shear fracture of rock and bedding.

3 The relative absence of caving related failure re-
corded within the strata.

4 Focal plane solutions for key events confirmed
the mode of rock failure and geometry.

5 The magnitude of events was typically low.

The location of events in a section about the cen-
tral zone of the longwall panel is presented in Figure
7 and the focal plane resolutions are presented in
Figure 8.

Figure 8. Failure mechanisms for 14 events. Shade
areas are under tension and the open areas are under
compression.

Computer modelling of the site was undertaken
and reported within Kelly et al (1999). The key
findings of the study were:

1 Shear fracture and bedding plane failure occur
well ahead of the faceline;
2 The geometry of the shear fractures was very

ee. es) ° 10 20
Distance (mn)

Figure 9. Rock Failure modes and fracture orient-
tions about the central zone of the faceline.

similar to that from the focal plane solutions;

3 Coupled fluid pressure within the rock failure cri-
teria is a major factor in the failure characteristics
of weak material.

The zones of rock fracture about the central zone

of the longwall panel is presented in Figure 9.
Overall, the computer simulation and the micro-

seismic monitoring show very similar and comple-
mentary results. At the time these results were con-
trary to the more conventional rock fracture
geometry based on tension fracture of the ground
behind the mining face, however subsequent inves-
tigations have confirmed these findings.

7 CASE STUDY OF A UTAH LONGWALL
MINE

NIOSH (National Institute for Occupational Safety
and Health) recently studied a mine located in Utah
USA at a depth of 500-600m. The strata about the
mine are typically strong interbedded siltstone and
sandstone of 60-120MPa together with thin coal
seams. Higher within the overburden a thick unit
named the Castlegate sandstone exists which is a
laminated sandstone in the strength range of 40—
60MPa. Above this more interbedded and carbona-
ceous material exists. A more detailed discussion of
the geology is presented by Ellenberger et al. (2001).
A microseismic monitoring study was conducted
in 1999 and was reported by Ellenberger et al.
(2001). The key aspects of this study were:
1 Microseismic activity occurred well ahead of the
mining face within the roof and floor;

300 200 100 =©0)—-100 -200 300-400
Distance In Front of Face (em)

Figure 10. Location on microseismic events in a
cross-section view relative to face position.

2 The fractures typically were not within the tradi-
tional caving zone;

3. The largest magnitudes of the events ahead of the
face ranged from M, 2.0 to 2.5.

4 A large scale seismic event was recorded (M,4.2)
within the overburden which was considered to
be associated with large scale overburden move-
ments.

The location of small magnitude events (typical
events) ahead of the faceline is presented in Figure
10 together with the location of the M:4.2 event.

Computer modelling of the site was conducted to

1317


that shear fracture of the rock and bedding plane
shear would be expected to occur in these areas.
These fractures are likely to generate the highest
stress drop and strain energy release.

It is also notable the caving related seismicity is
typically absent from the surveys. This may in part
be related to the microseismic systems, but a sig-
nificant factor is considered to be the relatively low
stress drops and energy release which result from
such fracture patterns. Bedding plane tensile failure
and tension fracture of small sections of rock as an-
ticipated in the caving zone are unlikely to be re-
corded. This is also true for small scale rock failure
known to have occurred in the immediate vicinity of
the gateroad entries which was not picked up at
some sites studied in Australia.

The magnitudes of the seismic events about
longwall mines are generally consistent with that
expected from computer modelling however, further
work is being undertaken to better define the rela-
tionships of microseismic response with the type
and scale of fracture generated.

The monitoring at the Springfield Pike Mine is
being analysed in terms of the scale of fracture rela-
tive to the energy in a similar manner as the exam-
ple of the Utah longwall mine.

Ongoing work is aimed at providing a better in-
terpretation of the capability of microseismic moni-
toring to define the nature of rock damage about
mining operations. Knowledge of the energy re-
leased by various failure modes will better define
the requirements of a microseismic system to moni-
tor the type of rock behaviour required for various
mine planning and safety issues.

It is also anticipated that the nature of the rock
failure and the stability of the strata in the area of
seismic activity will be better defined in ongoing
analyses.

Oct. 28, 2000 Roof Fall Seismic Activity

1200 1400 16:00 18:00 20:00 2200 00 20 400 800
ow

Time

-+—Microseismic ecindty Large sents

Fi 16. Microseismic events associated with rock
failure recorded over a two day period.

REFERENCES

Ellenberger, J.L & Heasley, K.A. 2000. Case study of a coal
mine Bump seismicity at the Arch No.37 Mine, Harlan
County, Southeastern Kentucky. NIOSH Internal report
4943 Nov. 2000, 1Spp

Ellenberger, J.L., Heasley, K.A., Swanson, P.L. & John
Mercier, J. 2001. Three dimensional microseismic monitor-
ing of a Utah Longwall. DC Rocks 2001: Rock mechanics

in the national interest, Proceedings of 38th US Rock Me-
chanics Symposium, Washington D.C July 7-10 2001.

Gale ,W.J. 1998. Experience in Computer Simulation of Cav-
ing, Rock Fracture and Fluid Flow in Longwall Panels.
Proceedings of International Conference on Geomechan-
ics/Ground Control in Mining and Underground Construc-
tion 14-17 July 1998, Wollongong NSW Australia Vol 2 pp
997-1007.

Hatherly, P., Lou X., Dixon, R., McKavanagh, B., Berry, M.,
Jecny Z & Bugden C. Roof and Goaf monitoring for strata
control in Longwall Mining. Final Report ACARP C3067,
1995.

lannacchione, A., Marshal, T., Burke, L., Melville R. & Lit-
senberger J.E. 2001. Safer Mine layouts for underground
stone mines subjected to excessive levels of Horizontal
stress. S.M.E. Annual Meeting, Preprint 01-88 Denver CO.
Feb 26-28 2001, 7 pp.

Itasca, FLAC: Fast Lagrandian Analysis of Continua User's
Guide (Minneapolis: Itasca Consulting Group, Inc., 1998).

McGarr, A. 1993. Factors Influencing the Strong Ground Mo-
tion from Mining-Induced Tremors. ings of the 3°
Intern. Symp. On Rockbursts and Seismicity in Mines (R.P.
Young, ed.) Kingston, Ontario. Balkema, Rotterdam, 1993,
pp. 3-12.

Kelly, M., Gale, W.J., Luo, X.,Hatherly, P., Balusu R, & Le
Blane G. 1998. Longwall Caving Process in Different Geo-
logical Environments Better Understanding through the
Combination of Modern Assessment Methods. Proceedings
of International Conference on Geomechanics/Ground Con-
trol in Mining and Und Construction 14-17 July
1998, Wollongong NSW Australia Vol 2 pp 573-589.

Kelly, M. & Gale, W.J. 1999. Ground Behaviour about Long-
wall faces and its Effect on Mining. ACARP Project
C5017. February 1999,

Sandford, J. 1998. Review of Longwall Mining Experience at
South Bulga Colliery. Proceedings of International Confer-
ence on Geomechanics/Ground Control in Mining and Un-
derground Construction 14-17 July 1998, Wollongong
NSW Australia Vol 2 pp 591-597.



be recorded (where as the hundreds of meters of
propagation are possible for shear events) and their
locations are typically behind the face.

5 COMPUTER MODELLING OF LONGWALL
EXTRACTION IN COAL AND SEISMIC
MONITORING CASE STUDIES

Computer modelling with the FLAC codes (Itasca,
1998) was undertaken at various mine sites to sim-
late the rock failure modes and caving characteristics
about the central zone of the longwall face. The
longwall mining process in the central portion of the
face is simulated as a two dimensional longitudinal
slice by progressively excavating a web of coal and
allowing the ground to cave behind the longwall
supports.

The model represents the central zone of the
longwall panel and is most appropriate for panels of
supercritical width to depth geometry. The super-
critical width to depth ratio of an extraction panel is
typically 1.2-1.5. In a supercritical geometry, the
overburden can cave and the goaf may load in the
central zone of the panel without any significant in-
fluence from the pillars at the edges of the panel.
The model does not simulate the behaviour close to
the gate-ends. While the two dimensional simplifi-
cations are easiest to use for supercritical panels, it is
still possible and appropriate to investigate the be-
haviour under subcritical panel widths. The results
from many models have been compared and vali-
dated against field measurements and have been
found to provide a good estimation of the ground
behaviour (Gale 1998, Sandford 1998, Kelly et al
1998).

The rock fracture, stress geometry, ground dis-
placement and support loading/convergence as min-
ing occurs is recorded in the model. An example of

Depth (am)

“O 0 Bo 2 ie 2 0
Distance (m)

Figure 5. Rock failure mode and fracture geometry
modelled about the faceline of a longwall panel.

the results of the modelling is presented in Figure 5
where the location and type of fractures about a
longwall panel are presented. The stress drop asso-
ciated with the fractures, which occurred during the
last mining cycle, is presented in Figure 6. These re-
sults provide one information source to assess the
ground failure mechanics and the types of rock frac-
tures associated with the micro seismicity.

~ 120

2 «6
Distance (m)

Stress drop duc to failure in current
mumung cy cle

Figure 6. Stress drop zones associated with rock
failure which occurred during the current mining cy-
cle.

6 CASE STUDY OF GORDONSTONE MINE

CSIRO conducted a microseismic monitoring study
at Gordonstone Mine (Queensland, Australia). The
geology of the mine is described by Kelly et al,
(1999). The German Creek Seam (3m thick) is
mined at 230m depth within weak (5-20MPa) lami-
nated siltstone and mudstone. Some stronger sand-
stone bands occur but are relatively thin. The micro-
seismic study was reported by Hatherly et al (1995),
Kelly et al (1999), The key findings from a rock
mechanics viewpoint, were:

1 The rock fracture was recorded well ahead of the

mining face within the roof and floor strata.

2 The dominant rock failure modes recorded were

a 1 yeas occured in pel centre

Tie
: ioeeteee rn
Ik i

fr
lig eo
a:

»
nme

oe 10 °
Distance rom Longval Face (1)

Figure 7. Microseismic events distribution about the
centre of the faceline relative to a fixed face position
in side view.

1316


assess the location and type of fractures developed

about the control zone of the longwall panel and

also to assess the impact of overburden behaviour
on multiple panels. The key findings were:

1 Shear fracture of the ground and bedding plane
shear would be expected as the primary failure
modes.

2 Such fracturing would occur well ahead of the
faceline as independent events within a stable
system.

3 Under certain mine geometries, a large scale fail-
ure zone would be anticipated higher into the
geological section as a result of overburden span-
ning characteristics. A combination of a high an-
gled rock fracture zone and bedding plane shear
was noted. The high angle fractures form within
a soft loading system and would propagate rap-
idly. High energy seismic responses would be
anticipated.

The rock failure zones anticipated about the face
area for a single panel geometry is presented in Fig-
ure 11. The stress drop zones associated with the
rock fracture is presented in Figure 12. The results
indicate shear fracture of the strata and bedding
plane shear are common failure modes which de-
velop the potential for many low energy events
about the face area. The potential for larger scale
events which relate to a larger mine geometry (re-
gional scale) and larger scale ground movements is
presented in Figure 13 where the effect on panel in-
teraction was found to allow failure zones to de-
velop in the overburden as panels interacted. The
stress drop zones associated with the rock failure is
presented in Figure 14 and indicates the potential
for large scale failure and seismic events where
ground movement in the overburden occurs over a
large (regional) scale.

bedding plane shear \

material

40 40 80
Distance (m)

Figure 11. Rock failure zones developed about the
central area of the faceline.

Analysis of the magnitude of the events indicates
that the M.4.2 event would be consistent with a rup-

Jongwatl shi

40 0) 40 80 120
Distance (m)

Figure 12. Stress drop zones developed about the
central area of the faceline.

ture zone of approximately 170-200m in diameter.
This is consistent with the modelled results and also
in general with the mine geometry. The smaller
events ahead of the faceline are consistent with frac-
tures in the range of 0.2-2m in diameter. The mod-
elling was done in isolation with the microseismic
monitoring and does not directly reflect the specific
mine geometry at the location of the monitored
M,4.2 event, however, the ground behaviour and
scale of events is consistent with the microseismic
monitoring and provides an enhanced understanding
of the interaction of seismicity and rock damage.

8 CASE STUDY OF SPRINGFIELD PIKE MINE

NIOSH is in the process of monitoring microseismic
activity at Commercial Stone’s Springfield Pike

regional

Figure 13. Regional scale rock failure mode about
mining panels.

1318


Rock damage characterisation from microseismic monitoring

W.J.Gale

SCT Operations Pty Ltd, Wollongong, New South Wales, Australia

K.A.Heasley, A.T.Iannacchione & P.L.Swanson

National Institute for Occupational Safety & Health, Pittsburgh, PA, USA

P.Hatherly & A.King

Commonwealth Scientific Industrial Research Organisation, Brisbane, Queensland, Australia

ABSTRACT: This paper outlines the concepts used to correlate rock failure with microseismic events and
presents examples of microseismic monitoring together with associated computer modelling of the rock fai-
ure. This study is motivated by the need to develop improved ways to reduce ground control hazards in un-
derground mining. Toward this end we present and compare results from numerical modelling and micro-
seismic monitoring studies conducted at several different mine sites. Emphasis is on integrating results
obtained with these tools to characterize, and thus increase our understanding of, important mine deformation
processes. The ultimate goal is to use this knowledge to design mine structures, and develop mitigation

measures, that minimize specific ground control hazards.

1 INTRODUCTION

The location of microseismic energy release associ-
ated with mining activities is well advanced and is
practiced regularly. In Australia the method has
been used in conjunction with in situ monitoring and
computer modelling to research ground failure char-
acteristics and caving mechanics about longwall
panels in underground coal mines. In the USA this
approach has been used in similar situations and also
within underground limestone mines to assess
ground failure in regard to mine design.

At the various field sites in Australia and the
USA, in situ instrumentation consisting of various
combinations of surface to seam extensometers,
pore pressure cells, rock stress change cells, support
pressure monitoring was used to better understand
strata movement and stresses. Computer modelling
of the stress distributions, rock failure modes and
ground displacements during longwall extraction
was undertaken to better understand the rock frac-
ture characteristics and caving mechanics developed
within various geological environments. Micro-
seismic monitoring was undertaken to locate rock
failure during mining and provide information on
the nature of the failure. The majority of this seis-
mic information related to source location, fre-
quency and relative magnitude. Focal plane solu-
tions were available from a limited number of
selected events.

The combined investigation approach adopted in
these studies, while individually independent, pro-
vided very complementary data and allowed confi-
dence in the findings from each investigation
method. The nature of rock failure about under-
ground extraction panels has been significantly en-

hanced by the use of such techniques. Results uti-
ising these techniques in various combinations have
been reported by: Gale (1999), Kelly et al., (1999)
Iannacchione et al., (2001) Ellenberger & Heasley
(2000).

One of the key issues to arise out of this work
with regard to microseismic monitoring was to what
extent can we characterise the physical nature of the
actual rock damage or the type of rock failure repre-
sented by the microseismicity. This is the focus of
current research as the nature of the failure is be-
coming of significant interest when interpreting the
implications of the microseismic events in terms of
mine design and safety. This has involved further
work in assessing the magnitude of energy or the
stress drop associated with the various rock failure
modes and what are the practical limitations of the
microseismic method in sensing various failure
modes known to occur. The effects of the seismic
network configuration and energy transmission
while of considerable importance to the problem are
regarded as a separate issue in this context and are
not part of this discussion.

This paper outlines the concepts used to correlate
the rock failure with micro seismic events and pre-
sents examples of micro seismic monitoring together
with associated computer modelling of the rock fail-
ure.

2 ROCK FAILURE MODES AND STRESS
MODIFICATION WITHIN THE ROCK MASS.

The rock in situ may be considered to experience
failure in a number of modes. These are:
1 Shear fracture through intact rock material

1313


Mine near Connelsville, Pennsylvania, USA. In
contrast with the first two examples, this is a room
and-pillar limestone mine. Overburdens ranges from
60m to 120m, with rooms 13.8m wide by 7.6m high
and pillars 10.7m square. The limestone is mined by
blasting 4m deep V-cuts in various entries along a
wide mining front. The Loyalhanna Limestone is
approximately 21.5m thick and is overlain by the
Mauch Chunck Formation containing interbedded
shales and calcareous sandstones and underlain by
the Pocono Sandstone. While the strength of intact
specimens are very high (UCS 130 to 200MPa), the
Loyalhanna Limestone contains a great deal of struc-
ture which greatly influences the overall rock mass

Depth (in)

400
500

local scale eer OOO

200 100 0 100 200
Distance (m)

300

Figure 14. Local and regional scale stress drop
zones about mining panels.

strength. The largest scale features are reverse faults
with several meters of displacement. Large trough
beds, which can extend from several meters to tens
of meters in length dip at angles ranging from 70 to
20 degrees. Jointing is generally widely spaced but
can often extend through the entire mining horizon.
Bedding planes can extend over large mining areas
and are often sought after to form smooth roof hori-
zons in the mine. These planes are close to horizon-
tal and are spaced at intervals ranging from several
centimetres to several metres. The smallest scale
structures are crossbeds which dip from 15 to 35 de-
grees and are spaced at intervals averaging one cen-
timeter.

Horizontal stresses at this mine are extremely
large (15 to 55 Mpa) and are thought to contribute to
the excessive stress conditions which have produced
several large roof falls. These roof falls are thought
to contain several of the failure mechanisms previ-
ously discussed. Typically, roof failure starts when
one of the stiffest and thinnest beds in the roof strata
concentrates enough horizontal compressional
Stresses to initiate both intact shear failure and adja-
cent tensile bedding plane failures. At this point,

low angle shears and the accompanying vertical ten-
sile failures can occur more easily in the remaining
intact isolated roof beams, which have had the verti-
cal confinement reduced. As various roof beams
fail, stresses are transferred to adjacent beams where
the process is repeated. Since these roof falls at the
mine range from 6 to 10m in height, a single roof
fall could easily have hundreds to thousands of in
vidual shear failures (seismic events) associated with
them.

This failure process can be illustrated by examin-
ing the microseismic activity from one of these falls.
On October 28 and 29, 2000, a roof fall occurred in
the southwest comer of the mine. This is presented
in Figure 15. In this area, a prominent low angle
shear had formed three months before generating
considerable microseismic activity. After a period
of relative quiet, early on October 28, approximately
50 microseismic events were recorded over a two
hour period as presented in Figure 16. It is assumed
that numerous thin beds within the immediate roof
failed along bedding and through the intact material.
This period was followed by another period of rela-
tive quiet lasting approximately three hours. Then
almost 50 events were recorded over a 20 minute pe-
riod. This is assumed to be the time when the major
portion of the roof collapsed. Again a period of rela-
tive quiet occurred for approximately 2 hours. Then,
a third period of high activity occurred over the next
hour followed by approximately 16 hours of rela-
tively small events at fairly constant rates. It is pos-
sible that during this time period both the edges and
top of the roof fall continue to increase in size.

Figure 15. Microseismic locations and mapped
rock damage above mine roadways.

9 DISCUSSION

In the monitoring studies undertaken about longwall
panels, the notable feature is the distance ahead of
the caving front that the fracturing is being recorded.
This is consistent with the modelling which indicates

1319


2 Tensile fracture through intact rock material
3. Shear fracture of bedding planes

4 Tensile fracture of bedding

5 Remobilisation of pre-existing fractures.

The failure criteria for these can be obtained
from relationships determined in laboratory testing.
The criteria for the failure modes above are pre-
sented in Figure 1 and Figure 2.

Shear Stress (t)

Figure 1. Mohr failure criteria and associated stress
modification.

Shear Stress (1)

LEGEND

0, = Maximum Principal Stress
03 = Minimum Principal Stress

‘AO = Normal Stress Change

Og = Maximum Stress after Rock Fracture
Cj = Intact Cohesion

C, = Residual Cohesion

AT = Shear Stress Change

Figure 2. Mohr representation of failure criteria.

The stress modification within the ground result-
ing from rock failure is different depending on the
mode of failure and the physical scale of the frac-
ture. The process of failure/fracture causes a reduc-
tion in the stress sustainable across the fracture from
that which was within intact material to that which
is governed by the strength properties of the rock
fracture. The difference from the initial stress to the
residual stress is termed the stress drop in the fol-

lowing discussion.. This concept is presented in
Figure 3 and it can be seen that the stress drop asso-
ciated with tensile fracture is significantly less than
shear failure. The general relationships would indi-
cate tensile stress drops are 8-12% of those gener-
ated by shear failure. Also, for stressfields where
the minimum stress is tensile, but not sufficient to
induce tensile failure, the stress drop associated with
shear fracture may be significantly less than if the
minimum stress is compressive.

Intact Rock Strength
Relationship a
Yt AG stress drop

(shear fracture)
for compressive

|

‘stress drop in

UCS#') shear for tensile
f\/ minor re
Residual Rock Strength
Relationship
0; (MPa)
Tensile ‘To Compressive
LEGEND
UCS = Unconfined Compressive Strength
To = Tensile Strength
6, = Maximum Principal Stress
0; = Minimum Principal Stress
AO = Normal Stress Change

Figure 3. Stress drop associated with various failure
modes.

Stress drops for bedding plane shear and bedding
plane tensile failure will be different again depend-
ing on the rock properties. Reactivation of pre-
existing planes will provide only small stress drops
associated with asperities or stick slip geometries
created by fracture networks.

The stress drop (Ao) can be calculated from the
various failure modes of the rocks within the stress-
fields developed about mining panels by computer
modelling, however a general approximation is ob-

tained from:
ms Ao =(0,-0;,), oF

do =KAr

where K = 2 in the ideal case as shown in Figure
1, or

Ao=Te

The resultant stored strain energy released due to
failure is:

(0.5) x (Ac) x (area of fracture) x (displacement
across fracture during rupture).

This can be related to the typical stress strain

1314


characteristics of rock in Figure 4. In this diagram
the area shaded is proportional to the energy re-
leased during rupture.

Bedding Plane|__bs Ste
‘Shear (inclined) [~

€ tensile € compressive
Tension
; Oy, Fracture
e
One:
v

Figure 4. Strain energy release for various rock fail-
ure modes.

The relevant parameters are displayed for a range
of failure modes for unconfined conditions. It is
apparent that shear failure modes will generate sig-
nificantly greater energy than tensile modes and it
will depend on the strength characteristics of the
particular rock type as to the energy release per area
of rupture.

It is assumed in the model, that the seismic events
measured result from the stress drop associated with
the formation of fractures. Such events are consid-
ered to be “instantaneous” rupture and displacement
surfaces which generate the P and S waves meas-
ured. The “instantaneous” rupture surface or net-
work is expected to propagate at a similar speed to
the shear wave velocity.

Remobilisation of existing fractures in a stick slip
mode would be recorded as the rapid energy release
associated with failure of asperities or areas which
have caused previous “lock up” along the fracture,
together with any associated rapid movement along
the fracture in the vicinity of the asperity/lock-up
zone. The seismic energy rele from these frac-
ture/slip sources represents, at most, a few percent of
the total energy released (McGarr, 1993). Depend-
ing upon the local stability conditions, these same
deformation processes can also occur without gener-
ating any detectable seismic energy.

Similarly, elastic deflection of overburden is not

expected to be recorded seismically, but such deflec-
tion may contribute to the loading system which
subsequently causes a sudden rupture of the ground.

3 SEISMIC RESPONSE TO GROUND FAILURE

The resultant effect of the failure on mining and in
seismicity is related to the ability of the rock to re-
distribute the stress drop about the boundaries of the
fracture. This will to a large extent be dependent on
the regional stiffness contrasts in the ground.

In the case of a “stiff” loading system (where the
stiffness of the ground generating the stresses caus-
ing rock fracture (regional stiffness) is greater than
the stiffness of the fractured rock zone), controlled
rock fracture geometries are expected in the form of
individual fractures in a stable mining geometry.

In the case of a “soft” system (where the-regional
ground stiffness is significantly less than the stiff-
ness of the fractured rock) then rapid failure and
networking of fractures to form a large unstable frac-
ture zone is likely. This type of failure is most likely
to be manifest in large scale mine geometries where
overburden movement is involved in the loading.

The seismic response envisaged for the stiff sys-
tem would be numerous independent low energy re-
sponses whereas the soft system would produce a
single or limited number of high energy responses as
a single fracture zone propagates. In reality, a range
of responses will be expected depending on the
strength and stiffness of the ground about excava-
tions.

4 SIGNATURES OF SHEAR AND TENSILE
FRACTURES

The nature of the failure that is occurring within the
rock mass can also be inferred from microseismic
data. The Commonwealth Scientific and Industrial
Research Organization (CSIRO) has undertaken 13
microseismic monitoring studies of longwall extrac-
tion at Australian longwall mines. Typically the
events that are recorded occur ahead of the face and
may occur within the floor strata as well as the roof.
These events exhibit strong P- and S-wave motions
and examination of their polarisation suggests that
shear failure is occurring.

Microseismic events caused by tensile failure
have proved much more difficult to detect but this is
to be expected given the reduced energy associated
with such events, and the need for the seismic wave
propagation to occur through more highly absorptive
broken ground. However some events that are inter-
preted to be due to tensile failure have been ob-
served, These typically have amplitudes about a fac-
tor of 10 less than the shear events, the P-wave
motion is dominant (but not exclusive) and the po-
larisation of the P-waves suggests that there is first
compression in all directions (i.e. the expansion as-

“sociated with tensile failure). It has been found that

these events need to be within 50m of geophones to

1315