Download Research on the Seismic Activity of the Lower Yangtze River

Earthquake Research in China
Volume 27，Number 2，2013
Research on the Seismic Activity of
the Lower Yangtze River-South
1
Yellow Sea Seismic Zone
Xie Zhuojuan，Lv Yuejun，Peng Yanju，Lan Jingyan，and Zhang Lifang
Institute of Crustal Dynamics，China Earthquake Administration，Beijing 100085 ，China
The Lower Yangtze River-South Yellow Sea Seismic Zone，located at the southeast of the
Northern China Seismic Zone， characterized by moderate-strong earthquakes， is an
intensive earthquake zone， which is controlled by a series of faults within the Lower
Yangtze River-South Yellow Sea Seismic Zone． This article counts and calculates the bvalue，V 4 and energy density value of medium-small earthquakes by taking full advantage
of the latest data from regional seismic stations，reviews data of historical earthquakes and
seismic structure，and discusses the relationship between spatial distribution of the b-value，
historical strong earthquakes and spatial distribution of energy density of medium-small
earthquakes，and further investigates the seismic activity of the Lower Yangtze RiverSouth Yellow Sea Seismic Zone． This article obtains seismic activity parameters of the
Lower Yangtze River-South Yellow Sea Seismic Zone as calculation parameters for
probabilistic seismic hazard analysis，and discusses the trend of this seismic zone in the
next one hundred years and deduces the potential seismic hazard region within this seismic
zone，which provides references and methods for long-term prediction on seismic activity．
The research results are significant to seismic zoning， seismic safety evaluation of
engineering sites and long-term prediction of seismic activity．
Key words:Seismicity; b-value; Annual mean occurrence rate; The Lower Yangtze RiverSouth Yellow Sea Seismic Zone
INTRODUCTION
The Lower Yangtze River-South Yellow Sea Seismic Zone is located at the southeast of the
North China Seismic Zone，including the lower reaches of the Yangtze River and the sea area of
the South Yellow Sea． The lower reaches of the Yangtze River are a region with developed
1
Received on March 9，2012． This project was sponsored by Institute of Crustal Dynamics，China Earthquake
Administration ( ZDJ2013-05 ) ，the sub-project from the Ministry of National Science and Technique's project
(2011ZX05056-001-02) ，and the Special Scientific Research Fund of Seismological Public Welfare Profession of
China (200708055) ．
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179
economy，intensive cities and population． In the sea area of the South Yellow Sea， large
quantities of offshore platform projects have been established year by year． According to historical
earthquake data，seismic activities are intensive in this seismic zone，characterized by moderatestrong earthquakes． Once an earthquake occurs，development of economy and stability of society
will be seriously affected． Therefore，research on seismic activity of this zone is significant．
Research has been made by predecessors on the seismic activity of this zone by means of
different methods． In detail， Wei Guangxing， et al． ( 1991 ) studied the regionalization of
seismicity based on tectonic setting; Xie Huazhang，et al． ( 1998 ) made research based on
segmentation of seismic active phases and periods; Li Qiang，et al． (2001 ) performed research
based on the theory of gray prediction model． As time goes on， new earthquakes occur
continuously． With the development of nationwide seismic station networks and improvement of
monitoring technology，the monitoring blank in partial sea areas was made up and a large amount
of data of medium-small earthquakes with high accuracy and completeness have been
accumulated． By checking on the historical earthquakes in the sea area performed by numerous
scholars ( Lv Yuejun，2008; Peng Yanju et al． ，2008) ，new insights have been gained regarding
the seismic tectonic setting ( Xu Xing et al． ，2011 ) and geophysical field of the South Yellow
Sea，providing rich data for research on the seismic activity of the Lower Yangtze River-South
Yellow Sea Seismic Zone ( hereinafter referred to as LYRSYS Zone) ． This article，by taking full
advantage of this new data，calculates the b-value，V 4 and energy density value of medium-small
earthquakes，discusses the seismic activity parameters and characteristics of this seismic zone by
combining with prior research，and analyzes the relationship between spatial distribution of the bvalue，spatial distribution of energy density of medium-small earthquakes， historical strong
earthquakes and the distribution of major active faults． Moreover， it discusses the trend of
seismicity in the next hundred years and predicts the potential earthquake risk areas． Seismic
activity of the LYRSYS could be comprehended further through the research of this article． The
conclusions of this article can serve as reference for analysis on seismic activity of this seismic
zone，as well as for seismic hazard analysis and long-term prediction of seismic activity．
1
THE REGIONAL SEISMOTECTONIC BACKGROUND
The Lower Yangtze River-South Yellow Sea Seismic Zone is located at 29. 50° ～ 38. 23° N
and 115. 74° ～ 126. 20°E，including the lower reaches of the Yangtze River and the sea area of
the South Yellow Sea． The Yellow Sea，in accordance with the characteristics of geology and
geophysical field，is divided into the South Yellow Sea and North Yellow Sea． The southern area
divided by the line which is connecting Chengshantou，the easternmost of Shandong Peninsula，
with Changshanchuan in Korean Peninsula is called the South Yellow Sea，the nature of which is
the same as the Yangtze block，as the main part of eastward extension of the Lower Yangtze
Platform，a stratum with thickness of 15km on the crystalline basement of the Presinian Period
( Hao Tianyao et al． ，2003; Xu Xing et al． ，2011) ． Therefore，tectonically，the LYRSYS Zone
is located at the Yangtze Paraplatform，the south rim of Lower Yangtze fold-thrust belt，where
sedimentation has occurred mainly since the Cenozoic． According to the tectonic setting，regional
geophysical field and deep geologic structure，the LYRSYS Zone is further divided into several
secondary tectonic units: Northern Suzhou Basin， Southern Suzhou Upheaval， Northern
Depression，Southern Depression，Central Upheaval，Wunansha Upheaval and Minzhe Upheaval
( Xu Xing et al． ，2011; Institute of Geology，CEA，2010) ，as shown in Fig． 1．
With respect to the regional seismotectonic background，the sea area consists of a series of
NEE and E-W trending depressions． Moderate-strong earthquakes happened mostly within these
depressions． Faults on the land area mostly strike NE or NEE as a whole，and late Pleistocene
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180
Fig． 1
The regional seismotectonic background and epicenter distribution of historical strong
earthquakes in the Lower Yangtze River-South Yellow Sea Seismic Zone
( Adapted from the data of 2010 from Institute of Geology，China Earthquake Administration (2010，
2011) )
faults are occasional． Earthquakes occur mainly in the intersection of NW and NE structures．
Therefore，seismicity within this zone is mainly controlled by the NE and NNE trending faults
inside the Northern Suzhou Depression in the lower reaches of Yangtze River，and the big faults
along or within the near west-east border between the Northern Depression and Southern
Depression in the South Yellow Sea． The size of these faults is relatively small and they are mostly
normal，characterized by moderate-strong earthquakes．
2
2. 1
SEISMIC DATA AND PREPROCESSING
Data Sources and Overview
The historical strong earthquake dataused for this research is from “Catalogue of Chinese
Historical Strong Earthquakes ( 23 rd century B． C． ～ 1911 A． D． ) ”( Department of Earthquake
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181
Disaster Prevention，China Seismological Bureau，1995 ) ． The data from 1912 to 1990 is from
“Catalogue of Modern Earthquakes in China ( 1912A． D． ～ 1990A． D． ， M S ≥ 4. 7 ) ”
( Department of Earthquake Disaster Prevention，China Seismological Bureau，1999 ) ，and the
data after 1990 is from the New Seismic Zonation Map． Small earthquake data is from the
database of China Seismic Network Center．
According to statistics，there have been 91 earthquakes with M S ≥4 
 in the LYRSYS zone
since 499 A． D． ，including，one M7. 0 earthquake，19 M6. 0 ～ 6. 9 earthquakes，40 M5. 0 ～ 5. 9
earthquakes，and 31 earthquakes with M4. 0 ～ 4. 9． Since 1970， there have been 6564
earthquakes with 1. 0≤M L ≤5. 0，including 2674 of M1. 0 ～ 2. 0，2916 of M2. 0 ～ 2. 9，875 of
M3. 0 ～ 3. 9，and 99 of M4. 0 ～ 4. 9．
2. 2
Location Accuracy and Regional M onitoring Capability
According to its changes with time，the location accuracy of earthquakes can be roughly
divided into five periods． Assessed value of changes with time of location accuracy of strong
earthquakes in the sea area of the Yellow Sea can be calculated according to the earthquake
location accuracy assessment formula ( Formula 1 ) ( Institute of Crustal Dynamics， China
Earthquake Administration，2011) ，as shown in Table 1．
(6 － I) × N i － 1． 0 × N 6
E = ∑
i = 1，
2，…，
5
(1)
i = 1，5
N × 5． 0
Where，E is the assessment value of earthquake location accuracy， N i is the number of
earthquakes with location accuracy of category i during a certain period within a certain region，
and N is the total number of earthquakes within a certain region． From Table 1，the assessment
value of earthquake location accuracy before 1935 is negative，the reliability degree of which is
quite low． Earthquake location accuracy after 1970 improved greatly． Earthquake catalogs used
for this research are of Category I in accuracy ( Category I≤10km) ( Department of Earthquake
Disaster Prevention，China Seismological Bureau，1995) ．
The monitoring capability of the network is calculated according to the data from the digital
seismic stations network of eastern coastal provinces in China since 1970，and the distribution of
digital seismic stations and their monitoring capability in the LYRSYS Zone are mapped as Fig． 2．
We can see that seismic stations in the inland region are distributed intensively and evenly，with
monitoring capability of about M L 1. 5． The monitoring capability is about M L 1. 5 ～ 2. 0 for the
border and nearby sea area，and M L 2. 0 ～ 2. 5 for remote sea area．
Table 1
Assessment value of earthquake location accuracy in the sea area of the Yellow Sea
Time range
1900 ～ 1935
1936 ～ 1969
1970 ～ 1980
1981 ～ 1990
1991 ～ Now
Assessment value
－ 0. 03
0. 30
0. 37
0. 97
1. 00
2. 3
Data Preprocessing and Completeness Analysis on Integrity
By determining the space and time range of aftershocks according to the space formula lgR =
0. 48M － 1. 57 ( Chen Ling et al． ，1998 ) ，as well as the time range table given by Console
(1979 ) ，and deleting the aftershocks from collected earthquake catalogues，we obtained the
catalog of 4055 earthquakes of 1. 0 ≤ M L ≤ 5. 0 for the LYRSYS Zone since January 1970．
Analysis on completeness of small earthquake data is done according to the G-R relationship based
magnitude-frequency distribution law ( Gutenberg B，et al． ，1956; Richter C． F． ，1958 ) to
determine the minimum magnitude of completeness M C in the region． From Fig． 3，the minimum
magnitude of completeness in the LYRSYS Zone is 2. 4．
In summary，considering the network monitoring capability，earthquake location accuracy，
182
Earthquake Research in China
deletion of aftershocks and the minimum magnitude of completeness，we carry out preprocessing
for the original seismic data and finally select 2564 medium-small earthquakes with 2. 0 ≤ M L ≤
5. 0 since 1970 for statistical analysis of earthquake activity in the LYRSYS Zone，including 1888
earthquakes of M L 2. 0 ～ 2. 9 earthquakes，597 of M L 3. 0 ～ 3. 9 and 79 of M L 4. 0 ～ 5. 0．
Fig． 2
Distribution of regional digital seismic stations and monitoring capability
Fig. 3
Magnitude-frequency diagram for 1. 0≤M L ≤5. 0 earthquake
3
ANALYSIS OF SEISMIC ACTIVITY AND SEISMIC HAZARD
This section analyzes firstly the characteristics of time distribution for earthquake activities in
the LYRSYS Zone，as the basis of analysis on future seismic activity trends，and then statistically
calculates the b-value and V 4 and maps the distribution of the b-value in the LYRSYS Zone with
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183
the space scan method of the b-value，and finally calculates the energy density of medium-small
earthquakes． It discusses the earthquake trend in the next hundred years for this zone，and
initially outlines the potential danger zone．
3. 1
Distribution Characteristics of Time for Earthquakes in the LYRSYS Zone
According to historical earthquake data，the earliest earthquake record in the Lower Yangtze
River-South Yellow Sea Seismic Zone is the August 5，499，M4
 earthquake in Nanjing． Fig． 4
( a) shows that there were only five earthquakes recorded before 1450，thus data is greatly
lacking． After 1450，records of earthquakes with M ≥ 4. 7 are basically complete． Fig． 4 ( b )
shows that this zone had undergone two active earthquake periods since 1450 ( 1491 ～ 1679 and
1839 ～ ?) ，and the second active earthquake period has undergone three active phases，each of
which is approximately more than 40 years． Currently，this zone is in the third active phase of the
second active earthquake period (1970 until now) ． It is estimated that this zone will still be in
the later stage of an active period in the next hundred years，with a possibility of a M6. 0 ～ 7. 0
earthquake．
In summary，the Lower Yangtze River-South Yellow Sea Seismic Zone could be divided into
three research periods: 1491 ～ 2011，1839 ～ 2011 and 1970 ～ 2011．
Fig． 4
M-t plot of earthquakes in the Lower Yangtze River-South Yellow Sea Seismic
Zone since 400AD ( a) and 1450AD ( b) ，respectively．
3. 2
Statistical Calculation of b -value and V 4 Value
The b-value means the tangent line slope in the magnitude-frequency formula． It represents
the ratio of earthquake frequency of various magnitudes in a certain region， calculated from
statistics of actual data and related to the completeness，reliability of data and statistical sample
size．
The Gutenberg-Richter frequency relationship is ( Gutenberg B，et al． ，1956; Richter C． F，
1958) :
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184
lgN = a － bM
(2)
Where，N is the number of earthquakes with magnitude more than M，a and b are the statistical
constant． Generally，the b-value could be obtained from historical earthquake data，but the
available range for the b-value is limited ( typically 5. 0 ≤ M S ≤ 8. 0 ) ，so the b-value is often
lower． As the time for modern earthquake data is too short，the b-value could not correctly reflect
the characteristics of seismic activity ( Huang Weiqiong et al． ，1989; Yan Jiaquan et al． ，1996;
Pan Hua et al． ，2006 ) ． In this section，the modern small earthquake data，combined with
historical strong earthquake data are converted to the accumulative annual mean earthquake rate to
get a b-value in order to make up for the lack of seismic data sample quantities ( Pan Hua et al． ，
2006; Wu Zaoying et al． ，2005;Zhang Lifang，
2008;Lei Jiancheng et al． ，
2010) ．
According to the preprocessing result，we select small earthquakes with M L (2. 4 ～ 5. 0) and
convert them to M S magnitude by the formula M S = 1. 0112M L － 0. 2591 ( Xie Zhuojuan et al． ，
2012 ) ，then use the earthquakes within the range of M S (2. 0 ～ 4. 9) for the statistical research to
obtain 1738 medium-small earthquakes． The accumulative annual mean occurrence rate of
earthquake ( V ) in different periods is calculated according to the characteristics of temporal
distribution of earthquakes in the Lower Yangtze River-South Yellow Sea Seismic Zone． The
calculation results are shown in Table 2．
Table 2 Accumulative annual mean earthquake rate in different periods in the Lower Yangtze
River-South Yellow Sea Seismic Zone
Magnitude(M S )
Time
range
1491 ～ 2011
2. 0
2. 5
3. 0
3. 5
4. 0
4. 5
N
V
1839 ～ 2011
N
N
5. 5
6. 0
6. 5
7. 0
56
31
19
9
1
0. 1077 0. 0596 0. 0365 0. 0173 0. 0019
42
V
1970 ～ 2011
5. 0
27
16
8
1
0. 2442 0. 1570 0. 0930 0. 0465 0. 0058
1757
1149
499
191
67
32
11
5
4
0
0
V 42. 854 28. 0244 12. 1707 4. 6585 1. 6341 0. 7805 0. 2683 0. 1220 0. 0976 0. 0000 0. 0000
According to magnitude and accumulative annual mean occurrence rate of earthquake ( V) in
different periods in Table 2，we fit the data in different periods bythe means of least squares
method． By taking comparative analysis with many programs，we select the best fitting program
and data and finally get the b-value and V 4 value of the Lower Yangtze River-South Yellow Sea
Seismic Zone． The statistic results are shown in Table 3 ( “△”means the standard deviation of a
certain value，R means fitting the correlation coefficient and SD means the regression standard
deviation) ． Fig． 5 shows the fitting results．
Table 3
Seismicity parameters in the Lower Yangtze River-South Yellow Sea Seismic Zone
a
Δa
b
Δb
R
SD
V4
3. 395
0. 073
0. 7945
0. 015
0. 998
0. 081
1. 649
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185
According to Table 3，the b-value of this zone is 0. 7945 and V 4 value is 1. 649，which are
slightly higher than those in the Earthquake Intensity Zoning Map ( Version IV) ( 1990 ) ，where
the b-value is 0. 66 and V 4 value is 1. 16，which is related to the increment of ratio of mediumsmall earthquakes due to the improvement of monitoring technology of earthquake stations and the
expansion of monitoring scope in recent years． The correlation coefficient is R = 0. 998 in linear
fitting，R → 1 represents the good linear relationship and fitting effect． Regression standard
deviation SD = 0. 081 indicates the high reliability of data． From Fig． 5，the degree of linearity for
the data in low magnitude from 1970 to 2011 is preferable，and degree of linearity for the data in
magnitude 6. 0 ～ 6. 5 from 1491 to 2011 is preferable． Therefore，the accumulative annual mean
occurrence rates of earthquake of various magnitudes at different periods are considered in the
linear fitting to seek the optimal fitting results．
Fig． 5
Accumulative annual mean occurrence rate of earthquake and statistical b value
in different periods in the Lower Yangtze River-South Yellow Sea Seismic Zone
The V 4 value，the annual mean occurrence rate of earthquakes with magnitudes more than
4. 0，represents the seismic activity level in a certain region． It is demonstrated in the research
that V 4 represents the expected value ( mean value of probability distribution) of the occurrence
rate of Poisson distribution events in the statistic region and determines the probability distribution
of occurrence times of earthquakes． According to the assumption of Poisson distribution，
distribution characteristics of annual occurrence times of earthquakes are the main determinant of
V 4 ． The V 4 value of the Lower Yangtze River-South Yellow Sea Seismic Zone could be calculated
by means of the V 4 statistical method ( Pan Hua et al． ，2011) based on annual occurrence rate．
Fig． 6 shows the relationship between distribution characteristics of occurrence times of
earthquakes in this zone and V 4 ． The horizontal axis is the earthquake occurrence time and the
vertical axis is the probability of annual occurrence． From Fig． 6 we find that the distribution of
annual earthquake occurrence time from actual data statistics is similar to Poisson distribution．
We deduce that，for the Lower Yangtze River-South Yellow Sea Seismic Zone，V 4 = 2. 13 and
standard deviation SD = 0. 2113，based on the definition and formula of Poisson distribution．
This article，based on the b-value and the earthquake occurrence rate of relatively reliable
magnitude ranges，comes to a conclusion that V 4 of the Lower Yangtze River-South Yellow Sea
Seismic Zone is 1. 649，and the expected value of Poisson distribution V 4 = 2. 13 by means of
fitting according to the distribution of annual mean occurrence rate of earthquakes over magnitude
4. 0 in reliable periods． The V 4 value of the Lower Yangtze River-South Yellow Sea Seismic Zone
is confirmed as 2. 13 among the various statistics，following the principle of not underestimating
Earthquake Research in China
186
Fig． 6
Fitting chart of Poisson distribution for the annual earthquakes frequency
in the Lower Yangtze River-South Yellow Sea Seismic Zone
the seismic activity level of the past 40 years ( Pan Hua et al． ，2011) ．
3. 3
Spatial Distribution Characteristics of the b -value
The last section deduces the b-value of the Lower Yangtze River-South Yellow Sea Seismic
Zone ． As the whole seismic zone is quite large in space，seismic activity parameters from statistics
are macroscopic and average，which have difficulty reflecting the seismic activity in different
regions． Therefore，this section analyzes the spatial distribution characteristics of b-value in this
seismic zone by means of the space scanning method．
The space scanning method of the b-value is to perform gridding at a certain spaced interval
(0. 02° is selected in this article) ，pick up the earthquake data within the statistical unit which
takes each grid center as the center of a circle and r as radius ( As this region includes sea areas，
earthquakes are relatively sparse． In order to satisfy a sufficient quantity of statistical samples，r
in this article extends to 40km ～ 50km) ，calculate the minimum magnitude of completeness M C
for each unit respectively，then calculate the b-value by mean of least squares method as the value
of central point of the statistical unit，and finally，calculate the value of all grid points in the
research region，use the interpolation method for discrete point and draw isolines to form the
spatial distribution of b-value，as shown in Fig． 7．
From Fig． 7 we find that:① the remote sea area and most of the land region within the Lower
Yangtze River-South Yellow Sea Seismic Zone are blank，indicating that earthquakes in this
region are extremely scarce，which does not satisfy the space scanning requirements，thus space
scanning of the b-value is not applicable; ② there are obvious space differences in spatial
distribution of the b-value in this seismic zone，reflecting the difference of seismic activity between
different regions as well as the spatial difference of seismic hazards; ③ regions with high b-value，
such as southern Huaiyin，mean relatively low stress accumulation，which are mainly subject to
small earthquakes in the future; ④ there are many regions with abnormal low b-value in this
seismic zone and strong earthquakes may occur in these regions． In detail，the b-value is quite
low in land regions such as north Guichi，Zhenjiang，Suzhou，Lishui county and Ningbo． The
offshore sea area at 121. 35° ～ 122. 02° E and 33. 42° ～ 33. 97° N，and the remote sea area at
123. 11° ～ 123. 96°E and 35. 89° ～ 36. 95°N． In these regions，b values are obviously lower than
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187
the mean value 0. 7945 of the whole zone，indicating these abnormal regions are under currently
high stress accumulation and may be subject to moderate-strong earthquakes．
Fig． 7
Spatial distribution of b-value in the Lower Yangtze River-South Yellow Sea Seismic Zone
3. 4
Spatial Distribution of Energy Density of M edium-small Earthquake
Studies show that the energy density pattern of medium-small earthquakes can reflect the
characteristics of epicenter distribution of strong earthquakes，the spreading of major active faults
and the potential strong earthquake risk zones ( Lv Yuejun，2008) ． In order to further investigate
the seismic activity of the Lower Yangtze River-South Yellow Sea Seismic Zone，this section
quantitatively analyzes the spatial distribution pattern of medium-small earthquakes in the Lower
Yangtze River-South Yellow Sea Seismic Zone since 1970 using the method of calculating the
energy density of medium-small earthquakes．
The calculation method for energy density of medium-small earthquakes is to carry out
gridding at a certain spaced intervals，taking r ij as the distance from epicenter to grid point． The
earthquake energy of the grid point ( i，j) in unit intervals is the sum of energy density of n
earthquakes located at ( i，j) ( Wang Jian，2001; Zhang Lifang et al． ，2008 ) ． Calculation is
done at the 0. 02 × 0. 02° grid interval in this section to get the density value of all grid points in
the Lower Yangtze River-South Yellow Sea Seismic Zone，and using the interpolation method to
estimate the value of discrete points and draw isolines，we obtain the strain energy density isogram
of medium-small earthquakes in the Lower Yangtze River-South Yellow Sea Seismic Zone ( as
shown in Fig． 8) ． Epicenter distribution of 18 strong earthquakes in this seismic zone since 1970
is also plotted in this figure．
From Fig． 8 we find that:① three earthquakes with magnitude over 6. 0 within this seismic
zone since 1970 occurred in the center or border of the area with high strain energy density，
which indicates that the energy density graph of medium-small earthquakes could reflect the
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188
distribution characteristics of epicenters of strong earthquakes; ② there are multiple areas with
abnormal strain energy density existing in this seismic zone． Energy of medium-small earthquakes
around Anqing，Guichi，Ningbo，Yancheng and Zhoushan islands is quite intensive，and also the
energy of medium-small earthquakes around the nearby sea area of the South Yellow Sea within
120. 9° ～ 121. 9°E and 34. 38° ～ 35. 51°N，as well as 123. 2° ～ 124. 1° E and 32. 1° ～ 33. 4° N，
and the remote sea area is quite intensive，which indicate that these areas，though there is so far
no record of strong earthquakes，may be prone to moderate-strong earthquakes in future．
Comparing Fig． 7 and Fig． 8，we found that spatial distribution of the b-value matches well
with the distribution of energy density of medium-small earthquakes． Generally，it shows that
strong earthquakes occur in the center or border of the region with low a b-value and high strain
energy density of medium-small earthquakes． Summing up the above analysis，we estimate the
earthquake trends of this seismic zone in the next hundred years and basically outline the potential
risk areas for strong earthquakes． The results of this study show that the Lower Yangtze RiverSouth Yellow Sea Seismic Zone is in the third active phase of the second earthquake active
period． It is estimated that this zone will be in the later stage of seismic active period in the next
hundred years，with a possibility of magnitude 6. 0 ～ 7. 0 earthquakes． For the land areas，such
as northern region of Guichi county ( region A) ，east region of Ningbo ( region B) ，Lishui county
( region C) ，offshore sea area within 118. 83° ～ 119. 64° E and 31. 10° ～ 31. 91° N ( region D )
Fig． 8
Strain energy density isogram of medium-small earthquakes and epicenter distribution of
strong earthquakes since 1970 in the Lower Yangtze River-South Yellow Sea Seismic Zone
( Grid interval of 0. 02 × 0. 02° )
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189
and the remote sea area within 123. 11° ～ 123. 96° E and 35. 89° ～ 36. 95° N ( region E ) ，there
are frequent medium-small earthquakes but no strong earthquakes and the b-value is very low，
therefore，these regions can be predicated as potential strong earthquake risk regions in the Lower
Yangtze River-South Yellow Sea Seismic Zone．
4
CONCLUSION
Through research and comparative analysis in this article，the conclusions are reached as
follows:
(1) Monitoring capability is about M L 1. 5 ～ 2. 0 for the inland region，border and nearby sea
areas within the Lower Yangtze River-South Yellow Sea Seismic Zone，and M L 2. 0 ～ 2. 5 for the
remote sea area． The minimum magnitude of completeness of the earthquake catalogue in this
seismic zone is 2. 4．
(2) Since 1970，the Lower Yangtze River-South Yellow Sea Seismic Zone has been in the
third active phase of the second earthquake active period which began at 1485 A． D． It is
estimated that this zone will still be in the later stage of an active period in the next one hundred
years，and the possibility of occurrence of magnitude 6. 0 ～ 7. 0 earthquakes exists．
(3) According to the latest earthquake data，the b-value of the Lower Yangtze River-South
Yellow Sea Seismic Zone is calculated to be 0. 7945 and V 4 value is 2. 13，which can serve as
calculating parameters for probabilistic earthquake hazard analysis． This is significant for seismic
safety evaluation of this seismic zone．
( 4) Under the condition of sufficient data samples，the statistic analysis of spatial distribution
of the b-value could better reflect the actual level and the future trend of seismic activity within
this seismic zone．
(5 ) By comprehensive analysis on the relationship between the spatial distribution of bvalue，spatial distribution of energy density of medium-small earthquakes， historical strong
earthquakes and the distribution of major active faults，five regions are delineated as strong
earthquake risk regions in the Lower Yangtze River-South Yellow Sea Seismic Zone． They are the
northern region of Guichi county ( region A) ，east region of Ningbo ( region B ) ，Lishui county
( region C) ，offshore sea area in 118. 83° ～ 119. 64° E and 31. 10° ～ 31. 91° N ( region D) and
remote sea area in 123. 11° ～ 123. 96°E and 35. 89° ～ 36. 95°N ( region E) ．
This paper has been published in Chinese in the journal of Earthquake，Volume 32，Number
4，2012．
REFERENCES
Chen Ling，Liu Jie，Chen Yong，et al． Aftershock deletion in seismicity analysis ［J］． Chinese Journal of
Geophysics，1998，41( Supplement) :244 ～ 252( in Chinese with English abstract) ．
Console R． ，Gasparini C． ，Simoni B De，et al． Preambolo al Catalogo Sismico Nazionale I criteri di informazione
del CSN［J］． Pnnali Geofis，1979，32:37 ～ 77．
Department of Earthquake Disaster Prevention，China Seismological Bureau． The Catalogue of Modern Earthquakes
in China (1912A． D． ～ 1990A． D． ，M S ≥ 4. 7) ［M］． Beijing: China Science and Technology Publishing
House，1999( in Chinese) ．
Department of Earthquake Disaster Prevention，Sate Seismological Bureau． The Catalogue of Chinese Historical
Strong Earthquakes ( 23 rd Century B． C． ～ 1911 AD) ［M］． Beijing: Seismological Press，1995 ( in
Chinese) ．
Gutenberg B，Richter C． F． Earthquake magnitude，intensity，energy and acceleration ［J］． Bull． Seism． Soc．
Am，1956，46: 105 ～ 143．
Hao Tianyao，Liu Jianhua，Suh Mancheol，et al． Deep structure character and geological evolution of the Yellow
Sea and its adjacent regions ［J］． Chinese Journal of Geophysics，2003，46 ( 6 ) :803 ～ 809 ( in Chinese with
190
Earthquake Research in China
English abstract) ．
Huang Weiqiong，Shi Zhenliang，Cao Xuefeng． Factors influencing the estimation of b value and the selection of b
value in hazard analysis ［J］． Acta Seismologica Sinica，1989，11 ( 4 ) :351 ～ 361 ( in Chinese with English
abstract) ．
Institute of Crustal Dynamics， China Earthquake Administration． Research Report on the Strong Earthquake
Tectonics and the Offshore Engineering Seismic Hazard in the Sea Area ［R］． Beijing，2011． 32 ～ 33 ( in
Chinese) ．
Institute of Geology， China Earthquake Administration． Research Report: Determination of Strong Earthquake
Structures and Zoning of Potential Earthquake Source in the Offshore Sea Area ［R］． Beijing，2011． 45 ～ 49
( in Chinese) ．
Lei Jiancheng，Gao Mengtan，et al． Effects of seismic belt division scheme on seismicity parameters ［J］． Acta
Seismologica Sinica，2010，32(4) :457 ～ 465( in Chinese with English abstract) ．
Li Qiang，Xu Guiming，Fan Guiying． Study on seismicity tendency for seismic zone from mid-lower reaches of the
Yangtze River to South Yellow Sea ［J］． Northwestern Seismological Journal，2001，23 ( 3 ) :265 ～ 268 ( in
Chinese with English abstract) ．
Lv Yuejun． A Study on Seismic Zonation of Bohai Sea ［D］． ( Doctor's degree thesis ) ． China University of
Geosciences，2008，11 ～ 16( in Chinese with English abstract) ．
Pan Hua，Li Jinchen． Study on uncertainties of seismicity parameters b and v 4 in seismic statistical zones ［J］．
Technology for Earthquake Disaster Prevention，2006，1(3) :218 ～ 224( in Chinese with English abstract) ．
Peng Yanju，Meng Xiaohong，Lv Yuejun，et al． The seismicity of China offshore seas and its relationship with
geophysical fields ［J］． Progress in Geophysics，2008，23 ( 5 ) : 1377 ～ 1388 ( in Chinese with English
abstract) ．
Richter C． F． Elementary Seismology ［M］． San Francisco，1958．
Wang Jian． Seismic pattern treatment method through calculation of seismic density at grid nodes［J］． Acta
Seismologica Sinica，2005，23(3) :262 ～ 267( in Chinese with English abstract) ．
Wei Guangxing， Guo Aixiang， Hu Zheng， et al． Subarea and its seismicity parameters of the Yellow SeaDetermination of the Yellow Sea Seismic Belt ［J］． Earthquake． 1991，6:68 ～ 72( in Chinese) ．
Wu Zaoying，Bo Jingshan，Liu Zhipin，et al． Statistical analysis on b value and earthquake yearly average
occurrence ratio in northeast seismic zone ［J］． Seismological Research of Northeast China，2005，21(3) :27
～ 32( in Chinese with English abstract) ．
Xie Huazhang，Tian Jianming． Tendency analysis of seismicity for seismic zone of Changjiang mid-downstream
reach and South Yellow Sea ［J］． Journal of Seismology，1998，3:1 ～ 6( in Chinese with English abstract) ．
Xie Zhuojuan，Lv Yuejun，Peng Yanju，et al． Study on the empirical relations between surface wave magnitude
and local earthquake magnitude in Chinese mainland and neighboring region ［J］． Bulletin of the Institute of
Crustal Dynamic，Beijing，2012(24) :74 ～ 89( in Chinese with English abstract) ．
Xu Xing，Yao Yongjian，Feng Zhiqiang，et al． Geophysical cognition of tectonic evolution in the Northern South
Yellow Sea ［J］． Progress in Geophysics，2011，26(4) :1266 ～ 1278( in Chinese with English abstract) ．
Yan Jiaquan，Han Wei，Gao Mengtan． Uncertainty of seismicity parameters and its affects to the seismic zoning
［J］． Earthquake Research in China，1996，12( Supplement) :71 ～ 77( in Chinese with English abstract) ．
Zhang Lifang， Lv Yuejun， Peng Yanju， et al． Analysis of seismological energy density of minor-moderate
earthquakes ［J］． Earthquake Research in China，2008，24 ( 4 ) : 407 ～ 414 ( in Chinese with English
abstract) ．
Zhang Lifang． Study on Estimate of Seismicity Parameters in Low and Moderate Seismicity Region ［D］． ( Master's
degree thesis) ． Institute of Crustal Dynamics，China Earthquake Administration，2008，20 ～ 40( in Chinese
with English abstract) ．
About the Author
Xie Zhuojuan，born in 1984，is an assistant research professor at the Institute of Crustal
Dynamics，CEA． She was granted a Master of Science degree in solid geophysics by Institute of
Crustal Dynamics in 2009． She engaged in seismic hazards and risk assessment research，mainly
including seismicity and seismic hazards． E-mail:xiezhuojuan@ sohu． com