Open Journal of Geology, 2013, 3, 38-40
doi:10.4236/ojg.2013.32B009 Published Online April 2013 (http://www.scirp.org/journal/ojg)
Plumb Line Variations (PLV) at China and Their Relation
with Earthquakes and Underground Materials Changes
Yongzhang Yang1,2, Jinsong Ping1,2, Zhengxin Li1,2
1National Astronomical Observatories, CAS, Beijing China
2Shanghai Astronomical Observatory, CAS Shanghai China
Received 2013
ABSTRACT
Plumb line variations (PLV) of points at Tangshan and Midu during 1985-1998 are determined by using the 46 batch
repeated gravity observations of the Beijing-Tangshan network and 32 batch repeated gravity observations of the
West-Yunan network. As for Tangshan region, relation between underground matter change (UMC) and the PLV on
ground are discussed and approach of determining the UMC by a derived regional PLV is found. The UMC before and
after earthquake（1995.10.05, Mb 5.0）in Tangshan region, as an example, is studied, giving the location and depth of
the mass center of underground disturbing body as well as its mass quantity.
Keywords: Plumb line Variations; Earthquake; Gravimetric Network; Underground Material Changes
1. Introduction
We know now that the “variations in the deflection of the
vertical” (or plumb line variations, PLV, as follows) is a
local phenomenon, they are different in different loca-
tions [1]. It is here to discuss them in the case of northern
China.
There are only a few astrometric instruments in China,
not enough to give a complete picture of the PLVs there.
Fortunately, repeated observations of a gravimetric net-
work in some regions (Figure 1 and Figure 4) can be
used, from which the PLV within the region has been
calculated [2].
2. Plumb Line Variations (PLV) at
Tangshan and Midu and Their Relation
with Earthquakes
2.1. PLV at Tangshan and Its Relation to the
Earthquakes Around (M > 4.0)
Tangshan is a place where earthquakes happen frequently
(Figure 1). The PLV, obtained by gravimetry technique,
is shown in Figure 2, in which an annual term is also
found [3]
The PLV time series is very close to its regression
straight line, thus its component on the line is considered
as a main one with which a comparison is made with the
earthquakes around. It appears that earthquakes (M > 4.0)
usually happen near the time when the PLV begins its
opposite moving (Figure 3) [3,4].
It is clear that the PLV at Tangshan is detectable and is
related to the earthquakes ev ent around.
Figure 1. Gravity network in Northern China and the
Earthquake (M > 4) in 1987-1998.
Figure 2. PLV time series (46 points: 1987.25 - 1998.67) at
Tangshan and its annual term (left).

Y. Z. YANG ET AL. 39
2.2. PLV at Midu and its Relation to the
Earthquakes Around (M > 5.0)
There is also a gravimetry network in West Yunnan,
China (Figure 4). Similar work is done including the
PLV at Midu (Figure 5) reduced by the observations of
the network (Figure 6),
Figure 3. Relation between PLV main component at Tang-
shan and the earthquakes around.
99 99.5 100 100.5101 101.5102
24
24.5
25
25.5
26
26.5
27
Gravimetric Network at West Yunnan
Longitude (degree)
Latitude (degree)
Midu
Figure 4. Gravimetry network at West Yunnan.
Figure 5. PLV time series (32 points: 1985.33 – 1998.75) at
Midu and its annual term (left).
It is clear that the PLV at Midu is evident, as well as
its relation with the around earthquakes.
3. Relation between Plumb Line Variation
in Tangshan Region and Underground
Material Changes
There are detectable PLVs in northern China, especially
those derived before and after an Earthquake. Figure 2
shows one example, which is the PLVs related to the
earthquake M5 .0 (1995.76) [4 ] .
In order to interpret Figure 7, a simulation is done in
case of the existence of a disturbed body (point source)
underground (Figure 8), from which one may say there
1985 19861987198819891990 1991 1992 19931994 1995 1996 1997 19981999
-0.4
-0.3
-0.2
-0.1
0
0.1
0.2
0.3
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18 19
20
YEAR
PLV main component (")
Comparison between PLV main component and earthquakes
Figure 6. Relation between PLV main component at Midu
and the earthquakes ar ound.
Figure 7. PLV before (upper part) and after (lower part)
earthquake (unit: arc sec ond (as)).
Figure 8. Simulation results of the PLVs on ground while
there is an underground disturbed body (Mass: M (+) posi-
tive; M (-) negative).

Y. Z. YANG ET AL.
40
Table 1. Underground disturbing body determined related
to the earthquake (1995.10.05; M5.0).
Time period before earthquake:
1995.42 – 1995.92
Underground
disturbing body
Order of body
(W to E) NO.1 NO.2 NO.3
Longitude of mass
center (E)
Latitude of mass
center (N)
Depth of mass center
(km)
Mass quantity
(unit: 1 × 10; The
mass of Eart h = 1)
116.77
39.76
6.75
2.24
117.24
39.76
6.20
1.86
117.73
39.76
6.85
1.94
Time period after earthquake:
1995.92 – 1996.42
Underground
disturbing body
Order of body
(W to E) NO.1 NO.2 NO.3
Longitude of mass
center (E)
Latitude of mass
center (N)
Depth of mass center
(km)
Mass quantity
(unit: 1 × 10; The
mass of Eart h = 1)
116.77
39.76
6.65
3.60
117.24
39.76
6.50
3.60
117.73
39.76
7.15
3.24
are three underground disturbed bodies nearby the
earthquake, and those derived after earthquake might be
in pair, but in opposite sign, with their counter parts be-
fore earthquake.
The loc ation, depth and mass o f a body are calcula ted,
listed in Table 1 [4].
It can be conclud ed that the PLVs detected in northern
China are the observational signals of an underground
disturbed body, with which parameters of the under-
ground distu rb e d bo dy ca n be calculated.
REFERENCES
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