An experimental study of thermally induced microfracturing and its relation to volcanic seismicity

In this study, unconfined samples of basalt, sintered and bonded perlite, insulating firebricks, and paraffin were subjected to thermal gradients ranging from 15°C/cm to 100°C/cm. Microshocks produced by thermoelastic stress relief were detected by means of miniature accelerometers. Histograms of microshock occurrences have similar shapes for all tests. Activity begins and increases abruptly following onset of heating or cooling of the sample. It reaches a maximum, and then decays hyperbolically, in approximate correlation with the time dependence of the thermal gradient. There are later swarms of activity apparently initiated by small thermal fluctuations in the sample. The log-log plots of cumulative number of microshocks versus amplitude were constructed and show kinks or knees. The value of the negative slope b for the linear approximations to these plots ranges from 1.2 to 2.7. These values are greater than those for normal earthquakes series and are similar to those observed for volcanic B-type earthquakes. The effects of a cellular crack pattern, nonuniformity of stress due to the thermal gradient, and sample inhomogeneity may explain these results.