COAL MINE SEISMICITY AND BUMPS: HISTORICAL CASE

STUDIES AND CURRENT FIELD ACTIVITY

John L. Ellenberger, Mining Engineer

Keith A. Heasley, Supervisory Physical Scientist

National Institute for Occupational Safety and Health

Pittsburgh Research Laboratory

Pittsburgh, PA

ABSTRACT

The National Institute for Occupational Safety and Health

(NIOSH) has continued the research role of the former U.S.

Bureau of Mines to develop techniques that will reduce the

hazards in the mining work place associated with coal bumps.

Current research focuses on both analyzing historical seismic

data from bump-prone operations and utilizing a mine-wide

seismic network to investigate the exact strata failure

mechanics associated with bump-prone geology. The

anticipated outcome of this research will be reduced bump

incidences through advanced engineering concepts and

designs which implement the new understanding of strata

behavior.

The analysis of the historic seismic data consists of

correlating observed mining seismicity with the geologic and

geometric parameters at the sites. The primary seismic

parameters are the timing, location and magnitude of a

recorded seismic event. These parameters are correlated with

such mining parameters as: the overburden, the size of the

immediate gob, the size of the district gob area, etc. This

detailed analysis of historical seismic data has provided an

informative quantifiable relationship between many of the

specific mining parameters and the induced seismicity.

The second aspect of the coal bump research is the

instrumentation of an appropriate field site to determine the

main roof, floor, and gob behavior associated with bump

behavior. The chosen field site is a deep-cover longwall mine

with competent geology in a historically bump-prone area.

The primary field instrumentation is a three-dimensional, fullwaveform, seismic array with both surface and underground

sensors surrounding an active multi-panel district. The

purpose of this seismic array is to determine the timing, the

exact location, and the mechanism (tensile fracture, bedding

plane slip, etc.) of the failure of the strata surrounding the

active and multi-panel gobs. The preliminary results presented

in this paper help to define the strata failure areas around the

longwall panel.

INTRODUCTION

Coal mine bumps, sometimes referred to as outbursts,

bursts, or bounces, have been recognized as a serious problem

in mining for more than 75 years. For the purposes of this

paper, a bump is defined as a sudden release of geologic strain

energy that results in the expulsion of coal from a rib or pillar

in a catastrophic manner. Beginning as early as the 1930's, the

U.S. Bureau of Mines conducted research relating to causes

and potential mechanism to avoid bumps in coal mines (1).

Current research being conducted by the National Institute for

Occupational Safety and Health (NIOSH) remains focused on

the reduction and elimination of bumps.

In the past, bumps have been acknowledged as having a

greater likelihood of occurrence at depths greater than 300 m

(1,000 ft), in the presence of strong roof and/or floor, and when

an unusually strong massive unit exists in the main roof (1, 2,

3). Miners working under one or more of these conditions need

to be constantly aware of the possibility of a coal bump.

Recently, the U.S. coal industry experienced several

consecutive years with no fatal accidents resulting from bumps,

until November of 1996, when three fatalities and five

additional serious injuries occurred in a two week period.

Coal bumps are often associated with seismic events that

are large enough to be registered by regional seismic networks.

However, not every potentially hazardous bump generates a

regional seismic event, nor does every mine-induced, regional

seismic event manifest itself as a coal outburst at the seam

level. In reality, coal bumps are just one subset of mineinduced seismicity, and like mine-induced non-bump events,

they can exhibit a wide range of magnitudes and energy release.

But, knowing that coal bumps are a subset of mine seismicity,

it seems reasonable that analyzing mine seismicity in general

may provide a better understanding of the causes and control of

mine bumps. Also, there are considerably more documented

mine-induced, non-bump, seismic events than there are

documented coal bumps. In fact, the seismic network in Utah

has found the most active seismic area since 1962 to be the

vicinity of the active coal mining with hundreds of mininginduced events recorded each year (4). With this number of
events, it would seem possible to develop statistically

significant relationships between mining parameters and the

occurrence of the mine-induced seismic events that would not

be evident using the much smaller subset of actual coal bumps.

This paper presents a correlation analysis between the

observed mining-induced seismicity and the geologic and

geometric parameters at bump-prone sites. The analysis

provides a number of results which support previous research

and a number of results which may question conventional

wisdom. The paper also presents some of the preliminary

results from a recent three-dimensional seismic system

installed at a bump-prone site.

CASE STUDY A

The first seismic study location, Site A, operates in a

coalbed varying from 2.4 to 3.6 m (8.0 to 12.0 ft) in thickness.

The immediate roof varies, generally consisting of a laminated

gray shale; however, in places, the immediate or main roof

varies from a weak, highly fossilized and slickensided black

shale to a strong siltstone, or sometimes, a sandstone channel

system.

This mine progressed through 10 longwall panels

without incident, then encountered bumps related to

increasing depth and a sandstone channel system that

resulted in abrupt changes in the physical characteristics of

the immediate roof. In the following longwall district,

multiple-seam interactions in conjunction with sandstone

channels appeared to manifest coal bumps (3). Information

in this current report covers the bumps encountered in the

final longwall district of the mine where neither sandstone

channels nor multiple-seam mining was evident.

At least six different gate road designs were utilized at

Site A. The coal pillar configuration historically used was

a three-entry yield-abutment design, later being changed to

a four-entry yield-abutment-yield design. Pillar dimensions

frequently varied from one gate road to the next. However,

all the three-entry systems were configured similarly, with

the abutment pillar placed adjacent to the longwall panel in

the headgate. In the earlier three-entry designs, the

dimensions of both pillars were increased in successive

designs, to account for increasing overburden (3). In the

current report, the pillar design was consistent across the

study area, and had evolved to a four-entry, yield-abutmentyield configuration (figure 1). Topographic relief varied

sharply across the panels included in this study, varying

from 150 to 660 m (500 to 2,200 ft).

Figure 1. Panel and face locations of regional and Local events at Mine A
Bump/Seismic Events

Figure 1 shows the face locations at the time when the

mine records listed 48 different events (the figure shows less

than 48 face positions because, on several occasions, more

than one event occurred in one day). These events were

initially recorded on the foreman’s reports because coal was

ejected from the face and/or large vibrations were felt from the

tailgate/gob area. A large number of theses events appear to

have been smaller local events where some coal was ejected

from the face, but no large ground vibrations were evident.

On the other hand, 22 of these bump/seismic events registered

a magnitude of 2.8 or greater on regional seismic monitoring

networks. These 22 regional seismic events and the mine

parameters at the time of the event are presented in table 1.

Analyzing this small database of mine-induced seismic events

produced a number of notable correlations.

Effect of Overburden on Mine Seismicity

Deep cover and/or strong lithologic units have long been

recognized as necessary contributors to coal bumps (1). With

this in mind, the distribution of the seismic events from Site A

with respect to overburden was produced as shown in figure

2. (In order to normalize the number of seismic events

occurring at a particular overburden range with relation to the

amount of panel that was mined at that overburden range, the

“event rate” was determined as the number of events that

occurred per 300 m (1,000 ft) of panel advance at that

overburden. For example, 5 events occurring in 2,200 m

(7,200 ft) of face advance under overburden ranging from 300

to 375 m (1,000 to 1,250 ft), would produce an event rate of

0.69 (5/7.2) per 300 m (1,000 ft).) Clearly, the seismic events

at this site are biased towards the deeper cover. In particular,

all of the events occurred when the overburden was greater

than 500 m (1,625 ft) although over one-third of the extraction

area was under less than this amount of cover.

Table 1. Summary of Bump/Seismic Events.

Date

Intensity

(UK,

VPI)1

Overburden, 

m Panel

Distance to

max.

overburden, m

Side

distance to

solid from

tailgate, m

Distance

to setup

room, m

Distance to

recovery

room, m

ALPS

stability

factor

Distance

to first

solid, m

Energy

(UK, VPI)1

7/31/1994 3 738 L-7 0 1402 1144 822 0.58 822 925

8/1/1994 3 738 L-7 -34 1402 1163 803 0.58 803 925

8/3/1994 3.5 738 L-7 -46 1402 1193 773 0.58 773 2465

10/5/1994 3.6 731 L-7 -823 1402 1954 12 0.58 12 2998

12/23/199

4

N/A, 3.5 592 L-8 186 1676 869 1097 0.74 869 n/a, 2465

1/12/1995 3.5 661 L-8 46 1676 1013 953 0.65 953 2465

1/12/1995 3 661 L-8 46 1676 1013 953 0.65 953 925

1/12/1995 3 661 L-8 46 1676 1013 953 0.65 953 925

1/19/1995 3.7 679 L-8 -24 1676 1085 881 0.63 881 3648

1/30/1995 3.7 687 L-8 -107 1676 1149 817 0.62 817 3648

2/4/1995 2.8, 3.3 668 L-8 -140 1676 1205 761 0.65 761 625, 1665

2/4/1995 2.8, 3.3 668 L-8 -140 1676 1205 761 0.65 761 625, 1665

3/11/1995 4 592 L-8 -472 1676 1527 439 0.74 439 6568

3/11/1995 3.4 592 L-8 -472 1676 1527 439 0.74 439 2026

7/22/1995 2.9, 2.9 592 L-9 216 1951 968 998 0.74 968 760, 760

8/5/1995 2.8 661 L-9 91 1951 1090 876 0.65 876 625

10/25/199

5

3.4, 4.1 574 L-10 433 2225 1003 963 0.77 963 2026, 7990

4/19/1996 3.7 566 L-11 277 2499 1266 700 0.78 700 3648

5/4/1996 3.2, 3.7 609 L-11 158 2499 1390 576 0.72 576 1369, 3648

5/13/1996 2.7, 3.5 653 L-11 232 2499 1497 469 0.66 469 513, 2465

5/13/1996 2.5, 3.5 653 L-11 232 2499 1497 469 0.66 469 347, 2465

5/16/1996 2 668 L-11 107 2499 1528 438 0.65 438 130

1 Seismic data were collected and compared from both the University of Kentucky (UK) and 

Virginia Polytechnic Institute (VPI) systems, single values representing UK data are presented first, when both stations recorded

events, UK then VPI are provided. Magnitude type (ML-MC) is unknown.
0-150

150-225225-300

300-375375-450

450-525

525-600

600-675

O ve rburde n (m )

Event Rate per 300 m (1,000 ft)

Figure 2. Event rate per 300 m (1,000 ft)

vs overburden at Mine A

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

2

0-300

300-600

600-900

900-1200

1200-1500

1500-1800

1800-2100

2100-2400

2400-2700

D istance to S olid (m)

Event Rate per 300 m (1,000 ft)

Figure 4. Event rate per 300 m (1,000 ft) vs distance

to solid in meters at Mine A

0

5

10

15

20

25

30

35

0-150

150-300

300-450

450-600

600-750

750-900

900-1050

Distance to Nearest Panel End (m)

Percent of Events

Figure 3. Distribution of events occurring at Mine A

relative to the nearest panel end of the longwall panels.

Events

0

10

20

30

40

50

60

10.0-3.81

3.81-2.15

2.15-1.44

1.44-1.08

1.08-0.86

0.86-0.72

0.72-0.62

0.62-0.55

ALPS Stability Factor of Gateroad Pillars

Percent

Figure 5. Percentage distribution of events by ALPS stability

factor of gateroad pillars

Distance to Nearest Panel End

Another observation that one may make from reviewing

figure 1 is that the face had progressed through a significant

portion of the panel when the seismic events occurred.

Similarly, few of the seismic events occurred when the face

was very close to the end of a panel. Figure 3 shows the

seismic events plotted against face distance to the nearest end

of the panel. With this figure and figure 1, it can be seen that

no events occurred during the first 900 m (3,000 ft) of face

advance in any of the panels in the district. And even taking

into account a number of events that occurred closer to the end

of the panels, over 95 percent of the bumps occurred greater

than 300 m (1,000 ft) from either end of the panels.

Side Distance to Solid

In other bump-prone mines, it has been suggested that the

width of the district gob (over multiple panels) has a distinct

influence on the occurrence of bumps. In this paper, we refer

to the total width of the district gob area, as measured from the

tailgate across the adjacent gob from previous panels to the

starting edge of the longwall district, as the "side distance to

solid." At this site, no seismic events were observed until the

distance to the side boundary of the district exceeded 1,200 m

(4,000 ft) (figure 4). In fact, panels L-5 and L-6 had more

overburden than panels L-7 through L-11 where the recorded

bumps occurred, but the side distance to solid was only 600

and 900 m (2,000 and 3,000 ft), respectively.

Relationship between Seismic Events and ALPS

Pillar Stability

In the past, the occurrence of bumps was shown to be

related to low gateroad stability factors as calculated by the

Analysis of Longwall Pillar Stability (ALPS) program (3, 5).

(The ALPS stability factor is essentially a measure of the

strength-to-load ratio of the gateroad pillar system.) In this

analysis, the ALPS stability factor was correlated with the

seismic activity. In order to accomplish this correlation, the

overburden above each abutment pillar was measured and the

ALPS stability factor was calculated. Next, factors for the

individual pillars were then grouped into 75 m (250 ft)

increments of overburden. Finally, the observed seismic

events were associated with the ALPS stability factor of the

tailgate pillar configuration for the face location at the time of

the event and the results were charted as shown in figure 5.

This histogram and table 1 show that the first seismic event

was not encountered until the gateroad ALPS stability factor

was 0.78, and that all other events occurred when the ALPS

stability factor was less than 0.78. (Since the pillar design

was essentially the same within the analyzed longwall district,
0

50

100

150

200

250

300

350

400

0-150

150-225

225-300

300-375

375-450

450-525

525-600

600-675

Overburden (m)

Energy (x1014 ergs)

0

10

20

30

40

50

60

70

80

Percent

Energy per Event

Percent of Energy

Percent of Events

Figure 6. Distribution of energy per event, percent of energy, and

percent of events at Mine A

the differences in the ALPS stability factors correlate directly

to depth; therefore, the distribution in figure 5 is essentially

identical to the distribution of events with respect to

overburden as shown in figure 1.)

Seismic Energy of the Events

Up until this point in the paper, only the occurrence of an

event has been analyzed and the relative size of the events has

been ignored. Of course, correlating the size, or energy, of the

events to the mining parameters may reveal some useful

information. The regional seismic events are typically reported

in Richter magnitude which is a logarithmic scale with relation

to the size/energy of the event. In order to get a better

perspective on the relative magnitude of these events, the

logarithmic Richter magnitudes were converted to pure energy

values using the following formula:

log ES = 1.96 ML + 9.05 (1)

where: log ES = energy released (ergs) and

ML = Richter magnitude 

This equation (6) has a number of assumptions built into

the derivation because of the empirical nature of the Richter

calculation. Therefore, the absolute energy values calculated

by the equation are somewhat debatable; however, for our

purposes, the calculated energy values can be considered

consistent in a relative sense. For the analysis, the average

energy per event and the percentage of energy released for each

overburden interval was calculated. The result is presented in

figure 6. In terms of energy released, it is notable that the

average energy per event was highest in the events that

occurred in the range of 450 to 525 m (1,500 to1,750 ft) and

that average energy per event decreased at greater depths.

Discussion

From this first look a small number of seismic events at

Mine A, a number of interesting observations were presented:

(1) all the bumps occurred under more than 450 m (1,500 ft) of

overburden; (2) all the bumps occurred where the panel had

advanced at least 900 m (3,000 ft) and the mined area was a

lateral distance of greater than 1200 m (4,000 ft) to solid coal;

and (3) the energy released per event did not continue to

increase with increasing overburden. These observations are

all informative and suggest some geometric limitations

associated with the seismic events.

Some caution needs to be used when looking at these

results and interpreting the individual effect of the mining

parameters. Looking at figure 1, it can be seen that the

deepest overburden generally occurs near the center of the

longwall district. This geometry forces a strong correlation

between the depth of cover and the middle of the panels.

Thus some of the above results concerning the mining

distance from the ends of the panels may just be a crosscorrelation with depth.

CASE STUDY B AND C

After finding some useful correlations with the limited

number of seismic events available at Mine A, it was decided

to continue analyzing seismic events in relation to mining

parameters at a couple other mines with considerably more

recorded seismic activity. These two mines have a long

history of longwall mining and have each extracted numerous

panels causing numerous seismic events to be registered by

the regional seismic system. For our analysis, all of the

regional events located within approximately a 5 km (3 mi)

radius of the mines was considered to be mine-induced.

(Realizing that the average horizontal location error for the

regional seismic events was approximately 1.5 km (1 mi), a

large area around the mines was included in order to collect

all of the mining related seismic events from the regional

database.) Further, to insure good quality of the event

detection, only the events with a Richter magnitude of 2.0 or

greater were considered. The event selection was narrowed

even more by including only those events occurring within a

six year period of active seismicity, this procedure resulted in

623 events associated with the two mines.

In order to perform the analysis with these mines, each

monthly longwall extraction area was considered to be a

separate mining sample. The center of this sample area was

determined and the appropriate parameters: advance footage,

overburden depth, distance from panel start, distance from

panel end and side distance to solid were assigned to the area.

Then, each seismic event within the month of the extraction

was assigned to that sample area. This resulted in 436 events

associated with Mine B and 187 events associated with Mine

C.

Event Rate Versus Overburden

The first parameter to be examined for the properties was

the event rate versus the overburden and this is plotted in

figure 7. Similar to Mine A, there is very little activity until
150-225

225-300

300-375

375-450

450-525525-600

600+

Overburden (m)

Energy (x10 13 ergs )

Mine B

Mine C

Figure 8. Average energy per event (x1013 ergs)

distributed by overburden

0

2

4

6

8

10

12

150-225

225-300

300-375

375-450

450-525

525-600

600+

Overburden (m)

Event Rate per 300 m (1,000 ft)

Mine B

Mine C

Figure 7. Event rate per 300 m (1,000 ft) versus

overburden at Mines B and C

the overburden reaches a lower limit of 375 m (1,250 ft). 

Also as seen at Mine A, the event rate is seen to decrease after

the maximum rate around 450 m (1,500 ft) of overburden.

Intuitively, the seismic event rate might be anticipated to

increase with increasing overburden. The data from all three

mines seems to conflict with this hypothesis. All of these

mines reach a maximum event rate between 450 and 525 m

(1,500 and 1,750 ft) of overburden and then the rate distinctly

declines at greater depths.

Energy Released Versus Overburden

The average energy per event by overburden range for

Mines B and C was calculated using the same equation

presented for Mine A and is plotted in figure 8. Here again,

the sample does not indicate that the seismic energy continues

to increase with depth as one might initially anticipate. In

mine C, the highest measured average energy is encountered

Figure 9. Mine outline showing locations of surface and undergreound geophones
when the overburden is between 300 and 450 m (1,000 and

1,500 ft), and in Mine B, the highest measured average energy

occurs when overburden is in the range of 450 to 525 m (1,500

to1,750 ft). In both mines, the energy per event generally

drops at greater depths (the only exception being Mine C at

greater than 600 m (2,000 ft) of overburden). When this drop

in the average energy per event is coupled with the drop in

event rate with depth (figure 9), it can be determined that over

75 percent of the measured seismic energy occurs in the

overburden range of 375 to 525 m (1,250 to 1,750 ft). 

Discussion

This brief analysis of mine-induced seismicity has

produced some interesting results. First, the seismic data

clearly show that very few detectable events occur with less

than 300 m (1,000 ft) of overburden, and it is not until greater

than 375 to 450 m (1,250 to 1,500 ft) that the majority of

seismic activity occurs. This behavior almost exactly matches

the expected occurrence and depth ranges for coal bumps, and

supports the possibility that reducing the overall mine

seismicity will similarly reduce the bump frequency.

Second, the seismic data indicate that both the event rate

and the event energy do not continue to increase with depth,

but actually decrease slightly above approximately 450 m

(1,500 ft) of cover. This result initially seems to be counter

intuitive, but upon further thought, it is not totally

unreasonable to hypothesis that the fracture processes around

a longwall panel may reach a steady state, or even decline,

above a certain depth. Certainly, the upper strata will have a

tendency to bend versus fracturing as it becomes more remote

from the extraction area. Also, the actual fracturing around

the panel may become more plastic and less brittle, thereby

releasing less seismic energy, with higher confinement stresses

at greater depths. This aspect of the data poses some

interesting questions and certainly needs to be investigated

further.

MICROSEISMIC FIELD SITE

The NIOSH project which is presently investigating bump

hazards is titled; "Coal Bump Reduction through Advanced

Mine Design". The basic research approach of this project is

to instrument a deep, bump-prone longwall mine and

determine the main roof, gob, and floor behavior using a threedimensional, microseismic system. This microseismic system

"listens" to the rock and determines the timing and location of

the failure of the rock strata surrounding the longwall. By

analyzing the observed rock failure, researchers can better

understand the caving of the massive main roof, the

compaction and load acquisition of the gob, the failure of the

floor, and the stress redistribution in the coalbed and

surrounding strata. With this knowledge, mines can be better

designed to reduce dangerous bump occurrences.

Throughout the first two years of this project, a

microseismic system was installed over a longwall district at

a deep western coal mine. The overburden at this mine

reaches 900 m (3,000 ft) and the lithology contains several

notably competent units. In particular, the 180 m (600 ft)

thick Castlegate sandstone, known for forming vertical cliffs

in the surrounding escarpments, is approximately 165 m (550

ft) above the coal seam. The coal bed at the mine ranges from

2.4 to 6.0 m (8 to 20 ft) in thickness with an extraction

thickness of 2.4 to 3.0 m (8 to 10 ft), and the geology

immediately surrounding the seam consists of thinner (< 3 m

(10 ft)) layers of siltstones, mudstones, shales, sandstones and

coal.

The microseismic system at the mine consists of 23

geophones surrounding the coal mine both underground and

on the surface. At present, the underground seismic array

consists of 14 geophones in the mains and bleeders around the

longwall panels (figure 9). On the surface over the mine,

another nine geophones are distributed above the panels. At

the mine office, a data analysis workstation receives the data

from both the underground and surface geophones. These data

are automatically analyzed in order to calculate the event

locations, and then the locations are displayed in relation to a

mine map on the computer screen for use by mine personnel.

At the time this paper was written, some 7,500 seismic

events had been recorded from the first three quarters of panel

2. As an initial step in analyzing this information, the

locations of the events were normalized to the advancing

longwall face. The results of this normalization are shown in

plan view in figure 10 and in a cross section parallel to the

advance direction in figure 11. This is only preliminary data

and the reader should understand that the exact location of

these events may change in the future as the location algorithm

is optimized based on field calibration data; however, the

relative location of the events will remain fairly consistent and

can be used at this time for some initial inferences.

From figures 10 and 11, it can be seen that the seismic

activity is generally well in front of the face. This matches

seismic data from other sites (7) and is interpreted to be the

shear failure of the strata due to the forward stress abutment

zone. From the plan view (figure 10), the seismic activity can

be seen to curve back over the gateroads with a little more

seismic activity located over the tailgate pillars than the

headgate pillars. This result is consistent with strata failure

above and below the yielding gateroad pillars, with the

increased activity around the tailgate pillars due to the side

abutment stress. From the side view (figure 11), it can be seen

that the seismic activity is occurring both above and below the

seam level. This response matches other field sites and is

consistent with a front abutment that is vertically symmetric

about the coal seam. Also, in figure 11, a slight angle (from

vertical) of the seismic events back into the gob above the

panel can be seen. This type of angle would be expected with

the arching of the principal stress over the caved area in the

gob.
1

Figure 10. Plan view of event locations, normalized to face location

Figure 11. Cross sectional view of event locations, normalized to face locations
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