applied across gougy argillite interbeds have shown
Progressive slip up to about 10 cm over a period of 1
month. It is notable that slips of this amount have
involved bedding planes that intersect the sill. Thus,
it is apparent that the shear planes sometimes pass
into the unmined vein.

2.4 Examples of bedding plane slip

A paint mark applied across bedding near the 5850-06
stope during 1998 underwent total slip of 18 cm
during the 6 weeks required to complete a cut. The
bedding slip plane was seen in the access ramp 4 m
from the ore body, indicating that the plane intersect-
ed the sill. During mining of the next cut, this slip
plane was viewed cutting across the vein and offset-
ting it a total of about 25 cm.

A rock burst extensively damaged the 5750-05
stope on August 28, 1998, in a Richtcr-magnitude 3.1
seismic event. Rock was expelled from both ribs and
the overhead cemented sandfill to an average depth of
1 m for a distance of about 25 m along the length of
the west half of the stope. The event occurred during
blasting, and no injuries resulted. Bedding slip be-
lieved to have resulted from this event could be docu-
mented by offsets in marks that had been painted
across all exposed argillite interbeds for 50 m in the
access ramp. A 2-cm slip was found 30 m from the
stope, and a 13-cm slip was found 27 m from the
stope. Because rock on one side of the slip detached
from the rib and may have moved independently of
the substrate, a greater amount of apparent slip could
not be confirmed. Evidence of slip could not be seen
on any other argillite interbeds, but many paint marks
had been destroyed by raveling during in the event.
Thus, the minimum net slip in the event was 15 cm.

Eastward and downward, the slip planes of the
August 28 event were projected to the location of the
5840-06 stope, which had been mined deeper than the
5750-05 stope, but which had become temporarily
inactive and inaccessible while the 5850-05 stope
continued to be advanced. It seems likely that slip on
these planes was controlled primarily by the presence
of the 5840-06 stope, but that the slip planes projected
westward so thal they intersected the 5750-05 area
deep in the sill. Based on this, minimum dimensions
of the slip zone can be estimated as being 90 m long
by 30 m along dip.

Damage to the stope in this event included
apparent compression of the sandfill to failure.
Damage that involved ribs can also be attributed to
shortening in the vertical dimension and buckling of
the affected rock mass, as discussed in the following
section. However, no bedding slip could be docu-

Cemented sandfill

Figure 6. Cross section of Lucky Friday vein identify-
ing mining-induced structures. 1, Bedding fault; 2, flat
extension fractures; 3, shear-activated zone of closely
spaced extension fractures; 4, thrust fault-shear rup-
ture in face; 5, flat fractures in sandfill; 6, galena-
filled stringers; 7, delaminated quartzite bedding.

mented in or near the stope that may have caused such
shortening, as all reference points were disrupted.

The various mining-induced fractures, bedding
plane slips, and shear ruptures seen at the Lucky
Friday Mine are summarized in Figure 6.

3 DAMAGE

Rock burst damage may affect either ribs, the cement-
ed sandfill, or the sill, and may involve several of
these locations in the same event (Fig. 7). A common
pattern of damage is one that affects the lower
footwall rib and adjacent sill and the diametrically
opposite upper hanging wall rib and the adjacent
sandfill. In these cases, damage in the hanging wall
frequently extends a short distance up the side of the
sandfill. Such damage resembles the pattern formed

1055


4 DISCUSSION AND SUMMARY

Progressive movements in the footwall of the Lucky
Friday vein generally cause a reduction in the physical
dimensions of the stope. Compression of sandfill is a
necessary and obvious consequence of downward
bedding slip. However, bedding slip may also reduce
various stope dimensions in a variety of other ways
(Fig. 8). Except for bursts that affect the hanging wall
rib, shortening of stope margins is likely to be a
consequence of normal slip on bedding planes. The
nature of the resulling damage depends partly on the
elevation at which a particular slip intercepts a stope.
If the slip plane intercepts the rib, only the sandfill
may be affected (Fig. 8A). However, a bedding plane
slip that fails to pass through relatively tabular rock
layers formed between a stope and weak, galena-filled
stringers may cause shortening and failure of the rock
while compressing the sandfill (Fig. 8B). Bedding
slips that intersect the underlying sill may concentrate
stress at the sill-footwall junction and fracture quart-
zite along its internal laminations, enabling the layers
to buckle in a rock burst (Fig. 8C). However, the
diametrically opposite corner, at the juncture of the
hanging wall rib and the sandfill, probably fails as a
result of weakening by earlier fracturing.

The high horizontal component of in situ stress
documented at the mine may be enough to cause min-
ing-induced extension fractures and shear ruptures to
form in the sill and for slip to take place on these frac-
tures without associated bedding plane slip. However,
bedding slip could also promote formation of fractures
and thereby contribute to sill bursts (Fig. 8D).

Movements associated with bursts that affect the
hanging wall rib have not been documented. It is like-
ly that these bursts are promoted by high vertical
stress in the hanging wall rib resulting from toppling
of overlying strata toward the mined-out vein (Fig.
8E).

Despite their steep dips, mining-induced structures
at the Lucky Friday Mine have similarities to those in
South African gold mines, discussed by Joughin &
Jager (1983), Pretorius (1966), and Adams et al.,
(1981). The mining-induced extension fractures that
form in the sill of the Lucky Friday ore body seem
identical to the fractures most closely associated with
the rock bursts that affect faces in South African
mines. Slip identified on extension fracture zones in
the Lucky Friday duplicates slip along such fractures
in South African stopes Joughin & Jager, 1983). The
shear ruptures seen in the Lucky Friday stopes are also
iden-tical in disposition and geometry to those found
in the South African mines.

In the Lucky Friday Mine, bedding in the footwall
approximates a common type of mining-induced
extension fracture documented by Joughin & Jager
(1983) in South Africa. Although ditatiog occurs on
both, shear is present only at the LuckyFriday Mine.
Unlike in the South African mines, where mining-
induced fractures parallel to the face seem to be
implicated in rock bursts (Joughin & Jager, 1983),
bursts in Lucky Friday stopes affect all margins and
several comers. However, planar discontinuities
parallel to the affected margins in the Lucky Friday
represent the same scenario as the extension fractures
parallel to the face in South African mines.

ACKNOWLEDGMENTS,

We gratefully acknowledge the cooperation and
assistance of Hccla Mining Company, owner and
operator of the Lucky Friday Mine for making this
research possible. Through these efforts , the NIOSH
Spokane Research Laboratory continues the mission
of the former U.S. Bureau of Mines in improving the
safety of miners.

REFERENCES

Adams, G.R., AJ. Jaeger, & C. Roering, C. 1981.
Investigations of Rock Fractures Around Deep
Level Gold Mine Stopes. In Proceedings, 25th
Symposium on Rock Mechanics. Cambridge,
MA: MIT, pp. 213-218.

Gay, N.C. & W.D. Ortlepp 1979. The Anatomy of a
Mining-Induced Fault Zone. Bull. Geol. Soc.
Amer. 90:47-58.

Hsiung, S.M., W. Blake, A.H. Chowdhury, & T.J
Williams 1992a. Effects of Mining-Induced Seis
mic Events on a Deep Underground Mine.
PAGEOPH 139(3/4):

Hsiung, S.M., A.H. Chowdhury, W. Blake, M.P.
Ahola, & A. Ghosh 1992b. Field Site Investiga-
tion: Effect of Mine Seismicity on a Jointed Rock
Mass. Nuclear Regulatory Commis. contract NRC
-02-88-005. San Antonio, Texas: Center for
Nuclear Waste Regulatory Analyses.

Jenkins, F.M., T.J. Williams, & C.J. Wideman 1990.
Analysis of Four Rockbursts in the Lucky Friday
Mine, Mullan, Idaho, USA. In D.A.J. Ross-Watt
& P.D.K. Robinson (eds), International Deep

1058


Figure 5. Inferred mining-induced thrust fault or
shear rupture cutting Lucky Friday vein. Extension
fractures lie about 30° to fault,

2.3 Identification and measurement of wall rock
movement

Movements taking place in wall rocks in response to
mining have been documented or inferred by various
investigators through the use of strain gages and pres-
sure cells, closure measurements, the appearance of
fractures in shotcrete, offsets in artificial markers
(paint marks and reflective reference fines), calcula-
tions of scismic source locations and fault plane solu-
tions, and repeated leveling and surveying traverses.
However, efforts to record movements carefully and
thoroughly over time have been frustrating because of
raveling of wall rock, disturbance of reference points,
and transi-tory access to active parts of the mine. As
a result, definitive data are scarce.

Slip on northwesterly striking faults has been
inferred from localized clusters of seismic events and
from visible evidence of differential squeezing of sills
and cement-ed sandfill. The largest seismic events,
up to Richter magnitude 4, have occurred along the
large faults that bound the ends of the ore body.
When rock bursts damage stopes during these larger
seismic events, the damage is not notably different
from damage seen after minor seismic events and is
not necessarily more severe. However, following
large events, damage and rock fall may be found
widely distributed about the mine, suggesting that the
large seismic events cause damage by triggering
smaller, localized events. Thc most common move-
ments identified in the wall rock at the Lucky Friday

Mine involve dilation and slip on sheared, gougy
argillite interbeds that separate thick, vitreous quart-
zite beds.

Closure measurements in stopes and development
openings have been made by numerous investigators
and Lucky Friday mine personnel, but much of this
work is unpublished. Gradual closure and step
increases that may or may not correspond to recorded
seismic events have been identified (Hsiung et al.,
1992a, 1992b; Whyatt et al., 1992; Williams et al.,
1992). Reported measurements have commonly been
in the range of 3 to 25 cm during the 6- to 8-week
period required to mine a single cut, These measure-
ments are consistent with physical evidence of
squeezing seen in the overhead sandfill.

‘The most extensive and detailed measurements
have never differentiated among movements resulting
from slip or dilation along bedding or other preexist-
ing structures, movement from dilation of mining-
induced extension fractures parallel to surfaces of
openings, or movement caused by mining-induced
buckling. In these studies, the data can be interpreted
in terms of both dilational and slip movements on
bedding planes, although dilation of mining-induced
fractures and buckling-type deformation may also
have be involved. Most data suggest that deformation
increases toward the vein

The more definitive observations have often been
the simplest. These are progressive fracture develop-
ment in shotcrete and offsets in reference marks paint-
ed across bedding planes. However, movements re-
corded by these means arc rather qualitative, because
it has never been possible to obtain thorough coverage
within stopes or along the length of individual ramps
for extended periods of time.

Differential movements in the ribs of access ramps
are most obviously shown by fractures that begin de-
veloping in shotcrete soon after it is applied. In addi-
lion to fractures that parallel bedding, en echelon frac-
tures cross bedding at about 45°. Both dilation and
slip along bedding planes are common, with dilation
more frequently seen than slip. The direction of
dilation is consistently toward the mined-out part of
the ore body. Dilation in the absence of slip may be
aresult of the buckling occasionally seen in the lower
portions of steeply inclined layers in pit walls,
although development of buckling-type folds
strained because of the narrowness of the available
open space. The most common sense of sli
al, although reverse movements have occasionally
been identified.

Near the vein where ramp sections are too short
lived to warrant application of shotcrete, other indica-
tors of movement have been used. Paint marks

1054


Figure 7. Rock burst damage in the 5750-05 stope. Note that both ribs and the overhead sandfill were damaged.

by dog-cared breakouts at the margins of drill holes in
highly stressed ground. Considerable stress must be
being transmitted through the sandfill, and damage to
the wall rock probably requires extensive fracture
development prior to driving the present cut. Other
distinctive types of damage involve failures of sandfill
or floor heave across the entire width of the sill
Damage to the ribs or at the contact between rib and
sandfill is particularly common at the corners between
the ramp crosscut and the stope. Damage and
extensive raveling at these locations suggests that
intense fracturing occurs here, probably during mining
of the previous cut, when such sites were probably
subjected to high stress.

3.1 Cause of damage

In many, or possibly all, cases, ground damaged in
rock bursts has undergone intensified compressive
stress prior to failure, and it is believed that such
compression plays a vital role in most or all rock
bursts. In cemented sandfill and in the sill, evidence
for compression is provided by flat-dipping extension
fractures and thrust faults. Similarity of the patterns
of diagonal damage at the junctions of (1) the sill and
the lower footwall rib and (2) the sandfill and the
hanging wall rib to dog-eared breakouts suggests
compression in a direction approximately parallel to
dipping beds. Compression within the ribs of hori-
zontal openings is shown by mining-induced folds
with flat-plunging axes that are occasionally seen at
these locations. These folds are the equivalent of

folds caused by the buckling that sometimes occurs at
the toes of inclined slabs in open pits.

3.2 Inferred damage mechanisms

Evidence that movement in the footwall commonly
causes a reduction in the physical dimensions of the
stope suggests that rock burst damage in particular
results from shortening along stope margins. Signifi-
cant shortening, as identified in the various studies
cited, has the potential for squeezing rock or cemented
sandfill at stope margins to the point of failure. The
resulting violence of such a failure (a rock burst) re-
quires that the rock or cement-ed sandfill be under
compression prior to failure and that initial resistance
to failure enabled the storage of elastic strain energy
in the surrounding rock.

Except where rock is so extremely fractured that
no systematically oriented fractures can be identified,
acharacteristic of burst sites is that the affected rock
has a generally tabular character, either because of
mining-induced fractures, delamination of quartzite
along bedding planes, or the existence of other pre-
existing structures. In contrast to Joughin & Jager
(1983), who concluded that the expulsion of rock in
bursts results from interleaving of rock layers by
compressional movement, we favor the view that
buckling of such layers leads to fracturing and expul-
sion (e.g., White et al., 1994; Maleki & White, 1997).
We note that interleaving of rock layers would
necessarily cause the appearance of buckling as layers
double up and slip past each other in the central part
of the affected volume of rock. Perhaps both pheno-
mena have a role in rock bursts.

1056,


Mining Conference: Technical Challenges in Deep
Level Mining. Johannesburg, S. Afr.: S. Afr. Inst.
of Min, and Metall., pp. 1201-1212.

Joughin, M.C. & A.J. Jager 1983. Fracture of Rock at
Stope Faces in South African Gold Mines. In
Rockbursts: Prediction & Control. London: Inst.
of Min. and Metall., pp. 53-66.

Maleki, H., & B.G. White 1997. Geotechnical
Factors Influencing Violent Failure in U.S.
Mines. In International Symposium on Rock
Support and Applied Solutions. Lillehammer,
Norway: pp. 208-221.

Pretorius, P.G.D. 1966. Contribution to Rock Me-
chanics Applied to the Study of Rockbursts. J. S.
Afri. Instit. Min, and Metall. 66:705-713.

White, B.G., J.K. Whyatt, & D.F Scott, 1995,
Geologic Factors in Rock Bursts in the Coeur
d’Alene Mining District: Structure. In H. Maleki,
P.F, Wopat, R.C. Repsher, & R.J. Tuchman (eds),
Proceedings: Mechanics and Mitigation of Vio-
lent Failure in Coal and Hard-Rock Mines. U.S.
Bur. Mines Spec. Pub. 01-95, pp. 217-230.

Whyatt, J.K., T.J. Williams, & M. Board 1992, Rock
Mechanics Investigations at the Lucky Friday
Mine, Part 2: Evaluation of Underhand Backfill
Practice for Rock Burst Control. U.S. Bur. Mincs
Rep. Invest. 9433, 10 pp.

Whyatt, J.K., BJ. White, & W. Blake 1995, Struc~
tural Stress and Concentration of Mining-Induced
Seismicity. In H. Maleki, P.F. Wopat, R.C. Rep-
sher, & R.J. Tuchman (eds), Proceedings: Me-
chanics and Mitigation of Violent Failure in Coal
and Hard-Rock Mines. U.S. Bur. Mines Spec. Pub.
01-95, pp.243-264.

Williams, T.J., J.K. Whyatt, & M.E. Poad 1992. Rock
Mechanics Investigations at the Lucky Friday
Mine, Part 1: Instrumentation of an Experimental
Underhand Longwall Stope. U.S. Bur. Mines Rep.
Invest. 9432, 26 pp.

1059


Rock Mechanics for Industry, Amadei, Kranz, Scott & Smeallie (eds)© 1999 Balkema, Rotterdam, ISBN 90 5809 0523

Differential wall rock movements associated with rock bursts, Lucky Friday
Mine, Coeur d’ Alene Mining District, Idaho, USA

B.G.White & J.K.Whyatt

Spokane Research Laboratory, National Institute for Occupational Safety and Health, Wash., USA

ABSTRACT: Various methods of monitoring slip movements on bedding planes, as well as examination of rock
burst damage in stopes, suggests that rock bursts in the Lucky Friday Mine are closely associated with these
movements. Slip displacements along bedding simultaneously reduce the physical dimensions of stopes and
increase compressive stress along stope margins. Such changes, in turn, contribute directly to rock bursts and
the sudden failure of volumes of rock and cemented sandfill surrounding stopes.

‘The distribution of seismicity and several mining-induced and premining structures resemble those described
in South African gold mines. These suggest fundamental similarities in seismic sources and mechanisms of rock

burst damage.

1 INTRODUCTION

The Coeur d’ Alene Mining District in northern Idaho
is the second largest silver mining district in the world
as well as a leading U.S. producer of lead and zinc.
The Lucky Friday Mine alone has contributed more
than 680,000 tonnes of lead, 86,000 tonnes of zinc,
and 108,000,000 oz of silver during 55 years of
operation.

At recent mining depths of nearly 2,000 m, the
Lucky Friday has been one of the most active mines in
North America in terms of seismic energy per ton of
ore mined (Jenkins ct al., 1990). Rock bursts at the
mine have necessitated cleanup of several hundred
tons of rock debris per month in recent years.

As used in this paper, rock bursts are defined as
violent outbursts of broken rock. Hundreds of tons of
pulverized rock may be expelled into openings during
such events, A sudden, sharp sound, a seismic signal,
an air blast, and large quantities of dust are typical
indicators of a rock burst. The existence of high
ground stresses at burst sites prior to these events are
common and may be characteristic.

Although rock bursts affect development openings,
this paper is focused on rock bursts in stopes. Ribs or
rib-sill junctures are most commonly damaged, but
sills and backs are occasionally affected also. In the
underhand stopes of the Lucky Friday Mine, damage

that affects overhead, cemented sandifill is basically
identical to bursts that affect rock, so these events are
also included as rock bursts.

The results of various investigations have been
reviewed and reconsidered. On the basis of this
reconsideraton, the authors propose that most, and
possibly all, rock bursts in Lucky Friday stopes are
associated with differential movements in the sur-
rounding wall rocks. These movements involve slip
on bedding planes and faults and contribute directly to
rock burst damage. Slip movements are belicved to
promote localized stress concentrations that cause
stress fracturing of wall rocks and cemented sandfill.

1.1 Geology and in situ stress

The Lucky Friday vein is hosted by slightly meta-
morphosed Precambrian sedimentary strata. Burst-
prone strata are dominated by strong, vitreous, thick-
bedded quartzite, but thin laminations in the quartzite
create planes of weakness, Thin, weak interbeds of
soft argillite separate the quartzite beds at intervals of
0.5 to 5 m. Early mapping of the stratigraphy showed
that rock bursts were prevalent in the quartzitic lithol-
ogies that characterize the upper and lower members
of the Revett Formation, but were uncommon in the
argillitic middle Revett member. However, the struc-
tural complexity of the mine has hindered under-

1051


Vein

Toppling

Figure 8. Interpretations of rock bursts in stopes caused by movement within the surrounding wall rock. Stippled
areas indicate rock burst locations; arrow with ball identifies inferred direction of shortening responsible for
damage at sites.

A, Bedding slip identifies inferred direction of shortening responsible for damage at sites;

B, bedding slip that fails to cut rock layers in rib compresses both the rib and the sandfill;

C, shortening parallel to bedding delaminates quartzite bed at sill and damages upper hanging wall rib at
sandfill;

D, bedding slip compresses sill;
E, inferred toppling in hanging wall increases vertical stress concentration along hanging wall rib.

1057


Figure 1. Plan view of Lucky Friday Mine, 5500
sublevel

standing of the processes thal cause damage.

On a larger scale (tens to hundreds of meters),
beds are tightly folded about stecp axes and are trun-
cated by the steeply dipping ore body. However,
strata generally dip into the vein from the footwall
side and away from the vein into the hanging wall.
Argillite interbeds have commonly been sheared by
premining tectonism, resulting in a weak clay gouge.
Widespread core drilling demonstrates that wall rocks
have been abundantly, but irregularly, fractured by
tectonism. These fractures are belicved to aggravate
rock burst problems in the stopes.

The Lucky Friday ore body is composed primarily
of massive galena with lesser amounts of quartz,
siderite, and other sulfides. Width of the ore body
ranges from 0.5 to 8 m, but averages about 1 m. Nar-
row galena stringers commonly parallel! the main vein.
These stringers evidently arc planes of weakness, as
they often form the walls of stopes. The northeasterly
striking part of the vein is approximately vertical,
while the more westerly trending portions dip steeply
southward.

Various investigators (Whyatt et al., 1995) have
consistently identified the direction of greatest in situ
stress, both at the Lucky Friday and in the district, as
nearly horizontal and trending northwest. This direc-
tion is nearly normal to the northeast-trending part of
the vein and oblique to the west-trending part.

2 MINING-INDUCED WALL ROCK
MOVEMENTS

The mechanics of wall rock movement can be sub-
stantially understood by comparing deformation at the
mine to deformation in the walls of highway cuts or in
the walls of open-pit mines. Strata that dip toward
open cuts, as in the footwall of the Lucky Friday vein,
are vulnerable to slippage, and buckling sometimes
occurs in the lower part of slipping layers. Layers that
dip away from walls, as in the hanging wall of an ore
body, fail by toppling. While sliding movements have
been frequently documented in the Lucky Friday
Mine, toppling is generally not evident at active oper-
ating levels and may be manifested primarily in the
concentration of vertical stresses in the hanging wall
sides of stopes. Actual movements are restrained by
the cemented sandfill and the narrow width of the
mined-out vein, but visible squeezing of the sandfill
confirms that such movement does take place.

2.1 Seismicity

An extensive seismic monitoring system installed
within the mine records seismic events and identifies
source locations. Approximately 90% of all scismi-
cily originates within a zone centering on the active
stopes and extending about 100 m into the footwall.
These events are believed to reflect primarily slip on
bedding planes. It is notable that the remaining 10%
of the seismic events, which mainly originate outside
of this zone, includes events with the greatest magni-
tudes and accounts for about 90% of all seismic
energy released in a mine (Williams, unpublished
data). The highcst-magnitude events seem to lic close
to known major faults and are interpreted as indica.
ting wall rock movernent toward the mined-out part of
the ore body (Fig, 1). As in other mining districts, the
larger seismic events usually produce significant
damage, while major damage to both stopes and
development openings may be associated with low-
energy seismicity. This suggests a minimal role for
seismic impulses in the generation of rock bursts and,
probably, a greater role for movements over time.
‘The distribution of seismicity about Lucky Friday
stopes resembles that in longwall stopes in South
African gold mines (e.g., Joughin & Jager, 1983), but
with one major difference. In contrast to seismicity in
the gold mines, which occurs symmetrically about a
stope, Lucky Friday seismicity occurs asymmetrically
and is essentially limited to the footwall side.
coincides with the marked asymmetry of the bedding
structure of Lucky Friday wall rocks and emphasizes

1052


Figure 2. Soft, white gouge on slip plane in argillite
interbed in hanging wall rib.

Kigure 3. Mining-induced extension fractures in
stope ribs.

the role of this structure in the generation of
seismicity.

2.2. Structures caused by mining-induced movement

Several types of recurrently seen structures are be-
lieved to indicate mining-induced wall rock move-
ments. These structures include mining-induced
thrust faults, or shear-ruptures; disrupted, mining-
induced extension-fracture zones; and bedding plane
faults containing a distinctive gouge. That is, in most
cases, shear planes contain compact, gray of greenish-
gray gouge. This color contrasts with the soft, white
gouge found along bedding planes near active stopes
and within inferred mining-induced shear ruptures

= ,
Figure 4, Disrupted zone of closely spaced mining-
induced extension fractures indicating activation of

zone as a flat fault.

(Fig. 2). The presence of the white gouge in
structures that are believed, for other reasons, to have
been caused by mining and the absence of the white
gouge at locations away from the ore body suggests
that this gouge results from mining-induced slip.
Flat-dipping extension fractures are locally seen in
the stope walls and in the advancing stope face (Fig.
3). These fractures are sometimes densely concentral-
ed within zones 5 to 10 cm thick, Concentration of
these fractures in the upper one-third of mining faces
confirms that they are caused by mining and that they
form in the immediate sill. Thin, tabular rock layers
within these zones are occasionally broken, rotated,
and disrupted (Fig, 4) Evidence of slip has been
shed by offsets of veins up to several tens of

centimeters. Thus, these extension-fracture zones
may evolve into flat-dipping thrust faults.

Flat-to-moderate-dipping thrust fault shear rup-
tures containing the soft, white gouge offset the ore
body by as much as 10 to 20 cm (Fig, 5). Indirect
evidence that the thrust faults result from mining is
seen in a consistent angle of about 30° between the
faults and adjacent mining-induced extension frac-
tures, even where the extension fractures locally pos-
sess greater-than-normal dips. These shear ruptures
may reflect a recurrent fault plane solution obtained
from seismic data. However, correlation of structures
with specific seismic events, as has been done for
some similar structures in South Africa (Gay &
Ontlepp, 1979), has not been possible.

1053