Shear-wave splitting anisotropy

Clitheroe, G. vander Hilst, R. D. 1998. Complex anisotropy in the Australian lithosphere from shear-wave splitting in broad-band SKS-records. 

Savage, M. 1999. Seismic anisotropy and mantle deformation What have we learned from shear wave splitting Reviews of Geophysics, 37, 65-106. 

Barruol, G. Kern, H. 1996. Seismic anisotropy and shear-wave splitting in lower crust and upper-mantle rocks from the Ivrea Zone-experimental and calculated data. Physics of the Earth and Planetary Interiors, 95, 175-194. 

Fouch et al. (1999) compared model simulations of asthenospheric flow and seismic observations, and found that a significant proportion of the shear-wave splitting can be explained by the deflection of asthenospheric flow around a cratonic root. In our study we are interested in the extra complication of anisotropy in flowing mantle plume material that hes beneath and around continental keels. 

Upper-mantle anisotropy and shear-wave splitting... 

Mt. Ruapehu volcano in the North Island of New Zealand has been the subject of some of the most comprehensive shear wave splitting studies of any volcano on Earth. Therefore, to illustrate the spatial versus temporal variation issue, anisotropy studies at Mt. Ruapehu will be explored. Temporal variation of shear wave splitting at Mt. Ruapehu was investigated by Miller and Savage (2001) and Gerst and Savage (2004). 

Shear wave splitting analysis is becoming a very popular method for determining seismic anisotropy in the cmst. This is in part due to the abundance of data and methods available and also to the relative insensitivity to the source-receiver geometry (other than deep enough earthquakes for the rays to arrive within the shear wave window) and independence from the need for dense networks of seismometers (see Seismic Network and Data Quality ). Another benefit of... 

Temporal variation of shear wave splitting parameters as a stress indicator has been employed at several other volcanoes. While most studies concentrate on either stress-induced anisotropy or structurally dominated anisotropy, several studies have found that the dominant mechanism can change between the two (Keats et al. 2011 Johnson and Poland 2013) and that more than one mechanism can be dominant over short distances, potentially creating an apparent temporal change that is actually an artifact of changing earthquake location (Zinke and Zoback 2000). 

To quantify the response of seismic anisotropy to pre-, CO-, and post-eruption subsurface magma movement, shear wave splitting data should first be used to map the areas affected by changing stress and identify the mechanism of seismic anisotropy in areas that do not have stress-controlled anisotropy. Methods have recently been developed for the inversion of... 

Roman et al. (2006) analyzed the focal mechanisms from a subset of 551 VT earthquakes at SHV with well-constrained double-couple fault-plane solutions, spanning October 1996 to July 2005, and observed temporal changes in the orientation of the p-axes. Several periods of a few months were identified where the dominant orientation of the p-axes switched from the predominant NE-SW to a NW-SE trend. This represents a 90° rotation from the assumed arc-normal regional maximum compressive stress. Analysis of seismic anisotropy through study of shear-wave splitting of regional earthquakes recorded on the MVO network has provided further evidence to support the presence of short-term variations in the local stress field at SHV. 

Shear wave splitting is a sensitive indicator of elastic anisotropy and therefore has a special place in rock physics (see, for example, Crampin and Lovell, 1991). 

The study of the shear wave anisotropy has created a special field of seismic research the investigation and application of the phenomenon of shear wave splitting (acoustic bifrigerence). 

FIGURE 6.21 Shear wave splitting in a transversely isotropic rock (a) principle of shear wave splitting and different velocities short arrows at shear wave indicate particle moticHi (polarization). (b) Example Amphibolite (Western Alps/Ivrea zone) anisotropy and ear wave splitting are related to texture and microcracks foliation is horizontal. Data from Siegesmund et al. (1993). 

The calculated tensor elements allow studies of anisotropy as well as of the velocity ratio Vp/Vs- Figure 6.36 gives an example of forward calculation for the shear wave anisotropy (Thomsen parameter y) as a function of fracture porosity and aspect ratio. Clearly expressed is the strong influence of aspect ratio on anisotropy. Such studies can help to interpret and understand shear wave splitting effects. 

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