Effects of the free surface on shear wavetrains

Evans, Russ

doi:10.1111/j.1365-246x.1984.tb05032.x

Cited by 138 publications

(92 citation statements)

References 11 publications

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“…• for the Poisson's ratio of 0.25) (Evans, 1984;Booth and Crampin, 1985). In only a pure strikeslip source with strike parallel to the fault zone was treated, and for this source type the formulation of the wave source was described.…”

Section: Methodsmentioning

confidence: 99%

A numerical analysis of seismic waves for an anisotropic fault zone

Nakamura

Takenaka

2006

Earth Planet Sp

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In this study we examine the effects of anisotropy on the seismic wavefield in a fault zone from computation of the synthetic seismograms for a simple fault zone model and a variety of seismic wave sources. The fault zone is modeled by a homogeneous vertical layer with transverse isotropy, induced by cracks, sandwiched between isotropic half-spaces (host rocks). The symmetry axis of the transverse isotropy is horizontal and perpendicular to the fault zone strike. We calculate the synthetic seismograms for this anisotropic fault zone model using a semianalytical method, the propagator matrix method. The synthetic seismograms show a later phase arriving after the main shear-wave in the horizontal component perpendicular to the fault zone strike at most stations near the fault zone. It is the slower shear-wave (q S 2 ) and its reverberation. The amplitude of this phase and the time delay from the main shear-wave arrival are proportional to the degree of anisotropy, which suggests that observing such phase in field measurements may imply the presence of an anisotropic fault zone. We also perform the shear-wave splitting measurements by applying the cross-correlation method to the synthetic seismograms for various sources. For a strike-slip source, the synthetic seismograms show that the wavefield is more affected by the velocity structure than by the degree of anisotropy, which makes it difficult to estimate the anisotropic (shearwave splitting) parameters. For normal and dip-slip fault sources with the strike parallel to or striking against the fault zone, the effects of anisotropy is so dominant that the anisotropic fault zone can be detected. These results suggest that the determination of the anisotropic properties in the fault zone would require an appropriate station deployment and the source type information.

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Section: Methodsmentioning

confidence: 99%

A numerical analysis of seismic waves for an anisotropic fault zone

Nakamura

Takenaka

2006

Earth Planet Sp

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“…• (Evans, 1984). The incidence angle depends on the V p/V s ratio and was calculated directly using the depth and epicentral distance of the earthquake.…”

Section: Datamentioning

confidence: 99%

Crustal Seismic Anisotropy at the Edge of the South-East Border of Caribbean and South American Plates

Espinoza¹,

Feliciangeli²

2013

Rev. Bras. Geof.

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ABSTRACT. Anisotropic properties in the NE of the Caribbean-South American plate are investigated based on the analysis of shear-wave splitting from new S local phase data. At the collisional front in the Caribbean and the South American plates and within the retro-arc region, fast wave polarization from local S phases show considerable variability. δt are small (<0.14 s) and no dependence with depth is observed suggesting the presence of an heterogeneous anisotropic region confined to the crust and that most of the upper-mantle wedge is isotropic. In the fore-arc, φ is fairly constant, the average value is sub-parallel to the Lesser Antilles trench and differs from teleseismic result, which is perpendicular to the trench, which suggests that the crust and mantle are decoupled. The difference between the δt found in S local events and teleseismic, suggests that there is another source of deep anisotropy, which can be attributed to mantle flow underneath or around the edges of the subducting Atlantic plate beneath the Caribbean. A mix combination of deformation mechanism such as micro-fractures EDA and vertical foliation on metamorphic rocks could be responsible for the large variability found for δt in S local phases in this region.Keywords: seismic anisotropy, deformation mechanisms, shear-wave splitting, crust, mantle. RESUMEN.Se investigan las características anisotrópicas en el NE de la placa Caribe-Sur América basados en el análisis de nuevos resultados del desdoblamiento de ondas de corte en fases locales S. En el frente de colisión entre placa Caribe y Suramerica y en la zona de retro-arco, la polarización de onda rápida en eventos locales S muestran considerable variabilidad. Los δt son pequeños (<0,14s) y no presenta dependencia con la profundidad, sugiriendo la presencia de una región anisotropícamente heterogenea confinada a la corteza, siendo además la cuña del manto superior isotrópico. En el ante-arco, φ es poco variable, el valor promedio es sub-paralello a la trinchera de las Antillas Menores y difiere del valor obtenido para fases telesísmicas el cual es perpendicular a la trinchera, lo cual sugiere que la corteza y el manto en esta zona se encuentran desacoplados. La diferencia entre los δt en eventos locales S y telesísmicos, sugiere que existe otra fuente de anisotropía profunda, que se puede atribuir a flujo mantélico debajo o alrededor de los bordes de la placa Atlántica en subducción debajo del Caribe. Una combinación de mecanismos de deformación, tales como micro-fracturas EDA y foliación vertical en rocas metamórficas podría ser responsable de la gran variabilidad encontrada en los δt en fases S locales en esta región.Palabras claves: anisotropía sísmica, mecanismos de deformación, doblamiento de ondas de corte, corteza, manto.

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“…Secondly we selected the seismic waves whose incident angles θ are less than 35 degrees, corresponding to the "shear-wave window" of Booth and Crampin (1985) to avoid the influence of SP converted wave at the free surface (Evans, 1984). To estimate the incident angle θ , we referred the almost constant V P /V S (∼1.70) model of the upper crust of the JMA2001 velocity model (Ueno et al, 2002).…”

Section: Observation and Data Selectionmentioning

confidence: 99%

Strong S-wave anisotropy in the aftershock region of the 2000 Tottori-ken Seibu, Japan, earthquake (Mw6.6)

2005

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The 2000 Tottori-ken Seibu earthquake (MW6.6) occurred in Tottori prefecture, western Japan on October 6, 2000. We conducted aftershock observation and analyzed S-wave anisotropy using the aftershocks MJMA2.0 to 3.5 observed at two stations near the aftershock region. The fast S-waves are polarized to almost E-W direction, which corresponds to the P-axis direction of the mainshock. The delay time of the split S-waves ranges between 20 and 100 ms. The crack density inside and outside the aftershock area is estimated from the delay time. The distribution of crack density shows a strong spatial variation and depends on the ratio of the path length inside the aftershock region against the whole path length. Assuming a uniform distribution of anisotropy inside and outside the aftershock region, the crack densities inside and outside are 0.017 and 0.007, respectively. It thus seems probable that in the aftershock region the distribution of cracks is intensive and cracks are opened due to the presence of fluids in seismogenic layers. This considerable spatial variation of anisotropy between inside and outside the aftershock region suggests that the aftershock region may have different mechanical properties from the surrounding area.

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Effects of the free surface on shear wavetrains

Cited by 138 publications

References 11 publications

A numerical analysis of seismic waves for an anisotropic fault zone

A numerical analysis of seismic waves for an anisotropic fault zone

Crustal Seismic Anisotropy at the Edge of the South-East Border of Caribbean and South American Plates

Strong S-wave anisotropy in the aftershock region of the 2000 Tottori-ken Seibu, Japan, earthquake (Mw6.6)

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