are cakulated both instrumentally and macroseismically in ah possible cases. The purpose of the present paper is partly to examine the correlation between focal depths, determined in the two ways mentioned, partly to examine possible reIations between the focaI depths and the averagecrustalstructure.This investigation of the depth distribution is ~ppl~en~ to earlierinvestigationsof magnitude distribution (B&h, 1978a), of time distribution (B&h, 197%) and of epicenter distribution (B&h, 1978d) of Swedish earthquakes. FOCAL DEPTH
Instrumental focaf depths (h) are calculated from differences of arrivaI times of crustal waves, i.e. from kinematic properties, whiIe macroseismic focaI depths (h,) are cakulated from the intensity-distancedependence, i.e. from dynamic properties. Detailsof the two calculation methods are given in B&h (1978a). Fig. 1 shows the frequency distribution of the two focaf
T30
Fig. I. Frequency distribution (n) cf focal depths of Swedish earthquakes from instrumental (h) and macr~~mie (h,) ~cu~ati~~ (modes marked). For h,, two curves are given: “actual” refers to the directly &served number per km, “percent” to the percentage distribution (% per km). For h, the two corresponding curves coincide. The suggested P-wave velocity distribution in the average Swedish crust is shown to the right in the figure.
depths for Sweden for the interval 1951-1976, ~cl~d~g 99 &values and 70 h,-values. In Fig. 1, the frequencies are ~rnoo~~ con~ut~vely by the rule fa + 2b + c)/4. The depth range 5-34 km is covered. The co~elatiun coefficient betwam the twu frequency curves (30 pairs of observations) is iO.77 f 0.07, which is highly significant. This high correlation refers to the statitical presentation of our depth data, whereas individual values may show large errors and often poor correlation. The high statistical correiation between h and hn is interesting, especially as the calculation of the two depth values depends on entirely different methods and data. The mean depths are h = 17.0 rt 6.8 km and h, = 14.8 f 6.0 km, The modes of the frequency ~t~b~tio~ curves are h = 14 km and h, = 15-16 curve has a cfear km. Besides the primary mode h = 14 km, the i~t~rnent~ ~terest~g because of the secondary mode h = 22 km. This is p~ic~~ly almost perfect ~eement with the modes of macrose~mi~ depths earlier for Fe~o~~d~a for the intervaI 189X---1930 ~~~~, 1954, detain fig. 4). VELOCITY
STRUCTURE
Figure 1 shows a suggested average crustal structure
for Sweden, for
T31
whose details reference is made to B&h (1978a, b). The main features are summarized as follows, referring only to the P-wave: (1) The superficial granitic layer O-l.4 km with P-wave velocity V =: 5.82 kmlsec (Pg2; B&h et al., 1976). This is the site of near-surface phenomena, like rockbursts, not included in our depth calculations (B&h and WahlstrSm, 1976; Bath, 1977). (2) The main granitic layer 1.4-19 km, V = 6.22 km/set (Pgl). (3) The low-velocity layer in the lower granite, tentatively in the depth range 12-16 km, V = 6.14 km/see. Most of our earthquakes are associated with this low-velocity layer. (4) Conrad discontinuity at a depth of 19 km. It should be noted that this level is associated with minima in the dep~-~equency curves. (5) The upper basaltic layer in the range 19-31 km, V = 6.64 kmlsec (P*). (6) A tentative low-velocity layer at 21-23 km, corresponding to the secondary mode of h. Contrary to the granitic low-velocity layer, the basaltic low-velocity layer is not substantiated by any reliable phase readings. (7) The lower basaltic layer at 31-38 km, V = 7.03 km/set (P*), well documented, especially by corresponding S-waves. (8) Moho discontinuity at a depth of 38 km. Moho is also not associated with any maximum in the depth-frequency curves. (9) The upper mantle P-wave velocity just below 38 km is 7.84 km/set (Pn). CONCLUSIONS
Our results can be summarized in the following points: (1) Statistically there is a high degree of correlation between instrumental and macroseismic focal depths of Swedish earthquakes. (2) Most Swedish earthquakes are probably associated with low-velocity layers, i.e. weakness layers. This is particularly true for the lower granite, where maxima of depth-frequency curves coincide with well documented Pwaves ascribed to a low-velocity layer. (3) Crustal d~cont~uities correspond to minima in the depth-frequency curves, suggest~g that the layers are in welded contact at these d~~ont~uities.
REFERENCES B&h, M., 1964. Seismicity of Fennoscandia and related problems. Gerlands Beitr. Geophys., 63: 173-208. B&h, M., 1977. A rockburst sequence at the Grilngesberg iron ore mines in central Sweden - Part II. Seismol. Inst., Uppsaia, Rep. No. 5-77, 21 pp. B&h, M., 1978a. Earthquakes in Sweden in 1951 to 1976. Swedish Geol. Survey, in press. B&h, M., 1978b. A seismicity map of Sweden. Tectonophysics, 49: Tl-TS.
T32 B&h, M., 1978c. Suspected periodicities of earthquakes in Sweden. Tectonophysics, 51: T55--T62. B&h, M., 1978d. Deep-seated fracture zones in the Swedish crust. Tectonophysics, 51: T47-T54. B&h, M. and Wahlstrijm, R., 1976. A rockburst sequence at the GrZingesberg iron ore mines in central Sweden. Seismol. Inst., Uppsala, Rep. No. 6-76, 30 pp. B%th, M., Kulh&rek, O., Leong, L.S., LindstrSm, D., Meyer, K., Rubio, M., Van Eck, T. and Wahlstriim, R., 1976. A seismic refraction investigation of superficial granitic layering. Seismol. Inst., Uppsala, Rep. No. 7-76, 21 pp.