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Earth and Planetary Science

Seismic structure of the Bohai Bay Basin, northern China:

Implications for basin evolution

Liang ZhaoT, Tianyu Zheng1

Institute of Geology and Geophysics, Chinese Academy of Sciences, Qijiahuozi, Deshengmenwai, Chaoyang District, P.O. Box 9825,

Beijing 100029, PR China

Received 1 July 2004; received in revised form 28 November 2004; accepted 20 December 2004

Available online 1 February 2005

Editor: R.D. van der Hilst

Abstract

As part of an extensive seismic research program, 33 portable broadband seismic stations were deployed along a line

crossing the Bohai Bay Basin, northern China. Three teleseismic events were selected to constrain the seismic structure along a

~280 km profile across the western edge of the basin. We determined the basin structure that described the observed shear

horizontal (SH) wave field. The synthetic SH wave was calculated using a finite difference (FD) method with its computational

domain localized in the basin area and input motions at the base of the model extrapolated from the displacement recorded at a

nearby hard-rock station. Synthetic seismographs calculated for the models match the observations well in both waveform and

travel time. Numerical tests indicate that the structural features of the preferred models are well resolved. The analysis of

relations between structures and stratigraphic units along the cross sections allows multiple deformational events in the basin to

be inferred. In conjunction with a profile across the southern edge that has been presented previously by Zhao et al. [14] [L.

Zhao, T.Y. Zheng, W.W. Xu, Modeling the Jiyang depression, Northern China, using a wave field extrapolation FD method and

waveform inversion, Bull. Seismol. Soc. Am. 94 (2004) 988–1001], the results reveal basin-wide extension with local inversion

features in the Bohai Bay basin.

D 2005 Elsevier B.V. All rights reserved.

Keywords: Cenozoic; SH wave modeling; basin seismology; sedimentary basin structure

0012-821X/$ - see front matter D 2005 Elsevier B.V. All rights reserved.

doi:10.1016/j.epsl.2004.12.028

T Corresponding author. Tel.: +86 10 62007354; fax: 86 10

62010846.

E-mail addresses: [email protected] (L. Zhao)8

[email protected] (T. Zheng).1 Tel.: +86 10 62363458; fax: +86 10 62010846.

1. Introduction

The Bohai Bay basin, located in the North China

block [1,2], is a large oil-producing basin that under-

went compound and complex tectonic events during

its formation and development. In the last two

decades, a number of papers (e.g., [3–9]) have been

published on the evolution of the basin.

Letters 231 (2005) 9–22

Fig. 1. (a) Location map of the studied area. The lines indicate three cross sections. AAV is modeled in this paper; BBV has been modeled by

Zhao et al. [14]; and RRV is a geological cross section modified from Ye et al. [5]. (b) Location map of stations (solid triangles) for cross section

AAV. The stars on the inset represent locations of seismic events, and the solid lines represent great circle paths.

L. Zhao, T. Zheng / Earth and Planetary Science Letters 231 (2005) 9–2210

Table 1

Event list

Event Origin, UT Latitude Longitude Depth

(km)

Mw

020303 3 Mar. 2002, 12:08:19 70.488 36.508 225 7.4

020325 25 Mar. 2002, 14:56:33 69.328 36.068 8 6.2

020412 12 Apr. 2002, 04:00:23 69.428 35.968 10 5.9

Epicentral location is given in degrees. Event data is from the USGS

on web.

L. Zhao, T. Zheng / Earth and Planetary Science Letters 231 (2005) 9–22 11

The basic knowledge of the Bohai Bay basin is

mainly from surface geological investigation and

seismic exploration (e.g., [7]). During late Permian

time, the North China block and the South China

block collided. After the collision, Eastern China

experienced widespread tectono-thermal reactivation

during the Late Mesozoic and Cenozoic, as indi-

cated by emplacement of voluminous late Mesozoic

granites and extensive Cenozoic volcanism [10].

The rifting of the eastern China took place between

late Cretaceous and Oligocene [3,11]. As a result,

many basins, including the Bohai Bay basin, formed

across a vast area of eastern China. It is generally

accepted that the most important tectonic control on

extension was probably the subduction of the

Pacific plate in the eastern margin of Asia (e.g.,

[12,13]).

Some basic images of the Bohai Bay basin remain

ambiguous, including the tectonic character of the

basin-mountains edge and the deep faults cutting

through the basement of the basin, which are of great

importance in understanding the evolution of the

basin. Therefore, further investigation is necessary to

image the basin edge structure and the basement of the

basin.

An effective approach to image the basin edge

structure and the basement of the basin is to use the

seismic wave propagating through the basin. With

this target in mind for the last 3 yr, we participated

in the Northern China Interior Structure Project

(NCISP). This project deployed dense broadband

portable seismograph arrays across the Bohai Bay

basin and the surrounding uplifts. Some of the

objectives of this project are: (1) to improve our

understanding of the seismic and geological struc-

tures of the Bohai Bay basin, and regional tectonics;

(2) to study deep seismic structures beneath the

North China Plate.

In this paper we describe the seismic structure

along a new cross section crossing the western edge of

the basin obtained by waveform inversion of the

seismic data using a hybrid method that consists of

shear horizontal (SH) wave field extrapolation, finite

difference (FD) calculation and travel time and

waveform inversion [14]. The formation of the basin

was then interpreted based on analysis of relations

between structures and stratigraphic units inferred

from our preferred seismic model.

2. Seismic structure of cross section AAV

2.1. Seismic dataset

We model the basin edge structures from a cross

section AAV (Fig. 1a). This cross section orients

NNW–SSE direction and transects the western edge

of the Bohai Bay Basin. For section AAV (See Fig.

1b), two types of portable broadband seismograph

units were deployed in an approximately 280 km

long transecting line with an average station spacing

of ~10 km. Seismic data recorded at 29 stations were

selected for the modeling. 18 of these 29 stations

were deployed with REFTEK data loggers and

Guralp CMG-3ESP sensors (50 Hz–30 s). The other

instruments were CAS DS24-3 data loggers and

BKD-2 broadband sensors (40 Hz–20 s). We name

the stations differently according to their recording

sensors and data logger. Stations with the Guralp

system are named as station number+abbreviated

address, while those with the CAS system are named

as station number. Data were recorded at a rate of 40

samples per second for all the stations. Seismic data

from three seismic events were selected for modeling

the seismic structure along section AAV (shown in

the inset of Fig. 1b). All observations are almost in

an identical azimuth along the profile AAV. The

event magnitudes (Mw) are all z5.5. Table 1 lists the

event parameters. Event locations and origin times

are taken from the USGS catalog on the web (http://

www.neic.usgs.gov). Zero-phase band-pass filters are

applied with a typical band-pass frequency range of

0.05–4.00 Hz.

2.2. Methodology

We employ the technique introduced by Zhao et al.

[14] to model the shear wave velocity structure of the

http://www.neic.usgs.gov

Fig. 3. Observed tangential displacements for event 020325

Stations from 181DC to 187NT were deployed at hard-rock sites

outside the basin, while stations from 154XW to 180DM were

inside the basin. Note that the waveforms recorded at the hard-rock

sites are very similar, while those recorded inside the basin are

noticeably different.


western edge of the basin. Fig. 2 schematically

illustrates the principle of this method. The method

consists of forward synthetic FD calculations, wave-

form and travel time inversions [15]. We briefly

review the method here. Readers are referred to Zhao

et al. [14] and Wen [15] for details of the method.

For the in-plane propagation of the teleseismic SH

wave, we assume that the bbasin site responseQ is themost significant cause of the waveform difference

between stations outside and inside the basin, while

the path effects from the earthquake source to the base

of the basin are similar. The validity of this

assumption can be verified with the observations.

Fig. 3 shows that the tangential displacements

recorded at hard-rock stations (182DL to 187NT)

are in excellent agreement with each other, whereas

those recorded inside the basin (station from 154XW

to 181DC) are evidently different. Therefore, it is

reasonable to extrapolate the displacement recorded at

a nearby hard-rock station to the bottom of the FD

region as input motions of the FD calculation (e.g.,

P0, P1, P2 in Fig. 2). In the FD calculation, the basin

model vector is velocity as a function of position

m(x)=m(x, z), and a grid size of 0.15 km is adopted to

discretize the model.

Fig. 2. Schematic illustration of the principle of the SH wave field extrapolation FD method [14]. The basin is confined inside a rectangle, where

the FD method is applied. When a hard-rock site station P is sufficiently far away from the source, the SH-wave recorded at P can be

extrapolated to the bottom interface of the FD region (e.g., P0, P1, P2) as input motions in the FD calculations. The solid circles represent the

control depth points of the basin model, and the dashed lines schematically indicate FD grid lines.

.

Table 2

Average velocity assigned for the stratified sediments and the

basement rocks [14]

Strata Vp

(km/s)

Vs

(km/s

Quaternary (Q) 1.32 0.60

Neogene (N) 2.20 1.25

Paleogene (E) 2.90 1.70

Pre-Tertiary (P-T) 3.80 2.20

Basement rocks 5.50 3.20


A waveform and travel time inversion method is

applied to invert the basin structure (e.g., Ji et al.

[16]). The waveform misfit is defined as square norm

of the difference between the synthetic and recorded

seismograms

f sð Þ ¼ � 12Rp tð Þobs p t þ sð ÞsyndtR

p tð Þ2obs þ p t þ sð Þ2synh i

dt; ð1Þ

where p(t)syn and p(t)obs are the synthetic and

recorded seismograms, respectively, and s is the time

shift between the two.

The misfit function between observation and

synthetic is represent as [17]

E m xð Þð Þ ¼ Wt

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi1

N � 1

XNi¼2

dsi � dslð Þ2vuut

þWf

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi1

N

XNi¼1

f 2i dsiÞ;ð

vuut ð2Þ

where N is the number of stations, dsl is the retardedtime between synthetic and record at the origin point

of FD region, Wt and Wf are weights for travel time

residual and waveform fit, respectively. Based on our

data quality, we assign Wt as 0.3 and Wf as 0.7.

In the inversion, the basin models are parameter-

ized as isovelocity layers with linearly dipping seg-

ments represented with control depth points (marked

as solid circles in Fig. 2). Considering the spacing of

our stations and a reasonable inversion calculation

Fig. 4. The initial model constructed from approximate teleseismic travel time residuals with respect to IASP91 for event 020325. The SH trave

time residuals were represented by solid circles plotted at the locations of corresponding receivers, with their diameters proportioning to the

amplitude of the time residuals. The scales of the time residual are plotted at the bottom.

)

time, we adopt a horizontal spacing of 5 km between

control depth points in the initial inversion calcu-

lation. Then, we employ the conjugate gradient

algorithm [18] to search for the geometry of the strata

interfaces that minimizes the misfit defined in Eq. (2).

In the case that higher resolution is needed to

constrain some local structures, we add more control

points or adjust the spacing between control points to

represent the local structure. Since we adopt a grid

size of 0.15 km to discretize the model, the minimal

space between control depths could be reduced to

0.15 km if necessary.

2.3. Starting model

Following Zhao et al. [14], we divided our basin

model into four major isovelocity layers, correspond-

ing to the Quaternary, Neogene, Paleogene and Pre-

Tertiary strata, respectively. The average shear veloc-

ities (Table 2) of the layers are obtained from the

compressive wave velocities and the Vp/Vs ratios of the

l

Fig. 5. Best-fitting models for cross section AAV obtained based on waveform inversion of teleseismic data (top panels), and comparisons of

synthetic displacements (dashed traces) and seismic data (solid traces) (bottom panels) for three events: (a) event 020203, (b) event 020325 and

(c) event 020412. The best-fitting model for event 020203 has a shorter length than the other two, since we have only good records from station

181DC to 165JD for event 020203. Waveform is normalized by its maximum amplitude. The black solid circles indicate the depth control points

in describing the basin structure. Black solid triangles represent the stations used in the modeling, with the numbers above indicating the index

of stations. The input motions to the FD calculations are extrapolated based on the seismic data observed at a hard-rock site station 182DL

(located at distance 0 km in the model).



four strata averaged from the drilling core and sonic

data [14]. The shear wave velocity of the basement

rock is assigned to be 3.20 km/s. The station 182DL is

chosen as the hard-rock station whose tangential

displacement is used for wave field extrapolations.

An initial geometry of the basin (Fig. 4) is first

constructed based on the travel time difference

between the observed first arrival (from event

020325) and theoretical predictions based on IASP91

[19]. In the initial model, four layers are assigned with

same thickness, and the depth of the basin beneath the

stations satisfies the following equation

X4l¼1

hi

4vl� hi

m basement

¼ si; ð3Þ

where si is the travel time difference between the

observation and prediction for station no. i, hi is the

Fig. 6. (a) The preferred model for cross section AAV, and (b) a geologicallines represent faults, and the strata are illustrated at the bottom of the fig

total depth of the basin beneath station no. i, ml is theaverage shear wave velocity of the lth layer, and

mbasement is the average shear velocity of the basement

rocks.

2.4. Results

Beginning with the initial model, we model the

observed waveforms from the three events listed in

Table 1 and invert the best-fitting velocity structure

models along cross section AAV. The best-fitting

models obtained from inverting seismic data recorded

for the three events are shown in Fig. 5, and the

synthetics (dashed lines) from the best-fitting models

are compared with the seismic data (solid lines).

Overall, the relative timing and waveform match the

data well, except for several stations (for example,

station 163). We note that, although the input motions

cross section RRV [5] in the vicinity of AAV (see Fig. 1). The dashedure.

Fig. 7. Numerical test for a model without structural features

beneath stations from 175 to 172. (a) Smoothed model, which is

obtained by smoothing the interface undulance of the best-fitting

model for event 020325 in Fig. 5a with the fault beneath stations

from 175 to 172. (b) Comparison between data (solid traces) and

synthetics for the smoothed model (dashed lines). The main misfits

between them are marked with black arrows. (c) Comparison

between data (solid traces) and synthetics for the preferred mode

(dashed lines).


at the hard-rock site stations are quite different for the

three events, the models inverted from the seismic

data for the three earthquakes are very similar, which

confirms the validity of our results. A preferred model

was obtained along cross section AAV (Fig. 6a) by

averaging the models obtained from the three event

data. To evaluate our results, we compare our final

inverted model with a geological cross section RRV[5] (Fig. 6b, its location is shown in Fig. 1a) in the

vicinity of section AAV. The overall agreement

between them confirms the reliability of our results.

Note that, the preferred model and the geological

cross section RRV resemble the initial model in

general, implying that travel time residuals place

strong constraints in waveform inversion.

2.5. Resolution tests

Our teleseismic data have frequency content from

0.05 to 1 Hz. The resolution test of Zhao et al. [14]

has illustrated that this frequency content would allow

detection of a 1.0 km high and 10 km wide local rise.

Generally, such a local rise would increase waveform

misfits of 5–20% and travel time residuals of 0.2–0.5

s (defined in Eqs. (1) and (2)) for the receivers above

the local rise.

The preferred model reveals an image of large-

scale velocity structure, in which structural features

such as rises and depressions are resolved. To evaluate

the reliability of these structural features, we present a

series of numerical experiments.

The first resolution test focuses on the structural

features using the data from event 020325. To

illustrate we take a local rise beneath stations 175ZC

to 172 as an example. We assume a modified model,

in which the interface undulance (beneath station

175ZC to 172) was smoothed as shown in Fig. 7a.

Forward calculation using the smoothed model

produces significant discrepancy between the syn-

thetics and the data for the associated stations (Fig.

7b). Compared to the result from the preferred model

(Fig. 7c), the smoothed model produces time residual

increasing from �0.05 s to 0.75 s and waveform

misfit increasing by about 48% from 0.25 to 0.37 for

station 175. For station 174 the time residuals increase

from 0.05 s to 0.98 s and waveform misfit increases

by about 100% from 0.17 to 0.34. When we invert

basin models from the initial model constructed from
l

Fig. 8. Waveform sensitivity to different features in the inverted basin model for cross section AAV. Strata are added from (a) to (d) step by step.

Bottom panels show comparisons between the data (thick lines) and synthetic waveforms (dashed lines) generated from themodels above. The time

residual and waveform misfit values corresponding to each of the steps are marked above the waveforms. The solid triangles indicate receivers.



the travel time difference, the inversions converge to

the optimal model after 32 iterations. In contrast, if we

forced the structure beneath stations 175 to 172 to be a

smooth shape in inversions, we are not able to obtain

a better fitting result after 100 iterations. This means

we could not obtain a better result if we used the

model without the structural features, thereby it

illustrates the robustness of the existence of the

undulance in the strata boundaries beneath stations

175ZC to 172.

The second resolution test is to illustrate the physical

relationship between the model features and the

observed motions. We take the inverted model beneath

stations 182DL to 166ZF as an example.We start with a

thin surface stratum and add strata step by step. Fig. 8

shows comparison between synthetic waveforms

(dashed lines) and the observed data for event

020325 (listed in Table 1). The results illustrate that

different stratum affect synthetics differently. Fig. 8a

shows large discrepancy between synthetics (dashed

Fig. 9. Relationship between inferred basin structural features and faulting

motion of hanging wall and its normal faults interpretation, and (b) an upw

candidate interpretations: back-thrust faults (left lower panel) or normal

corresponding interpretation model. bNQ and bEQ indicate the strata, the dasof the motions between the blocks.

line) and data when only a thin surface layer is present.

Adding more strata (Fig. 8a–d) improves the fit to both

the timing and waveform (misfit values corresponding

to each step are marked in Fig. 8). The intermediate

strata have strong effects on the timing and waveform

of the synthetics, but have less effect on the reflected

phases (Fig. 8b,c). The shape of the basement interface,

which has a strong effect on the reflected phases, plays

an important role in fitting the waveform.

3. Interpretations and discussions

3.1. Inferred tectonic features

We infer the characteristics of the faults inside the

basin based on the preferred seismic velocity model

obtained from the waveform and travel time inver-

sions. Fig. 9 schematically illustrates our approach

and assumption. As can be seen from Fig. 9a, we can

in the basin for two types of faults: (a) a structure with downward

ard arc-shape interface with low undulance (upper panel) and its two

faults (right lower panel). The misfit values are marked below the

hed lines represent faults, and the solid arrows denote the directions


readily infer that a downward motion of hanging wall

probably indicates a normal fault associated with

extension. However, it seems that there are two

possible interpretations for the upward arc-shape

interface with low undulance in Fig. 9b (it corre-

sponds to the Jizhong depression in Fig. 6a): one is a

back-thrust structure and the other is a normal-faults

structure. To check which interpretation is better, we

compare the misfit values of the two different

interpretation models by using forward calculation.

The back-thrusts model produces an average time

residual of 0.48 s and waveform misfit of 0.30, while

the normal faults model produces an average time

residual of 0.55 s and waveform misfit of 0.34. The

result shows that synthetics from the back-thrusts

model match the observation better than from the

normal faults. Based on this, we prefer the back-thrust

faults interpretation.

Fig. 10. Geological interpretation inferred from the preferred models for cro

was modeled in Zhao et al. [14]. The dashed lines represent faults, and the s

The strata are illustrated at the bottom of the figure.

Fig. 10a shows a possible interpretation of the

structural features in the preferred models obtained for

cross sections AAV. To obtain a general image of the

basin structure, we also interpret a previously modeled

cross section BBV [14] (the location of BBV is shown

in Fig. 1a) transecting the southern edge of the basin.

Both cross sections were cut through by a series of

large normal faults, which suggest that extension was

the dominant effect in controlling the evolution of the

basin.

This extension has actually produced relatively

parallel normal faults with extremely low-lying

grabens, i.e., the extension has broken up the basin

into several sub-basins. For cross section AAV, themajor normal faults contain F1, F2, F5, F6 and F7.

Controlled by these major faults, the west edge of the

basin is divided tectonically into alternating grabens

and horsts corresponding to the Jizhong depression,

ss sections AAV (a) and BBV (b) (see Fig. 1). The cross section BBVolid arrows denote the directions of the motions between the blocks.


the Cangxian horst, the Huanghua depression and the

Chenning horst going from west to east (e.g., [2,5,6]).

For cross section BBV, the major faults contain F11 to

F15 that have smaller throws compared to the faults in

AAV. The major graben in BBV is named the Jiyang

depression. For these sub-basins, except the Jizhong

depression, the characteristic structures in the other

depressions exhibit a series of similar extensional

pattern (normal fault system, tilted blocks). Together

with the major faults, the smaller faults such as F3, F4

and so on likely controlled the development of the

sub-basins.

The normal fault F1 is the primary fault with the

greatest throw of about 35 km in our two cross

sections. It deformed all the sediment strata and the

basement, and it is coincident with the previously

proposed location of the Taihang Shan piedmont Fault

(e.g., [7]). Trending NNE–SSW with length over 500

km [5], this steep fault F1 forms the western boundary

of the basin and separates the basin from the Taihang

Shan uplift. It is probably a major detachment surface

and was active during the Cenozoic extension.

Extension is the general character of the basin.

However the two cross sections show certain differ-

ences. For instance, compared with the marginal faults

in the southern edge of the basin, the Taihang Shan

piedmont fault (F1) has steeper scarps and greater

throws. This spatial difference of the faults that belong

to different ages probably implies that the basin

underwent multiply rifts with varying extensional

amplitudes.

3.2. Timing of Cenozoic extension

The timing of deformation in the basin is con-

strained by the age of the units involved in the

deformation [20]. The major normal faults and the

angular unconformity between strata not only exhibit

characteristics of growth faults but also imply the

timing that the faults played a role in extension.

For example, normal faults F1 and F2 deformed the

Pre-Tertiary strata and the Paleogene strata. Moreover,

the Paleocene strata become thicker in the Jizhong

depression than in the Cangxian horst. These obser-

vations suggest that: (1) faults F1 and F2 were active

and controlled the sediments during early Paleocene;

(2) the Jizhong depression received sediments asso-

ciated with the extension of the basin. Since the oldest

syn-rift deposits belong to the Paleogene strata for

both of the edges, we can infer that the triggering time

of extension was not later than early Paleocene.

Extension continued during Neogene and/or Quater-

nary due to the fact that most of the normal faults cut

through the Neogene strata, and some of them, such as

F2 and F3, even cut through the Neogene strata to the

Quaternary strata. However, extensional deformation

occurred on a much smaller scale than the Paleogene

strata, with most faults either entirely inactive since

the early Neogene or showing only minor amounts of

Neogene extension in comparison with their early

Tertiary histories [2]. This indicated that the major

extension in the most of areas ended at the end of

Paleogene.

The Paleogene and Neogene strata form the major

reservoir rocks of the Bohai Bay basin; however, the

major syn-rift deposits in the different depressions

belong to different strata. In the piedmont zones of the

Taihang Shan (e.g., the Jizhong depression) and the

Luxi uplift (e.g., the Jiyang depression), the Paleogene

strata are thick. Yet, in the Huanghua depression, the

Paleogene strata become thinner while the Neogene

strata become thicker. According to thickness change

of the Paleogene strata and the Neogene strata, it can

be inferred that the depocenter migrated from the

flanks to the center resulting in that the sediments

became progressively younger. As rifting continued

and the basin widened, the locus of rifting changed

from the flanks to the center of the basin [3].

3.3. Possible local compressional structures

Two localized back-thrust faults (C1 and C2 in Fig.

10) and doming folds are recognized in the Jizhong

depression. The hanging walls back thrusted onto the

footwall due to the local tectonic shortening. The

cross section image (Fig. 10a) suggests a minimum of

22% shortening along this cross section, calculated by

comparing line length change between deformed and

undeformed sections. These reverse faults deformed

all sedimentary sequences and basement rocks,

possibly providing evidence that the compressional

deformation took place during Neogene and Quater-

nary [5]. It was also reported that Mid-Tertiary

thrusting exists within the western part of the Liaohe

Depression, the northern Bohai Bay basin [21]. It is

possible that some of the thrust and reverse faults are


inverted Tertiary normal faults. However, it is

important to note that such kind of compressional

pattern is only local and is not basin-wide. For

example, the compression is extensive in the Jizhong

depression the western edge, but is insignificant in the

Huanghua depression and the Jiyang depression.

4. Conclusions

Using an SH wave field extrapolation FD method

combined with travel time and waveform inversion,

the western edge shear velocity structures of the Bohai

Bay basin are constrained by teleseismic data from 29

portable seismic stations. Comparison between syn-

thetics and the observed data shows good agreements.

The overall similarity between our preferred model

and a nearby geological profile confirms the reliability

of our results. Distinct characters of basin-wide

extension and local inversion are recognized in our

cross sections. The most striking evidence of exten-

sion relate to a series of extensional pattern such as

normal fault system, horst and graben. The results

presented in this paper support the assumption that the

Bohai Bay basin was formed after early Cenozoic

extension and late Cenozoic thermal subsidence and

inversion (e.g., [2–7]). It appears that extension was

the main mode during the basin evolution, accom-

panied by local compression and some renewed

extensional movement along faults.

In addition, the method employed in this paper

provides an example to constrain the basin edge

structures by applying portable seismic observations.

Fundamental tectonic questions of basins such as

tectonic history of basin formation and development

can be addressed based on the basin edge structure

images. This method would probably find wider

applications with increasing number of stations

deployed inside basins, especially in the cases when

the detailed seismic reflection data is lacking.

Acknowledgements

The review of Qian Song improved our first

manuscript. We thank Dr. Lianxing Wen for his FD

calculation code, his constructive advice and most

helpful reviews. We acknowledge the participants of

the Broadband Seismic Array Laboratory, IGGCAS.

This research is supported by Chinese Academy of

Sciences (No. KZCX 1-07) and China Earthquake

Data Sharing (2003-DZGX-2004). We appreciate the

proofreading of Dr. M. Hill, the careful and insightful

reviews by Dr. R. D. van der Hilst, Dr. T. Hearn and

Dr. Eric Hetland.

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