Qualitative interpretation of Bouguer anomaly in the southern part of ...

Geosciences Journal
Vol. 3, No. 1, p. 49-54, March 1999
Qualitative interpretation of Bouguer anomaly in the southern part
of the Korean peninsula
Kwang Sun Choi
G.V.R. Kumar
Ki Young Kim
Department of Earth Science Education, Pusan National University, Pusan 609-735, Korea
(e-mail: ksunchoi@hyowon.pusan.ac.kr)
Oil & Natural Gas Corporation Limited, Chennai 600-008, India (e-mail: svbalu@giasmda.vsnl.net.in)
Department of Geophysics, Kangwon National University, Chunchon 200-701, Korea
(e-mail: kykim@cc.kangwon.ac.kr)
ABSTRACT: Regional gravity anomaly maps of Korea have
been used for geologic information on the nature of crust and
isostasy. We made an attempt to interpret the Bouguer map
based on the nature, quality, and characteristics of anomalies
and integrate the data with the geologic and tectonic features.
Seismicity is also considered for the correlation with the
Bouguer map. It reveals that the gravity anomaly map helps
delineate major tectonic boundaries. It is observed that the
gravity high zones are associated with earthquake activity and
lineaments in their proximity.
Key words: Bouguer anomaly, tectonic boundary, seismicity
1. INTRODUCTION
Gravity methods are used for the analysis of geologic
structures and evolution of the earth's crust. Though
detailed surveys are always advantageous, most of the
objectives initially require a broad and regional approach
to establish the basic scientific framework. Regional
gravity anomaly maps are particularly useful for mapping
geographic distribution and configuration of the basement
rocks, structural and lithologic provinces, zones of crustal
weakness, mass imbalances within the lithosphere, geometrical
configuration of sedimentary basins, and the
distribution of extrusive and intrusive rocks.
In Korea, gravity data were first measured using a
pendulum in 1927 (Kumagai, 1953). Regional gravity
surveys have been carried out using modem gravimeters
since late 1950s (Woollard and Rose, 1963). An updated
Bouguer anomaly map has been compiled by Choi and
Shin (1996) using gravity measurements at more than
5000 stations in the southern part of the Korean peninsula
(Fig. 1). For gridding the data, gravity data of Fukuda (1990)
are used in the East Sea and OSU91A in the northern part
of the peninsula (Choi et al., 1993). It was concentrated in
filling the gap and merging new observations with existing
gravity data and prepared a new version. The gravity
measurements have been reduced to mean sea level using a
Bouguer density of 2.67 gm/cc. Terrain effects have been
corrected at each station for 100 x 100 knq 2 in area.
The Bouguer anomaly map (Fig. 1) with a contour
interval of 5 mGal shows several distinctive features of
gravity anomaly in the southern part of the Korean
peninsula. The major gravity lows from this map are
ascribed to Sobaeksan (S) area and Kangwon-do (K)
province, respectively. The large linear gravity trend with
a maximum value of 60 mGal in the offshore is due to
diminishing depth of crust (Choi, 1988). He also
prepared a residual anomaly map and found that negative
anomalies coincide well with the granitic and volcanic
areas, as the density of these rocks is comparatively less
than that of the Precambrian gneiss. Lee (1979) used
Bouguer anomalies to calculate the depth to the Moho
and his studies indicate that the Korean peninsula is
almost in isostatic equilibrium and the mean crustal
thickness is about 35 km. On the other hand, Kwon and
Yang (1985) indicated from their studies through linear
regression that the crust of the Korean peninsula seems
not to be in perfect equilibrium but slightly undercompensated.
Choi et al. (1993) analysed gravity data and
deciphered that the crustal thickness in and around Korea
is on the order of 16 to 41 km. Kyung (1989) studied the
relation between the topography and the seismicity of the
Korean peninsula. He also observed that the epicenters of
earthquakes are distributed dominantly along the areas of
high gradient of Bouguer and residual anomalies. Many
researchers have utilized the Bouguer anomaly map for
the nature of the crust and isostasy. Nonetheless, a
complete description of Bouguer anomaly interpretation
is not available in the previous works, in concordance
with the regional geology.
2. QUALITATIVE INTERPRETATION OF
BOUGUER ANOMALY MAP
2.1. General Features
The Bouguer anomaly map presents many closures

50 Kwang Sun Choi, G.V.R. Kumar, and Ki Young Kim
Fig. 1. Regioal Bouguer anomaly map in the southern part of Korean peninsula. Contour interval is 5 mGal. This map presents
many closures around gravity 'highs' and 'lows'. The isoanomaly lines follow a NNE-SSW trend and are in accordance with the
general geologic strike of Korea.
around gravity 'highs' and 'lows'. Some of the Bouguer
anomalies do not appear to have been caused by surface
geology. The isoanomaly lines in the map generally
follow a NNE-SSW trend and are in accordance with the
general geologic lineament of Korea.
A gravity low with a magnitude of -25 reGal and NE-
SW direction is seen near south of Taejon. This low
appears to be caused by a downwarp in the basement and
can be attributed to the crustal thickening. As the density
of granites is less than the Precambrian rocks of the
surrounding area, the huge thickness of these granites
gives rise to a low gravity anomaly. This explanation
corroborates with the geological explanation of Sobaeksan
massif in this area. The southeastern part of this low
is faulted as the contours show a steep gradient with its
downthrown side southeastwards. Northwestern and western
sides of this low are represented by disturbed pattern of
anomaly contours. There might be a possibility for extrusive/
intrusive activity in these areas causing localized density
variations.
A gravity low with a magnitude of -20 reGal with its
strike (almost NE-SW) is noticed near southeast of

Interpretation of Bouguer anomaly in Korea 51
Chunchon. This low is most likely due to the granitic
intrusion in the Precambrian basement. Although the
gravity anomalies in the above two major gravity lows
show a little resemblance in their magnitude and strike
direction, it is not possible to make any genetic relation
between these gravity lows because they could have been
formed at different geologic times.
In the southeastern part, in the proximity of Taegu,
pairs of gravity lows and highs are observed. Although
these are of low-relief, the highs can be attributed to
volcanic or sedimentary rocks of low porosity and the
lows to granites or Tertiary sediments. The average
values of density for low-porosity sedimentary rocks,
basalts, andesites, granites, and Tertiary sediments are
2.60, 2.90, 2.61, 2.55, and 2.03, respectively (Min and
Chung, 1985).
Around Seoul, a very varied nature of gravity anomaly
pattern having small and large amplitudes suggests that
the area comprises metamorphic terrain with rocks of
different densities.
A broad zone along the east coast with seaward
gradient close to 10 mGal/km trends N-S upto latitude
36~ and then NW-SE which reflects the offshore
transition of continental crust. The gradual increase in
magnitude of Bouguer anomalies indicates a gradual
decrease in thickness of the crust. The attitude of this
flock of contours trending N-S direction and having
increased values towards the coast also suggests that
there exists a fault along the eastern side of the east
coast.
There are many locations in the southeast, northeast,
and south-central parts of Korea where dislocations of
the narrow belts of high gradients are observed. These
dislocations can be interpreted as originating from some
major or minor strike-slip faults with horizontal movements.
Some of the prominent faults including Yongduri
(F1), Andong (F3), and Yangsan (F4) faults have been
marked with the help of typical nature of the gravity
contours that associate with the faulting (Fig. 2). The
fault F2 appears as an assumed one on the geologic map
of 1:1,000,000 scale (Korea Institute of Geology, Mining
and Materials, 1995), whereas F3 has no surface indication.
2.2. Zoning
After qualitatively analysing the Bouguer anomaly map
in terms of gravity lows, highs, and linear trends for
possible explanation of subsurface causatives, zoning is
introduced on the basis of nature, pattern and characteristics
of gravity anomalies to describe various tectonic
boundaries (Fig. 2). Seven zones have been clearly
identified. The description of each zone is provided
hereunder.
2.2.1. Kyonggi massif zone (Z1)
This zone is characterized by irregular pattern of
contours, relative lows and highs with small relief,
trending NNE-SSW. Precambrian rocks associated with
granites as intrusives and extrusives at some places are
the geological manifestations of the gravity signatures.
The zone may contain small and complex minor fold
structures at places because of the intrusive activity.
2.2.2. Nonmetamorphic zone (Z2)
This zone is characterized by long wavelength anomalies,
low ampfitude but large relief and trends NNE-SSW.
Jurassic granites in this area give rise to this type of
anomaly deportment.
2.2.3. Okchon belt zone (Z3)
This zone shows narrow anomalies of moderate amplitude.
It comprises mainly metamorphic rocks and trends
NNE-SSW.
2.2.4. Taebacksan zone (Z4)
This zone is characterized by a low gravity anomaly of
the order of -20 mGal. This low trends NE-SW with
small lows and highs. The Triassic granites with low
density are most likely responsible for the low values.
2.2.5. Sobaeksan zone (Z5)
This zone is characterized by large wavelength, very
low amplitude with high relief and trends NNE-SSW.
Massive granitic intrusion attests to the gravity signatures.
2.2.6. Kyongsang Basin (Z6)
This zone is classified as many separated lows and
highs with small magnitude and no particular trend. These
lows are explained due to low-density Bulguksa granites
and the highs to sedimentary rocks of the Cretaceous.
2.2.7. Yongdong-Kwangju depression zone (Z7)
This zone is of high magnitude with a large range and
no particular trend. Cretaceous continental sediments and
volcanic rocks are responsible for the gravity anomalies.
3. SEISMICITY
Earthquakes in Korea are classified as intraplate
seismicity (Lee and Na, 1983). Figure 3 shows epicenters
of both historical and instrumental earthquakes having
intensity equal to or more than V, which occurred in
Korea from A.D. 2 to 1987. It is apparent that higher
seismicity is associated with many of the faults and
tectonic boundaries. Most seismic activities appear to be
associated with the gravity-high zones and are confined
to the tectonic boundaries. Evidently, the gravity lows and
highs are forming pairs at many places and are separated by

52 Kwang Sun Choi, G.V.R. Kumar, and Ki Young Kim
Fig. 2. Zoning map of Bouguer anomaly. Seven tectonic boundaries and five faults are indicated with solid and dashed lines,
respectively. Zones 1 through 7 are Kyonggi massif (Z1), Nonmetamorphic zone (Z2), Okchon belt zone (Z3), Taebaeksan zone
(Z4), Sobaeksan zone (Z5), Kyongsang Basin (Z6), Yongdong-Kwangju depression zone (Z7), respectively.
faults (Fig. 2). The large size and areal extent of these
anomalies can reasonably be explained by these contacts
extending deep into the crust.
Had the loci of the available earthquake data been
plotted on the crustal thickness map, it would have been
clear whether these contact zones extend into the Moho.
Moreover, the focal mechanism solutions for the available
earthquake data provide the type of fault mechanism
existing in and around the contact zones.
Nonetheless, it is apparent that the tectonic boundaries in
Korea are associated with fault lineaments and these
fault zones mark the gravity-high and -low axes. It is
worth mentioning here that Kyung (1993) has examined
a close correlation between the seismicity and the
Bouguer gradient and concluded that deviatoric stresses
may be concentrated in such high gradient areas and
earthquakes may occur more easily in those areas than
others.

Interpretation of Bouguer anomaly in Korea 53
Fig. 3. Epicenter zoning map in the southern part of Korean peninsula. Epicenter zones I through IV trend NNE-SSW and coincide
with the general geological lineament, whereas Zone V trends N-S. Only epicenters having an MMI V or greater are marked
with empty circles (modified from Lee and Jin, 1989). The size of circle increases with the intensity scale.
4. EPICENTER ZONING MAP (EZM)
The seismicity map of Korea helps delineate epicenter
zones that are constructed solely on the basis of density
of epicenters (Fig. 3). It has been considered that at least
one epicenter having a modified Mercalli intensity (MMI)
V or greater should fall into the zone which is associated
either with gravity high or a tectonic lineament. It is
called 'epicenter zoning map' (EZM) and no resemblance
to seismic zoning map in which isoseismicity is plotted.
Lee and Jin (1989) evaluated seven seismic source zones
in Korea based on the correlation between seismicity and
geotectonic features. Lee (1998) also delineated seven
seismic source zones using digital elevation method.
However, the methodology adapted by them is different
from that of EZM. The EZM helps delineate areas that
are seismically active and deserve fimher studies. The EZM
of Korea shows five epicenter zones. Zones I through 1V
show a NNE-SSW trend and coincide with the general
geological strike of Korea, whereas Zone V shows a

54 Kwang Sun Choi, G.V.R. Kumar, and Ki Young Kim
N-S trend. Although the intensities are moderate and are
around MMI V, a considerable number of earthquakes
are marked in these zones.
5. DISCUSSION
Based on seismological studies Lee and Na (1983)
opined that the fault systems in the Kyongsang Basin are
seismically active, especially the Yangsan Fault where
neotectonic activity is observed. They also suggest that
seismicity is more obscurely related to Paleozoic and
Mesozoic faults because geochemical processes in the
crust generally augment the strength of rocks in the
fault area. Contrary to this observation, a seismicity map
shows large concentration of epicenters in the areas
occupied by Paleozoic and Mesozoic rocks. On the other
hand, our observations reveal that these high seismicity
areas are associated with gravity-high anomalies. These
gravity-high anomalies can be explained by crustal thinning
or the upper mantle density anomalies.
The large concentration of epicenters in Zones I
through IV suggests that mechanical strains arise from
displacement of separate blocks of lithosphere and the
strain is building up because of friction. As the volume of
combined forces exceeds the strength of friction, the
strain is released in the form of earthquakes and may
cause development of new faults within the crust if these
earthquakes have deep foci. This is true in Zone V, as the
trend of the zone is discordant with the general strike of
the area.
For these reasons, it is desirable (1) to acquire detailed
gravity data for an authentic crustal depth map, (2)
to establish seismograph stations in the vicinity of
seismically active areas, and (3) to carry out seismic
refraction studies across the major lineaments to study the
nature of crust.
6. CONCLUSIONS
The regional Bouguer anomaly map of Korea is useful
for delineating various tectonic features. The major
tectonic elements deciphered from this map corroborate
with the geological observations. Seven zones are identified
based on characteristic features of gravity anomaly
and their boundaries are associated with faults. The data
on earthquake epicenter delineate most of these tectonic
boundaries and gravity-high zones. It is important to
acquire geophysical data across the faults in the vicinity
of major tectonic lineaments.
ACKNOWLEDGMENTS: This work was partly supported by
the Institute of Research and Development for Energy Resources at
Kangwon National Univeristy. The second author wishes to place
on record his gratitude to KOSEF, for award of a Postdoctoral
Research Fellowship and to Oil & Natural Gas Corporation
Limited (ONGC), India for granting him leave. The authors are
grateful to the editor, S.K. Chough, for his helpful comments on
the manuscript and Prof. Czango Baag, Prof. Yeonghwa Kim, and
Dr. Jun Hee Lee at Kangwon National University for proffering
useful and instigating discussions. Messrs. H.G. Kim and N.C.
Woo deserve special appreciation for rendering help at various
stages of this work.
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Manuscript received May 4, 1998
Manuscript accepted February 11, 1999