Monitoring Coal Mine Seismicity with an 

Automated Wireless Digital Strong-Motion Network 

Peter Swanson, Research Geophysicist
 

NIOSH - Spokane Research Laboratory
 

Spokane, WA
 

Collin Stewart, Technical Services 

Bowie Resources LLC
 

Paonia, CO
 

Wendell Koontz, Senior Mine Geologist 

Mountain Coal Company
 

Somerset, CO 

ABSTRACT 

A seismic monitoring network has been installed in western 

Colorado (USA) in the vicinity of three underground coal mines to 

(i) distinguish and characterize seismic activity as either mining 

related or naturally occurring, (ii) implement a real-time event 

monitoring and notification tool, and (iii) collect data for use in 

research studies aimed at quantifying impacts from mining-related 

and natural seismicity. These potential impacts include dynamic 

rock mass failures such as coal bumps as well as strong shaking in 

the vicinity of critical structures such as impoundment dams, 

reservoirs, mine seals, mine openings, and steep slopes. Examples 

of two damaging seismic events are presented and the mining and 

geologic factors attending these dynamic failures are described. 

BACKGROUND 

Seismicity is a common feature of underground mining in 

geologic environments containing brittle rocks. Seismic events are 

transient releases of elastic energy that result in seismic waves 

being recorded at a distance. Processes that produce seismic events 

include fracturing of strata during caving, crushing of pillars, slip 

on bedding plane interfaces, joints and faults, shear-rupture, rock 

falls and blasting. Therefore, seismicity is expected at all mining 

operations, at some scale, since the operation involves breakage and 

removal of material. Experience in western U.S. coal mines shows 

that the largest dynamic failures can produce seismic events with 

magnitudes in the range of 2-3 with infrequent occurrences up to 4+ 

(1). Frequently, events with magnitudes up to 2 to 3 occur without 

any noticeable impact to mining operations or without even an 

awareness that such an event has occurred. Such events may occur 

in old workings or in the inaccessible gob areas of current workings 

where large-scale ground movements attend caving and subsidence. 

North Fork Valley area (CO) coal companies Bowie Resources 

LLC and Mountain Coal Company have joined together with 

NIOSH to develop a digital seismic monitoring network to collect 

background data on mining-related seismic activity and to 

implement a real-time hazard monitoring tool. In addition to the 

mine-safety technology transfer activity, NIOSH is participating in 

this project to gather data for use in its mine safety research 

program. Data collected with this network is being used to (i) 

identify and characterize dynamic failure mechanisms of potentially 

hazardous rock mass failures (e.g. coal bumps), (ii) assess the 

usefulness of seismic monitoring as an indicator of mine design 

performance, and (iii) determine empirical relations between 

levels of ground shaking, seismic event magnitude, and distance 

to the seismic sources. The latter can be used to help evaluate the 

potential responses of surface and underground structures to 

shaking and identify minimum distances between the seismic 

activity and these structures to keep levels of shaking below key 

damage thresholds. 

This paper describes the seismic network, its features for user 

access to data, and then gives two examples of seismic events 

resulting in some damage to mine workings. 

Setting 

The North Fork Valley (NFV) longwall coal mines are located 

in the rugged canyon-mesa terrain of western Colorado (Figure 1). 

Vertical relief over the span of a single mine is as much as ~ 1 km 

(3300 ft). The major geologic units present are representative of 

the Upper Cretaceous Mesa Verde Formation and include six coal 

seams with thickness from 2 to 6 m (6 ft to 20 ft). Bowie and 

West Elk longwalls are currently operating in the B seam where 

overburden extends up to ~ 0.7 km (2400 ft). Competent 

sandstone units are present with variable thickness and distance 

above and below the B seam. Yearly production from all NFV 

coal mines exceeds 17 million tons per year. 

NORTH FORK VALLEY SEISMIC NETWORK 

Seismic stations 

Two groups of five triaxial strong-motion seismometers were 

centered around the current and/or future workings of the Bowie 

and West Elk mines (Figure 2) providing coverage of an area of 

approximately 250 square kilometers (100 sq. miles). Strongmotion seismographs, or accelerographs, are specifically designed 

to measure ground motions near seismic sources that are strong 

enough to potentially impact the integrity of engineered 

structures. With the nominal 24-bit resolution of the internal data 

recorders (Figure 3; Kinemetrics Altus K2 with internal 

EpiSensor), background vibrations in the micro-g (10-6) range can 

be resolved. A fourth channel records an additional co-located 1-

Hz moving-coil (L4-C) seismometer, providing higher sensitivity 

to smaller and/or more distant seismic events. The data recorders 
 

 

 

 

 

 

 

 

 

are configured to (i) continuously stream data at 100 samples per 

second, and (ii) locally trigger and save event data to compact 

flash cards providing backup in case of telemetry failure. Time 

synchronization is achieved at each digitizing and recording unit 

using GPS receivers. 

Station sites were selected to provide suitable azimuthal 

coverage of the mining areas, a workable telemetry path, 

concealment from view, and, wherever possible, access to 

bedrock. Separate instrument and power vaults were installed by 

digging shallow holes and installing vertically-oriented 

corrugated steel culverts with locking lids. In the instrument 

vault, a flat concrete floor was poured onto exposed bedrock, onto 

which the strong-motion seismometer is bolted. Remote power 

sources were constructed with photovoltaic panels and deep-cycle 

batteries. Installation of the initial five stations at Bowie was 

completed in October 2005 with addition of West Elk stations in 

Figure 1. Location of North Fork Valley mining region. 2006. Automation of event processing began in May 2007. 

Figure 2. NFV seismic monitoring network. Stations north 

and south of the North Fork River are centered on Bowie 

and West Elk workings, respectively. 

Figure 3. Self-contained strong-motion recording unit. 

1 Mention of specific products or manufacturers does not imply 

endorsement by the National Institute of Occupational Safety and 

Health. 

Data communications 

License-free 900-MHz spread-spectrum radios equipped with 

10dB directional Yagi antennas are used to communicate with, 

and transfer data from, the strong-motion seismometers. The ten 

remote instrument sites are connected in four separate radio 

subnets operating in a bandwidth-sharing time-division multipleaccess (TDMA) mode. Radio interference issues have been 

experienced periodically with other nearby 900-MHz data-comm 

networks that are used for ventilation and methane drainage 

drillhole monitoring. The interference has been mitigated by 

appropriate antenna selection, positioning and RF power level 

adjustments. 

Continuous digital seismic waveform data is sent from each 

station to a central in-town site equipped with a DSL Internet 

connection (Figure 4). A dedicated server demultiplexes the 

serial data from each radio subnet and converts it to TCP/IP data 

streams. The serial/IP data network allows complete control over 

the instruments from distant Internet-connected user locations. 

Figure 4. Serial-IP communications network. 

The initial data acquisition arrangement transmitted data 

directly across the Internet for processing at remote locations. 

Reliability of this method was discovered to be, in part, a function 

of the ISP choice. Subsequently, as a guard against Internetrelated impacts to the time-critical data processing streams, the 

seismic waveform data collection and processing were moved to 

the radio reception site and then mirrored, with suitable buffering 

times, to remote locations. 
 

 

 

 

 

 

 

 

 

 

 

 

 

Data Processing 

A large portion of the software for data collection, processing, 

analysis and display is based on the U.S. Geological Survey’s 

(USGS) Earthworm system (2). The freely available, opensource, software is configured to automatically detect, process, 

archive and analyze seismic events from numerous types of 

seismic sensors and make the raw data and processed results 

available over a computer network and the Internet. 

Following software-based picking of arrival times at each 

station, event locations are calculated using Hypoinverse (3) with 

the layered velocity model in Figure 5. This model was initially 

developed for the West Elk property using a combination of sonic 

log and seismic reflection data (4) and has been slightly modified 

as a starting model for the general NFV network. Magnitude 

estimates are obtained from three separate sources: (i) the 

USGS’s National Earthquake Information Center (NEIC) in 

Golden, CO (limited to the largest events), (ii) Mesa State 

Seismic Network (Grand Junction, CO), and (iii) those calculated 

within the automated processing. 

Figure 5. Simplified layered velocity model. 

DATA PRODUCTS AND DATA DISTRIBUTION 

Raw and processed data are made available to users in several 

different ways. For those without direct connection to the 

networked data acquisition and processing computers, access is 

made available through a password protected Internet web page 

and time-critical information is also distributed via email and 

pager notifications. The web page provides access to helicorder 

plots, triggered-event waveform files, event location and 

magnitude calculations from the automated processing, and 

summary peak ground motion data for each station. A display 

client also provides a near real-time image of event locations, 

occurrence times and magnitude estimates superimposed on 

geographical data. 

Helicorder Plots 

Helicorder, or webicorder, plots (Figure 6) provide a 

convenient analog display of the digital waveform data similar to 

the old pen and ink seismograph drum recorders. They allow a 

quick assessment of overall levels of seismic activity occurring 

over the previous 24 hours. Raw and/or filtered seismic records 

are plotted drum-style in continuously updated GIF image files 

that are accessible via hyperlinks on the web interface. Figure 6 

shows a magnitude 2.5 seismic event, that occurred in the NFV 

area at a distance of 8 km (5 miles) from this station, and other 

smaller mining-related events. A magnitude 4.6 aftershock to the 

April 18, 2008 M 5.2 earthquake near West Salem, IL, 

approximately 1700 km (1056 miles) away from this station, is 

also evident. 

Figure 6. Low-pass filtered helicorder-style plot for 

vertical seismometer at station LLM. 

Triggered Event Files 

Waveform data are saved in separate files when a usercontrolled triggering condition is satisfied and can be viewed with 

a number of different software utilities. Several different file 

formats (e.g. SUDS, SAC, mini-SEED) are supported. Example 

waveforms, contained in a triggered data file from a coal bump, 

are shown in Figure 7. Signals were recorded at distances ranging 

from 0.5 to 16.9 km (0.3 to 10.6 mi). 

Event Location Display 

A local web server is set up to provide continuous reporting of 

seismic activity in the vicinity of the NFV network. Client 

software provides near real-time map displays of event locations, 

magnitudes and times of occurrence based on the automated 

processing results. Event locations are superimposed on GIS data 

tied to geo-referenced (lat/long) coordinates. Both server and 

client software are based on the CISN model designed for 

California earthquake monitoring and emergency management 

24/7 operations centers (http://www.cisn.org/). Custom 

modifications to this Java-based software were made to provide 

additional control of the quality of the displayed events. 

Such quality control is desirable when results from wholly 

automated processing are accessed by non-seismologist users. 

Some event locations and magnitude assignments can be 

erroneously calculated because no software can entirely replace 

the judgment of a human analyst. Such erroneous results occur, 

for example, with low signal to noise ratios, or when multiple 

seismic events occur close together in time – a frequent 

occurrence when several nearby mining operations are active. In 

addition to selecting events for display on the basis of magnitude 

and age of the event, new selection capabilities include horizontal 

and vertical location error, number of phases used in the location 

calculation, and RMS residual error. These additional display 

control parameters increase the robustness of the event reporting 
 

 

 

 

and reduce the display of poorly constrained events when 

operating under wholly automated processing conditions. 

Figure 7. Triggered waveform file for a coal bump (M 2.9) 

observed on vertical component velocity seismograms. 

Email and Pager Notifications 

Email and paging provides timely notice of events of special 

interest. Emails are used to notify users when specified levels of 

acceleration are met or exceeded at a given station. This notice 

can be used to prompt inspection of critical structures or initiate 

other actions. In practice, it has been found useful, as a simple 

awareness measure, to set the threshold lower than any critical, or 

actionable, level in order to generate a stream of email that is 

proportional to the rate of large-event seismic activity. This also 

provides periodic confirmation that the notification system is 

working as intended. 

When one is not readily reachable by email, a second system 

email module is used to send messages to a nationwide paging 

service. In addition to acceleration thresholds, additional criteria 

for this type of notification are satisfied when an event with a 

minimum magnitude occurs within a certain distance of a given 

station. Individual users can customize their own notification 

criteria by specifying values for acceleration threshold, minimum 

magnitude, distance from stations and station names. 

RESULTS AND DISCUSSION 

Local Network Results vs USGS/NEIC Locations 

Event locations of a few of the largest events calculated by the 

USGS/NEIC are compared with local NFV network results. 

Figure 8 shows locations of six events, occurring between May 

and Nov. 2007, with magnitudes ranging between 2.5 and 3.4. 

NEIC locations were calculated using data from available 

regional network stations and a generic global earth velocity 

model (5). Event locations based on the local network data 

coincide closely with areas of mining activity underway at those 

times. The average difference between the two calculated 

locations is 12.2 km (3.7 mi). 

Figure 8. Comparison of event locations reported by 

NEIC/USGS (filled squares) with those calculated using 

local NFV network data (circles). 

Figure 8 also shows locations of eight naturally occurring 

earthquakes near Mt. Lamborn, one of several post-Laramide 

volcanic remnants in the area. With magnitudes ranging between 

1.5 and 2.4, these events were too small to be reported by the 

NEIC. They occurred between Oct 19, 2007 and Jan 15, 2008 and 

continue to occur periodically in this area at an estimated depth of 

~ 4 km (2.5 mi). 

Mining-Induced Events vs Natural Earthquakes 

The events near Mt. Lamborn (Figure 8) provide an 

opportunity to contrast seismic signatures of mining-related 

events with naturally occurring earthquakes. By comparing 

signals at identical seismic stations, the influences of any 

differing instrument characteristics and local siting conditions are 

eliminated. Examples of three-component triaxial acceleration 

waveform data (vertical, north, east) recorded at station WTM are 

shown in Figure 9 for a mining-related event and a natural 

earthquake. The upper three traces were produced by a 

magnitude 2.2 non-damaging event in the vicinity of the longwall 

at West Elk (Jan. 26, 2008,15:36 UTC) and the lower three traces, 

recorded at the same station, were produced by an event near Mt. 

Lamborn (Jan 13, 13:05 UTC) with magnitude 2.4. The distances 

between the sources and station WTM for these two events are 

approximately 14 and 19 km, respectively. Similar results are 
 

 

 

 

 

 

 

observed when comparing the other Mt. Lamborn earthquakes to 

other mining-induced events recorded at these same distances. 

Several notable differences in waveform character are obvious 

from Figure 9. The Mt. Lamborn event exhibits a classic 

earthquake signature with abrupt P-wave arrival on the vertical 

component Z, followed by a sharp, later arriving, S-wave pulse on 

the horizontal components N and E. It clearly illustrates why 

arrival times of P waves are typically picked on the vertical 

component and S waves are picked on the horizontals. The 

waveform signature of the mining-related event appears quite 

different: (i) the demarcation of P and S phases is not nearly as 

well defined, (ii) there appear to be several additional phases 

present, or complexity that is absent in the earthquake 

seismogram, (iii) the duration of shaking of the mining event 

appears to be longer despite the slightly smaller magnitude (2.2. 

vs 2.4) and (iv) the frequency content across the entire waveform 

is lower for the mining event than for the natural earthquake. 

Comparison of Figure 9 with Figure 7 suggests that much of 

the difference in waveform character between these two events 

may be attributable to propagation path effects. 

Figure 9. Comparison of triaxial acceleration data observed
 

for mining-related seismic event (top 3 traces) and naturally 

occurring earthquake (bottom 3 traces) recorded
 

at station WTM.
 

Wave Propagation Path Effects 

The strong influence of propagation distance on waveform 

character is evident in the increasing elongation of the signals in 

Figure 7 as recording distance increases from station MFB (0.5 

km) to SEF (16.9 km). This effect is also seen with the M 4.6 

Illinois earthquake in the helicorder record (Figure 6). Typical 

rupture times for an earthquake of this size are measured in 

seconds, yet it produced a 10+ minute long signal at 1700 km 

distance. As the distance between source and receiver grows, 

there is an increasing number of different paths of variable 

lengths that waves can travel to reach the receiver; the different 

travel times along the different paths is one reason why the signal 

duration increases with distance traveled. Another factor is the 

existence of several different types of waves that travel at 

different speeds (P, S, surface waves, trapped modes, etc) so they 

arrive increasingly separated in time as distance increases. Even 

a station immediately adjacent to a seismic source can expect to 

see seismic signals last considerably longer than the duration of 

the dynamic process that produced the elastic waves as waves 

reflect off of different interfaces and reverberate within and 

between different layers. This is an important point to consider 

when using information from seismic waves as part of a forensic 

accident analysis. The length of the observed seismogram should 

not be directly interpreted as the duration of the dynamic process 

that gave rise to the seismic waves; the duration is a strong 

function of wave propagation path effects. 

The calculated depths of the Mt. Lamborn events are 

approximately 4 km (2.5 mi) whereas the depths of most miningrelated events are thought to be restricted to the upper ~1 km (0.6 

mi) or so. Consequently, waves from these natural events (Figure 

9) spent only a small fraction of their propagation time in the 

complex, rapidly varying, attenuating, low-velocity structure of 

the upper 1 km. Waves from the mining-induced events spend a 

much greater proportion, if not 100 %, of their propagation times 

within this near-surface region and this may account for a lot of 

the difference in visual appearance. However, as described later, 

there are also important notable differences in the manner in 

which seismic waves are generated by these two types of sources. 

Distribution of Seismic Activity 

More than 10,000 events with magnitudes ranging between 0 

and 3.4 were detected, recorded and automatically processed 

during the period May 2007 – March 2008 (Figure 10). In 

addition to the Mt. Lamborn events, the few (~6) other naturally 

occurring earthquakes which were processed were located well 

outside the NFV array. Figure 10 shows all automatically located 

NFV events without regard to the size of location error. Some of 

the scattered events lying outside the main mining areas have 

large location errors and some may be well-located events that are 

associated with old mine workings. A few of these scattered 

events are known to be unsuccessful attempts by the system to 

locate sources of thunder. The automatic processing, without 

human intervention, is largely successful in placing events into 

the appropriate mine and allows resolution according to specific 

working area (i.e. longwall vs development sections, etc). The 

clustering of events to the east of the Bowie stations on the north 

side of the valley are from a third NFV area longwall coal mine. 

As an example on the scale of an individual longwall panel, 

event locations located during one month of B-seam longwall 

retreat mining at the Bowie mine are shown in Figure 11. The 

variation in seismic activity with mining of this panel was quite 

significant. Activity strongly increased as the depth of cover rose 

above 460 m (1500 ft) and stress interactions occurred with the 

previously mined D seam 90 m (300 ft) above. 

Damaging Seismic Events 

Experience in the NFV, as well as other mining regions, shows 

that seismic activity is affected by the presence of thick strong 

roof and floor strata, high overburden, and geologic anomalies. 

These factors are commonly encountered in western underground 
 

 

 

 

coal mines and, in the right combination, can create conditions 

conducive to coal bumps (6; 7). 

Two damaging seismic events, or coal bumps, are described 

that occurred under high overburden with comparatively strong 

brittle strata in the roof and floor. Faults were present in both 

cases. The faults, as with other geologic anomalies, make it 

difficult to anticipate the structural response through numerical 

modeling, or even empirical experience, because the spatial extent 

and physical characteristics of the anomalies are generally, or 

even completely, unknown. 

Figure 10. Calculated seismic event locations (May 2007 

-Mar 2008) as determined by automated processing. 

Figure 11. Automatically processed event locations (~2600) 

during one month of longwall retreat under 460 m (1500 ft) of 

overburden and multiseam mining conditions. 

Independent of their possible role in initiating dynamic 

failures, faults can generally affect both stress and structural 

stability. Stress is affected because suitably oriented faults can 

provide a low-compliance deformation, or slip, boundary. 

Movement, or deformation, along zones of faulting means that 

stresses, particularly abutment loads which were expected in a 

uniform elastic medium to be shed to the surrounding rock mass 

away from the active mining area, are instead concentrated closer 

to working areas. Secondly, faults, particularly steeply dipping 

faults, reduce load-bearing capacity of pillars, solid blocks of coal 

and other strata subjected to mining-induced stress. Finally, 

faults, either directly or by rendering stiff load transferring strata 

ineffective, also reduce load system stiffness which serves to 

enhance unstable failure modes (8). Any or all of these effects of 

faulting may have played some role in creating conditions leading 

to these dynamic failures, but the relative contribution of each 

role has not been determined. 

In the first example, development of a 5-entry east-west 

submains was in progress on the Bowie B seam. Overburden is 

approximately 530 m (1750 ft) and the base of the 30 m (100 ft) 

thick “C” sandstone starts 27 m (90 ft) above the B seam. The 

submains had just finished crossing under the old D-seam northsouth mains 90 m (300 ft) above when a magnitude 2.9 seismic 

event (Figure 7), the largest in a series of three events, produced 

the distribution of damage shown in Figure 12. Damage consisted 

of floor heave and pillar spallation where the spalls occurred 

preferentially on north-facing ribs. The heaviest damage was 

centered not at the development faces but in an area of steeply 

dipping northwest-striking faults more than 150 m (500 ft) outby. 

At the time, development faces did not exhibit signs of heavy 

loading. None of the faults displayed evidence of fresh 

macroscopic slip movement. 

Figure 12. Distribution of floor heave and rib spall attending 

M 2.9 coal bump during B-seam submains development. 

Prior to mining the B-seam, two- and three-dimensional stress 

analyses were performed using MULSIMTI and FLAC (9) with a 

focus on stress transfer from old D seam workings onto the B 

seam. None of the numerical models indicated any significant 

failure was to be expected in this area, even when reducing the 

strength of the B-seam coal due to the presence of the faults. 

However, the modeling did anticipate crushing of pillars in old Dseam workings under high cover. Analysis of the largest events 

that have occurred during mining of the Bowie B seam over the 

past year (Swanson and Collins, in progress) suggest that many of 

the larger events are occurring 300 ft above on pillars left in the 

mains and gateroads of the mined-out D seam. With the existing 

sparse seismic array, vertical resolution of calculated event 

locations is insufficient to distinguish between events occurring 

on the B or D seams. A dynamic trigger from nearby failure in 

the D seam workings above, or slip on known nearby fault 

structures, is being examined as a possible initiating event that 

may have elevated stresses dynamically to the point of failure. 

However, the role of stress and material property variation due to 

known and unknown complexities in the geologic and mining 

environment can not be completely discounted. 

As a second example, a bump occurred with a magnitude 1.9 

seismic event at West Elk that affected headgate pillars adjacent 

to the longwall gob (Figure 13). Overburden ranged from 730 m 
 

 

(2400 ft) at the east end of the panel end to 425 m (1400 ft) over 

the face. Competent sandstone units of up to 60 m (200 ft) 

thickness are located 12-15 m (40-50 ft) above and below the B 

seam. Several discrete steeply dipping faults trend northeast in a 

zone that intersects the headgate in the middle of the 

damaged pillar zone. These faults are part of a consistently 

oriented fault set that periodically intersects the workings in this 

part of the mine. Although pillar spall was the dominant mode of 

failure, floor heave and roof failure were also present to a lesser 

extent. Two rows of support cans along the length of the middle 

headgate entry provided a space that was largely sheltered from 

the damage (Figure 14) and mine operations were not 

significantly affected by this event. 

Figure 13. Gateroad pillar damage produced during
 

M 1.9 seismic event.
 

Figure 14. Response of can-supported gateroad 

entry to floor heave, and pillar and roof damage 

produced by M 1.9 coal bump. 

Seismic signals from a triaxial accelerometer that was 

temporarily deployed on the surface above this panel at a threedimensional distance of 860 m (2800 ft) from the headgate 

damage zone are shown in Figure 15. Temporary stations are 

deployed to specifically capture strong-motion accelerations from 

large events that occur when mining under deep cover. This data 

is being collected at West Elk and Bowie to develop empirical 

relations between seismic magnitude, distance from the seismic 

source, and level of shaking. Initial results from these studies 

have been used to support stability analyses of at least one critical 

structure, with resulting impact on the regulatory decision-making 

process. 

Figure 15. Ground acceleration produced by M 1.9 coal 

bump at a distance of 860 m (2800 ft). 

Fault Slip and First Motions 

Seismic events can be categorized as shear-like or implosional 

on the basis of the directions of the initial movement of the 

ground at each station (i.e. direction of P-wave first motion). 

Implosional events, which reflect incremental closure of mine 

openings, are characterized by first motions that move toward the 

source at all of the stations in a well-distributed seismic network 

(in the case of vertical sensors above a mine, the directions of first 

motion would all be down). Events with a strong shearing 

component, similar to most natural earthquakes (e.g. Figure 9), 

exhibit a mixture of first motions that move both toward and 

away from the source (or up and down on vertical sensors). 

While some mining-related seismic events in the NFV exhibit 

the mixed polarity (up and down) first-motion patterns that 

accompany a significant component of shear slip motion, the vast 

majority of events, including the above-described bumps, display 

first motions that move toward the seismic source at all stations. 

In other words, the seismic signatures appear implosional and not 

shear like. This does not mean there was no co-seismic 

deformation along the fault(s) for the two damaging events, but 

the absence of the shear-slip first-motion pattern suggests that the 

contribution of fault slip was less than that of closure of the mine 

openings. Other coal mine seismic networks of this scale in the 

western U.S. report similar observations of implosional firstmotion patterns (e.g. 98% of events reported in (1)). The 

implosional elastic-wave radiation pattern is another likely reason 

the waveforms for the mining-induced seismic event of Figure 7 

differ from that of the natural earthquake. 
 

 

 

 

 

 

 

 

A mixture of shear slip and volume decrease may be present in 

these events but it is not well resolved using first motion analyses. 

A description of dynamic ground movements using seismic 

moment tensor analysis (10), which can provide such information, 

is underway but insufficiently developed for the present paper. 

Moment tensor analysis shows that events with pure shear slip are 

usually located at a distance well out into the solid rock mass 

rather than adjacent to a working face (11). For events that 

display a mixture of implosive and shear failure, the moment 

tensor components representing the strike of the shearing plane 

tend to align parallel to the nearby face direction. 

SUMMARY 

Bowie Resources LLC, Mountain Coal Company and NIOSH 

have cooperated on a joint project to develop a digital wireless 

seismic monitoring network to collect background data on mining 

-related seismic activity in western Colorado and to implement a 

hazard monitoring tool. The ten station strong-motion array 

provides real-time monitoring capability using wireless serial and 

IP communications networks. Data collected with this network is 

being used to (i) identify and characterize dynamic failure 

mechanisms of potentially hazardous rock mass failures, (ii) 

assess the usefulness of seismic monitoring as an indicator of 

mine design performance, and (iii) determine empirical relations 

between levels of ground shaking, seismic event magnitude, and 

distance to the seismic sources. 

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Pechmann, J. Ake, and A. McGarr, Coal-mining 

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Trail Mountain area, Emery County, Utah, Bull. Seism. Soc. 

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