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?

Abstract

S U M M A R Y It is 10 years since shear-wave splitting, thought to be diagnostic of some form of seismic anisotropy, was first positively identified in the Earth's crust. From the beginning it was argued that the splitting was probably associated with the presence of stress-aligned cracks (inclusions) in the crust, and that this would provide the opportunity for monitoring the in situ geometry of cracks and stress in a variety of different circumstances and in a variety of different applications. The early promise was not immediately realized, and the first 10 years were spent mainly in observing the phenomena in a variety of different situations. However, 1990 appeared to mark a turning point for anisotropy. Papers at the Fourth International Workshop on Seismic Anisotropy and elsewhere have announced major progress in understanding, interpreting, and particularly processing shear-wave splitting, with direct applications to hydrocarbon production, and a possible (but disputed) application to monitoring stress changes before earthquakes. However, there is still much that we do not understand about the phenomenon, and we are clearly only just beginning to appreciate the enormous information content of the shear wavetrain and its potential applications to science and engineering in the Earth's crust.This paper briefly reviews the past 10 years, and speculates on how best we can exploit this new window of opportunity for exploring the internal structure of the crust. In particular, what causes the shear-wave splitting? what use can we make of the phenomenon? and what should we do next?

Seismic imaging of the geothermal field at Krafla, Iceland using shear-wave splitting

“…To avoid potential ambiguity, it is worthwhile to mention that in an anisotropic medium, aligned fast shear-wave polarization orientations are independent of the initial polarization of the shearwave at the source and are mainly caused by the medium's anisotropy (e.g. Crampin et al, 1986;Peacock et al, 1988;Crampin and Lovell, 1991).…”

“…In the seismic case, the polarization direction of the fast split shear wave parallels the strike of the predominant cracks regardless of its initial polarization at the source (Crampin et al, 1986;Peacock et al, 1988). The differential time delay between the arrival of the fast and the slow shear waves (typically a few tens of milliseconds) is proportional to crack density, or number of cracks per unit volume within the rock body traversed by the seismic wave (Hudson, 1981;Crampin, 1987;Crampin and Lovell, 1991). In cracked reservoirs such as Krafla the anisotropy is likely caused by aligned systems of open, fluid-filled micro-fractures.…”

A review of shear-wave splitting in the compliant crack-critical anisotropic Earth

“…The underlying reason for those developments was that observations of three-component seismograms had shown evidence of shear-wave splitting [2,3], and the only possible source of anisotropy that satisfies all observations is fluid-saturated stress-aligned microcracks [19,38,39]. Almost all suitable three-component seismograms display shear-wave splitting in almost all in situ rocks.…”

Changes of Strain before Earthquakes: The Possibility of Routine Monitoring of Both Long-Term and Short-Term Precursors.

“…1. The fluid-filled intergranular EDA-cracks are so closely spaced, even in what is thought of as intact rock, that the distributions of microcracks are comparatively close to the fracture criticality (meta-stability) of heavily-fractured rock (Crampin, 1994). Differential horizontal stress acting on the rockmass partially aligns EDA-cracks, so that below about 1 km, where the minimum differential stress is horizontal, EDA-cracks are aligned into sub-parallel sub-vertical orientations approximately perpendicular to the direction of minimum horizontal compressional stress and striking sub-parallel to the dominant direction of maximum horizontal stress.…”

“…Shear-wave splitting, analogous to optical bi-refringence in crystals and diagnostic of some form of effective anisotropy, is widely observed in the Earth's crust, whenever suitable three-component digital seismic instruments record shear waves at appropriate sampling rates (Crampin, 1994). Shear waves are sensitive to the geometry of the microcracks in the rock along the ray paths, and worldwide observations of shear-wave splitting show that almost all rocks in the crust are pervaded by distributions of stress-aligned fluid-filled intergranular microcracks and intergranular pores leading to what is known as extensive-dilatancy anisotropy, or EDA (Crampin et al, 1984;Crampin, 1993a).…”