  Negative defect activation volumes are extremely rare in solids. Here, we
report for the first time that this holds in a couple of hydrated rocks for
dielectric relaxation by exploring the complex impedance spectra at various
pressures and temperatures. The present findings mean that the relaxation time
of the relevant relaxation mechanisms decreases upon increasing pressure, thus
it may become too short at higher pressure and hence lead to the emission of
transient electric signals before fracture. This may constitute the
long-standing laboratory confirmation for the explanation of the generation of
electric signals prior to an earthquake, as recently pointed out by Uyeda et al
[Tectonophysics 470 (2009) 205-213].
