S. Yoshida et al., ELECTRIC-POTENTIAL CHANGES ASSOCIATED WITH SLIP FAILURE OF GRANITE - PRESEISMIC AND COSEISMIC SIGNALS, J GEO R-SOL, 102(B7), 1997, pp. 14883-14897
Electric potential changes were measured for stick-slip events in gran
ite samples with a three-block direct shear arrangement at 8 MPa norma
l stress. Two electrodes were mounted on the left- and right-hand bloc
ks, and the electric potential difference between each electrode and t
he ground was measured with a high input impedance recording system of
frequency range from DC to 100 Hz. As well as coseismic electric sign
als of about 1.5 V which appeared the moment of the dynamic slip event
, preseismic signals were detected just before the slip event. The cos
eismic signal rises stepwise with opposite polarities at the two elect
rodes and exponentially decays with a time constant of epsilon/s, wher
e epsilon is the permittivity and s is the conductivity of the rock sa
mple. We conducted a simple test of rapid stress drop without slipping
and observed almost the same electric signal as the coseismic signal.
This suggests that the electric signal is generated by the piezoelect
ric effect. We proposed a generation model based on the piezoelectric
effect and the resultant relaxation process and obtained a theoretical
frequency response, which is in agreement with experimental data. The
preseismic signal appears about 2-3 s before the dynamic event with a
n amplitude of about 50 mV. The local strains along two sliding surfac
es were also measured to monitor the growth of the rupture nucleation
zone. When the growth of the rupture nucleation zone occurred on the l
eft sliding surface, a clear preseismic signal was detected at the ele
ctrode mounted on the left granite block. When the growth occurred on
the right-hand surface, a signal was detected at the electrode on the
right block. This shows that the preseismic electric signal is caused
by stress change in the rupture nucleation zone. These preseismic and
coseismic signals were also detected with an antenna, which was placed
away from the sample surface.