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22
I
nt. J. Mining and Mineral Engineerin
g
, Vol. 6, No. 1, 2015
Copyright © 2015 Inderscience Enterprises Ltd.
Focal mechanism solutions for roof collapse in deep
mine
Chun-lai Wang*, Hui Lu, Fu-li Wang,
Wei-qiang Li, Ming Luo, Lu Liu
and Zhi-jiang Lu
Faculty of Resources & Safety Engineering,
China University of Mining & Technology,
Beijing, Xueyuan Road,
Ding 11, Haidian District,
Beijing, P.C. 100083, China
Email: tswcl@126.com
Email: luhui@qq.com
Email: wangfuli@qq.com
Email: liweiqiang@qq.com
Email: luoming@qq.com
Email: liulu@qq.com
Email: luzhijiang@qq.com
*Corresponding author
Ai-xiang Wu and Xiao-hui Liu
School of Civil & Environment Engineering,
University of Science & Technology Beijing,
Xueyuan Road, No. 30, Haidian District,
Beijing, P.C.100083, China
Email: 190823624@qq.com
Email: 1756576238@qq.com
Abstract: Microseismic monitoring is an effective means of forecasting
instabilities in ruptured rock masses during deep mining. Based on the complex
geological conditions of a mine in Southwest China, a digital 24-channel
microseismic monitoring system was established to monitor microseismic
events during deep mining. In the monitoring period, the focal mechanism
solutions of roof collapse accidents were analysed using a double couple
model. The results show that the focal mechanism solutions of the double
couple model can be used to explain roof ruptures and instabilities. Two
seismic source events were located at the F4 active fault, and the other seismic
source event was located at the rock stratums demarcation line between little
eighth orebody and C1b stratum. It was concluded that the active microseismic
events were caused by deep mining. The method can be used to forecast roof
collapse.
Keywords: deep mining; roof collapse; microseismic monitoring; double
couple model; focal mechanism solution.
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or roof collapse in deep mine 23
Reference to this paper should be made as follows: Wang, C-l., Lu, H.,
Wang, F-l., Li, W-q., Luo, M., Liu, L., Lu, Z-j., Wu, A-x. and Liu, X-h. (2015)
‘Focal mechanism solutions for roof collapse in deep mine’, Int. J. Mining and
Mineral Engineering, Vol. 6, No. 1, pp.22–34.
Biographical notes: Chun-lai Wang has received his PhD in Mining
Engineering from University of Science & Technology, Beijing, in 2011.
He has over 13 years work experience in deep mining-induced rock burst
and rational support design, microseismic monitoring, and deformation
measurements around the orebody.
Hui Lu is a postgraduate of Mining Engineering.
Fu-li Wang is a postgraduate of Mining Engineering.
Wei-qiang Li is a undergraduate of Mining Engineering.
Ming Luo is a undergraduate of Mining Engineering.
Lu Liu is a postgraduate of Mining Engineering.
Zhi-jiang Lu is a postgraduate of Mining Engineering.
Ai-xiang Wu is a Professor at the School of Civil & Environment Engineering
at University of Science & Technology, Beijing. He is currently leading several
projects in the subject of rock mechanics such as the National Key Technology
P&D Program supporting project of the ‘Twelfth Five-Year-Plan’ (No.
2012BAB08B02).
Xiao-hui Liu is a doctoral student of Mining Engineering.
1 Introduction
Mining, especially deep mining, can induce microseismic activity. Seismic activities
caused by underground mining are collectively referred to as mining earthquakes
(Gibwica, 1990). In deep well safety mining, the prevention of dynamic disasters during
mining is one of the most serious and difficult issues to resolve. As mining depth has
increased and the scope of mining projects has expanded, this problem has grown (Brink,
1988).
The focal mechanism is a mechanical expression of the seismic source region in the
seismogenic process (Mcgarr, 1984; Durrheim et al., 1998). Dynamic mining disasters
caused by faulting are closely related to the direction of the rupture surface and the role
of stress in the rock mass internal rupture surface. However, it is possible that
microseismic event waveforms, which are induced by dynamic mining disasters, can be
analysed to determine the seismogenic rupture surface orientation. Based on the point
source model of fault rupture, the vibration direction and amplitude parameters of a
preliminary wave particle were obtained via microseismic monitoring. The principal
compressive stress and principal tension stress direction were obtained from the strike,
dip direction, and dip angle of a fault rupture point source model. Then, the fault rupture
direction, rupture speed, and stress drop parameters were obtained. Microseismic
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monitoring provides an effective means of expressing the mechanics of mining-induced
microseismicity. Several studies have been conducted abroad at locations such as a Polish
coal mine, a South African gold mine, a nickel mine in Canada, and an Australian copper
mine. Some studies have shown that mining-induced earthquake activity is closely related
to mining activity by examining the close relationship between mining depth, mining
process, and mining intensity (Gibowicz and Kijko, 1994; Jan and Alexander, 2008;
Joughin and Pethö, 2005; Potvin and Hudyma, 2001; Stanislaw and Beata, 2008; Stewart
et al., 2001; Vallejos and Mckinnon, 2011). In China, studies have been conducted at
some mines, such as the Huize, Fankou, Sanhejian, Dongguashan, Hongtoushan, and
Huafeng coal mines; these studies focussed on the relationship between seismic activity
in the mining process and the engineering space position (Wang et al., 2009; Li et al.,
2005; Ding and Li, 2005; Yang et al., 2007; Zhao et al., 2005; Jiang and Luo, 2002).
A few studies have found a focal mechanism solution for rock mass rupture and
instability caused by microseismic events and mining activity. Cao and Dou (2008)
studied the focal mechanism by using a point source model for the roof fracture
equivalent; based on the double-couple point source model, the main form was tensile
failure near the seismic source region. Li et al. (2005) found that the unloading secondary
stress field induced mine earthquakes from seismic P-waves. He and Dou (2012) and He
et al. (2012) proposed and built a criterion of rock burst induced by roof breakage
(RBRB) model and explained the layer dislocation rock burst and analysed the energy
radiation characteristics. Using focal mechanisms and fracture dislocation theory, this
research greatly expanded the description of the seismic source and the seismic source
characteristics associated with structural and mining conditions. However, these studies
examined the mining process because mining lacks a clear focal mechanism solution. The
ultimate goal of this study is to describe the mechanical behaviour of dynamic disasters
using microseismic event parameters.
At present, several scholars have an accurate understanding of the focal mechanism
solution for microseismic activity. However, this research in this area is still in its initial
stages due to the conditions and exploiting activities of ore body; with regard to the focal
mechanisms of microseismic activity and early warning prevention, further research is
needed.
2 The exploitation status and monitoring conditions
2.1 Mining conditions and method
The Southwest mine is one of the deepest mines in China. Currently, its depth reaches
1500 m, the conditions of ore body, such as water infiltration, its depth, and the difficulty
in accessing it, are extremely complex and without precedent in China. The in situ stress
is high in this mine, which is located in the valley region of the Yunnan-Guizhou Plateau.
An overhand horizontal slice paste filling mining method is used and is divided
into two steps, as shown in Figure 1. One, step stopes are located on the roof drift and
two sides, and another, stope is located on the next roof drift. The level height is 60 m,
the sublevel height is 12 m, and the layer height is 3 m. The stopes are vertically layered
with the strike of the ore body. The widths of the stope and pillar are 5 m and 6 m, and
the length is equal to the orebody thickness. The pillar is first stoped and then filled with
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pasting. The room is stoped after the adjacent pillars are stoped. Gob is used to fill the
pillars, and paste is used to fill the rooms.
2.2 Monitoring system
In August 2007, a digital 24-channel microseismic monitoring system (built by the ISSI
Company in South Africa), which consisted of 12 sensors (6 triaxial geophones and
six uniaxial geophones, they are velocity geophones) and 4 QS seismic data acquisition
instruments, was used to monitor microseismic events in deep mining, as shown in
Figure 2. The MS monitoring system routinely manufactures two kinds of geophones,
with natural frequencies of 4.5 Hz and 14 Hz. The 4.5 Hz geophone has a usable
frequency bandwidth of between 3Hz and 2000Hz but must be installed to within 2
degrees of its preset orientation with respect to the vertical. The 14Hz geophone is omni-
directional and can be installed at any angle, with a usable frequency bandwidth of
between 8Hz (–3 dB point) and 2000 Hz. The monitoring system was designed, installed,
debugged, test run, and processed by Wang et al. (2008, 2009, 2010) and Wu et al.
(2009). The system was applied to the forecasting of rock mass instabilities in deep mines
under complex geological conditions. The system provided the parameters used for
identifying early warnings in key rock mass rupture locations.
Figure 1 The paste filling mining method of overhand horizontal slice (see online version
for colours)
3 Theoretical basis
The double couple is composed of two equally sized parts. The single couple is co-planar
to the double couple but is oriented in the opposite direction with a moment that is
mutually orthogonal to the force direction. In the same medium, a pair of opposite forces
is considered in the adjacent two portions under the transient role condition, the
combination of their forces is zero, and they have the same force magnitude and
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direction, as shown in Figures 3 and 4. The total displacement is the sum of the
displacement produced by each force (Pujol and Herrmann, 1990):
33 2 3 2
[ ( /2) ( /2)/ ,
ii i
uFG e G e
ε
ξ
ε
ξ
εε
=× + − − (1)
where ui is the limit value, F3 tends to infinity, ε tends to zero, and εF3 is a finite value.
The following relationship can be obtained:
3
32
2
,
i
i
G
uM
ξ
∂
=×
∂ (2)
where 32 3.
M
F
ε
= Therefore, the total displacement caused by the double couple is as
follows:
40
0
22
0
22
0
3
0
3
11
(,) ( )d
4
11
()
4
11
()
4
11
()
4
11
().
4
r
N
ra
IP
IS
FP
FS
uxt R M t
r
RMtr
r
RMtr
r
RMtr
r
RMtr
r
βτττ
πρ
α
πρα
β
πρβ
α
πρα
β
πρβ
=−
+−
+−
+−
+−
∫
(3)
Figure 2 Microseismic monitoring system of mine in Southwest China (see online version
for colours)
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Figure 3 P wave polarisations along slipping plane (see online version for colours)
Figure 4 The main axial description of double couple model (see online version for colours)
Here, the radiation patterns of the P- and S-wave are given by the following formulas:
9sin 2 cos 6(cos 2 cos cos sin )
4sin 2 cos 2(cos 2 cos cos sin )
3si n 2 cos 3(cos 2 cos cos sin )
sin 2 cos
cos 2 cos cos sin .
N
IP
IS
FP
FS
RR
RR
RR
RR
R
θϕ θϕθ θϕ
θϕ θϕθ θϕ
θ
ϕ
θ
ϕ
θθ
ϕ
θϕ
θϕθ θϕ
=− −Φ
=− −Φ
=− − − Φ
=
=−Φ
(4)
The amplitude strength of the seismic wave is described as the distance from the origin of
the focal sphere. This distance can be used to make a radiation rose diagram for seismic
waves.
The parameters of the two nodal planes, such as the spatial attitude and the space
azimuth of the P, B, and T axes, need to be provided for the double couple point source
model to determine a focal mechanism solution for the seismic source. The B-axis
represents the intersection line of the two nodal planes, also known as the zero-axis (or
the N-axis), and the particle displacement is zero on this axis. The P-axis and T-axis
represent the plane that is perpendicular to the B-axis, and they are equal to the angle of
the two surfaces, which are located in the expansion wave quadrant P axis and the
compression wave quadrant T axis. As shown in Figure 4, the P-axis is the pressure shaft,
and the T-axis is the tension shaft.
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Double couple point source models, which determine the focal mechanism solutions
and tectonic stress fields associated with mining-induced dynamic disasters are widely
used to study natural earthquakes and mine earthquakes (Aki and Richards, 1980; Butler
et al., 1993). The double couple point source model can objectively reflect the stress
field, seismic stress, and energy release of a seismic source. The model can provide a
more objective interpretation of the focal mechanism of the dynamic disaster by
observing the nearest field. However, in the mining environment, the focal mechanism
solutions and radiation pattern of the seismic sources in the near-field and intermediate
field can be analysed. In most cases, mine dynamic disasters are caused by shearing and
regional fault plane sliding, the planes forced onto one surface. Under deep and high-
stress mining conditions, dynamic disasters result from the unloading of the secondary
stress field of stress sources. The secondary stress field is produced by the joint action of
the tectonic stress field and the gravity stress field. Unloading stresses appear more
importantly, and the performance is more prominent role. It is useful to study the double
couple focal mechanism, which can be used to judge stress field changes in the mining
area, the rock mass sliding direction, and any warnings of mining dynamic disasters.
4 Examples
In light of the complex geological conditions of the No. 8 ore body, brittle fractures are
easily produced in the rock mass near the mining area. Therefore, roof collapses are
induced by mining disturbances. The end of the ore body is located above 1451 m.
Currently, the 1331 m and 1391 m levels are upward ongoing stoped. Three collapses
were recorded between August 2007 and December 2007, as shown in Table 1.
Table 1 Roof collapse records
No. Time Position
Collapse
volume/t Coordinates (x, y, z)
1 9 August, 2007 5 room, 3 panel, 1511 level 200 (610, 126, 1575)
2 12 September, 2007 9 room, 2 panel, 1487 sublevel 100 (650, 160, 1496)
3 20 November, 2007 3 panel, 15 layering, 1499 sublevel 100 (610, 130, 1499)
x coordinate omits front four digits, and y coordinate omits front three digits.
5 Results and discussion
Using the double-couple point source model and the three incidents of roof collapse,
focal mechanism solutions were analysed for rock mass rupture instabilities. Rupture
surface fault plane solutions were obtained from the three roof collapses, as shown in
Figure 5, No. 1 (9 August, 2007). The descriptions of determining nodal planes, P, T, and
N axis were given as follows. Each geophone’s location relative to the hypocenter was
then projected onto the circular diagram with a symbol representing the type of motion,
up (black) or down (white), first recorded there. Geophones that fall within the “missing”
upper hemisphere (above the horizontal) were translated appropriately onto the low-
hemisphere projection. The first step: the first nodal plane was found by rotating the first
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motion stereonet plot until a great circle that separates population of open and closed
circle was found. The second step: this plane was one of the two nodal planes. With the
plot still rotated, count 90 along the equator away from the plane and make a dot. This
dot represents the normal to the plane. The third step: next, rotate the first-motion data
again to find a second plane that separates poplation of open and closed circles. The
second plane must also pass through the normal to the first plane. The fourth step: now,
rotate the first motion stereonet until the N tick was aligned with the N tick on the
Schmidt net. The strike direction of the two planes by counting small circles clockwise
around the edge was read off. In reality, fault plane solutions were representation of slip
on a fault and the pressure (P) and tension (T) axed represent the axes of maximum
shortening and maximum extension. The pressure or P axis was located in the middle of
the dilatational quadrant, whereas the tension or T axis was located in the middle of the
compressional quadrant. The B axis was located at the intersection of the fault and
auxiliary planes.
As shown in Figure 5, No. 1, the projection of the nodal plane was expressed by an
FBCG arc; its strike is 39°, its dip is SE, and its dip angle is 40° (N40E). The pole
of the nodal plane is at the point A, and the angle is 90° from the maximum arc FBCG.
The first and second section faces are orthogonal, and the maximum arc necessarily
passes through the point A. This arrangement can be represented by the WBEE arc,
which has a strike of 257° and dip angle of 57°N. The value of the second node at the
point C, if the count begins at the WBAE 90° arc, is located in the FBCG arc. The
intersection of the two sections is at point B, and the B-axis is the zero axis. The axis
perpendicular to the zero-axis plane is represented by the fourth axis, PATC. This axis is
perpendicular to the two nodal planes, A and C, as well as the P-axis and T-axis, which
are the compression and tension axes. The P-axis is inclined at 45° with respect to the A
and C axes and is in the tension quadrant.
Figure 5 Focal mechanism solution diagram of roof collapse in deep mining (see online version
for colours)
As shown in Figure 5 and Table 2, No. 1, the P-axis was determined by a strike angle of
150° and an elevation angle of 9°, and the T axis was determined by a strike angle of 36°
and an elevation angle of 69°. It is usually assumed that the P axis is in the direction of
the maximum compressive stress and that the T-axis corresponds to the direction of
maximum tension. If the first nodal plane FBCG is a sliding surface, then the point C (the
axis perpendicular to the auxiliary plane) is a sliding vector and has an inclination of 59°.
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If the second section plane (WABE) is a sliding surface, then the point A is a sliding
vector and has an inclination of 113°. Although the two circular arcs are not completely
separated by the compression and expansion of the recurring quadrant, the quality
of recognition depends on the number and distribution of the actual trigger sensors;
it can be used to correct polarity of the speed of the ray tracing model and microseismic
events recorded. The sliding vector is usually assumed to be parallel to the fault plane
solution of shear stress. The P-axis and T-axis do not strictly correspond to the
microseismic stress. In the uneven continuum medium, the relationship between the
fault plane solution and the direction of the main stress is quite complex. However,
there are some advantages to using P-axial and T-axial solutions instead of a planar
solution. The P-axial and T-axial can confirm that a non-fault surface is caused by the
tension and compression quadrants. This shows that the maximum amplitude direction, or
even a number of observation points, that correspond to the P-axis compression
orientation and the T-axis tensioning orientation, given the approximate azimuth, can be
selected.
Induced microseismic events are often controlled by the secondary stress field, which
is caused by the larger tectonic stress field and man-made excavation disturbances.
As shown in Figure 5, for 9 August, 2007 (No. 1), the hypocenter coordinates are in the
eighth small ore body and C1b rock formations boundaries. The dividing line has a strike
angle of 27° and a dip angle of 60°. According to the comprehensive analysis of focal
mechanism diagrams and focal mechanism solutions, the principal stress direction is 130°
and the dip direction is 117° along the ore body. The pre-original rock stress
measurement showed that the maximum principal stress direction is 129° and that the
stress value is 22 MPa, mainly due to horizontal compression. The stress and
metallotectonic directions are the same. As shown in Figures 5 and 6, in which section A
of No. 1 (9 August, 2007) is the FBCG nodal plane, focal mechanism solutions can
predict accidents caused by the effects of human stoping. As shown in No. 3 of Figure 5
(12 September, 2007), focal mechanism solutions show that the strike direction of nodal
plane A (FBCG) is 340°. The nodal plane and the fault F4 are on the same plane,
which has a strike of 333°. The focal dislocation mechanism was obtained so that the
focal dislocations were judged by the mainly strike-slip normal faults and supplemented
dip-slip normal faults, and the direction of dislocation and fault structures is consistent.
Analysis of the focal mechanism solutions shows that the fault is in an activity state. The
strike-slip normal faults were generated to the SE by the main compressive stresses,
impacting the regional tectonic stress field. The P-axis inclination is horizontal, which
shows that focal NW trending horizontal compression in the focal region is more
significant and that EW tensile stresses are also important.
6 Conclusions
• Using the double couple model, three roof collapses were monitored in the No. 8 ore
body. Focal mechanism solutions were obtained that consisted of a nodal plane A,
nodal plane B, P-axis, T-axis, and N-axis.
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Table 2 Focal mechanism solution of roof collapse in deep mining
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Figure 6 Location and distribution of focal mechanism solution in deep mining (see online
version for colours)
• Based on the parameters of the microseismic monitoring events, three roof collapses
were located, and focal microseismic events were obtained in the active faults or
rock formations’ dividing line. The law of fault activity motion was determined.
Microseismic event numbers 1 and 2 were near the fault F4, and number 3 was on
the eighth small ore body and the C1b rock formation boundaries. The results show
that the rock formations’ dividing line and fault were caused by microseismic events
resulting from artificial excavation.
• Based on the dynamic mining process monitored by the microseismic monitoring
system, the study of double couple focal mechanisms provides a useful way to
determine regional stress field changes and the rock mass sliding direction and to
provide early warnings for dynamic mining disasters.
Acknowledgements
We thank the financial support from the National Natural Science Foundation of China
(No. 51374217), the Specialised Research Fund for the Doctoral Program of Higher
Education (No. 20120023120008), the National Key Technology P&D Program
supporting project of the ‘Twelfth Five-Year-Plan’ (No. 2012BAB08B02), the
Fundamental Research Funds for the Central Universities (No.2011QZ01), National
Students’ Innovation and Entrepreneurship Training Program (No. Y20131110,
Y20141106), and the National Students’ Start and Practice Project (No. 201411413069).
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