1. INTRODUCTION 

Roof falls and roof caving events represent a serious 

hazard to miners working underground. The 

development of roof fall hazard recognition, 

monitoring, and control techniques is highly 

dependent on a sound understanding of the failure 

processes responsible for roof falls and roof caving 

events. Recent NIOSH studies in a limestone mine 

[1] have found that tracking the signatures 

emanating from strata transitioning from stable to 

unstable states aids in identifying roof failure 

mechanisms and monitoring for roof stability 

conditions. In the earliest stages of this transition, 

the rock strata fail by fracturing in both shear and 

tension. Generally, one or more significant large 

events and dozens of smaller events are observed on 

a daily basis during unstable roof states. As more 

fracturing occurs and existing fractures coalesce, 

the ground softens, increasing the potential for roof 

collapse. Slip can also occur along fracture planes 

or pre-existing bedding planes as strata move 

toward the developing roof fall cavity. Finally, the 

impact of the falling rocks onto the mine floor 

produces a wide range of seismic signatures that 

help characterize the rock fall mass and the 

condition of the surface upon which the rock has 

impacted. 

1.1. Microseismic Monitoring and Analysis 

This study analyzed microseismic emissions from 

three mine sites: Springfield Pike Quarry, Moonee 

Colliery, and Willow Creek Mine. Springfield Pike 

and Willow Creek used a microseismic system 

developed by NIOSH [2] and were analyzed using 

techniques discussed by Iannacchione et al. [3]. 

The Moonee field site employed a microseismic 

system developed in South Africa by ISS 

International. Emissions from the Moonee Colliery 

were collected by the operating mining company, 

Coal Operations Australia Limited, and provided to 

NIOSH for this study. 

In all three study sites, care was taken to determine 

accurate event locations. Typical rock fracture 

events have distinct first arrivals which allow for 

easy P wave identification, an essential 

characteristic for accurate event location. The size 

of rock failure events is established by calculating 

 

 

Characteristics of Mining-Induced Seismicity 

Associated with Roof Falls and Roof Caving Events 

Iannacchione, A.T., Esterhuizen, G.S. and Bajpayee, T.S. 

NIOSH, Pittsburgh Research Laboratory, Pittsburgh, Pennsylvania, USA 

Swanson, P.L. 

NIOSH, Spokane Research Laboratory, Spokane, Washington, USA 

Chapman, M.C

Virginia Polytechnic Institute and State University, Blacksburg, Virginia, USA 

 

ABSTRACT: The National Institute for Occupational Safety and Health (NIOSH) evaluated microseismic activity from three 

field sites to compare and contrast the characteristics of microseismic emissions from very different geologic, stress, and mining 

environments. Recently, NIOSH has embarked on a research program to evaluate the use of microseismic monitoring information 

to identify roof fall failure processes and to assess its potential to warn of unstable roof conditions. Large roof instabilities, such as 

roof falls and certain roof caving events, have proven difficult to anticipate representing an increased risk to miners working in 

these inherently hazardous areas. When local failure processes are better understood, appropriate control measures can be 

engineered to mitigate these hazards. This study used microseismic emissions to help identify three local rock failure processes. It 

was also shown that analysis of microseismic emissions can aid in assessing the degree of instability associated with these local 

rock failure processes.
the moment magnitude (M). The moment 

magnitude scale, developed by Hanks and 

Kanamori [4], is consistent with the more familiar 

Richter magnitude scale and is currently used by 

seismologists as a measure of seismic source size. 

Moment magnitude is based on the static seismic 

moment, Mo, by the relationship: 

(1) 

where Mo is expressed in SI units (N-m). 

The static seismic moment [5] is determined from 

the observed Fourier displacement amplitude 

spectrum of body waves, the rock density at the 

source, the body wave velocity, the body wave 

radiation pattern, and the distance between the 

seismic source and the receiver. The seismic 

moment is also important because it is a measure of 

the dynamic inelastic deformation, or nonrecoverable deformation, associated with a seismic 

source. 

1.2. Rock Failure Process 

In this study, three general classes of rock failure 

mechanisms are identified as progressive, episodic, 

and continuous. The roof in room-and-pillar mines 

is generally stable until a localized increase in stress 

or a loss in strength disturbs the local equilibrium 

condition and initiates failure. After the onset of 

rock failure, additional failures are possible as the 

adjacent rock layers adjust to the changing 

conditions. In this way, a progression of rock 

failure is established until the surrounding rock 

mass reaches a new state of equilibrium or until it 

results in a roof fall. This process is termed 

progressive rock failure. 

When mining creates expansive extraction zones 

where stresses and deformations are concentrated at 

panel edges, such as those found in longwall coal 

mining, the roof rock fails in a continuous 

incremental fashion. As support is removed from 

beneath the overlying strata, the strata break apart 

and eventually cave into the void made by mining. 

In this case, the failure process is highly dependent 

on the extraction effort. If the extraction effort is 

slow, the failure process is correspondingly slow. 

Rock failure progresses more rapidly as the mining 

rate increases. In this way, the failure process 

assumes a continuous incremental pattern that 

closely matches the daily extraction rate. 

An intermediate condition exists between the 

progressive and continuous failure process. When 

mining takes place in panels of limited size or under 

thick competent strata, the stresses and deformation 

gradient at the face are reduced in comparison to 

continuous incremental failure. In this case, strata 

failure and gob formation do not immediately occur 

after an increment of face advance, and the 

expanding area of supported overlying strata 

becomes larger. As a result, when failure does 

finally occur, it does so over a wide area. This type 

of failure can be thought of as an end member of the 

continuous failure process where the increments, or 

episodes, are very large and not tied very closely to 

the short-term mining rate. With additional mining 

the failure progresses, much like the room-andpillar rock failure process, until the overhanging 

rock mass becomes unstable and falls as a large 

mass. Hence, this type of rock failure is episodic in 

nature. These three rock failure processes provide 

an opportunity to examine microseismic 

information as it relates to the stability of 

underground structures. 

2. FIELD DATA 

NIOSH evaluated microseismic activity from three 

field sites: Springfield Pike Quarry, located in 

southwestern Pennsylvania; Willow Creek Coal 

Mine, located in eastern Utah’s Wasatch Plateau; 

and Moonee Colliery Coal Mine, located in New 

South Wales 100 km north of Sydney, Australia. 

Each of these sites exhibits a rock failure process 

that resulted in different microseismic emission 

characteristics. Of the three sites, only the 

microseismic system at Moonee Colliery was 

designed and utilized to provide real-time roof 

instability warning information. 

2.1. Springfield Pike Quarry: A Case of 

Progressive Failure 

The Springfield Pike Quarry is an underground 

room-and-pillar mining operation in the Loyalhanna 

Limestone. Overburden in the study area is 

approximately 100 m. The Loyalhanna Limestone 

makes up the mining horizon and approximately 

2 m of the immediate roof. Above the Loyalhanna 

is the Mauch Chunk Formation containing 

alternating layers of weak claystones, shales, 

siltstones, and thin sandstones. The Loyalhanna 

Limestone has an unconfined compressive strength 

ranging from 130 to 200 MPa and a high horizontal 

stress field ranging from 14 to 55 MPa [6]. 

log( ) 6.0, 3

2 M = Mo −
At the site, 4.5-Hz three-component geophones are 

used in a single-component configuration. The 

geophones are mounted on the roof some 8 m above 

the mine floor. During the course of seismic 

monitoring at the study site, several thousand 

seismic events, including several roof falls, were 

recorded and located [6]. Events involve medium 

size rock impacts and relatively small rock fracture 

events. In general, the smallest rock fracture events 

are typically recorded only by geophones in the 

immediate vicinity. 

On February 20-21, 2000, a series of rock impact 

events were associated with a major roof fall. This 

fall extended over 50 m in length, 13 m in width, 

and as much as 9 m in height. The seismic 

characteristics of these impacts are similar to those 

previously discussed by Iannacchione, et al., [3] in 

that they contain higher seismic moments than rock 

fractures, are long in duration, and emergent in 

form. Although it cannot be definitively determined, 

the authors believe the majority of the energy 

contained in these events comes from the rocks 

impacting the mine floor as opposed to the release 

of the rock from the roof. 

Five rock impact events and 136 rock fracture 

events were recorded at this site (Figure 1). Almost 

all of the rock fracture events occurred over a 

14-hour period from 2:17 p.m. February 20 until 

4:15 a.m. February 21. The five rock impact events 

occurred over a 94-minute period and ended with a 

final major event at 1:24 a.m. on February 21. The 

locations of the geophones, rock fractures, rock 

impacts, and roof falls are shown in Figure 2. 

The moment magnitude (M) of 32 rock fracture and 

five rock impact events with the lowest location 

error measurements are shown in Figure 1. The 

moment magnitude of the rock fracture events 

ranged from -1.5 to -0.7, averaging -1.1. The 

0

50

100

150

2/20/00 0:00 2/20/00 8:00 2/20/00 16:00 2/21/00 0:00 2/21/00 8:00 2/21/00 16:00 2/22/00 0:00

Date

Cumulative Frequency

-2.0

-1.5

-1.0

-0.5

0.0

0.5

Moment Magnitude, M

Event Number Rock Fractures, M Rock Impacts, M

Fig. 1. A plot of the cumulative frequency of the 136 rock fracture events and five rock impact events associated with the 

February 21, 2000, roof fall at the Springfield Pike Quarry. Moment magnitudes were also calculated and displayed for 32 

of the 136 rock fracture and for all five rock impact events. 

Fig. 2. Location of the 136 rock fracture events, five rock 

impact events, five geophones, and the February 21, 2000, 

roof fall at the Springfield Pike Quarry. 
moment magnitude of the rock impact events 

ranged from -0.6 to 0.2, averaging -0.2. The last 

rock impact event at 1:24 a.m. on Feb. 21 was the 

largest (M = 0.2) and most likely represented the 

biggest mass of falling rock. The magnitude of the 

event was probably lessened when debris buildup 

from the previous impacts partially absorbed the 

energy transfer [3]. 

The rock failure process in bedded formations with 

excessive levels of horizontal stress is initiated 

when the stiffest and thinnest beds in the roof strata 

begin to buckle from the horizontal loading [1]. 

When these layers buckle (Figure 3a), shear and 

tensile rupture between layers and low-angle shears 

through the intact rock layers can occur (Figure 3b). 

Eventually the beam begins to cantilever, initiating 

a tensile failure along the fixed contact area at the 

edge of the roof fall (Figure 3c). From this point 

forward, reductions in super- and sub-adjacent 

layers lower confinement, resulting in additional 

low-angle shear failures. One-by-one the individual 

roof beams are strained to failure, shedding their 

load to adjacent layers which are also strained to 

failure. The rock failure process is progressive in 

nature and reduces when the shape of the roof 

cavity assumes the more stable arch shape. The 

roof cavity shape is defined by the sub-vertical 

tensile failures (Figure 3d). Rock failure can be 

reinitiated at the axial ends of these roof fall cavities 

where elevated stress conditions sometimes occur. 

2.2. Moonee Colliery: A Case of Episodic Failure 

The Moonee Colliery is a longwall mining 

operation in the Great Northern Coalbed of the 

Newcastle Coal Measure. Overburden ranges from 

90 m in the north to 170 m in the south of the mine 

[7]. The immediate roof comprises 1.6 m of coal 

and claystone. These layers are overlain by the 

Teralba Conglomerate with a thickness of 30 to 

35 m. Unlike most longwall mines, the Teralba 

Conglomerate typically does not continuously cave 

as the longwall advances. Instead, it can hang in 

place until extensive unsupported spans exist. 

When it does cave, it can fall as a series of impacts 

well behind the longwall face or as one continuous 

mass [8]. The non-continuous caving of the roof is 

most likely influenced by low overburden, narrow 

panels, and strong abutment strength of the adjacent 

solid longwall panels. During mining of the first 

longwall panel, 41 distinct roof falls occurred with 

an average hanging span of 46 m. Many of these 

spans encompassed the entire width of the 100-mwide panels. 

Roof fall caving events began to occur at Moonee 

after the initial 200 m of longwall face advance of 

the first panel. Sometimes massive roof falls 

resulted in dangerous windblasts. For example, six 

miners were injured on Jan. 22, 1998, from a 

windblast event associated with the fifth longwall 

Tensile failure occurs along the 

arched roof cavity

Beam begins to shear

Time initial

Tensile

Beam cantilevers

Shear

Tensile

Mine opening

Shear

Time final

Shear propagates and beam dilates 

b)

c)

d)

e)

a)

Tensile failure occurs along the 

arched roof cavity

Beam begins to shear

Time initial

Tensile

Beam cantilevers

Shear

Tensile

Mine opening

Shear

Time final

Shear propagates and beam dilates 

b)

c)

d)

e)

a)

Fig. 3. Generalized sequence in which individual roof beams fail and develop into large roof falls under elevated 

horizontal stress conditions. 
panel roof fall event [9]. This roof fall produced a 

fallen material geometry similar to half a cone with 

stepped surfaces (Figure 4). The top of the roof fall 

failure surface arched approximately 16o over the 

panel from the longwall face and the two gate 

entries, reaching a maximum thickness of 15 m in 

the center of the panel, 35 m from the longwall face 

(Figure 4). The top of the roof fall cavity was made 

up of both horizontal and vertical planes that 

formed a step-like surface. The horizontal planes 

were most likely associated with local bedding 

structures within the conglomerate, while the 

vertical planes were associated with the local 

jointing, spaced a few meters apart [10]. Above the 

fallen material, the conglomerate strata continued to 

bridge across the panel, leaving a 2- to 3-m high air 

gap. The back side of the fall, facing the previous 

caved rocks, was approximately parallel to the 

longwall face but arched toward the longwall face 

[10]. Edwards [8] and Mills and Jeffrey [10] have 

indicated that the general pattern described above 

was typical of longwall panel No.1’s roof fall. 

Microseismic monitoring was introduced to 

Moonee soon after the Jan. 22 roof fall as a way to 

predict the onset of caving with sufficient warning 

to enable miners on the face to take shelter in a safe 

location prior to the associated windblast [11]. This 

system used 14-Hz three-component geophones. 

Four geophones are mounted in 10-m roof 

boreholes around the longwall and were 

continuously moved to surround the longwall face. 

During the course of this multi-year seismic 

monitoring project, tens of thousands of seismic 

events, including numerous roof fall caving 

episodes, were recorded and located [7, 12]. For 

example, there were 118 rock fracture events 

(Figure 5) in the eight-day period that occurred after 

roof fall No. 20 and prior to roof fall No. 21. 

Approximately 70% of these seismic events 

occurred during the last three days of advance. In 

this case, there was only a weak correlation between 

advance rate and seismicity. It seems reasonable to 

ascribe this seismicity to the stepped fracture 

surface developing in the overlying conglomerate 

that would soon outline the fallen material for roof 

fall No. 21 (Figure 6). 

By April 3, the hanging roof extended 97 m 

encompassing a 9,830 m2

 area (Figure 6). The 

average rock fracture event occurred approximately 

16 m above the extraction horizon. Also, a small 

Caved zone

Longwall panel

Conglomerate roof rock

Jan. 22 roof 

fall material

Stepped surfaces

Center line of longwall panel Mining direction

16°

15 m

45 m Gate entry pillar

A B

35 m

KEY

Caved zone

Longwall panel

Conglomerate roof rock

Jan. 22 roof 

fall material

Stepped surfaces

Center line of longwall panel Mining direction

16°

15 m

45 m Gate entry pillar

A B

35 m

KEY

Fig. 4. Generalized sketch of the Jan. 22 roof fall (Fall No. 5), longwall panel No.1, Moonee Mine. 

0

20

40

60

80

100

120

26-Mar 27-Mar 28-Mar 29-Mar 30-Mar 31-Mar 1-Apr 2-Apr 3-Apr 4-Apr

Date

Cummulative Frequency

-2.0

-1.5

-1.0

-0.5

0.0

0.5

1.0

Moment Magnitude, M

Moment Magnitude

Fig. 5. A plot of the cumulative frequency of the 118 rock 

fracture events associated with the period between Roof 

Fall No. 20 and 21 at the Moonee Colliery. Moment 

magnitudes were also calculated and displayed for all 118 

rock fracture events. 
cluster of rock fracture events occurred along a 

northwest-southeast trending dyke 150 m from the 

longwall face. These events may be associated with 

fracturing surrounding this stiff intact dyke. The 

moment magnitude of 118 rock fracture events, 

shown in Figure 5, ranged from -1.7 to 0.6 and 

averaged -0.9. Three rock fracture events with 

moment magnitudes between 0.3 and 0.6 occurred 

approximately three minutes prior to roof fall No. 

21. 

2.3. Willow Creek Mine: A Case of Continuous 

Failure 

The Willow Creek Mine, abandoned since 2001, 

was a longwall coal mining operation in the 

Castlegate “D” Coalbed. Overburden in the study 

area ranges from 750 to 900 m. The immediate roof 

strata are made up of less than 3 m of thin 

claystones, shales, sandstones, and coalbeds. 

Above this is between 150 to 180 m of Blackhawk 

Formation composed of strong interbedded 

coalbeds, siltstones, and sandstones with 

unconfined compressive strengths ranging from 60 

to 120 MPa. Capping the Blackhawk is 120 to 180 

m of massive cliff forming Castlegate sandstone. 

The Willow Creek longwall panels are over a 

thousand meters long and range in width from 162 

to 244 m. The immediate roof strata cave tightly 

behind the longwall shields as the longwall 

advances. The continuous caving of the immediate 

roof is undoubtedly influenced by significant 

overburden, the wide panels, and the marginal 

abutment strength of the adjacent longwall gate 

entries. 

A three-dimensional microseismic array used 14 

horizontally oriented and 9 vertically oriented 

surface geophones (4.5 Hz). The underground 

geophones were mounted directly on the roof and 

the surface geophones were placed in shallow holes. 

During the course of seismic monitoring, five 

thousand high-quality seismic events were recorded 

and located [13]. The seismicity associated with a 

typical day of longwall advance is shown in 

Figure 7. During the early hours of the morning, 

when maintenance activities idle the longwall, 

seismicity is very low. Seismicity increases 

considerably a short time after mining commences. 

Fig. 6. Location of the 118 rock fracture events, geophones, roof falls, longwall face, and dyke at the Moonee Colliery. 
During this period, events occur at a somewhat 

constant rate until production ceases and then 

relative quiet returns to the longwall panel. 

Correlations between advance rate and seismicity 

rate were documented by Heasley et al. [13] and 

Westman et al. [14]. 

The location of 111 rock fracturing events from 

May 23, 2000, cluster along the longwall face area 

(Figure 8). There is a noticeable concentration near 

the headgate entries. Additionally, the events occur 

both above and below the mining horizon. In 

examining the data for the entire longwall panel, 

Heasley et al. [13] found that the events generally 

occurred in advance of the longwall face, 

distributed both above and below the mining 

horizon. 

A majority of the events are associated with the 

forward abutment stress that fractures the rock in 

the longwall face area. The forward abutment stress 

acts along the longwall face area, producing 

fractures that eventually outline blocks of various 

size and shapes. These blocks are held together in 

this area because the longwall face and shields act 

to supply some level of confinement. As the 

longwall face advances and the confinement is 

removed, the blocks begin to separate and collapse 

forming the broken rock mass known as the 

longwall gob. 

A noteworthy observation is that microseismic 

emissions are obvious in the front abutment stress 

area but not so apparent in the longwall gob zone. 

 This is most likely an issue of event detection 

sensitivity. A minimum of eight stations with highquality arrivals were required to satisfy the criterion 

for locating events plotted in Figure 8. Average 

source to receiver distance was 1.1 km. Thus, 

smaller events which did not satisfy this condition 

were not detected. Numerical modeling studies by 

Gale et al. [14] support the notion that events 

representing incremental fracture in the frontal 

abutment zone should be larger than events in the 

gob zone. In the two-dimensional models, fracturing 

behind the longwall shield was generally not 

associated with microseismic activity because of 

low stress drop conditions, a corresponding lack of 

shear fracturing, and poor transmission of elastic 

energy through the broken rock mass. 

The moment magnitudes of 30 selected rock 

fracture events are also shown in Figure 7. This 

subset of the May 23 events was selected because of 

their low location value residuals. The moment 

magnitudes of the rock fracture events ranged from 

-0.2 to 0.9 with an average of 0.5. 

3. MICROSEISMIC EMISSIONS AND 

STABILITY MONITORING 

The three case studies demonstrated that the local 

rock failure process, active at each site, could be 

determined from a combination of observational 

information and microseismic monitoring. The 

microseismic information was able to detect and 

characterize the rock deformations in a way that 

was otherwise not possible. In each case a distinct 

pattern of seismicity occurred. 

3.1. Stability Assessment at Springfield Pike 

At the Springfield Pike Quarry, a period of quiet 

was interrupted by the onset of microseismic 

activity lasting approximately 14 hours and 

culminating in a series of rock impacts followed by 

the return to quiet. Approximately 12.5 hours of 

microseismic activity preceded the rock impact 

events that occurred in the last 94 minutes of this 

time period. In this case, it appears that the 

microseismic information could be used to provide 

warning of impending major roof instabilities. 

3.2. Stability Assessment at Moonee 

The episodic rock failure process at Moonee 

Colliery was in many ways similar to the 

Springfield Pike Quarry with the exception that it 

was rarely very quiet in the longwall panel area. 

The almost continuous advance of the longwall face 

0

20

40

60

80

100

120

5/23/00 12:00 AM 5/23/00 8:00 AM 5/23/00 4:00 PM 5/24/00 12:00 AM

Date

Cumulative Frequency

-0.40

-0.20

0.00

0.20

0.40

0.60

0.80

1.00

Moment Magnitude, M

Event Number Rock Fracture

Fig. 7. A plot of the cumulative frequency of the 111 

rock fracture events associated with longwall mining on 

May 23, 2000, at the Willow Creek Mine. Moment 

magnitudes were also calculated and displayed for 30 of 

the 111 rock fracture events. 
acted to destabilize the overlying roof strata above 

the longwall panel. The majority of the 

microseismic activity did not cluster around the 

face. Instead, the microseismic activity apparently 

represents the initiation and development of the 

stepped failure surface and accommodation of stress 

adjustment over a wide area. This final activity just 

prior to the roof collapse represented a dramatic 

increase in the occurrence, rate, and, to a lesser 

degree, magnitude of microseismic events 

(Figure 5). In fact, Moonee Colliery developed a 

system of alarms that warned of impending roof 

falls from observed trends in microseismic activity. 

These alarms were: 

1. Trend Alarms – based on interpreted trends of 

microseismic activity. Alarms were typically 

long-term warnings, usually a few hours, that 

provided sufficient time for the longwall crew 

to evacuate the face. 

2. Frequency or Magnitude Alarms – based on a 

sudden flurry of microseismic activity. 

Alarms were typically short-term warnings, 

usually a few minutes, that provided sufficient 

time for the longwall crew to move to the 

safest area of the face. 

3. Auto Alarms – based on the capture of six 

events or greater in a 10-second period. In 

this case, an alarm was automatically signaled. 

Hayes [7] reports that the microseismic system was 

able to give sufficient warning of impending roof 

falls that could cause windblast in approximately 

90% of the cases. 

3.3. Stability Assessment at Willow Creek 

The continuous failure process at the Willow Creek 

Mine is very different from the progressive and 

episodic processes at Springfield Pike and Moonee. 

Microseismic activity was associated with the 

constant caving of the longwall panel. This 

constant activity was a sign of “normal” mining 

conditions. While the microseismic information 

was never used to issue warning of specific 

hazardous ground conditions, deviations from a 

continuous failure process may represent the onset 

of abnormal caving. Recognition of this deviation 

Figure 8. - Location of 111 rock fracture events, geophones, longwall caving zone (gob), and longwall face at the Willow 

Creek Mine. 
from normal provides the basis of a hazard warning 

methodology. Abnormal caving behavior is often 

associated with coal bumps, weighting of the 

longwall shields, large roof collapses in the gob, 

and, in some cases, damaging windblasts. 

4. SUMMARY AND CONCLUSIONS 

This paper reviews the behavior of two roof fall 

events and one case of continuous roof caving. 

Significant differences are observed in the 

collective seismic character from each site. These 

differences are related to the interaction of local 

geologic, mining, and stress conditions. It would be 

very difficult to develop an adequate understanding 

of the important failure processes operating at each 

of these sites without the microseismic information. 

Three rock failure processes were identified as 

progressive, episodic, and continuous. 

Microseismic information collected from three field 

sites consisted of: 

• At Springfield Pike, 136 rock fracture events 

occurred over a 14-hour period. The moment 

magnitudes of these rock fracture events 

ranged in size from -1.5 to -0.7. During the 

final 94 minutes of this failure event, five 

significant rock impact events occurred. The 

moment magnitudes of these rock impact 

events ranged in size from -0.6 to 0.2. 

• At Moonee, 118 rock fracture events occurred 

over an eight-day period between Roof Fall 

No. 20 and No. 21. The moment magnitudes 

of these rock fracture events ranged in size 

from -1.7 to 0.6. 

• At Willow Creek, 111 rock fracture events 

occurred during a normal production day. The 

moment magnitudes of 30 selected rock 

fracture events ranged in size from -0.2 to 0.9. 

In each of the three case studies, the character of the 

microseismic information helped to identify the 

rock failure process: 

• Progressive – Periods of quiet are interrupted 

by the onset and progression of microseismic 

activity which lasted in this case for 

approximately 14 hours and culminated in a 

series of rock impacts followed by the return to 

quiet. 

• Episodic – The majority of the microseismic 

activity represents the initiation and 

development of the stepped failure surface that 

outlines the eventual roof fall material. The 

final surge in activity is associated with the 

completion of this surface and the collapse of 

the roof. 

• Continuous – Microseismic activity in the 

forward abutment stress zone was associated 

with the constant caving of the longwall panel. 

It was also demonstrated that the microseismic 

information provided a useful assessment of the 

stability of the roof rock. In the case of the 

progressive rock failure process, the onset of 

microseismic activity signaled the beginning of 

unstable conditions. It was also demonstrated that 

while activity continued, rock impacts were 

possible. In the case of the episodic rock failure 

process, trends in the microseismic activity were 

used to warn of roof falls with a high degree of 

success. Finally in the case of the continuous rock 

failure process, the constant uniform activity in the 

forward abutment stress area was a sign of “normal” 

conditions. Deviations from normal strata response 

can provide useful stability information. 

Microseismic activity, occurring during and before 

roof fall and roof caving processes, can be 

measured and analyzed to characterize local failure 

processes. Enormous potential exists for 

application of this technology to assess the stability 

conditions of underground structures, so that safer 

mine layouts, monitoring systems, and support 

systems can be engineered. 

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Marshall, and J. Litsenberger. 2004. Relationship of roof 

movement and strata induced microseismic emissions to 

roof falls. Mining Engineering 56: 12, 53-60. 

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and Seismicity in Mines – RaSiM5, South African Institute 

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International Symposium on Rockbursts and Seismicity in 

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