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Seismic anisotropy and its relation with crust structure and stress field in the Reggio Emilia Region (Northern Italy)
Language
English
Status
Published
JCR Journal
JCR Journal
Peer review journal
Yes
Title of the book
Issue/vol(year)
/ 167 (2006)
Pages (printed)
1035–1043
Issued date
2006
Abstract
Shear wave splitting is measured at 14 seismic stations in the Reggio Emilia region above local
background seismicity and two sequences of seismic events. The good quality of thewaveforms
together with the favourable distribution of earthquake foci allows us to place strong constraints
on the geometry and the depth of the anisotropic volume. It is about 60 km2 wide and located
between 6 and 11 km depth, inside Mesozoic age carbonate rocks. The splitting results suggest
also the presence of a shallower anisotropic layer about 1 km thick and few km wide in
the Pliocene–Quaternary alluvium above the Mesozoic layer. The fast polarization directions
(N30◦E) are approximately parallel to the maximum horizontal stress (σ 1 is SSW–NNE) in
the region and also parallel to the strike of the main structural features in the Reggio Emilia
area. The size of the delay times suggests about 4.5 per cent shear wave velocity anisotropy.
These parameters agree with an interpretation of seismic anisotropy in terms of the extensivedilatancy
anisotropy model which considers the rock volume to be pervaded by fluid-saturated
microcracks aligned by the active stress field. We cannot completely rule out the contribution
of aligned macroscopic fractures as the cause of the shear wave anisotropy even if the parallel
shear wave polarizations we found are diagnostic of transverse isotropy with a horizontal axis
of symmetry. This symmetry is commonly explained by parallel stress-aligned microcracks.
background seismicity and two sequences of seismic events. The good quality of thewaveforms
together with the favourable distribution of earthquake foci allows us to place strong constraints
on the geometry and the depth of the anisotropic volume. It is about 60 km2 wide and located
between 6 and 11 km depth, inside Mesozoic age carbonate rocks. The splitting results suggest
also the presence of a shallower anisotropic layer about 1 km thick and few km wide in
the Pliocene–Quaternary alluvium above the Mesozoic layer. The fast polarization directions
(N30◦E) are approximately parallel to the maximum horizontal stress (σ 1 is SSW–NNE) in
the region and also parallel to the strike of the main structural features in the Reggio Emilia
area. The size of the delay times suggests about 4.5 per cent shear wave velocity anisotropy.
These parameters agree with an interpretation of seismic anisotropy in terms of the extensivedilatancy
anisotropy model which considers the rock volume to be pervaded by fluid-saturated
microcracks aligned by the active stress field. We cannot completely rule out the contribution
of aligned macroscopic fractures as the cause of the shear wave anisotropy even if the parallel
shear wave polarizations we found are diagnostic of transverse isotropy with a horizontal axis
of symmetry. This symmetry is commonly explained by parallel stress-aligned microcracks.
References
Babuˇska, V. & Cara, M., 1991. Seismic Anisotropy in the Earth, Kluwer
Acad., Norwell, Massachusetts, p. 217.
Booth, D.C. & Crampin, S., 1985. Shear-wave polarizations on a curved
wavefront at an isotropic free surface, Geophys. J. R. astr. Soc., 83(1),
31–45.
Bowman, J.R.&Ando, M.A., 1987. Shear-wave splitting in the upper mantle
wedge above the Tonga subduction zone, Geophys. J. R. astr. Soc., 88, 25–
41.
Ciaccio, M.G. & Chiarabba, C., 2002. Tomographic models and seismotectonics
of the Reggio Emilia region, Italy, Tectonophysics, 344, 261–
276.
Crampin, S., 1978. Seismic-wave propagation through a cracked solid: polarization
as a possible dilatancy diagnostic, Geophys. J. R. astr. Soc., 53,
467–496.
Crampin, S., 1981. A review of wave motion in anisotropic and cracked
elastic-media, Wave Motion, 3, 343–391.
Crampin, S., 1999. Calculable fluid-rock interactions, J. Geol. Soc., 156,
501–514.
Table 4. Average splitting parameters and standard deviations for each station.
Results obtained from events of the 2000 shallower (6 km) seismic
sequence are denoted by ∗.
Station No of Average Average
analysed events φ φ δ τ δ τ
COD 27 38◦ 5◦ 0.12 s 0.02 s
FAN 27 38◦ 4◦ 0.10 s 0.05 s
MAN 27 21◦ 5◦ 0.13 s 0.03 s
SIL 25 21◦ 8◦ 0.09 s 0.03 s
ZAV 25 17◦ 4◦ 0.06 s 0.02 s
BAG 14 15◦ 5◦ 0.09 s 0.01 s
COR 14 24◦ 4◦ 0.08 s 0.02 s
CST 4 Null
FAB 6 Null
GUA 12 Null
NOV 14 Null
PDF 12 Null
REG 14 45◦ 7◦ 0.12 s 0.02 s
SAB 14 Null
COD∗ 12 Null
FAN∗ 12 32◦ 8◦ 0.03 s 0.01 s
MAN∗ 12 Null
SIL∗ 12 Null
ZAV∗ 11 16◦ 2◦ 0.05 0.02 s
Crampin, S. & Lovell, J.H., 1991. A decade of shear-wave splitting
in the Earth’s crust: what does it mean? What use can we make
of it? and what should we do next?, Geophys. J. Int., 107, 387–
407.
Crampin, S. & Chastin, S., 2003. A review of shear-wave splitting in the
crack-critical crust, Geophys. J. Int., 155, 221–240.
Crampin, S. & Peacock, S., 2003. Seismic evidence for fluid-driven deformation,
J. Geodyn., 36, 67–77.
Davis, J.C., 1986. Statistics and Data Analysis in Geology, John Wiley,
Hoboken, NJ, p. 646.
Evans, R., 1984. Effects of the free surface on shear wavetrains, Geophys. J.
R. astr. Soc., 76(1), 165–172.
Evans, J.R.&Bruce, R.J., 1995. Shear-wave splitting from local earthquakes
at the Geysers geothermal field, California, Geophys. Res. Lett., 22, 501–
504.
Frepoli, A. & Amato, A., 1997. Contemporaneous extension and compression
in the Northern Apennines from earthquake fault-plane solution,
Geophys. J. Int., 129, 368–388.
Jurkevics, A., 1988. Polarization analysis of three-component array data,
Bull. seism. Soc. Am., 78, 1725–1743.
Montone, P. & Mariucci, M.T., 1999. Active stress along the NE external
margin of the Apennines: the Ferrara arc, northern Italy, Geodynamics,
28, 251–265.
Pieri, M. & Groppi, P., 1981. Subsurface geological structure of the Po
Plain, Italy, AGIP-CNR Progetto Finalizzato Geodinamica, pub. 414,
Roma.
Pieri, M. & Groppi, G., 1998. Subsurface geological structure of the Po
Plain, Pubbl. 414 P.F. Geodinamica C.N.R., 1–23.
Pondrelli, S., Morelli, A., Ekstr¨om, G., Mazza, S., Boschi, E.&Dziewonski,
A.M., 2002. European-Mediterranean regional centroid-moment tensors:
1997–2000, Phys. Earth planet. Int., 130, 71–101.
Selvaggi, G. et al., 1996. The October 15, 1996, Reggio Emilia seismic
sequence: active compression tectonics in the Po Plain Italy, Geophys. J.
Int., 144, 1–13.
Silver, P.G. & Chan, W.W., 1991. Shear wave splitting and subcontinental
mantle deformation, J. geophys. Res., 96, 16 429–16 454.
Vannucci, G., Pondrelli, S., Argnani, A., Morelli, A., Gasperini,
P. & Boschi, E., 2004. An Atlas of Mediterranean seismicity,
Annals of Geophysics supplement to 47, 247–
334.
Zatsepin, S.V. & Crampin, S., 1997. Modelling the compliance of crustal
rock: I—response of shear-wave splitting to differential stress, Geophys.
J. Int., 129, 477–494.
Zhang, Z. & Schwartz, S.Y., 1994. Seismic anisotropy in the shallow crust
of the Loma Prieta segment of the San Andreas fault system, J. geophys.
Res., 99, 9651–9661.
Zinke, J.C. & Zoback, M.D., 2000. Structure-related and stress-induced
shear-wave velocity anisotropy: observations from microearthquakes near
the Calaveras Fault in Central California, Bull. seism. Soc. Am., 90, 1305–
1312.
Zollo, A., De Matteis, R., Capuano, P., Ferulano, F. & Iannaccone, G., 1995.
Costraints on the shallow crustal model of the Northern Apennines from
the analysis of microearthquake seismic records, Geophys. J. Int., 120,
646–662.
Acad., Norwell, Massachusetts, p. 217.
Booth, D.C. & Crampin, S., 1985. Shear-wave polarizations on a curved
wavefront at an isotropic free surface, Geophys. J. R. astr. Soc., 83(1),
31–45.
Bowman, J.R.&Ando, M.A., 1987. Shear-wave splitting in the upper mantle
wedge above the Tonga subduction zone, Geophys. J. R. astr. Soc., 88, 25–
41.
Ciaccio, M.G. & Chiarabba, C., 2002. Tomographic models and seismotectonics
of the Reggio Emilia region, Italy, Tectonophysics, 344, 261–
276.
Crampin, S., 1978. Seismic-wave propagation through a cracked solid: polarization
as a possible dilatancy diagnostic, Geophys. J. R. astr. Soc., 53,
467–496.
Crampin, S., 1981. A review of wave motion in anisotropic and cracked
elastic-media, Wave Motion, 3, 343–391.
Crampin, S., 1999. Calculable fluid-rock interactions, J. Geol. Soc., 156,
501–514.
Table 4. Average splitting parameters and standard deviations for each station.
Results obtained from events of the 2000 shallower (6 km) seismic
sequence are denoted by ∗.
Station No of Average Average
analysed events φ φ δ τ δ τ
COD 27 38◦ 5◦ 0.12 s 0.02 s
FAN 27 38◦ 4◦ 0.10 s 0.05 s
MAN 27 21◦ 5◦ 0.13 s 0.03 s
SIL 25 21◦ 8◦ 0.09 s 0.03 s
ZAV 25 17◦ 4◦ 0.06 s 0.02 s
BAG 14 15◦ 5◦ 0.09 s 0.01 s
COR 14 24◦ 4◦ 0.08 s 0.02 s
CST 4 Null
FAB 6 Null
GUA 12 Null
NOV 14 Null
PDF 12 Null
REG 14 45◦ 7◦ 0.12 s 0.02 s
SAB 14 Null
COD∗ 12 Null
FAN∗ 12 32◦ 8◦ 0.03 s 0.01 s
MAN∗ 12 Null
SIL∗ 12 Null
ZAV∗ 11 16◦ 2◦ 0.05 0.02 s
Crampin, S. & Lovell, J.H., 1991. A decade of shear-wave splitting
in the Earth’s crust: what does it mean? What use can we make
of it? and what should we do next?, Geophys. J. Int., 107, 387–
407.
Crampin, S. & Chastin, S., 2003. A review of shear-wave splitting in the
crack-critical crust, Geophys. J. Int., 155, 221–240.
Crampin, S. & Peacock, S., 2003. Seismic evidence for fluid-driven deformation,
J. Geodyn., 36, 67–77.
Davis, J.C., 1986. Statistics and Data Analysis in Geology, John Wiley,
Hoboken, NJ, p. 646.
Evans, R., 1984. Effects of the free surface on shear wavetrains, Geophys. J.
R. astr. Soc., 76(1), 165–172.
Evans, J.R.&Bruce, R.J., 1995. Shear-wave splitting from local earthquakes
at the Geysers geothermal field, California, Geophys. Res. Lett., 22, 501–
504.
Frepoli, A. & Amato, A., 1997. Contemporaneous extension and compression
in the Northern Apennines from earthquake fault-plane solution,
Geophys. J. Int., 129, 368–388.
Jurkevics, A., 1988. Polarization analysis of three-component array data,
Bull. seism. Soc. Am., 78, 1725–1743.
Montone, P. & Mariucci, M.T., 1999. Active stress along the NE external
margin of the Apennines: the Ferrara arc, northern Italy, Geodynamics,
28, 251–265.
Pieri, M. & Groppi, P., 1981. Subsurface geological structure of the Po
Plain, Italy, AGIP-CNR Progetto Finalizzato Geodinamica, pub. 414,
Roma.
Pieri, M. & Groppi, G., 1998. Subsurface geological structure of the Po
Plain, Pubbl. 414 P.F. Geodinamica C.N.R., 1–23.
Pondrelli, S., Morelli, A., Ekstr¨om, G., Mazza, S., Boschi, E.&Dziewonski,
A.M., 2002. European-Mediterranean regional centroid-moment tensors:
1997–2000, Phys. Earth planet. Int., 130, 71–101.
Selvaggi, G. et al., 1996. The October 15, 1996, Reggio Emilia seismic
sequence: active compression tectonics in the Po Plain Italy, Geophys. J.
Int., 144, 1–13.
Silver, P.G. & Chan, W.W., 1991. Shear wave splitting and subcontinental
mantle deformation, J. geophys. Res., 96, 16 429–16 454.
Vannucci, G., Pondrelli, S., Argnani, A., Morelli, A., Gasperini,
P. & Boschi, E., 2004. An Atlas of Mediterranean seismicity,
Annals of Geophysics supplement to 47, 247–
334.
Zatsepin, S.V. & Crampin, S., 1997. Modelling the compliance of crustal
rock: I—response of shear-wave splitting to differential stress, Geophys.
J. Int., 129, 477–494.
Zhang, Z. & Schwartz, S.Y., 1994. Seismic anisotropy in the shallow crust
of the Loma Prieta segment of the San Andreas fault system, J. geophys.
Res., 99, 9651–9661.
Zinke, J.C. & Zoback, M.D., 2000. Structure-related and stress-induced
shear-wave velocity anisotropy: observations from microearthquakes near
the Calaveras Fault in Central California, Bull. seism. Soc. Am., 90, 1305–
1312.
Zollo, A., De Matteis, R., Capuano, P., Ferulano, F. & Iannaccone, G., 1995.
Costraints on the shallow crustal model of the Northern Apennines from
the analysis of microearthquake seismic records, Geophys. J. Int., 120,
646–662.
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