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On the anomalous land uplift and aseismic creep prior to the 1976 Tangshan earthquake
107
7’ertorzop~~~;ts, 85 ( 1982) IO7- I2 I Elsevier
Scientific
Publishing
Company,
Amsterdam-
Printed
in The Netherlands
ON THE ANOMALOUS LAND UPLIFT AND ASEISMIC CREEP PRIOR TO THE 1976 TANGSHAN EARTHQUAKE
ZHANG
YING-ZHEN
Cenrre for Analysis (Received
and Prediction,
November
State Seismological
Bureau, Beijing (China)
26, 1981)
ABSTRACT
Zhang
Ying-zhen,
earthquake.
1982. On the anomalous In: A.L.
Hales
and
land uplift
Z. Suzuki
and aseismic
(F&tom),
creep prior
Earthquake
to the 1976 Tangshan
Prediction.
Tectonoph.wics,
85:
107-121. A spatio-temporal prior
analysis
to the 1976 earthquake
magnitude
based on the data of eleven repeated indicates
of SO mm, occurred
Aseismic
in the epicentral
creep superimposed uniform
strain
sites have been calculated
of strain
and the parameters
accumulation
Tangshan,
offsets
are,
0.9X 10 dip
respectively.
directions
over a greater
A correlation that
the uplift
aseismic
8 cm.
7/yr. Creep at the fault amounted
experienced
creep
the period
aseismic
creep-inverse
The
the Tangshan
of Tangshan
values of displacement method,
tbe optimal
rate
of strain
and 1.4 cm/yr. fault,
and dips SX7’E. and is
accumulation respectively,
although
of smaller
and dip-slip amounts
to
in the strike and dimensions.
has
fault. the early development
phenomena
of shallow
may be described
(or decrease
at
values
years have been determined.
fault, strikes N47”E
uplift and creep with that of the Tangshan
earthquake
land deformation
average
The Jiyunhe
have been a manifestation’of
may be one of the precursory
preceding
region
1969. with a
of the fault lies 2 km deep. The strike-slip
to 18.6 cm/yr
1969-1975.
of the above-mentioned
processes
normal
rate of creep than the Tangshan
might
theoretical
of the creep faults in different
boundary
104 cm and
the Tangshan
has been found in the vicinity
and, using the least squares
a right-lateral
X km long and 6 km wide. The upper
around
from 1968 through
faults.
and elastic dislocation,
the various dislocation
The creep fault under
strain
and the Jiyunhe
accumulation
leveihngs
for 2 years,
area.
on the accumulated
and Baodi along both the Tangshan Assuming
that an uplift lasting
earthquake
of the earthquake earthquakes.
as: strain
suggests and that
The sequence
accumulation-land
of
uplift-
in creep rate)-earthquake.
INTRODUCTION
Earthquakes and aseismic creep along a fault may, under pertinent conditions, be associated with each other. Numerous experiments of rock frictional sliding show that, prior to stick-slip along a fault, steady slip occurs (Scholz et al., 1972; Byerlee and Summers, 1975; Brace, 1978). However, evidence of fault creep based on actual 0040- 195 l/82/~-~/$02.75
% 1982 Elsevier Scientific
Publishing
Company
108
geodetic
data
prior
to an earthquake
understand
the pre-quake
earthquake
as well as the fault
levellings
(1954-1975).
parameters
is still lacking.
deformation Elastic
creep
dislocation
theory
involved
in the creep. Aseismic
is an attempt
to
area of the 1976 Tangshan
over the region,
of the creep faults and to calculate
displacements
This paper
over the epicentral
based
has been
of 11
used to determine
the theoretical creep related
on the data
the
values of the vertical to the earthquakes
has
also been investigated. The data used are from the adjustment results by the Geodetic Brigade. State Seismological Bureau. The Shan-Jin bench mark4, near Qinhuangdao, has been chosen as the origin.
THE MAIN CENTRAL
CHARACTERISTICS
OF THE PRE-EARTHQUAKE
DEFORMATION
IN THE EPI-
AREA
The pre-quake as follows:
(I) The anomalous
deformation
of the 1976 Tangshan
earthquake
may be summarized
upheaval at the early stage
There are over 140 bench marks within the area, which, taking Tangshan as the centre point, is 200 km long from east to west and 150 km wide from north to south. The levelling
routes
are indicated
by discontinuous
lines
and point-dash
lines
in
Fig. I (for more information on the bench marks see the paper by Chen, 1979). They have all undergone changes in elevation in recent years with, however, different modes of change with time. The character of the changes differs from that of the well-known NNigata earthquake (Japan) and the 1966 Xingtai earthquake (China). In the two cases mentioned above, either there is only one levelling route passing through
the epicentral
area (e.g. the Xingtai
earthquake)
or a levelling
route nearby
the epicentral area (e.g. the NNigata earthquake). In the case of the Tangshan earthquake the variation with time of the bench marks’ elevation is rather complicated. The 140 bench marks have been subdivided into six regions, in each of which the variations were similar. These regions are shown in Fig. 1. Figure 2 shows the variation of the average elevation in each of the six regions with time. The positive elevation difference of region A bench marks (located around the epicentral area)
increased
slightly
in 1967, relative
to 1954. There
was a rapid
increase
in
elevation in 1968 and 1969, reaching, on average, more than 30 mm. In Fig. 2 there are no data for the bench marks of type A’ prior to 1959. However, their changes are similar to those of region A bench marks after 1959. The changes of the region C and D bench marks, east of the epicentral area, are similar to those of the region A bench marks, although with considerably lower amplitude. The negative elevation difference of the region E and F bench marks west and southwest of the epicentral area
109
390 / 119O
1l8o
Fig.
I. Distribution
enclose
of levelling
routes and bench marks in the Tangshan
the regions A, B. C, D. E. F and A’ for which elevations
figure) were averaged information
for Fig, 2. The dash-dot-dash
for 1972, 1973 and 1974 was available
region. The heavy dashed
relative to the Fixed point (upper
lines and dotted
lines show the levelling
lines
right of
routes.
No
on the lines shown dotted.
increased with every passing year after 1969, due to exploitation of ground water in the neighborhood of these lines. The uplift in the epicentral area can also be observed in Fig. 3 which shows vertical
uplift and subsidence
between
1954-1959
located between Fengnan and Ninghe Tangshan earthquake), with a maximum
and in 1969. The centre of uplift is of the epicentre of the (i.e. southwest amplitude of up to 50 mm. In Fig. 3, thin
solid lines and dashed lines indicate faults and inferred faults, respectively. These faults form the so-called Tangshan rhombic tectonic block (in geological terms). The heavy solid and dashed curves indicate isolines of elevation difference. Isoseismals for the Tangshan should
earthquake
be noted in respect
(a) It lies within
are also shown
in this figure.
The following
points
to the uplift area:
the frame of the rhombic
tectonic
block of Tangshan,
implying
that the deformation might have been related to the tectonic block. (b) With uplift averaging only 30 mm, with a maximum of 50 mm, the area occupies at least 4000 km*. (c) The uplift began 9 years prior to the earthquake and lasted until 1970, when subsidence began. (d) The earthquake occurred 6 years after the change from uplift to subsidence.
110
54
60
56
66
64
70
72
74 76
rI
\
I
/
/
/ -
-
-
IOmm
Fig. 2. Average
variation
points in each group, region E, were plotted
with time of elevations
for regions A, B, C. D. E. F and A’ (See Fig. I). The first
I954 for regions A, B, C and D, I959 for region A’, 1967 for region F. and 1969 for at zero
OBuodi
Fig. 3. Changes
in elevation
between
Thin solid and dashed
lines indicate
and
by dash-dot
VIII
difference
are shown between
1959 and 1969, faults, and isoscismals faults and inferred lines.
Heavy
solid
faults, respectively. and
1959 and 1969, the solid lines representing
units of the isolines are mm.
dashed
of the Tangshan Isoseismals
lines indicate
uplift and the dashed
earthquake.
for intensities
isolines
IX
of elevation
lines subsidence.
The
III
(2) The struin accumulation Before the Tangshan earthquake, accumulation of strain was clearly observed some faults, such as on the Tangshan fault and the Jiyunhe fault. The levelling route crossing
data
from
the Tangshan
1969 to 1975, (data
before
1969 are lacking),
along
on a
fault show that north of, and far away from the fault,
the elevation of the land surface has been rising, while on the south of the fault the elevation has been sinking (see Fig. 4). However, BM2052, close to the fault on the northern side, subsided more than 50 mm relative to bench fault. The angle of tilt between these bench marks changed 1975 by 1.2, 2.6, 5.8, 8.2, 9.2, 8.6 (X 10e6), respectively.
BEFORE
THE
mark 71 south of the for the years 1970 to
In Fig.4,
the numbers
EARTHQUAKE
-30 e-e O-071 v---v w---*73 +-..-•+
70 72 74
.----175
I
1
-50 mm
IOkm t
AFTER
TflE ~ARTHOUAKE (North Sfde)
AFTER
IL Fig. 4. Vertical
displacement
THE EAffTHPUAKE (South Side)
EART~~~uAKE
of bench marks
m~i_t
for the Tangshan
fault before and after the earthquake
of
112
bench marks in both upper and lower figures are the same, values in the upper figure are for the years before quake. During result
the quake
the earthquake,
the fault break came directly
that after the earthquake,
subsided
considerably.
and in the lower figure,
the northern
for the years after the
up to the surface,
with the
side lifted up and the southern
There were also levelling
routes
crossing
the Jiyunhe
side fault.
They too show an uplift of the northern side and subsidence to the south of the fault. The points farther away from the fault on the southern side near the Tianjin subsidence area have been disturbed. PRE-QUAKE
CREEP
ALONG
THE FAULTS
(I) The dislocation model and the culculution methods The fact that, on the respective levelling lines across the Tangshan and faults, the northern side rose and the southern side subsided and the movement near the fault increased in the same direction from year to year, explained as a long-term accumulation of strain superimposed on fault creep
Jiyunhe vertical may be (Fig. 5).
If curve C in Fig. 5 indicates changes in the elevation difference as in the upper section of Fig. 4, curve C may be regarded as the result of superposition of curves A and B, where A represents the elastic strain across the fault, r/,. and B the creep at
UR \ \ \ x2 \
\
;t-
Fig. 5. Superposition
of strain accumulation
on fault creeping.
113
the fault, UK. The observed u,
= u,
where regional region.
+ u,
value, U,, of displacement
of the points
is then:
+ u,
(1)
UO is a constant,
representing
the displacement
stress or to other conditions,
including
U, is considered
For simplicity,
line in Fig. 5). U, may be obtained fault model is adopted,
at each point
meteorological
as an approximately
uniform
from elastic dislocation
U, may be expressed
due to either
changes
over a large strain (dotted
theory. If the rectangular
as (Mansinha
and Smylie,
1971):
U, = U,(X,,X,,X,,a,8,L,d,D,AU,,AU,,S,.S~.S~)
(2)
where X,, X, and X, are the geographical coordinates of the observation sites with ground origins. The X, axis points north, the X, axis points east, and the X, axis points downward. (Y is the fault strike, 0 the fault dip, L is half the length of the fault, d is the depth of the upper boundary of the fault, D = d + W is the depth of the lower boundary of the fault, W is the width of the fault surface, AU, and AU, are the offsets, respectively, along the strike and the dip. S,, S, and S, are the coordinates of the mid-point of the fault surface. Substituting the analytic expression (eq. 2) as given by Mansinha and Smylie, into eq. 1, it is possible to obtain AUs and AU,, as well as the theoretical value, U,, for the respective
dislocation
the least root mean values, that is:
with different
square
fault parameters.
of the residuals
between
the optimum values of r/,, U,, and the parameters N is the number of the observation sites.
Based on the principle
the theoretical
of
and observed
in eq. 2 are determined.
In eq. 3,
(2) Results of cukulution (u) For the Tungshun fault
In Fig. 4, the observed
values
for the past
years
at the Tangshan
fault
show
differences in elevation relative to 1969. Hence it is assumed that the offset and strain accumulation of the fault in 1969 are zero. Figure4 also shows that the changes of strains and creep in 1970 are comparatively small, hence the theoretical values are calculated from the observed values of 1971 to 1975, as given in Table I. For the year 1975, immediately preceding the earthquake, variations of the mean square root of residuals with different fault parameters are also computed. From Fig. 6 we can see that the theoretical and observational values fit quite well. The speed of slip of the fault along strike and dip directions and the rate of strain accumulation each year can be obtained from the results of Table I and are given in Table II.
47 47 47 47
;1
L/2
1.6 2.4 3.3 4
(km)
2 2 2 2
(km)
d
D
8 8 8 8
I X.6
1.6 1.7 1.8 0.4 1.4
(cm/H
(cm/H
20.8 22.2 22.9 8.6
Speed of dip slipping
Speed of strike slipping
creep fault and of the strain accumulation
87 87 87 87
(“f
B
3.2 4.9 6.7 8
(cm)
buD
SE
51.5 51.5 51.5 51.5
(km)
0.01 0.02 0.05 0.03
114.5 114.5 L14.5 114.5
3 -1 0.9
1
0.5
4t (cm/km)
(km)
s.2
Rate of strain accumulation (X K--7)
-41.6 -63.8 - 86.7 -154
&ml
ws
d (km) 1.7 1.7
L/2
0.7
1
1969-1973
1969-1975
Period
(km) 2.5 2.5
Wf
D
.- 107 -150
AUs (cm)
35.7 50
Au, (cm)
hi.5 61.5
$1 (km)
33.5 33.5
% (km)
0.09 0.07
(cm/km)
Us7
Parameters of the creep fault near Baodi, strain accumulation and the root mean square of residuals of the observed values
TABLE XII
1969-1971 1971-1972 1972-1973 1973-$975 average
PWiOd
Speed of the Tangshan
TABLE XI
1969-1971 1969- 1912 1969- 1973 1969- 1975
Period
v,
4.7 3.1
(cm)
3.6 2.2 3.0
3.1
icm)
G
0.50 0.37
(cm)
d
0.39 0.32 0.45 0.46
(cm)
(J
Parameters of the Tangshan creep fault, strain accumulation, and the root mean square of residuals of theoretical values from the observed values
TABLE I
From Table II, we can see little difference between the rate of fault creep near Tangshan from 1971 to 1973, while one year prior to the earthquake it decreased. The rate of strain peak, thereafter
accumulation
decreasing
increased
in 1975 relative
from
1969 to 1973, when it reached
its
to 1973.
Comparison of Figs. 1 and 2 indicates that the change of levelling data at points in region B is smaller than that for region A, but that the trends of the change are the same. This means that the causes of the changes are probably the same and hence the data should conform with the solution given by eq. 1. To explain this point, the parameters
obtained
each year for the creep fault (listed in Table I) have been used
to calculate the yearly theoretical values at the levelling points of regions A and B (except those points given in Fig. 4). The annual theoretical values of region A levelling points fit well with those observed. For those of region B, a constant of 1.3 cm must be added. This implies
Fengrun
Tangshan
-20 13 II
15 ‘40
16
-10 18 25-l
0 26 71
Zengpawan 37
IOkm 39
0 118 1(11)11 19 20)52 99 39
04
-20 -30
1975
-40 -50 -60
Fig. 6. Comparison Tangshan
fault.
.--.
OBSERVED
O--o
THEORETICAL
of calculated
VALUE VALUE
elevations
with
the observed
elevations
of bench
marks
along
the
116
OBSERVED ll975-19691
VALUE
Fig. 7. Contours rectangle
of equal
represents
displacements
THEORETICAL (1975-1969)
o_24
vertical
the projection
in cm are represented
displacements
for the Tangshan
of the creep by dashed
VALUE
fault
on
o_2,,
creep
the ground.
lines for subsidence
fault.
The shaded
narrow
of equal
vertical
Contours
and solid lines for uplift.
that if 1969 is taken as the initial year, the variation in the elevation of region B points differs from that of region A points. The added constant is roughly equal to the average difference in elevation between the two regions (Fig. 3). Figure 7 shows the contour lines of the observed and theoretical values of vertical displacement at all observational points near the Tangshan creep fault. From the figure we can see that the theoretical contours conform rather well with those observed, except for the southwestern end of the fault in the vicinity of Ninghe. (b) For the Jiyunhe fault The vertical crustal deformations that of 1969. Assuming
of 1973 and 1975 are rather obvious
that this is caused
by strain
accumulation
relative
to
and fault creep,
then the results of Tables III and IV may be obtained using the same method as was used for the Tangshan fault. From Table IV it can be seen that for the Jiyunhe fault, as for the Tangshan
TABLE
fault, the creep rate was faster from 1969 to 1973 and slower in
IV
Speed of creep fault near Baodi and of the strain accumulation Speed of strike slipping
Speed of dip slipping
Rate of strain
(cm/y0
(cm/yr)
(Xl0
1969- I973
26.7
8.9
2.2
1969- I975
21.5
1.2
~ 1.0
average
24.1
8.0
0.6
Period
7)
accumulation
117
the year prior to the earthquake.
A comparison
creep fault near Baodi is smaller
than that at Tangshan,
Based on the fact that the mean 0.35-0.50
square
of Tables
root of residuals
cm, it may be said that the selected
conditions DISCUSSION
II and IV reveals that the but its creep rate is higher. lies within
fault model has simulated
the range
of
the actual
well. AND CONCLUSION
(I) The abnormal up/$ in rdation to the earthquake As mentioned above, the area of anomalous uplift not only conforms with the rhombic block, but it is also within the confines of the isoseismal of intensity IX of the Tangshan earthquake (M = 7.8) with similar strike direction (Fig. 3). Furthermore, the radius of the uplifted area and the time span from the beginning of the uplift to the occurrence of the quake basically agree with those calculated from the empirical formulas of Dambara (1966) and Fujii (1974), respectively (Fig. 8). In addition, during the anomalous uplift in 1968 and 1969, the frequency of small earthquakes (M, 2 2) in the eastern section of the Beijing-Tianjin-Tangshan region increased abnormally (Fig. 9). From the analysis of the correlation between the anomalous uplift and the seismic activity, epicentral intensity, magnitude and occurrence time of the Tangshan earthquake,
it seems that the uplift prior to the earthquake
a dilatation
of the rock medium
It may be considered source’s development.
as an early manifestation
5
Fig. 8. The correlation of surface
of earthquake
deformation
6
in the process
7
of
of compression. of the earthquake
8
MAGNITUDE
MAGNITUDE
duration
might be an indication
in its focal region as a consequence
magnitude
precursory
with (a) the radius
phenomena.
of the uplifted
area and (b) the time
(2) The creep fault prior to an earthquake In TableV,
we compare
in relation to the earthquake fault
the parameters
with those of the earthquake
of the creep fault before
fault obtained
by inversion
the earthquake
of the ground
deformation
data after the earthquake, and from the initial motion of P-waves. The faults and their centers, obtained by various methods, are shown in Fig. 10a. Figure lob shows a vertical profile. In TableV and Fig. 10 we can see that the creep fault before the earthquake is very similar to the fault caused by the earthquake. From the agreement between the fault parameters and the slip direction of the creep fault and the earthquake fault, we may conclude
that the earthquake
the creep fault. After the Tangshan
earthquake,
fault has been brought
two events with M, = 5.5 and M,
on Aug. 24 and Dec. 2, 1976, respectively, largest
events
about
in the Beijing-Tianjin
near Baodi (Fig.
region
outside
by the offset of = 5.7 occurred
11). They are the two
Tangshan
during
the period
1976-1979, and the earthquakes of M, S- 3.0 after the M = 7.8 earthquake occurred mainly along the belt Ninghe-Baodi-Sanhe, striking NW. The strike of this belt is nearly the same as that of the creep fault near Baodi (Fig. 11). The coordinates the midpoints of the fault are 39”43’N and 117” 16’E.
mm 3oi
Elevation
changes
of
the
Frequency
of earthquake
56
60
bench
mark
25. 20. 15lo5-
I
I 1954
,,,,,,,I
[ML ? 2 ]
I,,,,,,,
Fig. 9. Difference
58
in elevation
62
,
64
66
60
of bench marks compared
70
72
I
I
I
74
76
with the frequency
of small earthquakes.
of
I19
TABLE
V
Comparison
of the Tangshan
creep fault with the earthquake
Coordinates
Dipping
Strike
Fault
fault
of the fault 118’16’E
’
N47”E
87”SE
39”38’N.
Earthquake
fault *
N49’E
76OSE
39”33’.5N,
Earthquake
fault 3
N41’E
85OSE
39”.6N,
Creep
fault
of the
mid-point
1 1E007’.7E
118”.2E
’ Used in this paper. * Obtained
by inversion
3 Obtained
from the initial motion
of geodetic
data after the earthquake
(Chen.
1979).
of P-waves (Chu. 1976).
From the above results, we can see that prior to the earthquakes of M= 7.8 in Tangshan and M, > 5.0 in Baodi, aseismic fault creep was present, with a uniform rate for several years preceding the earthquakes, but decreasing immediately before them. The creep would have been confined to the shallow portion of the crust, 2-8 km below the earth’s surface, as shallow faults are liable to creep (Byerlee and Summers, 1975). It seems reasonable to consider, therefore, that aseismic fault creep may be one of the precursory phenomena of shallow tectonic earthquakes. Because the average
Fig.
dislocation
IO. a. The creep
pre-quake
and seismic
fault and the earthquake
creep fault and its mid-point
fault and its mid-point, b. The Tangshan
the focus of the M=8/9
earthquake:
of the Tangshan
fault in Tangshan.
on the ground:
(3) the epicenter
fault, its mid-point
moment
earthquake,
(I) indicates
(.?). and the rectangle
deduced
the projection
of the
4, that of the earthquake
of the M= 7.8 earthquake.
and the focus. (I ) indicates (4) the earthquake
the creep fault and the slip direction:
fault and the displacement
direction.
( 3)
b
Fig.
I I. a. Distribution
M=4.0-4.9; Dotted
shaded
of ML a3 earthquakes circles
lines, the average
M=S.O-5.9,
position
b. The creep fault and its mid-point
from the geodetic
from Aug. 1976 to Dec. 1977. Dots:
dotted
circle
epicenter
of the Ninghe
M=3.0-3.9:
circles:
earthquake,
M=6.2.
of small earthquakes. near Baodi.
data obtained
after the earthquake,
are much larger
than
those
from seismic wave data, Chen (1979) suggested that either before or after the earthquake an aseismic slip, on nearly the same scale as that of the main shock, occurred within the crust beneath the epicentral area. Such a conjecture is consistent with the result mentioned above. In summing up the characteristics of the deformation in the epicentral area prior to the Tangshan earthquake, the stages of the process of development of the Tangshan earthquake may be described as: a long period of strain accumulationsurface uplifting-aseismic creep-reversal of surface deformation (or the slowing down of the rate of creep and strain accumulation)-the occurrence of the earthquake. Although the mechanics and physical model of this process need to be further studied, such an understanding of the developmental process of an earthquake is no doubt beneficial for the studies on earthquake prediction and control.
121
(3) Certain points which deserve attention Based on the deformation that
there would
deformation
in earthquake
data of the Tangshan
be a major
earthquake
was not sufficiently
in the case of the NNigata
prediction
region,
in this region.
significant
to justify
(1964) and Xintai
it was predicted However,
further
after
attention.
(1966) earthquakes,
in 1970 1970 the
Furthermore, the earthquake
occurred immediately after the change from uplift to subsidence. In Tangshan the change from uplift to subsidence occurred in 1969, but no earthquake followed. Thereafter, the attention given to the Tangshan region was inadequate. Stress-strain curves of brittle materials have often been related to the process of crustal deformation with a sequence such as continual deformation (rising or sinking)-accelerated deformation-reversal of deformation-occurrence of the earthquake. Shallow earthquakes do in fact nearly always follow this typical sequence of development. However, the Tangshan earthquake differs in its development from this sequence, in that prior to the earthquake, aseismic creep had already taken place and released a part of the strain energy. Thus the process was quite different from the brittle sequence. Further region,
investigations
on the physical
for all types of earthquakes,
precursory
phenomena
properties
of rock media
and their relationship
are required
for earthquake
in the source
to the characteristics
of
prediction.
ACKNOWLEDGEMENT
In the preparation of this paper, Xu Dao-yi is acknowledged.
the assistance
from comrades
Chen Yun-tai
and
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and
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Earthquake
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at
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in Tangshan.
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(up and
down
movement)
in the neighborhood
of the
12: 1945.
between
the magnitude
and the anomalous
creeptime
before earthquakes.
27 (3): 197-213.
L. and Smyhe,
D.E., 1971. The displacement
fields of inclined
faults. Bull. Seismol. Sot. Am..
61: 1433-1440. Scholz.
C.H..
implications
Molnar.
P. and Johnston,
for the earthquake
T., 1972. Detailed
mechanism.
J. Geophys.
studies
of frictional
Res., 77: 6392-6406.
sliding
of granite
and
7’ertorzop~~~;ts, 85 ( 1982) IO7- I2 I Elsevier
Scientific
Publishing
Company,
Amsterdam-
Printed
in The Netherlands
ON THE ANOMALOUS LAND UPLIFT AND ASEISMIC CREEP PRIOR TO THE 1976 TANGSHAN EARTHQUAKE
ZHANG
YING-ZHEN
Cenrre for Analysis (Received
and Prediction,
November
State Seismological
Bureau, Beijing (China)
26, 1981)
ABSTRACT
Zhang
Ying-zhen,
earthquake.
1982. On the anomalous In: A.L.
Hales
and
land uplift
Z. Suzuki
and aseismic
(F&tom),
creep prior
Earthquake
to the 1976 Tangshan
Prediction.
Tectonoph.wics,
85:
107-121. A spatio-temporal prior
analysis
to the 1976 earthquake
magnitude
based on the data of eleven repeated indicates
of SO mm, occurred
Aseismic
in the epicentral
creep superimposed uniform
strain
sites have been calculated
of strain
and the parameters
accumulation
Tangshan,
offsets
are,
0.9X 10 dip
respectively.
directions
over a greater
A correlation that
the uplift
aseismic
8 cm.
7/yr. Creep at the fault amounted
experienced
creep
the period
aseismic
creep-inverse
The
the Tangshan
of Tangshan
values of displacement method,
tbe optimal
rate
of strain
and 1.4 cm/yr. fault,
and dips SX7’E. and is
accumulation respectively,
although
of smaller
and dip-slip amounts
to
in the strike and dimensions.
has
fault. the early development
phenomena
of shallow
may be described
(or decrease
at
values
years have been determined.
fault, strikes N47”E
uplift and creep with that of the Tangshan
earthquake
land deformation
average
The Jiyunhe
have been a manifestation’of
may be one of the precursory
preceding
region
1969. with a
of the fault lies 2 km deep. The strike-slip
to 18.6 cm/yr
1969-1975.
of the above-mentioned
processes
normal
rate of creep than the Tangshan
might
theoretical
of the creep faults in different
boundary
104 cm and
the Tangshan
has been found in the vicinity
and, using the least squares
a right-lateral
X km long and 6 km wide. The upper
around
from 1968 through
faults.
and elastic dislocation,
the various dislocation
The creep fault under
strain
and the Jiyunhe
accumulation
leveihngs
for 2 years,
area.
on the accumulated
and Baodi along both the Tangshan Assuming
that an uplift lasting
earthquake
of the earthquake earthquakes.
as: strain
suggests and that
The sequence
accumulation-land
of
uplift-
in creep rate)-earthquake.
INTRODUCTION
Earthquakes and aseismic creep along a fault may, under pertinent conditions, be associated with each other. Numerous experiments of rock frictional sliding show that, prior to stick-slip along a fault, steady slip occurs (Scholz et al., 1972; Byerlee and Summers, 1975; Brace, 1978). However, evidence of fault creep based on actual 0040- 195 l/82/~-~/$02.75
% 1982 Elsevier Scientific
Publishing
Company
108
geodetic
data
prior
to an earthquake
understand
the pre-quake
earthquake
as well as the fault
levellings
(1954-1975).
parameters
is still lacking.
deformation Elastic
creep
dislocation
theory
involved
in the creep. Aseismic
is an attempt
to
area of the 1976 Tangshan
over the region,
of the creep faults and to calculate
displacements
This paper
over the epicentral
based
has been
of 11
used to determine
the theoretical creep related
on the data
the
values of the vertical to the earthquakes
has
also been investigated. The data used are from the adjustment results by the Geodetic Brigade. State Seismological Bureau. The Shan-Jin bench mark4, near Qinhuangdao, has been chosen as the origin.
THE MAIN CENTRAL
CHARACTERISTICS
OF THE PRE-EARTHQUAKE
DEFORMATION
IN THE EPI-
AREA
The pre-quake as follows:
(I) The anomalous
deformation
of the 1976 Tangshan
earthquake
may be summarized
upheaval at the early stage
There are over 140 bench marks within the area, which, taking Tangshan as the centre point, is 200 km long from east to west and 150 km wide from north to south. The levelling
routes
are indicated
by discontinuous
lines
and point-dash
lines
in
Fig. I (for more information on the bench marks see the paper by Chen, 1979). They have all undergone changes in elevation in recent years with, however, different modes of change with time. The character of the changes differs from that of the well-known NNigata earthquake (Japan) and the 1966 Xingtai earthquake (China). In the two cases mentioned above, either there is only one levelling route passing through
the epicentral
area (e.g. the Xingtai
earthquake)
or a levelling
route nearby
the epicentral area (e.g. the NNigata earthquake). In the case of the Tangshan earthquake the variation with time of the bench marks’ elevation is rather complicated. The 140 bench marks have been subdivided into six regions, in each of which the variations were similar. These regions are shown in Fig. 1. Figure 2 shows the variation of the average elevation in each of the six regions with time. The positive elevation difference of region A bench marks (located around the epicentral area)
increased
slightly
in 1967, relative
to 1954. There
was a rapid
increase
in
elevation in 1968 and 1969, reaching, on average, more than 30 mm. In Fig. 2 there are no data for the bench marks of type A’ prior to 1959. However, their changes are similar to those of region A bench marks after 1959. The changes of the region C and D bench marks, east of the epicentral area, are similar to those of the region A bench marks, although with considerably lower amplitude. The negative elevation difference of the region E and F bench marks west and southwest of the epicentral area
109
390 / 119O
1l8o
Fig.
I. Distribution
enclose
of levelling
routes and bench marks in the Tangshan
the regions A, B. C, D. E. F and A’ for which elevations
figure) were averaged information
for Fig, 2. The dash-dot-dash
for 1972, 1973 and 1974 was available
region. The heavy dashed
relative to the Fixed point (upper
lines and dotted
lines show the levelling
lines
right of
routes.
No
on the lines shown dotted.
increased with every passing year after 1969, due to exploitation of ground water in the neighborhood of these lines. The uplift in the epicentral area can also be observed in Fig. 3 which shows vertical
uplift and subsidence
between
1954-1959
located between Fengnan and Ninghe Tangshan earthquake), with a maximum
and in 1969. The centre of uplift is of the epicentre of the (i.e. southwest amplitude of up to 50 mm. In Fig. 3, thin
solid lines and dashed lines indicate faults and inferred faults, respectively. These faults form the so-called Tangshan rhombic tectonic block (in geological terms). The heavy solid and dashed curves indicate isolines of elevation difference. Isoseismals for the Tangshan should
earthquake
be noted in respect
(a) It lies within
are also shown
in this figure.
The following
points
to the uplift area:
the frame of the rhombic
tectonic
block of Tangshan,
implying
that the deformation might have been related to the tectonic block. (b) With uplift averaging only 30 mm, with a maximum of 50 mm, the area occupies at least 4000 km*. (c) The uplift began 9 years prior to the earthquake and lasted until 1970, when subsidence began. (d) The earthquake occurred 6 years after the change from uplift to subsidence.
110
54
60
56
66
64
70
72
74 76
rI
\
I
/
/
/ -
-
-
IOmm
Fig. 2. Average
variation
points in each group, region E, were plotted
with time of elevations
for regions A, B, C. D. E. F and A’ (See Fig. I). The first
I954 for regions A, B, C and D, I959 for region A’, 1967 for region F. and 1969 for at zero
OBuodi
Fig. 3. Changes
in elevation
between
Thin solid and dashed
lines indicate
and
by dash-dot
VIII
difference
are shown between
1959 and 1969, faults, and isoscismals faults and inferred lines.
Heavy
solid
faults, respectively. and
1959 and 1969, the solid lines representing
units of the isolines are mm.
dashed
of the Tangshan Isoseismals
lines indicate
uplift and the dashed
earthquake.
for intensities
isolines
IX
of elevation
lines subsidence.
The
III
(2) The struin accumulation Before the Tangshan earthquake, accumulation of strain was clearly observed some faults, such as on the Tangshan fault and the Jiyunhe fault. The levelling route crossing
data
from
the Tangshan
1969 to 1975, (data
before
1969 are lacking),
along
on a
fault show that north of, and far away from the fault,
the elevation of the land surface has been rising, while on the south of the fault the elevation has been sinking (see Fig. 4). However, BM2052, close to the fault on the northern side, subsided more than 50 mm relative to bench fault. The angle of tilt between these bench marks changed 1975 by 1.2, 2.6, 5.8, 8.2, 9.2, 8.6 (X 10e6), respectively.
BEFORE
THE
mark 71 south of the for the years 1970 to
In Fig.4,
the numbers
EARTHQUAKE
-30 e-e O-071 v---v w---*73 +-..-•+
70 72 74
.----175
I
1
-50 mm
IOkm t
AFTER
TflE ~ARTHOUAKE (North Sfde)
AFTER
IL Fig. 4. Vertical
displacement
THE EAffTHPUAKE (South Side)
EART~~~uAKE
of bench marks
m~i_t
for the Tangshan
fault before and after the earthquake
of
112
bench marks in both upper and lower figures are the same, values in the upper figure are for the years before quake. During result
the quake
the earthquake,
the fault break came directly
that after the earthquake,
subsided
considerably.
and in the lower figure,
the northern
for the years after the
up to the surface,
with the
side lifted up and the southern
There were also levelling
routes
crossing
the Jiyunhe
side fault.
They too show an uplift of the northern side and subsidence to the south of the fault. The points farther away from the fault on the southern side near the Tianjin subsidence area have been disturbed. PRE-QUAKE
CREEP
ALONG
THE FAULTS
(I) The dislocation model and the culculution methods The fact that, on the respective levelling lines across the Tangshan and faults, the northern side rose and the southern side subsided and the movement near the fault increased in the same direction from year to year, explained as a long-term accumulation of strain superimposed on fault creep
Jiyunhe vertical may be (Fig. 5).
If curve C in Fig. 5 indicates changes in the elevation difference as in the upper section of Fig. 4, curve C may be regarded as the result of superposition of curves A and B, where A represents the elastic strain across the fault, r/,. and B the creep at
UR \ \ \ x2 \
\
;t-
Fig. 5. Superposition
of strain accumulation
on fault creeping.
113
the fault, UK. The observed u,
= u,
where regional region.
+ u,
value, U,, of displacement
of the points
is then:
+ u,
(1)
UO is a constant,
representing
the displacement
stress or to other conditions,
including
U, is considered
For simplicity,
line in Fig. 5). U, may be obtained fault model is adopted,
at each point
meteorological
as an approximately
uniform
from elastic dislocation
U, may be expressed
due to either
changes
over a large strain (dotted
theory. If the rectangular
as (Mansinha
and Smylie,
1971):
U, = U,(X,,X,,X,,a,8,L,d,D,AU,,AU,,S,.S~.S~)
(2)
where X,, X, and X, are the geographical coordinates of the observation sites with ground origins. The X, axis points north, the X, axis points east, and the X, axis points downward. (Y is the fault strike, 0 the fault dip, L is half the length of the fault, d is the depth of the upper boundary of the fault, D = d + W is the depth of the lower boundary of the fault, W is the width of the fault surface, AU, and AU, are the offsets, respectively, along the strike and the dip. S,, S, and S, are the coordinates of the mid-point of the fault surface. Substituting the analytic expression (eq. 2) as given by Mansinha and Smylie, into eq. 1, it is possible to obtain AUs and AU,, as well as the theoretical value, U,, for the respective
dislocation
the least root mean values, that is:
with different
square
fault parameters.
of the residuals
between
the optimum values of r/,, U,, and the parameters N is the number of the observation sites.
Based on the principle
the theoretical
of
and observed
in eq. 2 are determined.
In eq. 3,
(2) Results of cukulution (u) For the Tungshun fault
In Fig. 4, the observed
values
for the past
years
at the Tangshan
fault
show
differences in elevation relative to 1969. Hence it is assumed that the offset and strain accumulation of the fault in 1969 are zero. Figure4 also shows that the changes of strains and creep in 1970 are comparatively small, hence the theoretical values are calculated from the observed values of 1971 to 1975, as given in Table I. For the year 1975, immediately preceding the earthquake, variations of the mean square root of residuals with different fault parameters are also computed. From Fig. 6 we can see that the theoretical and observational values fit quite well. The speed of slip of the fault along strike and dip directions and the rate of strain accumulation each year can be obtained from the results of Table I and are given in Table II.
47 47 47 47
;1
L/2
1.6 2.4 3.3 4
(km)
2 2 2 2
(km)
d
D
8 8 8 8
I X.6
1.6 1.7 1.8 0.4 1.4
(cm/H
(cm/H
20.8 22.2 22.9 8.6
Speed of dip slipping
Speed of strike slipping
creep fault and of the strain accumulation
87 87 87 87
(“f
B
3.2 4.9 6.7 8
(cm)
buD
SE
51.5 51.5 51.5 51.5
(km)
0.01 0.02 0.05 0.03
114.5 114.5 L14.5 114.5
3 -1 0.9
1
0.5
4t (cm/km)
(km)
s.2
Rate of strain accumulation (X K--7)
-41.6 -63.8 - 86.7 -154
&ml
ws
d (km) 1.7 1.7
L/2
0.7
1
1969-1973
1969-1975
Period
(km) 2.5 2.5
Wf
D
.- 107 -150
AUs (cm)
35.7 50
Au, (cm)
hi.5 61.5
$1 (km)
33.5 33.5
% (km)
0.09 0.07
(cm/km)
Us7
Parameters of the creep fault near Baodi, strain accumulation and the root mean square of residuals of the observed values
TABLE XII
1969-1971 1971-1972 1972-1973 1973-$975 average
PWiOd
Speed of the Tangshan
TABLE XI
1969-1971 1969- 1912 1969- 1973 1969- 1975
Period
v,
4.7 3.1
(cm)
3.6 2.2 3.0
3.1
icm)
G
0.50 0.37
(cm)
d
0.39 0.32 0.45 0.46
(cm)
(J
Parameters of the Tangshan creep fault, strain accumulation, and the root mean square of residuals of theoretical values from the observed values
TABLE I
From Table II, we can see little difference between the rate of fault creep near Tangshan from 1971 to 1973, while one year prior to the earthquake it decreased. The rate of strain peak, thereafter
accumulation
decreasing
increased
in 1975 relative
from
1969 to 1973, when it reached
its
to 1973.
Comparison of Figs. 1 and 2 indicates that the change of levelling data at points in region B is smaller than that for region A, but that the trends of the change are the same. This means that the causes of the changes are probably the same and hence the data should conform with the solution given by eq. 1. To explain this point, the parameters
obtained
each year for the creep fault (listed in Table I) have been used
to calculate the yearly theoretical values at the levelling points of regions A and B (except those points given in Fig. 4). The annual theoretical values of region A levelling points fit well with those observed. For those of region B, a constant of 1.3 cm must be added. This implies
Fengrun
Tangshan
-20 13 II
15 ‘40
16
-10 18 25-l
0 26 71
Zengpawan 37
IOkm 39
0 118 1(11)11 19 20)52 99 39
04
-20 -30
1975
-40 -50 -60
Fig. 6. Comparison Tangshan
fault.
.--.
OBSERVED
O--o
THEORETICAL
of calculated
VALUE VALUE
elevations
with
the observed
elevations
of bench
marks
along
the
116
OBSERVED ll975-19691
VALUE
Fig. 7. Contours rectangle
of equal
represents
displacements
THEORETICAL (1975-1969)
o_24
vertical
the projection
in cm are represented
displacements
for the Tangshan
of the creep by dashed
VALUE
fault
on
o_2,,
creep
the ground.
lines for subsidence
fault.
The shaded
narrow
of equal
vertical
Contours
and solid lines for uplift.
that if 1969 is taken as the initial year, the variation in the elevation of region B points differs from that of region A points. The added constant is roughly equal to the average difference in elevation between the two regions (Fig. 3). Figure 7 shows the contour lines of the observed and theoretical values of vertical displacement at all observational points near the Tangshan creep fault. From the figure we can see that the theoretical contours conform rather well with those observed, except for the southwestern end of the fault in the vicinity of Ninghe. (b) For the Jiyunhe fault The vertical crustal deformations that of 1969. Assuming
of 1973 and 1975 are rather obvious
that this is caused
by strain
accumulation
relative
to
and fault creep,
then the results of Tables III and IV may be obtained using the same method as was used for the Tangshan fault. From Table IV it can be seen that for the Jiyunhe fault, as for the Tangshan
TABLE
fault, the creep rate was faster from 1969 to 1973 and slower in
IV
Speed of creep fault near Baodi and of the strain accumulation Speed of strike slipping
Speed of dip slipping
Rate of strain
(cm/y0
(cm/yr)
(Xl0
1969- I973
26.7
8.9
2.2
1969- I975
21.5
1.2
~ 1.0
average
24.1
8.0
0.6
Period
7)
accumulation
117
the year prior to the earthquake.
A comparison
creep fault near Baodi is smaller
than that at Tangshan,
Based on the fact that the mean 0.35-0.50
square
of Tables
root of residuals
cm, it may be said that the selected
conditions DISCUSSION
II and IV reveals that the but its creep rate is higher. lies within
fault model has simulated
the range
of
the actual
well. AND CONCLUSION
(I) The abnormal up/$ in rdation to the earthquake As mentioned above, the area of anomalous uplift not only conforms with the rhombic block, but it is also within the confines of the isoseismal of intensity IX of the Tangshan earthquake (M = 7.8) with similar strike direction (Fig. 3). Furthermore, the radius of the uplifted area and the time span from the beginning of the uplift to the occurrence of the quake basically agree with those calculated from the empirical formulas of Dambara (1966) and Fujii (1974), respectively (Fig. 8). In addition, during the anomalous uplift in 1968 and 1969, the frequency of small earthquakes (M, 2 2) in the eastern section of the Beijing-Tianjin-Tangshan region increased abnormally (Fig. 9). From the analysis of the correlation between the anomalous uplift and the seismic activity, epicentral intensity, magnitude and occurrence time of the Tangshan earthquake,
it seems that the uplift prior to the earthquake
a dilatation
of the rock medium
It may be considered source’s development.
as an early manifestation
5
Fig. 8. The correlation of surface
of earthquake
deformation
6
in the process
7
of
of compression. of the earthquake
8
MAGNITUDE
MAGNITUDE
duration
might be an indication
in its focal region as a consequence
magnitude
precursory
with (a) the radius
phenomena.
of the uplifted
area and (b) the time
(2) The creep fault prior to an earthquake In TableV,
we compare
in relation to the earthquake fault
the parameters
with those of the earthquake
of the creep fault before
fault obtained
by inversion
the earthquake
of the ground
deformation
data after the earthquake, and from the initial motion of P-waves. The faults and their centers, obtained by various methods, are shown in Fig. 10a. Figure lob shows a vertical profile. In TableV and Fig. 10 we can see that the creep fault before the earthquake is very similar to the fault caused by the earthquake. From the agreement between the fault parameters and the slip direction of the creep fault and the earthquake fault, we may conclude
that the earthquake
the creep fault. After the Tangshan
earthquake,
fault has been brought
two events with M, = 5.5 and M,
on Aug. 24 and Dec. 2, 1976, respectively, largest
events
about
in the Beijing-Tianjin
near Baodi (Fig.
region
outside
by the offset of = 5.7 occurred
11). They are the two
Tangshan
during
the period
1976-1979, and the earthquakes of M, S- 3.0 after the M = 7.8 earthquake occurred mainly along the belt Ninghe-Baodi-Sanhe, striking NW. The strike of this belt is nearly the same as that of the creep fault near Baodi (Fig. 11). The coordinates the midpoints of the fault are 39”43’N and 117” 16’E.
mm 3oi
Elevation
changes
of
the
Frequency
of earthquake
56
60
bench
mark
25. 20. 15lo5-
I
I 1954
,,,,,,,I
[ML ? 2 ]
I,,,,,,,
Fig. 9. Difference
58
in elevation
62
,
64
66
60
of bench marks compared
70
72
I
I
I
74
76
with the frequency
of small earthquakes.
of
I19
TABLE
V
Comparison
of the Tangshan
creep fault with the earthquake
Coordinates
Dipping
Strike
Fault
fault
of the fault 118’16’E
’
N47”E
87”SE
39”38’N.
Earthquake
fault *
N49’E
76OSE
39”33’.5N,
Earthquake
fault 3
N41’E
85OSE
39”.6N,
Creep
fault
of the
mid-point
1 1E007’.7E
118”.2E
’ Used in this paper. * Obtained
by inversion
3 Obtained
from the initial motion
of geodetic
data after the earthquake
(Chen.
1979).
of P-waves (Chu. 1976).
From the above results, we can see that prior to the earthquakes of M= 7.8 in Tangshan and M, > 5.0 in Baodi, aseismic fault creep was present, with a uniform rate for several years preceding the earthquakes, but decreasing immediately before them. The creep would have been confined to the shallow portion of the crust, 2-8 km below the earth’s surface, as shallow faults are liable to creep (Byerlee and Summers, 1975). It seems reasonable to consider, therefore, that aseismic fault creep may be one of the precursory phenomena of shallow tectonic earthquakes. Because the average
Fig.
dislocation
IO. a. The creep
pre-quake
and seismic
fault and the earthquake
creep fault and its mid-point
fault and its mid-point, b. The Tangshan
the focus of the M=8/9
earthquake:
of the Tangshan
fault in Tangshan.
on the ground:
(3) the epicenter
fault, its mid-point
moment
earthquake,
(I) indicates
(.?). and the rectangle
deduced
the projection
of the
4, that of the earthquake
of the M= 7.8 earthquake.
and the focus. (I ) indicates (4) the earthquake
the creep fault and the slip direction:
fault and the displacement
direction.
( 3)
b
Fig.
I I. a. Distribution
M=4.0-4.9; Dotted
shaded
of ML a3 earthquakes circles
lines, the average
M=S.O-5.9,
position
b. The creep fault and its mid-point
from the geodetic
from Aug. 1976 to Dec. 1977. Dots:
dotted
circle
epicenter
of the Ninghe
M=3.0-3.9:
circles:
earthquake,
M=6.2.
of small earthquakes. near Baodi.
data obtained
after the earthquake,
are much larger
than
those
from seismic wave data, Chen (1979) suggested that either before or after the earthquake an aseismic slip, on nearly the same scale as that of the main shock, occurred within the crust beneath the epicentral area. Such a conjecture is consistent with the result mentioned above. In summing up the characteristics of the deformation in the epicentral area prior to the Tangshan earthquake, the stages of the process of development of the Tangshan earthquake may be described as: a long period of strain accumulationsurface uplifting-aseismic creep-reversal of surface deformation (or the slowing down of the rate of creep and strain accumulation)-the occurrence of the earthquake. Although the mechanics and physical model of this process need to be further studied, such an understanding of the developmental process of an earthquake is no doubt beneficial for the studies on earthquake prediction and control.
121
(3) Certain points which deserve attention Based on the deformation that
there would
deformation
in earthquake
data of the Tangshan
be a major
earthquake
was not sufficiently
in the case of the NNigata
prediction
region,
in this region.
significant
to justify
(1964) and Xintai
it was predicted However,
further
after
attention.
(1966) earthquakes,
in 1970 1970 the
Furthermore, the earthquake
occurred immediately after the change from uplift to subsidence. In Tangshan the change from uplift to subsidence occurred in 1969, but no earthquake followed. Thereafter, the attention given to the Tangshan region was inadequate. Stress-strain curves of brittle materials have often been related to the process of crustal deformation with a sequence such as continual deformation (rising or sinking)-accelerated deformation-reversal of deformation-occurrence of the earthquake. Shallow earthquakes do in fact nearly always follow this typical sequence of development. However, the Tangshan earthquake differs in its development from this sequence, in that prior to the earthquake, aseismic creep had already taken place and released a part of the strain energy. Thus the process was quite different from the brittle sequence. Further region,
investigations
on the physical
for all types of earthquakes,
precursory
phenomena
properties
of rock media
and their relationship
are required
for earthquake
in the source
to the characteristics
of
prediction.
ACKNOWLEDGEMENT
In the preparation of this paper, Xu Dao-yi is acknowledged.
the assistance
from comrades
Chen Yun-tai
and
REFERENCES
Brace,
W.F..
1978. Recent
precursory Byerlee,
A.E.P.S.,
laboratory
J.D. and Summer,
high pressure. Chen, Yun-Tai earthquake Chu. Chun. Ho-pei Dambara.
and Huang.
Li-Zen,
by geodetic
measurements.
1976. The background T..
Matushiro
Chinese
Mansinha,
mechanics
and
prediction.
Earthquake
Chinese
and fault surface
in granite
at
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