A DECADE OF SHEAR-WAVE SPLITTING IN THE EARTHS CRUST - WHAT DOES IT MEAN - WHAT USE CAN WE MAKE OF IT - AND WHAT SHOULD WE DO NEXT

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?