Shear-wave polarizations near the North Anatolian Fault -I. Evidence for anisotropy-induced shear-wave splitting

Booth, David C.; Crampin, Stuart; Evans, Russ; Roberts, Gareth

doi:10.1111/j.1365-246x.1985.tb05156.x

Cited by 86 publications

(14 citation statements)

References 16 publications

Supporting

Mentioning

Contrasting

Order By: Relevance

“…Because cracks, faults, and fractures are confined to the brittle upper crust (0 -15 km), the dt associated with such aligned structures are probably within the noise of teleseismic shear wave splitting. This is consistent with the typically observed crustal dt of 0.1-0.3 s [Booth et al, 1985[Booth et al, , 1990Kaneshima et al, 1988;Kaneshima, 1990;Shih and Meyer, 1990;Crampin, 1994;McNamara et al, 1994]. However, partially molten tabular intrusions (dikes) are a possible cause of teleseismic splitting because the velocity contrast between magma and the surrounding rock is large, and parallel dikes could penetrate through the entire lithosphere, leading to considerable dt.…”

Section: Anisotropy Due To Parallel Dikes or Magma-filled Lensessupporting

confidence: 88%

On the relationship between extension and anisotropy: Constraints from shear wave splitting across the East African Plateau

et al. 2004

View full text Add to dashboard Cite

[1] East Africa is a tectonically complex region owing to the presence of a rigid craton, paleothrust belts and shear zones, active magmatism and rifting, and possibly even a mantle plume. We present new splitting results of teleseismic shear phases recorded by 21 broadband seismic stations in Tanzania, seven broadband stations in Kenya, and three permanent broadband Global Seismic Network stations in Kenya and Uganda. Inconsistent apparent splitting is observed beneath the craton and along its southern and southeastern flank in Tanzania. Splitting at stations elsewhere in the rifts and orogenic belts is more consistent. We test between different models of anisotropy for stations with inconsistent splitting (single layer with horizontal fast axis, single layer with dipping fast axis, and two layers with horizontal fast axes). However, we show that these more complicated models do not reasonably explain the data. The data are explained better by a laterally (and/or in some places vertically) varying single-layer model with a horizontal fast axis. We arrive at a conceptual model of anisotropy in Tanzania and Kenya that is controlled by (1) active shear along the base of the plate associated with asthenospheric flow beneath and around the moving craton keel, (2) asthenospheric flow from a plume north of central Kenya, (3) fossilized anisotropy in the lithosphere due to past orogenic events, and possibly (4) aligned magma-filled lenses beneath the rifts. Our most robust conclusion is that we can rule out an extension-induced lattice preferred orientation of olivine as a dominant factor, which is surprising given the long history of extension in the region. This indicates that mantle-lithospheric extension in East Africa occurs via dikeintrusion and/or ductile thinning within narrow rift zones and is possibly facilitated by a mechanical lithospheric anisotropy imparted by fossilized north/south structural or mineralogical fabrics.

show abstract

Section: Anisotropy Due To Parallel Dikes or Magma-filled Lensessupporting

confidence: 88%

On the relationship between extension and anisotropy: Constraints from shear wave splitting across the East African Plateau

et al. 2004

View full text Add to dashboard Cite

show abstract

“…Recently, many researchers have proposed the presence of seismic anisotropy in the upper crust [e.g., Crampin et al, 1984;Booth et al, 1985;Buchbinder, 1985;Kaneshima et al, 1987Kaneshima et al, , 1988aKaneshima et al, , 1989Peacock et al, 1988]. Most of them have analyzed three-component seismograms from crustal earthquakes with various azimuths and incident angles, finding out parallel or subparallel alignments of the particle motion upon the leading shear wave arrivals.…”

Section: Introductionmentioning

confidence: 99%

An analysis of split shear waves observed above crustal and uppermost mantle earthquakes beneath Shikoku, Japan: Implications in effective depth extent of seismic anisotropy

Kaneshima¹,

Ando²

1989

J. Geophys. Res.

View full text Add to dashboard Cite

Shear wave splitting has been detected in three‐component seismograms from a number of small earthquakes taking place either within the upper crust or the uppermost mantle beneath the Shikoku area, Japan. The faster shear waves are polarized nearly in E‐W at the stations in the central part of Shikoku, and in NEN‐SWS at the southern stations. The arrivals of slower shear waves polarized orthogonally to the faster ones are also clearly recognized in many seismograms at one of the stations. The faster shear wave polarizations at the stations in the central part of Shikoku and the pattern of travel time differences between split shear waves are successfully explained in terms of crustal anisotropy generated by vertical alignment of stress‐induced microcracks. The difference in polarization directions suggests that the tectonic stress state beneath the southern Shikoku region is significantly different from that beneath the central part of Shikoku. A quantitative analysis of travel time differences between split shear waves suggests that the presence of seismic anisotropy which causes the observed shear wave splitting is limited to the upper 10 or 15 km of the crust. Nevertheless, the possibility that anisotropy is concentrated in a much thinner surface layer cannot be ruled out. The average aspect ratio of crack might be greater than of the order of 10−2, or the cracks may not be purely vertical but have some fluctuations in the dip angle.

show abstract

“…The nest is located within the W-B zone, which is expected to be close to the interface of the subducting slab angle of $-to-P conversion (35 ø for Vs/Vp •-1.7) (Table 1), assuming that P and S waves traveled the same paths (constant Poisson's ratios); therefore the effect of S-to-P conversion at the surface on observed shear wave particle motion should be small [Nuttli, 1961;Booth and Crampin, 1985]. At LSL, the calculated incident angle is 36ø; its data are used but with caution.…”

Section: Shear Wave Splitting: Nest Eventsmentioning

confidence: 99%

“…Determination of time delay was difficult in some cases because several phase matchings, i.e., similar linearity, occurred in the process of shifting. The time delay was then confirmed by particle motion diagrams [e.g.,Booth et al, 1985], and/or measured on the seismograms of the two horizontal components rotated to the fast and slow directions(Figures 3b and 3d)[e.g.,Bowman and Ando, 1987]. The largest average time delays, 0.42 -!-0.04 and 0.39 ñ 0.05 s at 99% confidence level, are at seismographs BCA and BCH, respectively (Figure 6 and…”

mentioning

confidence: 99%

Polarities of P and S waves, and shear wave splitting observed from the Bucaramanga Nest, Colombia

Shih¹,

Schneider²,

Meyer³

1991

J. Geophys. Res.

View full text Add to dashboard Cite

The Bucaramanga nest, Colombia, is an intense seismic source centered at 6ø48'N, 73ø10'W and 161-krn depth beneath the eastern Andes. During a 3-week period, 161 microearthquakes (mb -Q 4.3) were recorded by a 260-by-140-km array of 12 three-component University of Wisconsin (UW) seismographs centered above the nest. Eighty-eight percent of the events recorded were from the nest, providing a unique opportunity to study natural events that have different source parameters but nearly the same ray paths. Shear wave splitting analysis shows that the average polarization direction of the first-arriving shear waves from the nest events at nine of the seismographs is N12øE q-18 ø. After corrections based on the amount of discrepancy between the observed average P wave azimuth and the expected event-to-station azimuth, average S wave polarization is N6øE q-15 ø. Variation in time delays with azimuth and epicentral distance appears to be due to the varying angles between the ray path and the principal axes of the anisotropic system. Shear wave splitting observed from 17 events along the Wadati-Benioff zone outside the nest shows that the time delays increase with increasing depth, and defines a wedge-shaped anisotropic volume above the subducting plate. The anisotropic layer has a maximum thickness of approximately 60 km at the depth of the nest and is thinning updip. The anisotropy most likely results from aligned anisotropic minerals such as olivine and orthopyroxene in a, localized mantle flow in the lithosphere above the Wada,ti-Benioff zone, a region weakened by active fluid migration and mobilized by the heating and shearing along the suducting slab. related to subduetion of Nazca (E-W) and Caribbean (WNW-ESE) plates under the South American plate (Figure 1). The present Wadati-Benioff (W-B) zone seems to be closely associated with the Nazca plate, which is converging faster (6-8 cm/yr) toward the South American plate than is the Caribbean plate (1-2 cm/yr). The seismicity of the nest has been studied by many authors [Santo, 1969; Isacks and Molnar, 19713 Dewey, 19723 Pennington et al., 1979; Pennington, 1981; Schneider et al., 1987]. In a local microseismicity study, Schneider et al. [19871 found that the volume of the nest is approximately 8 x 4 x 4 km • and is centered at 161 km depth. The dip of the W-B zone is less than 30 ø from the trench to the nest and increases to near-vertical in the vicinity of the nest [Schneider et al., 1987]. The focal mecha. nisms of nest events are variable; two distinct mechanisms were found from teleseisms [Isacks and Molnar, 1971; Dewey, 1972; Pennington, 1981], but no distinct orientations of stress axes were noted from the locally recorded events [Schneider et al., 1987] used in this analysis. This paper focuses on the azimuths of the particle motions of both compressional and shear waves from intermediate-depth microearthquakes. We determine the azimuth of approach of compressional waves, the polarization direction of the first-arriving shear waves, and the time delays bet•vee...

show abstract

Shear-wave polarizations near the North Anatolian Fault -I. Evidence for anisotropy-induced shear-wave splitting

Cited by 86 publications

References 16 publications

On the relationship between extension and anisotropy: Constraints from shear wave splitting across the East African Plateau

On the relationship between extension and anisotropy: Constraints from shear wave splitting across the East African Plateau

An analysis of split shear waves observed above crustal and uppermost mantle earthquakes beneath Shikoku, Japan: Implications in effective depth extent of seismic anisotropy

Polarities of P and S waves, and shear wave splitting observed from the Bucaramanga Nest, Colombia

Contact Info

Product

Resources

About