Abstract

It is now well-established that earthquakes change the seismic velocity of the near surface. There is certainly some understanding of what mechanisms are responsible for these changes, but there remain many questions. Here we attempt to answer the question of the relative importance of different connection mechanisms between cracks and how these change with applied load. To study this, we first perform nonlinear wave-mixing experiments in two sandstone samples at a variety of applied uniaxial stresses. The two samples differ in the relative orientation of their microstructures. We find that although the samples show velocity anisotropy we do not see aligned structures in scanning electron microscope images. By measuring the changes in velocities with applied stress we find that most cracks close during our experiments independent of crack orientation. By contrast, we find that the nonlinear wave interactions vary strongly with applied load and with crack orientation. We analyze these differences and relate them to an emerging model of nonlinear wave interactions with microstructures.