Russian Journal of Earth Sciences
Vol 1, No. 4, June 1999
Determination of instability of the fault zone model
G. A. Sobolev and A. V. Ponomarev
Abstract
A series of experiments was carried out with three-layer
models constructed of
granite chippings, quartz sand, and cement; the inner layer of
the models had lower
strength. The models were loaded on a servocontrolled press under
biaxial compression
and supplementary mechanical vibration with 40 and 120-sec periods.
The variations in
press rigidity allowed both to obtain friable macrodestruction
of the inner layer and to
conduct the experiment in the post- peak stress state with accumulation of
microdestruction. Stress, deformation, and acoustic emission were recorded
in the course
of the experiment, and ultrasonic sounding was also periodically employed.
The main
object of research was to find symptoms of transition of the model into
the stage of
unstable deformation in the absence of the rheological curve.
We have established that this
transition can be revealed by several indicators, i.e., the appearance
of asymmetry in the
compression-extension phases of vibration cycles, accumulation of diversity of
deformations recorded by spatially distributed sensors, distortion
of the waveform during
ultrasonic sounding, the growth of the "seasonal" amplitude of the
acoustic emission
showing an increase in the strain sensitivity of the medium,
the appearance of the acoustic
silence and subsequent activization. We presume that these
symptoms can be used to
estimate the onset of the instability stage in the fault zones
of the Earth's crust when
analyzing the weak seismicity, Earth tides and vibroseis data.