Plumb Line Variations (PLV) at China and Their Relation with Earthquakes and Underground Materials Changes

doi:10.4236/ojg.2013.32B009

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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

Email: [email protected]

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

24.5

25.5

26.5

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

0.1

0.2

0.3

18 19

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.

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

[1] Z. X. Li, H. Li，Y. F. Li and Y. B. Han, “Non-tidal

Variations in the Deflection of the Vertical at Beijing

Observatory,” Journal of Geodesy, Vol. 78, No. 10, 2005,

pp. 588-593.doi:10.1007/s00190-004-0421-2

[2] H. Li, G. Y. Fu and Z. X. Li, “Plumb Line Deflection

Varied with Time Obtained by Repeated Gravimetry,”

Acta Seimologica, 2001, pp. 66-71.

[3] Z. X. Li and H. Li, “Earthquake-Related Gravity Field

Changes at Beijing-Tangshan Gravimetric Network,”

Studia Geophysica et Geodaetica, Vol. 53, 2009, pp.

185-197. doi:10.1007/s11200-009-0012-z

[4] Z. X Li and H. Li, “Relation between Plumb Line Varia-

tion in Tangshan Region and Underground Material

Changes. Acta Seimologica,” Vol. 33, No. 6, 2011, pp.

817-827.