Premonitory Variations in
            <i>S</i>
            -Wave Velocity Anisotropy before Earthquakes in Nevada

A new technique is presented for modelling the elastic constants of cracked structures with application to systems with weak concentrations of parallel cracks, and of simple biplanar and triplanar cracks. The velocities and V,lVs ratios of these anisotropic structures are used to provide quantitative models for some earthquake precursors. These results indicate the great importance of crack geometry to the behaviour of precursors. The velocities of saturated cracks appear to favour the dilatancy-diffusion model of precursory phenomena. Synthetic seismograms are calculated for propagation through possible dilatancy zones. The seismograms show some characteristic features which may be useful for the investigation of earthquake dilatancy .

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“…Gupta (1973) attempted to recognize S-wave splitting in dilatancy zones, but his observational criteria for recognizing Fig. 14(b).…”

Section: P R O P a G A T I O N In M U L T I P L A N A R C R A C K S Ymentioning

confidence: 99%

Seismic-wave propagation through a cracked solid: polarization as a possible dilatancy diagnostic

1978

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“…The cause of the anisotropic zones has been researched by many scientists. Two possible models have been proposed for the explanation of the observed shear-wave splitting: one is the olivine alignment model (Hess, 1964;Sugimura and Uyeda, 1967;Francis, 1969;Fuchs, 1977), the other is the crack alignment model (Nur and Simmons, 1969;Gupta, 1973;Crampin, 1978;Crampin et al, 1980). Seismic anisotropy beneath the ocean is commonly observed and is probably related to seafloor spreading (e.g., Bibee and Shor, 1976;Fuchs, 1977).…”

Section: Introductionmentioning

confidence: 99%

Shear-wave splitting analysis of the upper mantle at the Niigata-Kobe Tectonic Zone with the data of the Joint Seismic Observations at NKTZ

Iidaka

Hiramatsu

2009

Earth Planet Sp

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We conducted seismic observations with a spatially high density seismic network at the Niigata-Kobe Tectonic Zone, central Japan. The seismic network was used for the analysis of shear-wave splitting. Large lateral variations were found in the polarization direction data: the northern part of the research area yields polarization directions of NW-SE (Region A), the central part of the research area with the polarization direction of NNE-SSW (Region B), the eastern part of the research area with the polarization direction of NE-SW (Region C), and the southern part of the research area with the polarization direction of E-W (Region D). The polarization directions in Regions B and C could be explained by the preferred orientation of olivine caused by the flow of the subducting Philippine Sea plate. However, the cause of anisotropic region which was related to the heterogeneous structure was also plausible. The polarization direction in Region A might be related to the flow caused by both of the subducting Philippine Sea and Pacific slabs. The polarization direction at the Region D could not be produced by the flow in the wedge and might be related to an anisotropic region beneath the slab. The lateral variation of the polarization direction does not support a model that the NKTZ is a collision region.

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“…These ideas eventually led to observations of temporal changes (with hindsight) in Band-1 time delays before many earthquakes, including one successful stress forecast of an M5 earthquake in Iceland . Note that Gupta (1973a) had earlier claimed to have observed temporal variations in shear-wave splitting before earthquakes. Unfortunately, Gupta (1973a) had misinterpreted laboratory experiments (Gupta 1973b) and identified crustal observations of shear-wave splitting so far outside the effective shear-wave window that they were completely uninterpretable in terms of our current understanding of shear-wave splitting Crampin 1994).…”

Section: Interpretation-aligned Cracksmentioning

confidence: 99%

A review of a quarter century of International Workshops on Seismic Anisotropy in the crust (0IWSA–12IWSA)

Crampin

Gao

2008

J Seismol

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Premonitory Variations in S -Wave Velocity Anisotropy before Earthquakes in Nevada

Cited by 67 publications

References 17 publications

Seismic-wave propagation through a cracked solid: polarization as a possible dilatancy diagnostic

Seismic-wave propagation through a cracked solid: polarization as a possible dilatancy diagnostic

Shear-wave splitting analysis of the upper mantle at the Niigata-Kobe Tectonic Zone with the data of the Joint Seismic Observations at NKTZ

A review of a quarter century of International Workshops on Seismic Anisotropy in the crust (0IWSA–12IWSA)

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