Time-dependence of Seismic Wave Speeds in Volcanic and Geothermal Systems

That seismic wave speeds in geothermal systems might change with time is an expected consequence both of natural processes and of industrial activities such as fluid withdrawal and injection, and indeed detections of such changes have been reported, based on repeated microearthquake seismic tomography. Seismic tomography suffers, however, from limitations that cast doubt on some of these reports. Specifying a three-dimensional model of seismic wave speed in even a small volume requires at least thousands of parameter values, but because of the limited distribution of both earthquakes and seismometers, real data sets virtually never contain enough information. Seismic rays tend to occur in bundles that sample the Earth very unevenly, and standard inversion methods impose assumptions that result in structural features that mimic these bundles. Any changes in the ray distributions, such as those caused by seismicity variations, can therefore produce spurious apparent changes in the wave speeds.