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Crustal shear-wave velocity and Poisson's ratio distribution in northwest Spain
.I. Geodynomics
Vol.25,
No. I, pp. 3545, 1998 1997 ElsevierScienceLtd All rights merwd. Printed in Great Britain
0
Pergamon PII: s026&3707@7)oooo2-1
0264-3707/98 $19.00+0.00
CRUSTAL SHEAR-WAVE VELOCITY AND POISSON’S RATIO DISTRIBUTION IN NORTHWEST SPAIN JULIA TfiLLEZ and DIEGO C6RDOBA Departamento de Geofisica, Facultad de Ciencias Fisicas, Universidad Complutense, 28040-Madrid, Spain (Received 30 January 1996: accepted 23 November 1996)
Abstract-We have modelled shear wave data from a seismic refraction experiment in the northwest of the Iberian Peninsula to derive shear-wave velocity and Poisson’s ratio models in the crust and upper mantle. The models show that the uppermost crust has high Poisson’s ratios, g. It is low, 0.21-0.24, in the middle crust and high, (~>0.25, and increasing with depth in the lower crust. Poisson’s ratio is 0.27 in the uppermost mantle. These results may be interpreted in terms of high quartz content in the rocks of the upper and middle crust and of high feldspar and low quartz content in the lower crust of NW Spain. 0 1997 Elsevier Science Ltd
INTRODUCTION
For a long time, seismic refraction surveys of the lithosphere have concentrated almost exclusively on the interpretation of compressional (P) waves. This is due to the small signal-tonoise ratio of the crustal shear (S) waves recorded and to the lack of three-component receivers. However, the interpretation of shear waves, especially when combined with that of compressional waves, provides essential information on the structure, composition and physical properties of the crust and upper mantle. The understanding of both compressional (V,(z)) and shear (V,(z)) wave velocity models enables us to calculate the Poisson’s ratio distribution a(z) which depends on: temperature, pressure, rock composition (e.g. Kern, 1982a; Tarkov and Vavakin, 1982), the presence of partial melting and on the existence of fluid inclusions (e.g. Spetzler and Anderson, 1968; Christensen, 1984). In the summer of 1982, a seismic refraction experiment was carried out in NW Spain. The description of the planning and execution of the field experiment can be found in C6rdoba er al. (1987). During this experiment not only P-waves but also high amplitude S-waves were obtained, which were recorded by three-component instruments. Because of the high quality of these signals, they were used to improve the models obtained from the P-wave interpretations (Cbrdoba et al., 1988; TCllez et al., 1993; TCllez, 1993) and to derive Poisson’s ratio models for the crust using the relation v; - 2v; u= 2(v; - Vf) In this paper we present shear-wave velocity and Poisson’s ratio models obtained from the 35
36
J. Tellez and D. Cdrdoba
interpretation of the two seismic profiles which best illustrate the most important features of the earth’s crust in NW Spain. These profiles, lines A and B in Fig. 1, were recorded in the northwestern comer of the Iberian Massif, which represents a major outcrop of the Hercynian erogenic belt in Europe. Profiles A and B traverse the Galicia-Tras-os-Montes and Central Iberian zones, according to the zonation of the Iberian Massif proposed by Farias et al. (1985).
DATA AND INTERPRETATION
METHOD
Figure 2 illustrates the excellent quality of the shear wave arrivals as well as the most important features of the data. As for the P-waves, the seismic response of the S-waves of the upper crust (S,) and crust-mantle boundary (SJ) is clearly observed on the record sections. A converted Moho phase (P,S-S,P) is also identifiable in both profiles. Figure 2 shows that the amplitudes for P- and S-arrivals are of the same order of magnitude. Another striking feature of the data is that all three components display similar P- and S-wave energy although underwater explosions were used in this experiment. P-to-S conversion at the water bottom (e.g. Ergin, 1952; Kim and Seriff, 1992) and the conversion of P-waves at the free surface (Fertig, 1984) can be considered as the most possible causes for S-wave generation by
Fig. 1. Simplified structural map of NW Spain (after Matte, 1983); bold lines locate the seismic profiles interpreted in this study. (1) Upper Precambrian; (2) Lower Palaeozoic; (3) Ophiolites; (4) Catazonai nappes; (5) Aluminous granites; (6) Calc-alkaline granites.
37
Crustal shear-wavevelocity (Spain)
underwater explosions. The S-wave record sections have been plotted for interpretation with a reduction velocity of 6/1.73 =3.46 km/s and a reduced time scale equal to that of the P-wave record sections divided by 1.73. This allows a qualitative estimate of the Poisson’s ratio in the crust and upper mantle. If the Poisson’s ratio were 0.25, the P-wave travel time curves would coincide with the corresponding S-wave phases. Differences between both of these travel time curves indicate deviations of the Poisson’s ratio from 0.25 and, therefore, deviations of the S-wave velocities in the crust and upper mantle from Vdl.73. In order to better correlate the S-phases, we use the record sections for all three components, vertical, horizontal-radial and horizontal-transversal, and the motion product of the vertical and radial components (TCllez, 1993). Examples of all three components are presented. All record sections shown in this paper appear -with tracewise normalized amplitudes, i.e. the maximum amplitude of each seismogram is scaled to a fixed value. The P-wave velocity models derived by Ttllez (1993) for lines A and B were used as starting models for the interpretation of S-waves, assuming a Poisson’s ratio of a=0.25. The velocitydepth models were iteratively modified until an optimal adjustment between the observed and
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Fig. 2. Tracewise normalized and band-pass filtered (2-20 Hz) record sections showing the whole P- and S-wave field for the (a) Radial component for profile A; (b) Radial component for profile B. Reduction velocity=6 km/s.
J. Tellez and D. Cordoba
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model-derived S-wave travel times and amplitudes was achieved. In the model adjustment the S-wave velocities, rather than the layer boundaries, were altered. Synthetic seismograms were calculated using the reflectivity method (Fuchs and Miiller, 1971; Braile and Smith, 1975) and the ray-tracing algorithm (e.g. Cerveny, 1985). A source with a homogeneous radiation pattern (e.g. equal P and S energy in all directions) was used to accommodate the high amplitude observations for both P and S arrivals.
INTERPRETATION OF LINE A Figure 3a shows the P-wave record section of the vertical component for profile A. The lines drawn in the record section indicate the travel time curves corresponding to the P-wave velocity model shown in Fig. 4a. In the horizontal S-wave record section for this profile (Fig. 3b) clear S, and S$ phases are easily recognized. Phase S, is visible up to a distance of about 80 km. Strong S-waves reflected from the Moho boundary can be correlated (phase S,S) from a distance of about 30 km to the end of the record sections. Between phases S, and S,S some arrivals can be correlated, labeled as phase S,S. In contrast to the P-wave field, the shear wave refraction from the upper mantle (S,) cannot be identified. (a) 6
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DISTANCE INKM Fig. 3. Tracewise normalized record sections for profile A with predicted travel times from the models of Fig. 4. (a) P-wave vertical component band-pass filtered l-15 Hz. Reduction velocity=6 km/s. (b) S-wave transverse component band-pass filtered I-10 Hz. Reduction velocity=3.46 km/s.
Crustal shear-wave velocity (Spain)
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Fig. 4. (a) P- and S-wave velocity models for profile A. The S-wave velocities have been multiplied by 1.73. (b) Poisson’s ratio model for profile A. M=Moho. A preliminary comparison of the travel times of the P- and S-waves indicates some deviations of the S-wave velocities from Vdl.73 kmk (corresponding to a Poisson’s ratio of 0.25). Phase S, arrives later than phase Pp, i.e. later than expected for a Poisson’s ratio of 0.25, which is an indication that the theoretical S-wave velocities (VJ1.73) in the uppermost crust are too large. Phase S,S arrives earlier than the corresponding phase P,P. Phase S,S arrives roughly at expected travel times from an average Poisson’s ratio in the crust of about 0.25. A satisfactory agreement between the theoretical and observed travel times and the amplitude ratios between the S phases is obtained with the final S-wave velocity model plotted in Fig. 4a together with the P-wave model. This V,(z) distribution accounts for the observations described above. The S-wave velocities in Fig. 4a have been multiplied by 1.73, so that both curves will coincide where the Poisson’s ratio has a value of (+=0.25. The S, arrivals can be explained by an upper crust composed of two layers. Near the surface the velocity is 2.5 km/s and it increases to a value of 2.7 km/s at a depth of 1.5 km. In the crystalline basement, at a depth between 1.5 and 8 km, the velocity increases from 3.4 to 3.6 km/ s. In the middle crust, which extends to a depth of 22 km, the S-wave velocities are higher than Vdl.73, as can be seen in Fig. 4a. This increase in the S-wave velocities is necessary in order to fit the travel times of the phase S,S. The small S-velocity contrast at a depth of 22 km produces the observed low amplitude arrivals of phase S,S. In the lower crust, V, increases to 3.85 km/s just above the Moho. The S-velocity of 4.6 km/s in the uppermost mantle has been estimated by modelling the amplitudes of SHS and converted Moho waves. Combining the P- and S-wave velocity models in Fig. 4a, a Poisson’s ratio mode1 for the crust and uppermost mantle beneath line A can be calculated (Fig. 4b). The highest values of u are found in the uppermost crust, near the surface. The average u for the crystalline basement is
J. Ttllez and D. Cordoba
40
i/ 40
DISTANCE IN KM Fig. 5. Synthetic seismograms of the radial component for profile A, calculated from the models in Fig. 4a and Fig. 4b. Reduction velocity=6 km/s. Full wavefield. Compare with data in Fig. 2a.
about 0.25. In the middle crust the Poisson’s ratios are low, ~~0.25, as a consequence of the relatively high V, at a depth between 8 and 22 km. In the lower crust, uis high, i.e. a>0.25, and increases continuously with depth from about 0.25 to 0.29, due to a velocity gradient stronger for P- than for S-waves. The Poisson’s ratio is 0.27 in the uppermost mantle. We find that changes greater than 0.01 in the values of (T produce significant disagreements between the calculated and the observed travel times and amplitude ratios, therefore, the Poisson’s ratio determined is accurate to 0.01. The recorded section comprising all phases of the radial component of the ray synthetic seismograms obtained for profile A is shown in Fig. 5. To facilitate the comparison, reduction velocity, amplitude normalization and the scaling of the axes of the synthetic seismograms coincide with those of the observed data (Fig. 2a).
INTERPRETATION
OF LINE B
The seismogram sections recorded for profile B (Fig. 6) are similar to the records of profile A but, in contrast to profile A, there is a clear intra-crustal phase P,P (and also S,S) observed in profile B. As can be seen in Fig. 6, all the crustal phases of the P-wave field can be identified on the S-wave record section: an S, phase, arriving later than Pg, can be correlated up to a distance of about 60 km with an apparent velocity which increases with distance. A phase S,S, arriving about 1 s later than S,, can be recognized with large amplitudes at a distance range between about 30 and 80 km. A phase S,S can be correlated from a distance of 65 km to the end of the line, its quality depending on the component and the distance. This phase arrives earlier than the corresponding phase P$? Phase S,S can be clearly recognized in all three components with large amplitudes at a distance between 40 and 135 km. The initial S-wave velocity-depth model was modified, as necessary, to obtain an optimal adjustment between observed and calculated S-wave data. The final S-wave model (Fig. 7a) is characterized by a gradually increasing velocity in the uppermost crust down to a depth of 5 km. At the surface the velocity is 2.6 km/s and it increases to a value of 3.52 km/s at a depth of 5 km. S-wave velocity remains virtually constant down to a depth of 10 km, where S velocity jumps from 3.5 to 3.75 km/s. These values are required in order to explain the travel times and
Crustal shear-wave velocity
41
(Spain)
amplitudes of the S,S arrivals. The S-wave velocity is higher than Vdl.73 in the middle crust, which extends to a depth of 22 km; here the S-wave velocity jumps from 3.75 to 3.8 km/s; this
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Fig. 6. Tracewise normalized record sections for profile B with predicted travel times from the models of Fig. 7. (a) P-wave vertical component band-pass filtered l-20 Hz. Reduction velocity=6 km/s. (b) S-wave vertical component band-pass filtered 1-15 Hz. Reduction velocity=3.46 km/s. (c) Product of the radial and vertical components. Reduction velocity = 3.46 km/s.
J. TCllez and D. C6rdoba
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(a)
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:
:I : , I
354oJ
Fig.
7. (a) P- and S-wave velocity models for profile B. The S-wave velocities have been multiplied by I .73. (b) Poisson’s ratio model for profile B. M=Moho.
discontinuity causes the generation of phase S,S. In the lower crust, V, increases with depth to a value of 3.85 km/s. As in profile A, subcritical P,P and S&3 reflections of great amplitude suggest a sharp crust-mantle transition. The observation of converted Moho reflections (Fig. 2) confirms that the Moho is a first-order discontinuity. The synthetic seismograms computed with the reflectivity method for the final model (Fig. 7) are displayed in Fig. 8 with the same scaling, identical reduction velocity and amplitude normalization as the data in Fig. 2b.
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DISTANCE IN KM Fig. 8. Synthetic seismograms of the radial component for profile B, calculated from the models in Fig. 7a and Fig. 7b. Reduction velocity=6 km/s. Full wavefield. Compare with data in Fig. 2b.
Crustal shear-wavevelocity (Spain)
43
The a(z) distribution for line B (Fig. 7b) derived from the P- and S-wave velocity models has high values of (Tin the uppermost crust, especially near the surface where Poisson’s ratio is as high as 0.30. The low values of c between 5 and 10 km deep are a consequence of the presence of a P-wave low-velocity layer and the lack of an S-wave low-velocity zone (LVZ). Poisson’s ratio is low, less than 0.25, in the middle crust, and high and increasing with depth in the lower crust. As in profile A, the Poisson’s ratio value of 0.27 in the uppermost mantle is clearly confirmed by that estimated from a separate interpretation of the converted Moho reflections (Ttllez and Cbrdoba, 1996). DISCUSSIONAND CONCLUSIONS The shear wave velocity and Poisson’s ratio models for lines A and B presented in this paper show some differences in the distribution of the S-wave velocities and the g values but are remarkably coincident in their main features: Poisson’s ratio is high (>0.25) in the uppermost crust, low (~0.25) in the middle crust and high and increasing with depth in the lower crust. The differences in the uppermost layers of the crust can be explained by the variations in the superficial geological features crossed by the profiles (see more detailed results for the superficial structure in Cordoba and Banda, 1987). The knowledge of both V,(z) and V,(z) models, and therefore of the Poisson’s ratio distribution, allows one to draw conclusions on the composition and petrology of the crystalline crust of Northwest Spain. In order to do this, we compared our estimated V, and V, values with the P- and S-wave velocities measured in laboratory investigations on rocks and minerals (e.g. Christensen, 1982; Gebrande, 1982; Kern, 1982b). Laboratory measurements of velocities have shown a correlation between u and the degree of basicity of the rock: u increases together with basicity (e.g. Tarkov and Vavakin, 1982). In general, the P- and S-wave velocity distributions in the upper and middle crust in NW Spain are consistent with rocks of granitic/gneissic composition; a high quartz content explains the low Poisson’s ratio, 0.21-0.24, observed (e.g. Kern, 1982a). The highest values of u observed in the uppermost crust probably reflect the presence of soft and unconsolidated sediments near the surface (e.g. Gregory, 1976). The presence of crustal low-velocity zones is usually associated with factors such as compositional changes, high temperatures and fluids at high pore pressure. If we consider simultaneously the P- and S-wave velocities observed in profile B, we can rule out high temperature and high pore pressure as possible explanations of the LVZ, since these factors would reduce not only V, but also V, (e.g. Christensen, 1982) and therefore, the observed decrease in Poisson’s ratio would not take place. Possible explanations for the P-wave LVZ are highly quartz-rich rocks, such as granite or quartzite, and the presence of pore fluids at low pore pressures (Spencer and Nur, 1976; Mueller, 1977). The existence of a P-wave LVZ, without a corresponding S-wave LVZ, has also been observed in other areas of the Variscan belt (e.g. Holbrook et al., 1988). The high Poisson’s ratios in the lower crust of NW Spain are well modelled by rocks with high feldspar and low quartz content (e.g. Kern, 1982b). More information on factors such as temperature, pressure, fluid content, anisotropy, pore pressure, etc., would be required to obtain a more precise picture of the crustal composition in NW Spain. However, the results obtained in this study demonstrate that the interpretation of shear waves in seismic refraction experiments provides additional information for the understanding of the structure and composition of the crust. The determination of both shear wave velocities and Poisson’s ratios imposes significant constraints on the structural, physical
J. TCllez and D. Cordoba
44
and petrological models of the crust which could have been considered interpretation of P-waves.
viable from the mere
REFERENCES Braile L. W. and Smith R. B. (1975) Guide to the interpretation of crustal refractions profiles. Geophys. J. R. Astron. Sot. 40, 145-I 76. Cerveny V. (1985) The application of ray tracing to the numerical modelling of seismic wavefield in complex structures. In Seismic Shear Waves (Dohr G., ed.), pp. l-124. Geophysical Press, London. Christensen N. I. (1982) Seismic velocities. In Handbook of Physical Properties of Rocks, Vol. 2 (Carmichael R. S., ed), pp. l-228. CRC Press, Boca Raton, Florida, U.S.A. Christensen N. I. (1982) Pore pressure and oceanic crustal seismic structure. Geophys. J. R. Astron. Sot. 79,4 1 l-423.
Cordoba D. and Banda E. (1987) Estructura de 10s mazicos de Cabo Ortega1 y Ordenes a partir de datos sismicos. In Geologia de 10s Granitoides y rotas asociadas de1 Macizo Hespe’rico (Bea F., Carnicero A., Gonzalo J. C., Lopez Plaza M. and Rodriguez Alonso M. D., eds), pp. 427437. Editorial Rueda, Madrid. Cordoba D., Banda E. and Ansorge J. (1987) The Hercynian crust in northwestern Spain: a seismic survey. Tectonophysics 132, 321-333. Cordoba D., Banda E. and Ansorge J. (1988) P-wave velocity-depth distribution in the Hercynian crust of northwest Spain. Phys. Earth Planet. Inter: 51, 235-248. Ergin K. (1952) Energie ratio of the seismic waves reflected and refracted at a rock-water boundary. Bull. Seis. Sot. Am. 42,349-371. Farias I?, Gallastegui G., Gonzalez F., Marquinez J., Martin L. M., Martinez J. R., Pablo J. G. de and Rodriguez L. R. (1985) Aportaciones al conocimiento de la litoestratigrafia y estructura de Galicia Central, IX Reunion Geologia de1 Oeste Peninsular, Porto, pp. 411-431. Fertig J. (1984) Shear waves by an explosive point-source: The earth surface as generator of converted P-S waves. Geophys. Prospect. 32, l-l 7. Fuchs K. and Miiller G. (1971) Computation of synthetic seismograms with the reflectivity method and comparison with observations. Geophys. J. R. Astron. Sot. 23,417433. Gebrande H. (1982) Elasticity and inelasticity. In Physical Properties of Rocks, vol. lb (Angenheister G., ed), pp. l-99. Landolt-Bbmstein. Springer, New York. Gregory A. R. (1976) Fluid saturation effects of dynamic elastic properties of sedimentary rocks. Geophysics 41,895-92 1. Holbrook W. S., Gajewski D., Krammer A. and Prodehl C. ( 1988) Interpretation of wide-angle compressional and shear wave data in Southwest Germany: Poisson’s ratio and petrological implications. J. Geophys. Res. 93, 1208 l-l 2 106. Kern H. (1982) Elastic-wave velocity in crustal and mantle rocks at high pressure and temperature: The role of the high-low quartz transition and of dehydration reactions. Phys. Earth Planet. tnter: 29, 12-23.
Kern H. (1982b) Elasticity and inelasticity. In Physical Properties of Rocks, vol. lb (Angenheister G., ed), pp. 99-140. Landolt-Bornstein, Springer, New York. Kim N. W. and Seriff A. J. (1992) Marine PSSP reflections with a bottom velocity transition zone. Geophysics 57, 16 1- 170. Matte Ph. (1983) Two geotraverses across the Ibero-Armorican arc of Western Europe. Am. Geophys. Union, Geodyn. Ser: 10,53-81. Mueller St. (1977) A new model of the continental crust. In The Earth S Crust - Its Nature and Physical Properties (Heacock J. G.. ed). Geophysical Monograph 20, pp. 289-3 17. American
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velocity (Spain)
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Geophysical Union, Washington, D.C. Spencer J. W. and Nur A. (1976) The effect of pressure, temperature and pore water on velocities in Westerly granite. J. Geophys. Res. 81,899-904. Spetzler H. and Anderson D. L. (1968) The effect of temperature and partial melting on velocity and attenuation in a simple binary system. J. Geophys. Res. 73,6051-6060. Tarkov A. P. and Vavakin V. V. (1982) Poisson’s ratio behaviour in various crystalline rocks: application to the study of the Earth’s interior. Phys. Earth Planet. Inter. 29,24-29. Tellez J. (1993) Analisis e interpretation de ondas P y S de perfiles sismicos. Aplicacion al noroeste de la Peninsula Iberica, Ph.D. thesis, Universidad Complutense, Madrid. Tellez J., Matias L. M., Cordoba D. and Mendes-Victor L. A. (1993) Structure of the crust in the schistose domain of Galicia-Tras-os-Montes (NW Iberian Peninsula). Tectonophysics 221, 8 l-93. Tellez J. and Cordoba D. (1996) Observation of converted Moho reflections in the north-west of the Iberian Peninsula. Geophys. J. Znt. 124,7-17.
Vol.25,
No. I, pp. 3545, 1998 1997 ElsevierScienceLtd All rights merwd. Printed in Great Britain
0
Pergamon PII: s026&3707@7)oooo2-1
0264-3707/98 $19.00+0.00
CRUSTAL SHEAR-WAVE VELOCITY AND POISSON’S RATIO DISTRIBUTION IN NORTHWEST SPAIN JULIA TfiLLEZ and DIEGO C6RDOBA Departamento de Geofisica, Facultad de Ciencias Fisicas, Universidad Complutense, 28040-Madrid, Spain (Received 30 January 1996: accepted 23 November 1996)
Abstract-We have modelled shear wave data from a seismic refraction experiment in the northwest of the Iberian Peninsula to derive shear-wave velocity and Poisson’s ratio models in the crust and upper mantle. The models show that the uppermost crust has high Poisson’s ratios, g. It is low, 0.21-0.24, in the middle crust and high, (~>0.25, and increasing with depth in the lower crust. Poisson’s ratio is 0.27 in the uppermost mantle. These results may be interpreted in terms of high quartz content in the rocks of the upper and middle crust and of high feldspar and low quartz content in the lower crust of NW Spain. 0 1997 Elsevier Science Ltd
INTRODUCTION
For a long time, seismic refraction surveys of the lithosphere have concentrated almost exclusively on the interpretation of compressional (P) waves. This is due to the small signal-tonoise ratio of the crustal shear (S) waves recorded and to the lack of three-component receivers. However, the interpretation of shear waves, especially when combined with that of compressional waves, provides essential information on the structure, composition and physical properties of the crust and upper mantle. The understanding of both compressional (V,(z)) and shear (V,(z)) wave velocity models enables us to calculate the Poisson’s ratio distribution a(z) which depends on: temperature, pressure, rock composition (e.g. Kern, 1982a; Tarkov and Vavakin, 1982), the presence of partial melting and on the existence of fluid inclusions (e.g. Spetzler and Anderson, 1968; Christensen, 1984). In the summer of 1982, a seismic refraction experiment was carried out in NW Spain. The description of the planning and execution of the field experiment can be found in C6rdoba er al. (1987). During this experiment not only P-waves but also high amplitude S-waves were obtained, which were recorded by three-component instruments. Because of the high quality of these signals, they were used to improve the models obtained from the P-wave interpretations (Cbrdoba et al., 1988; TCllez et al., 1993; TCllez, 1993) and to derive Poisson’s ratio models for the crust using the relation v; - 2v; u= 2(v; - Vf) In this paper we present shear-wave velocity and Poisson’s ratio models obtained from the 35
36
J. Tellez and D. Cdrdoba
interpretation of the two seismic profiles which best illustrate the most important features of the earth’s crust in NW Spain. These profiles, lines A and B in Fig. 1, were recorded in the northwestern comer of the Iberian Massif, which represents a major outcrop of the Hercynian erogenic belt in Europe. Profiles A and B traverse the Galicia-Tras-os-Montes and Central Iberian zones, according to the zonation of the Iberian Massif proposed by Farias et al. (1985).
DATA AND INTERPRETATION
METHOD
Figure 2 illustrates the excellent quality of the shear wave arrivals as well as the most important features of the data. As for the P-waves, the seismic response of the S-waves of the upper crust (S,) and crust-mantle boundary (SJ) is clearly observed on the record sections. A converted Moho phase (P,S-S,P) is also identifiable in both profiles. Figure 2 shows that the amplitudes for P- and S-arrivals are of the same order of magnitude. Another striking feature of the data is that all three components display similar P- and S-wave energy although underwater explosions were used in this experiment. P-to-S conversion at the water bottom (e.g. Ergin, 1952; Kim and Seriff, 1992) and the conversion of P-waves at the free surface (Fertig, 1984) can be considered as the most possible causes for S-wave generation by
Fig. 1. Simplified structural map of NW Spain (after Matte, 1983); bold lines locate the seismic profiles interpreted in this study. (1) Upper Precambrian; (2) Lower Palaeozoic; (3) Ophiolites; (4) Catazonai nappes; (5) Aluminous granites; (6) Calc-alkaline granites.
37
Crustal shear-wavevelocity (Spain)
underwater explosions. The S-wave record sections have been plotted for interpretation with a reduction velocity of 6/1.73 =3.46 km/s and a reduced time scale equal to that of the P-wave record sections divided by 1.73. This allows a qualitative estimate of the Poisson’s ratio in the crust and upper mantle. If the Poisson’s ratio were 0.25, the P-wave travel time curves would coincide with the corresponding S-wave phases. Differences between both of these travel time curves indicate deviations of the Poisson’s ratio from 0.25 and, therefore, deviations of the S-wave velocities in the crust and upper mantle from Vdl.73. In order to better correlate the S-phases, we use the record sections for all three components, vertical, horizontal-radial and horizontal-transversal, and the motion product of the vertical and radial components (TCllez, 1993). Examples of all three components are presented. All record sections shown in this paper appear -with tracewise normalized amplitudes, i.e. the maximum amplitude of each seismogram is scaled to a fixed value. The P-wave velocity models derived by Ttllez (1993) for lines A and B were used as starting models for the interpretation of S-waves, assuming a Poisson’s ratio of a=0.25. The velocitydepth models were iteratively modified until an optimal adjustment between the observed and
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Fig. 2. Tracewise normalized and band-pass filtered (2-20 Hz) record sections showing the whole P- and S-wave field for the (a) Radial component for profile A; (b) Radial component for profile B. Reduction velocity=6 km/s.
J. Tellez and D. Cordoba
38
model-derived S-wave travel times and amplitudes was achieved. In the model adjustment the S-wave velocities, rather than the layer boundaries, were altered. Synthetic seismograms were calculated using the reflectivity method (Fuchs and Miiller, 1971; Braile and Smith, 1975) and the ray-tracing algorithm (e.g. Cerveny, 1985). A source with a homogeneous radiation pattern (e.g. equal P and S energy in all directions) was used to accommodate the high amplitude observations for both P and S arrivals.
INTERPRETATION OF LINE A Figure 3a shows the P-wave record section of the vertical component for profile A. The lines drawn in the record section indicate the travel time curves corresponding to the P-wave velocity model shown in Fig. 4a. In the horizontal S-wave record section for this profile (Fig. 3b) clear S, and S$ phases are easily recognized. Phase S, is visible up to a distance of about 80 km. Strong S-waves reflected from the Moho boundary can be correlated (phase S,S) from a distance of about 30 km to the end of the record sections. Between phases S, and S,S some arrivals can be correlated, labeled as phase S,S. In contrast to the P-wave field, the shear wave refraction from the upper mantle (S,) cannot be identified. (a) 6
-2 10
-4 0
10
20
30
40
50
60
70
80
90
100
110
120
130
140
DISTANCE INKM Fig. 3. Tracewise normalized record sections for profile A with predicted travel times from the models of Fig. 4. (a) P-wave vertical component band-pass filtered l-15 Hz. Reduction velocity=6 km/s. (b) S-wave transverse component band-pass filtered I-10 Hz. Reduction velocity=3.46 km/s.
Crustal shear-wave velocity (Spain)
(4 4 0
5
VELOCITY (km/r) 6 I 1,,..1..111....
(b) 8
9
0.20
POISSON’S RATIO 0.25
0.30
0.35
0
-l
10 -
lo-
3 1s:
x “a
20 -
20:
:
!i 25-
25 -
30 -
35 -
Fig. 4. (a) P- and S-wave velocity models for profile A. The S-wave velocities have been multiplied by 1.73. (b) Poisson’s ratio model for profile A. M=Moho. A preliminary comparison of the travel times of the P- and S-waves indicates some deviations of the S-wave velocities from Vdl.73 kmk (corresponding to a Poisson’s ratio of 0.25). Phase S, arrives later than phase Pp, i.e. later than expected for a Poisson’s ratio of 0.25, which is an indication that the theoretical S-wave velocities (VJ1.73) in the uppermost crust are too large. Phase S,S arrives earlier than the corresponding phase P,P. Phase S,S arrives roughly at expected travel times from an average Poisson’s ratio in the crust of about 0.25. A satisfactory agreement between the theoretical and observed travel times and the amplitude ratios between the S phases is obtained with the final S-wave velocity model plotted in Fig. 4a together with the P-wave model. This V,(z) distribution accounts for the observations described above. The S-wave velocities in Fig. 4a have been multiplied by 1.73, so that both curves will coincide where the Poisson’s ratio has a value of (+=0.25. The S, arrivals can be explained by an upper crust composed of two layers. Near the surface the velocity is 2.5 km/s and it increases to a value of 2.7 km/s at a depth of 1.5 km. In the crystalline basement, at a depth between 1.5 and 8 km, the velocity increases from 3.4 to 3.6 km/ s. In the middle crust, which extends to a depth of 22 km, the S-wave velocities are higher than Vdl.73, as can be seen in Fig. 4a. This increase in the S-wave velocities is necessary in order to fit the travel times of the phase S,S. The small S-velocity contrast at a depth of 22 km produces the observed low amplitude arrivals of phase S,S. In the lower crust, V, increases to 3.85 km/s just above the Moho. The S-velocity of 4.6 km/s in the uppermost mantle has been estimated by modelling the amplitudes of SHS and converted Moho waves. Combining the P- and S-wave velocity models in Fig. 4a, a Poisson’s ratio mode1 for the crust and uppermost mantle beneath line A can be calculated (Fig. 4b). The highest values of u are found in the uppermost crust, near the surface. The average u for the crystalline basement is
J. Ttllez and D. Cordoba
40
i/ 40
DISTANCE IN KM Fig. 5. Synthetic seismograms of the radial component for profile A, calculated from the models in Fig. 4a and Fig. 4b. Reduction velocity=6 km/s. Full wavefield. Compare with data in Fig. 2a.
about 0.25. In the middle crust the Poisson’s ratios are low, ~~0.25, as a consequence of the relatively high V, at a depth between 8 and 22 km. In the lower crust, uis high, i.e. a>0.25, and increases continuously with depth from about 0.25 to 0.29, due to a velocity gradient stronger for P- than for S-waves. The Poisson’s ratio is 0.27 in the uppermost mantle. We find that changes greater than 0.01 in the values of (T produce significant disagreements between the calculated and the observed travel times and amplitude ratios, therefore, the Poisson’s ratio determined is accurate to 0.01. The recorded section comprising all phases of the radial component of the ray synthetic seismograms obtained for profile A is shown in Fig. 5. To facilitate the comparison, reduction velocity, amplitude normalization and the scaling of the axes of the synthetic seismograms coincide with those of the observed data (Fig. 2a).
INTERPRETATION
OF LINE B
The seismogram sections recorded for profile B (Fig. 6) are similar to the records of profile A but, in contrast to profile A, there is a clear intra-crustal phase P,P (and also S,S) observed in profile B. As can be seen in Fig. 6, all the crustal phases of the P-wave field can be identified on the S-wave record section: an S, phase, arriving later than Pg, can be correlated up to a distance of about 60 km with an apparent velocity which increases with distance. A phase S,S, arriving about 1 s later than S,, can be recognized with large amplitudes at a distance range between about 30 and 80 km. A phase S,S can be correlated from a distance of 65 km to the end of the line, its quality depending on the component and the distance. This phase arrives earlier than the corresponding phase P$? Phase S,S can be clearly recognized in all three components with large amplitudes at a distance between 40 and 135 km. The initial S-wave velocity-depth model was modified, as necessary, to obtain an optimal adjustment between observed and calculated S-wave data. The final S-wave model (Fig. 7a) is characterized by a gradually increasing velocity in the uppermost crust down to a depth of 5 km. At the surface the velocity is 2.6 km/s and it increases to a value of 3.52 km/s at a depth of 5 km. S-wave velocity remains virtually constant down to a depth of 10 km, where S velocity jumps from 3.5 to 3.75 km/s. These values are required in order to explain the travel times and
Crustal shear-wave velocity
41
(Spain)
amplitudes of the S,S arrivals. The S-wave velocity is higher than Vdl.73 in the middle crust, which extends to a depth of 22 km; here the S-wave velocity jumps from 3.75 to 3.8 km/s; this
(a)
-2 10
@I
a* z Lz6 E4 $2 ii0 c2 -4
0
10
20
30
40
50
0
10
20
30
40
50
60
70
60
70
DISTANCE
80
90
100
110
120
130
140
80 IN KM
Fig. 6. Tracewise normalized record sections for profile B with predicted travel times from the models of Fig. 7. (a) P-wave vertical component band-pass filtered l-20 Hz. Reduction velocity=6 km/s. (b) S-wave vertical component band-pass filtered 1-15 Hz. Reduction velocity=3.46 km/s. (c) Product of the radial and vertical components. Reduction velocity = 3.46 km/s.
J. TCllez and D. C6rdoba
42
(a)
VELOCITY 5
6
-
8
9
0.20
POISSON’S RATIO 0.25
0.30
0.35
0.40
I
20:
E
7
I II
15,
s A x
(b)
(km/s)
:
a” 2s: *hi 30: .
---
P ve +1.73
:
:I : , I
354oJ
Fig.
7. (a) P- and S-wave velocity models for profile B. The S-wave velocities have been multiplied by I .73. (b) Poisson’s ratio model for profile B. M=Moho.
discontinuity causes the generation of phase S,S. In the lower crust, V, increases with depth to a value of 3.85 km/s. As in profile A, subcritical P,P and S&3 reflections of great amplitude suggest a sharp crust-mantle transition. The observation of converted Moho reflections (Fig. 2) confirms that the Moho is a first-order discontinuity. The synthetic seismograms computed with the reflectivity method for the final model (Fig. 7) are displayed in Fig. 8 with the same scaling, identical reduction velocity and amplitude normalization as the data in Fig. 2b.
40
SO
60
70
80
90
100
DISTANCE IN KM Fig. 8. Synthetic seismograms of the radial component for profile B, calculated from the models in Fig. 7a and Fig. 7b. Reduction velocity=6 km/s. Full wavefield. Compare with data in Fig. 2b.
Crustal shear-wavevelocity (Spain)
43
The a(z) distribution for line B (Fig. 7b) derived from the P- and S-wave velocity models has high values of (Tin the uppermost crust, especially near the surface where Poisson’s ratio is as high as 0.30. The low values of c between 5 and 10 km deep are a consequence of the presence of a P-wave low-velocity layer and the lack of an S-wave low-velocity zone (LVZ). Poisson’s ratio is low, less than 0.25, in the middle crust, and high and increasing with depth in the lower crust. As in profile A, the Poisson’s ratio value of 0.27 in the uppermost mantle is clearly confirmed by that estimated from a separate interpretation of the converted Moho reflections (Ttllez and Cbrdoba, 1996). DISCUSSIONAND CONCLUSIONS The shear wave velocity and Poisson’s ratio models for lines A and B presented in this paper show some differences in the distribution of the S-wave velocities and the g values but are remarkably coincident in their main features: Poisson’s ratio is high (>0.25) in the uppermost crust, low (~0.25) in the middle crust and high and increasing with depth in the lower crust. The differences in the uppermost layers of the crust can be explained by the variations in the superficial geological features crossed by the profiles (see more detailed results for the superficial structure in Cordoba and Banda, 1987). The knowledge of both V,(z) and V,(z) models, and therefore of the Poisson’s ratio distribution, allows one to draw conclusions on the composition and petrology of the crystalline crust of Northwest Spain. In order to do this, we compared our estimated V, and V, values with the P- and S-wave velocities measured in laboratory investigations on rocks and minerals (e.g. Christensen, 1982; Gebrande, 1982; Kern, 1982b). Laboratory measurements of velocities have shown a correlation between u and the degree of basicity of the rock: u increases together with basicity (e.g. Tarkov and Vavakin, 1982). In general, the P- and S-wave velocity distributions in the upper and middle crust in NW Spain are consistent with rocks of granitic/gneissic composition; a high quartz content explains the low Poisson’s ratio, 0.21-0.24, observed (e.g. Kern, 1982a). The highest values of u observed in the uppermost crust probably reflect the presence of soft and unconsolidated sediments near the surface (e.g. Gregory, 1976). The presence of crustal low-velocity zones is usually associated with factors such as compositional changes, high temperatures and fluids at high pore pressure. If we consider simultaneously the P- and S-wave velocities observed in profile B, we can rule out high temperature and high pore pressure as possible explanations of the LVZ, since these factors would reduce not only V, but also V, (e.g. Christensen, 1982) and therefore, the observed decrease in Poisson’s ratio would not take place. Possible explanations for the P-wave LVZ are highly quartz-rich rocks, such as granite or quartzite, and the presence of pore fluids at low pore pressures (Spencer and Nur, 1976; Mueller, 1977). The existence of a P-wave LVZ, without a corresponding S-wave LVZ, has also been observed in other areas of the Variscan belt (e.g. Holbrook et al., 1988). The high Poisson’s ratios in the lower crust of NW Spain are well modelled by rocks with high feldspar and low quartz content (e.g. Kern, 1982b). More information on factors such as temperature, pressure, fluid content, anisotropy, pore pressure, etc., would be required to obtain a more precise picture of the crustal composition in NW Spain. However, the results obtained in this study demonstrate that the interpretation of shear waves in seismic refraction experiments provides additional information for the understanding of the structure and composition of the crust. The determination of both shear wave velocities and Poisson’s ratios imposes significant constraints on the structural, physical
J. TCllez and D. Cordoba
44
and petrological models of the crust which could have been considered interpretation of P-waves.
viable from the mere
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Cordoba D. and Banda E. (1987) Estructura de 10s mazicos de Cabo Ortega1 y Ordenes a partir de datos sismicos. In Geologia de 10s Granitoides y rotas asociadas de1 Macizo Hespe’rico (Bea F., Carnicero A., Gonzalo J. C., Lopez Plaza M. and Rodriguez Alonso M. D., eds), pp. 427437. Editorial Rueda, Madrid. Cordoba D., Banda E. and Ansorge J. (1987) The Hercynian crust in northwestern Spain: a seismic survey. Tectonophysics 132, 321-333. Cordoba D., Banda E. and Ansorge J. (1988) P-wave velocity-depth distribution in the Hercynian crust of northwest Spain. Phys. Earth Planet. Inter: 51, 235-248. Ergin K. (1952) Energie ratio of the seismic waves reflected and refracted at a rock-water boundary. Bull. Seis. Sot. Am. 42,349-371. Farias I?, Gallastegui G., Gonzalez F., Marquinez J., Martin L. M., Martinez J. R., Pablo J. G. de and Rodriguez L. R. (1985) Aportaciones al conocimiento de la litoestratigrafia y estructura de Galicia Central, IX Reunion Geologia de1 Oeste Peninsular, Porto, pp. 411-431. Fertig J. (1984) Shear waves by an explosive point-source: The earth surface as generator of converted P-S waves. Geophys. Prospect. 32, l-l 7. Fuchs K. and Miiller G. (1971) Computation of synthetic seismograms with the reflectivity method and comparison with observations. Geophys. J. R. Astron. Sot. 23,417433. Gebrande H. (1982) Elasticity and inelasticity. In Physical Properties of Rocks, vol. lb (Angenheister G., ed), pp. l-99. Landolt-Bbmstein. Springer, New York. Gregory A. R. (1976) Fluid saturation effects of dynamic elastic properties of sedimentary rocks. Geophysics 41,895-92 1. Holbrook W. S., Gajewski D., Krammer A. and Prodehl C. ( 1988) Interpretation of wide-angle compressional and shear wave data in Southwest Germany: Poisson’s ratio and petrological implications. J. Geophys. Res. 93, 1208 l-l 2 106. Kern H. (1982) Elastic-wave velocity in crustal and mantle rocks at high pressure and temperature: The role of the high-low quartz transition and of dehydration reactions. Phys. Earth Planet. tnter: 29, 12-23.
Kern H. (1982b) Elasticity and inelasticity. In Physical Properties of Rocks, vol. lb (Angenheister G., ed), pp. 99-140. Landolt-Bornstein, Springer, New York. Kim N. W. and Seriff A. J. (1992) Marine PSSP reflections with a bottom velocity transition zone. Geophysics 57, 16 1- 170. Matte Ph. (1983) Two geotraverses across the Ibero-Armorican arc of Western Europe. Am. Geophys. Union, Geodyn. Ser: 10,53-81. Mueller St. (1977) A new model of the continental crust. In The Earth S Crust - Its Nature and Physical Properties (Heacock J. G.. ed). Geophysical Monograph 20, pp. 289-3 17. American
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velocity (Spain)
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Geophysical Union, Washington, D.C. Spencer J. W. and Nur A. (1976) The effect of pressure, temperature and pore water on velocities in Westerly granite. J. Geophys. Res. 81,899-904. Spetzler H. and Anderson D. L. (1968) The effect of temperature and partial melting on velocity and attenuation in a simple binary system. J. Geophys. Res. 73,6051-6060. Tarkov A. P. and Vavakin V. V. (1982) Poisson’s ratio behaviour in various crystalline rocks: application to the study of the Earth’s interior. Phys. Earth Planet. Inter. 29,24-29. Tellez J. (1993) Analisis e interpretation de ondas P y S de perfiles sismicos. Aplicacion al noroeste de la Peninsula Iberica, Ph.D. thesis, Universidad Complutense, Madrid. Tellez J., Matias L. M., Cordoba D. and Mendes-Victor L. A. (1993) Structure of the crust in the schistose domain of Galicia-Tras-os-Montes (NW Iberian Peninsula). Tectonophysics 221, 8 l-93. Tellez J. and Cordoba D. (1996) Observation of converted Moho reflections in the north-west of the Iberian Peninsula. Geophys. J. Znt. 124,7-17.