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. One of these open questions is how cracks and other
microstructures within the rock control these changing velocities. Here
we look at the nonlinear interaction of two waves, one of which (the
PUMP) simulates the effect of an earthquake and the other (the probe)
senses the changes in the travel time caused by the passage of the PUMP
wave. We use a sandstone sample that is established to have a nonlinear
response that depends on the orientation of the sample layering. We
study two samples with different orientations of this layering, which we
infer to be different orientations of the micro-structure. We show that
the dependence of these changes on applied load are exponential, with a
characteristic load of 11.4-12.5~MPa that is independent
of sample orientation and probe wavetype (P or S); this value agrees
with results from the literature.