Study on structure of the Earth’s crust in Thua Thien-Hue province and adjacent areas by using gravity and magnetic data in combination

517 
Vietnam Journal of Marine Science and Technology; Vol. 19, No. 4; 2019: 517–526 
DOI: https://doi.org/10.15625/1859-3097/19/4/14903 
Study on structure of the Earth’s crust in Thua Thien-Hue province and 
adjacent areas by using gravity and magnetic data in combination 
Pham Nam Hung
*
, Cao Dinh Trong, Le Van Dung, Thai Anh Tuan, Mai Xuan Bach, 
Nguyen Anh Duong 
Institute of Geophysics, VAST, Vietnam 
*
E-mail: pnhungigp@yahoo.com 
Received: 29 September 2019; Accepted: 15 December 2019 
©2019 Vietnam Academy of Science and Technology (VAST) 
Abstract 
This paper presents the structural characteristics of the Earth’s crust in Thua Thien-Hue province and 
adjacent area based on interpretation of gravity and magnetic data in combination. Research results have 
shown that: The depth of crystalline basement varies complicatedly, in the range of 0–11 km. The depth of 
Conrad surface increases from Northeast (12 km) to Southwest (18 km) and the depth of Moho surface is 
23–34 km; The density of sedimentary layer changes from 2.61 g/cm3 to 2.65 g/cm3. Meanwhile, the density 
of granitic layer is in the range of 2.68–2.73 g/cm3. The basaltic layer has the density value of 2.88–2.93 
g/cm
3 and the average density of lower layer of the Earth’s crust is about 3.30 g/cm3; The depth of second-
order faults, Red River and A Luoi - Rao Quan, is through the Earth’s crust. Meanwhile, the depth of 
influence of third-order faults, Chay river, Dong Ha - Phu Vang, Vinh Linh, Hue - Son Tra and Tam Ky - 
Phuoc Son, is within the thickness of the Earth’s crust. 
Keywords: Gravity, magnetic, fault, the Earth’s crust, Thua Thien-Hue. 
Citation: Pham Nam Hung, Cao Dinh Trong, Le Van Dung, Thai Anh Tuan, Mai Xuan Bach, Nguyen Anh Duong, 
2019. Study on structure of the Earth’s crust in Thua Thien-Hue province and adjacent areas by using gravity and 
magnetic data in combination. Vietnam Journal of Marine Science and Technology, 19(4), 517–526. 
Pham Nam Hung et al. 
518 
INTRODUCTION 
The structural characteristics of the Earth’s 
crust in the whole territory of Vietnam at 
1/1,000,000 scale was established by Cao Dinh 
Trieu, 2005 [1] on the basis of analysis of 
obtained gravity data (with reference to 
aeromagnetic and satellite magnetic data). At a 
scale of 1/500,000, Lai Hop Phong (2009) [2] 
studied the structural characteristics of the 
Earth’s crust in Northern Vietnam on the basis 
of analysis of ground Bouguer gravity data 
combined with two obtained deep seismic lines. 
The researches at 1/200,000 scale are mainly 
carried out by Pham Nam Hung (2013) in Ca 
river - Rao Nay [3] and Phan Thanh Quang in 
Da river hydroelectric ladder region [4]. 
Thus, the structural characteristics of the 
Earth’s crust in the mainland of Vietnam and 
coastal areas were studied at a small scale 
(1/1,000,000). The researches at the scales of 
1/500,000 and greater have not been adequately 
conducted. Meanwhile, the previous studies 
have not fully met the requirements of seismic 
zoning in the whole territory of Vietnam at 
1/500,000 scale and in separate regions at 
1/200,000 scale. Therefore, the state-level 
independent project (code: DTDL.CN.51/16) 
has set up the task of studying the structural 
characteristics of the Earth’s crust (deep fault 
system and crystalline basement surfaces, 
Conrad surface and Moho surface) in Thua 
Thien-Hue at 1/250,000 scale. This paper 
presents the major results of the project: 
“Assessment of earthquake hazard for 
territorial development planning, and ensuring 
the safety of hydropower plants, irrigation 
works and cultural relics in Thua Thien-Hue”. 
The research process is conducted 
according to the following steps: 
Analyze and transform gravity and 
magnetic data according to lines and planes for 
fault identification, establish the model of 
direct problem for solving the inverse gravity 
problem and studying the structural 
characteristics of the faults. 
Establish 11 sections along 11 lines by 
using the method of solving 2.5D inverse 
gravity problem (fig. 1). 
Fig. 1a. Bouguer gravity anomaly and lines of analyzing of gravity data 
in Thua Thien-Hue area at the scale of 1:250,000 
Study on structure of the Earth’s crust 
519 
Fig. 1b. Aeromagnetic anomaly in Thua Thien-Hue area at the scale of 1:250,000 
Establish the structural diagrams of basic 
boundary surfaces of the Earth’s crust 
(crystalline basement, Conrad and Moho 
surfaces) based on the linear research results 
and the establishment of multi-dimensional 
linear correlation functions. 
Establish the diagram of faults in the 
study area based on gravity and magnetic data. 
The gravity and magnetic data used in this 
study include Bouguer gravity anomaly [5, 6] 
and satellite gravity anomaly [7], 
aeromagnetic anomaly [8] and satellite 
magnetic anomaly [9]. 
METHODOLOGY 
Method of analyzing gravity and magnetic 
data for fault determination 
The faults in Thua Thien-Hue province and 
adjacent areas are determined by using gravity 
and magnetic data according to the following 
indications and criteria [1, 3, 6, 10, 11]: 
1. The indications of fault based on 
gravity and magnetic anomaly and anomaly 
field transformation include: 
Faults associated with extended gradient 
field. 
The boundary between two areas with 
different anomaly structure directions, or the 
intersection of anomalies with different 
structure directions. 
The movement of extended linear 
anomalies and the appearance of flexures of 
contour line. 
These are the initial indications according 
to primary identification method. 
2. Using the calculation results of 
theoretical model: In case of three-dimensional 
and two-dimensional problems when applying 
methods of upward continuation, downward 
continuation, calculations of gradient at 
different upward continuation heights, 
horizontal gradient, vertical gradie ... e 2 
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Fig. 2. Geological-geophysical sections (along 11 lines) in the study area 
Study on structure of the Earth’s crust 
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Fig. 2. Geological-geophysical sections (along 11 lines) in the study area (next) 
4. Indications based on the calculation 
results of linear normalized full vertical gradient 
are particular points when calculating the field 
reduction through geological object. Particular 
points often appear in the object or coincide with 
the outer edges of geological object causing 
anomaly in the predetermined shape. 
Research methodology of structural 
characteristics of the Earth’s crust 
The process of analyzing Bouguer gravity 
data along the study lines is conducted as 
follows [1, 3, 6, 10, 11–13]: 
1. Calculate the section of horizontal 
gravity gradient; 
2. The section of normalized full gradient; 
3. The section of structure/density 
coefficient based on the problem of horizontal 
cylinder model. 
The results of analysis of above sections 
allow determining the location as well as the 
angle of fall and dip direction of the fault. 
Based on these analysis results, we can also 
build an initial model for solving the 2.5D 
gravity problem. 
Pham Nam Hung et al. 
522 
4. Solve the 2.5D inverse gravity problem 
(using GM-SYS software from Geosoft) for 
studying the structural characteristics of the 
Earth’s crust along the study lines. 
The results of analysis of 11 gravity lines 
are shown in fig. 2, providing a primary picture 
of density structure of the Earth’s crust in the 
study area. 
STRUCTURAL CHARACTERISTICS OF 
THE EARTH’S CRUST IN THE STUDY 
AREA 
In the study area, there are very few 
published data on the depth to basic boundary 
surfaces of the Earth’s crust, including deep 
seismic data and magnetotelluric data. 
Therefore, we use the gravity lines analyzed in 
fig. 2 as a basis to build multi-dimensional 
linear correlation functions, with the aim of 
determining the depth of basic boundary 
surfaces of the Earth’s crust in the study area. 
The calculation results have determined the 
correlation functions between the depth of 
surfaces and the value of transformed gravity 
fields as follows: 
The depth of Moho surface in the study 
area is established on the basis of multiple 
regression equation: 
( ) 28.42 0.13 0.02MH B DH km g g (1) 
Correlation coefficient: RMH = 0.83 
Where: Dg is the value of residual gravity 
anomaly at the upward continuation levels of 
10 km and 20 km. 
The depth of Conrad surface is determined 
by multiple regression equation as follows: 
( ) 15.63 0.03 0.05CR B DH km g g 
(2) 
Correlation coefficient RCR = 0.82 
Where: Dg is the value of residual gravity 
anomaly at the upward continuation levels of 5 
km and 10 km. 
The depth to the crystalline basement 
surface is determined by regression equation as 
follows: 
( ) 2.02 0.03 0.05KT B DH km g g (3) 
Correlation coefficient RKT = 0.84 
Where: 
Dg is the value of residual gravity 
anomaly at the upward continuation levels of 2 
km and 5 km. 
Structure of crystalline basement 
The structural morphology of crystalline 
basement surface in the study area is presented 
in fig. 3. The result shows that: 
The distribution of crystalline basement 
surface is relatively complicated, increasing 
gradually towards the Southwest and Northeast 
directions. The isometric uplift structures (with 
crystalline basement exposed on the surface) 
are commonly distributed in the study area. 
The maximum depth of crystalline 
basement surface in the mainland does not 
exceed 5 km. Meanwhile, the depth of this 
boundary surface can reach 2.0–3.5 km in the 
territory of Laos and 11 km in the Red river 
basin. 
The difference in density between the 
sedimentary layer above (2.61–2.65 g/cm3) and 
the granitic layer below (2.68–2.73 g/cm3) is 
about 0.07 g/cm
3
. 
Structure of Conrad surface 
The depth to the Conrad surface ranges 
between 12–18 km, increasing gradually from 
Northeast to Southwest. Conrad surface is 
uplifted highest at about 12 km in Nong Son-
Quang Nam zone, about 16 km along the 
coast and subsided along the Vietnam - Laos 
border. The difference in density between the 
granitic layer above (2.68–2.73 g/cm3) and 
the basaltic layer below (2.89–2.93 g/cm3) is 
about 0.09 g/cm
3
. 
Structure of lower boundary surface of the 
Earth’s crust (Moho surface) 
The depth of the Moho surface varies from 
23 km to 34.5 km, increasing gradually from 
Northeast to Southwest. In the mainland of 
Vietnam, the depth of Moho surface changes 
from 27 km along the coast to 31–34 km along 
the Vietnam - Laos border. The difference in 
density between the basaltic layer above (2.89–
2.93 g/cm
3
) and the mantle layer below (3.30 
g/cm
3
) is about 0.42 g/cm
3
. 
Study on structure of the Earth’s crust 
523 
Fig. 3. Crystalline basement surface depth (km) in Thua Thien-Hue province 
and adjacent area at the scale of 1:250,000 
Figure 4. Conrad surface depth (km) in Thua Thien-Hue province 
and adjacent area at the scale of 1:250,000 
Pham Nam Hung et al. 
524 
Deep fault system 
The deep faults in the study area are 
presented in the fig. 6 and table 2, indicating 
that: 
There exist 10 deep faults with the depth 
of influence of 15 km or greater (within and 
through the Earth’s crust): 
There are 2 regional second-order faults 
(local first-order) with depth through the 
Earth’s crust (25–30 km and greater, depending 
on the depth of Moho surface): Red river and A 
Luoi - Rao Quan; 
There are 8 regional third-order faults 
(local second-order) with depth of influence 
within the Earth’s crust, to Conrad surface (15–
20 km and greater, depending on the depth of 
Conrad surface): Chay river, Dong Ha - Phu 
Vang, Vinh Linh, Hue - Son Tra and Tam Ky - 
Phuoc Son. 
The major development direction of the 
faults is Northwest - Southeast (Red river, A 
Luoi - Rao Quan, Chay river, Dong Ha - Phu 
Vang, Vinh Linh, Hue - Son Tra and Tam Ky - 
Phuoc Son). Some faults follow the sub-
latitudinal direction (SePon - Cam Lo, Cu De 
river and Vu Gia river) and the sub-longitudinal 
direction. 
The faults are mainly normal faults with 
nearly vertical dip angle (60–75o). 
Fig. 5. Moho surface depth (km) in Thua Thien-Hue province 
and adjacent area at the scale of 1:250,000 
Study on structure of the Earth’s crust 
525 
Fig. 6. Tectonic faults in Thue Thien-Hue province and adjacent area at the scale of 1:250,000. 
Legend of fig. 6: (1) Red river fault; (2) A Luoi - Rao Quan fault; (3) Chay river fault; (4) Dong 
Ha - Phu Vang fault; (5) Vinh Linh fault; (6) SePon - Cam Lo fault; (7) Hue - Son Tra fault; (8) 
Cu De river fault; (9) Tam Ky - Phuoc Son fault; (10) Vu Gia fault 
Table 2. Fault parameters according to geophysical data 
No. Fault name Rank of fault Strike direction 
Dip direction, 
Dip angle (degree) 
Maximum depth (km) 
1 Red River II NW-SE NE 
60 80
76
 25–30 
2 A Luoi - Rao Quan II NW-SE NE 
61 82
70
 30–35 
3 Chay River III NW-SE NE 
67 83
76
 15–20 
4 Dong Ha - Phu Vang III NW-SE NE 
59 80
71
 15–20 
5 Vinh Linh III NW-SE S 
62 74
66
 15–20 
6 SePon - Cam Lo III Sub-latitudinal S 
65 78
72
 15–20 
7 Hue - Son Tra III NW-SE NE 
62 82
72
 15–20 
8 Cu De River III Latitudinal S 
60 78
71
 15–20 
9 Vu Gia River III Latitudinal S 
72 76
74
 15–20 
10 Tam Ky - Phuoc Son III NE-SW NE 
64 76
71
 15–20 
Pham Nam Hung et al. 
526 
CONCLUSION 
Based on the data used, the research 
methodology and the results of structural 
characteristics of basic boundary surfaces of 
the Earth’s crust, the paper can draw some 
conclusions as follows: 
1. The crystalline surface in the study 
area varies quite complicatedly, from exposed 
on the ground to the maximum depth of over 
11 km. The depth of Conrad surface changes 
from 12 km to 18 km, gradually increasing 
towards the inland area and decreasing 
towards the sea. Meanwhile, the depth of 
Moho surface has the greatest value of about 
34 km in the mainland and only reaches 23–24 
km in the Red river basin. 
2. The density of sedimentary layer 
changes from 2.61 g/cm
3
 to 2.65 g/cm
3
. 
Meanwhile, the density of granitic layer ranges 
between 2.68 g/cm
3 
and 2.73 g/cm
3
. The 
basaltic layer has the density value of 2.88–
2.93 g/cm
3 
and the average density of lower 
layer of the Earth’s crust is 3.30 g/cm3. 
3. There are 10 deep faults, including 2 
regional second-order faults (local first-order) 
with depth through the Earth’s crust (Red river 
and A Luoi - Rao Quan) and 8 regional third-
order faults (local second-order) with depth of 
influence within the Earth’s crust (Chay river, 
Dong Ha - Phu Vang, Vinh Linh, Hue - Son 
Tra and Tam Ky - Phuoc Son). The faults are 
mainly normal faults and developing in the 
Northwest - Southeast direction with nearly 
vertical dip angle (60–75o). 
Acknowledgment: We thank two anonymous 
reviewers for helpful comments. The article has 
been completed with the support of the 
National-level Independent Project 
DTDL.CN.51/16. 
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