  Microseismicity in subsurface geologic environments, such as sandstone gas
reservoirs, is expected in the presence of liquid or gas injection. Although
difficult to predict, the potential for microseismic events is important to
field-scale projects, such as geologic storage of CO2 whereby the gas is
injected into natural sandstone formations. We conjecture that a primary factor
causing microseismicity is the existence of metastable states in granular
porous medium and provide experimental evidence for its validity. External
perturbation trigger abrupt relaxation events, which, with a certain
probability, can grow into macroscopic microseismic events. Here the triggering
perturbation is produced by cooling to a cryogenic temperature. As the "sensor"
for the abrupt relaxation events we use thin Al films deposited on the
sandstone surface. We show that as the temperature is varied, the films'
resistance exhibits sharp jumps, which we attribute to mechanical restructuring
or microfractures in the fabric of the sandstone. We checked the
superconducting characteristics of the Al thin films on the sandstone and found
microwave-induced Shapiro steps on the voltage-current diagrams. Such quantized
steps provide indicates that the film is made of a network of nanobridges,
which makes it ever more sensitive to abrupt relaxation events occurring in the
substrate, i.e., in the underlying sandstone.
