1
Department of Earth Sciences
University of Southern California
2
Department of Geosciences
Pennsylvania State University
Email:
tjordan@usc.edu
poster/oral: oral
|
Induced seismicity recorded by local arrays in the deep gold mines of South Africa
is
yielding new insights into the physics of earthquake nucleation and rupture.
Seismicity
data for five mines in the Far West Rand district display clear bimodal
distributions
indicative of two distinct classes of events, which we have designate Type A and
Type B
(Richardson and Jordan, 2002). Type-A events are tightly clustered in time and
space
and generally occur within 30 m of an active mining face; they have spectra
comparatively enriched in high frequencies, their focal mechanisms often involve
isotropic components, and they show an upper magnitude cutoff at MW < 1.
We associate
these events with "fracture-dominated" ruptures of competent rock at low normal
stress,
induced by dynamic stresses during blasting and quasi-static stress perturbations
from the
excavation and closure of individual stopes. In contrast, Type-B events are
temporally
and spatially distributed throughout the active mining region. We interpret them
to be
"friction-dominated" ruptures occurring on existing faults or other weak geologic
structures at near-lithostatic normal stresses. They have double-couple focal
mechanisms
and scaling properties that agree with extrapolations from tectonic earthquakes.
For
example, the energy/moment ratios for Type-B events yield apparent stresses in the
range
.01-1 MPa with a distinct increase in apparent stress with moment, consistent with
the
observations of small earthquakes at Cajon Pass by Abercrombie (1995) and Long
Valley
by Prejean and Ellsworth (2000). Although the data show considerable scatter, the
increase in apparent stress scales approximately as MW1/3. We observe a lower
magnitude
cutoff in the Type-B events at MW ~ 0. We interpret this cutoff in terms of a
critical patch
size for nucleation of shear failure, which yields a critical slip distance Dc =
104 m. This
result is consistent with an upper frequency cutoff fmax near 200 Hz observed on
accelerograms recorded in the near field of large events. To explain the scaling
of
apparent stress for Type-B ruptures, we have developed a simple slip-weakening
model
for small earthquakes in which the specific fracture energy increases as MW
1/3 from a
nucleation value Gc ~ 103 J/m2 at MW = 0 to a saturation value Gc ~ 105 J/m2 at MW = 4. This model implies that the rupture velocity also scales as MW1/3. We show that the model satisfies constant-stress-drop scaling and correctly predicts the variation of peak particle velocity with magnitude. The implications for tectonic earthquakes will be discussed. |