Influencing factors and evaluation application of regional crustal stability in the Bohai Strait

Sai Mei, Hong-xian Chu,*, Ling-yu Dong, Zhong-hua Fang, Pan-feng Li, Fei-fei Zhang,Rui Shan, Tie-hu Zhao

a Qingdao Institute of Marine Geology, China geological survey, Ministry of Natural Resources, Qingdao 266071, China

b Laboratory for Marine Mineral Resources, Qingdao National Laboratory for Marine Science and Technology, Qingdao 266071, China

Keywords:

Regional crustal stability

Trans-straits Passage

Coastal geological survey Engineering

GIS

Bohai Strait

China

A B S T R A C T

Due to the extremely complex geological conditions in the Bohai Strait, active faults is very developed and earthquake activity occurs frequently.It is of great importance to evaluate the regional crustal stability for the planning and construction of the Trans-straits Passage.In this study, the authors carry out the evaluation by means of buffer analysis, topology analysis and overlay analysis based on ArcGIS software.The evaluation mainly analyzed six assessment factors of Bohai Strait including neotectonic activity,active faults distribution, seismicity, tectonic stress fields, Bouguer gravity anomaly and geological disasters.In brief, the crustal stability of the eastern North Yellow Sea Basin is best, while that of the Bohai Basin and southwest Bohai Strait is poor.Meanwhile, small and frequent earthquakes in the southern Strait and geological disasters developed in the northern and central Strait such as tidal ridge and shallow gas which may adversely impact on the Trans-straits Passage construction.

1.Introduction

Circum-Bohai-Sea region is one of the three core economic zones in China with Beijing-Tianjin-Hebei region as its central part and the Liaodong Peninsula and Shandong Peninsula as its wings.For a long time, due to the separation of the Bohai Strait, the disconnection of Bohai Sea northern and southern sides has greatly limited passengers, cargo transportation as well as economic development.It is of great importance in economic and military strategic aspects,therefore, the government decides to accelerate the construction of the Bohai Strait Trans-straits Passage from Dalian to Yantai and build a 5700 km railway artery across 11 provinces, autonomous regions and municipalities between Heilongjiang and Hainan (Wang MS, 2015).The idea for the Bohai Strait Trans-straits Passage construction is based on the favorable geographical conditions to build an all-weather,multi-functional transportation trunk line connecting the north and south of the Bohai Sea between Penglai and Lüshun.However, the geological conditions are extremely complex in the Bohai Strait with several active faults developed and frequent seismic activities.

The evaluation of regional crustal stability is based on the interaction and influence of internal and external geological dynamics, rock mass media and geological disasters induced or superimposed by human activities on the construction of the project.As for the relative stability of the current crust and surface, the geological disasters of internal and external dynamics are the research objects.The research goal is to avoid the zones prone to seismic activity, fault activity and geological disasters, and to find relatively stable zones taken as project construction sites.Since the preliminary demonstration of the major project is related to the economic rationality, technical feasibility, safety and reliability of the project construction, the evaluation of regional crustal stability has been paid much attention by the construction planning department and engineering geology community.(Hu HT, 2001; Wu ZH, et al., 2005; Tang HM, et al., 2009;Guo CB, et al., 2009; Yao X, et al., 2015; Yin P, et al., 2018).

Based on the previous studies (Zhao TH, et al., 2014; Li PF, et al., 2015; Zheng HX, et al., 2015; Sun YB, et al., 2015;Shi HJ, et al., 2015; Liu ZY, et al., 2016), this paper refers to the latest achievements of the Bohai Strait Trans-straits Passage fault investigation project in 2018 and comprehensively summarizes the internal and external dynamic geological processes in six aspects for study and analysis, including the current crustal deformation features,active fault distribution features, seismic activity features,tectonic geostress field features, the Bouguer gravity anomaly regional stress field features and shallow geological disaster features.Furthermore, the crustal stability comprehensive evaluation of the Bohai Strait Trans-straits Passage has been carried out to provide basic data and decision-making basis for the project construction.

2.Influencing factors of the regional crustal stability evaluation

Liu ZY et al.(2016) selected five factors that are fault activity, seismicity, topographical variation, geophysical field and geostress field from the perspective of internal dynamic geological process to evaluate the crustal stability of the Trans-straits Passage.As the evaluation of regional crustal stability is a comprehensive process involving multiple factors and angles, the comprehensive impact of internal and external dynamic geological processes should also be considered.

The shallow seabed geological disasters such as tidal ridges, erosion ridges and active faults seriously affect the safety of marine projects, which are also important factors of regional geological stability.Therefore, the main influencing factors of evaluation in this paper include fault and its activity, seismic activity, crustal deformation rate,current geostress concentration, regional Bouguer gravity anomaly features and shallow seabed geological disasters.

2.1.Faults and its activity

Yanshan, Jiao-liao and Luxi mountains in the Bohai Strait were generally uplifted in the early to middle Miocene, and the structures were stable in the late Miocene and Pliocene.The surface was strongly denuded and planated, forming the Tangxian planation surface (7-3 Ma b.p.).In the Quaternary period, there were frequent and intermittent uplifts, and 4-grade river terraces were generally developed.Actually, the Bohai bay Basin and the north Yellow Sea Basin show periodic sedimentation, generally depositing the Neogene and Quaternary with a thickness of more than 1000 m, forming the discontinuous surface between the Oligocene-Miocene and the upper Pliocene-Quaternary.Quaternary deposits are also rhythmic, for the intermittent uplift of mountain areas and the periodic subsidence of the basin.

There are three main tectonic active fault zones in Bohai Strait.The Yingkou-Weifang NNE fault zone is a continuous active tectonic zone with a dextral lateral reversedisplacement activity fault and weak intensity; the Beijing-Penglai NWW fault zone is also a continuous active tectonic zone with a sinistral lateral displacement activity fault and strong intensity; the northeast Yellow River Estuary-Northwest Miao Fault Zone is a neogenic tectonic fault zone with dextral lateral displacement activity and strong intensity.The latter two constitute a pair of conjugated active tectonic zones dominated by the Yellow River Estuary-Northwest Miao fault zone, which is closely related to seismic activity.

On the basis of the existing data collected and the multichannel seismic survey data in Bohai Strait (Sun YB, et al.,2015), the active fault distribution in Bohai Strait was clarified (Fig.1) and the main active fault properties were summarized (Table 1).

Fig.1.The active fault distribution in Bohai Strait.

Table 1.Major active faults in Bohai Strait.

2.2.Seismic activity

Seismic activity is one of the most important factors affecting regional stability and also the most destructive factors for engineering construction.The indicators that reflect the degree of seismic activity are magnitude, basic intensity and crustal strain energy of the earthquake.Based on the continuous accumulation of seismic data and numerous indepth seismic geological researches, the received distribution of the earthquake area is closer to the actual reflection.The potential hypocenter area division and its level of seismic zoning evaluation not only reflect the historical earthquakes in the area, but also predict the trend of future earthquakes in certain predictability.Thus, the potential hypocenter area ranks by the evaluation index of seismic activity.

The historical characteristics of regional strong earthquakes are firstly expressed that the frequency of earthquakes with magnitude 7 above is relatively high and concentrated.In the Bohai Strait (38° -39° N, 119° -119° E),there are four earthquakes ofM＞7, which form the most earthquake-concentrated area in the North China.Meanwhile,the magnitude of an earthquake occurrence is relatively low in other sea areas, such as in the north of the Liaodong Peninsula and the south of the Laizhou Bay.There are only moderate earthquakes with about 5-magnitude and 6-magnitude earthquakes occurred in the Yellow Sea area in the study area.

Based on the potential source area division scheme of the fifth generation of China Earthquake Parameter Zoning Map(2010), the hypocenter areas are divided according to the research results of seismic structure and seismic activity environments in this area and surrounding area.Total of 85 potential source areas are divided, including 2 potential hypocenter areas of 8.0 magnitude, 4 potential hypocenter areas of 7.5 magnitude, 17 potential hypocenter areas of 7.0 magnitude, 28 potential hypocenter areas of 6.5 magnitude, 18 potential hypocenter areas of 6.0 magnitude, and 2 potential hypocenter areas of 5.5 magnitude, as shown in Fig.2.

Fig.2.Potential hypocenter area division map in the Bohai Strait.

2.3.Characteristics of crustal deformation

The present crustal deformation rate is closely related to crustal structure, stress state and lithospheric dynamic conditions.Using high-precision GNSS data and the latest high-precision GPS calculating software GAMIT/GLOBK version 10.50, the velocity field in the research area was solved, and a total of 729-day data were downloaded from the 183rdday of 2011 to the 180thday of 2013.

After calculating the GPS data, the horizontal velocity of the Circum-Bohai-Sea Region is about 6.8±0.8 mm/a, and the average horizontal velocity of the North China region obtained by Wang M et al.(2003) is about 7.1±1.0 mm/a.Both results showed good consistency.The overall trend of the two velocity fields is consistent, both showing a weak southeastward movement (Fig.3).The middle part of the Yingkou-Weifang fault zone and the eastern part of the Zhangjiakou-Penglai fault zone present a weak compression movement with a velocity of about 1.5 mm/a.

Fig.3.The regional velocity field under the Eurasian plate frame.

2.4.Characteristics of tectonic stress field

The study of the tectonic stress field simulation is mainly to simulate the current tectonic activity and its evolution,which is of great significance for predicting the crust stability in future engineering application.Based on the systematic analysis of the active faults, crustal structure and GPS observation results of current crustal movement in the Bohai Sea region, the variation law of current crustal deformation field and tectonic stress field formed by crustal movement in the Bohai Sea and adjacent regions, is analysed on the ANSYS software, which is commonly used in finite element simulation.

The model unit type is PLANE183.Referring to previous research results (Gao WB and Luo D, 2013; Xie QH, et al.,2012), the Young's modulus of the surrounding rock around the fault is taken as 5e9Pa, and all the faults in the model are treated as contact friction units.The Young's modulus of the fault is 1/3 Young's modulus of the surrounding medium and the Poisson ratio is slightly larger than that of the surrounding medium, with the friction coefficient as 0.4.The final finite element model contains 45339 elements and 133599 nodes.For the determination of the model boundary conditions, the existing GPS point displacement is mainly used, and the boundary loads are applied to the east-west and north-south direction respectively, and all the displacement boundary conditions on four boundaries of the model are obtained.

Controlled by regional active block difference and boundary fault zones, the stress field has obvious zoning characteristics (Fig.4).The tectonic stress field in the Chaoyang-Beipiao fault and the adjacent area is northwestsoutheast horizontal compressional stress; that in the Yingkou-Weifang fault and the adjacent area is east-west horizontal compressional stress; that in the south of the Zhangjiakou-Penglai fault generally transforms from northeast and northeast-eastward direction in the west section to near east-west direction in the east section.According to the hypocenter mechanism solution of moderate and small earthquakes and the results of geostress measurement, the direction of the maximum principal stress in the Bohai Sea is dominated by northeast-eastward and near east-west direction(Cui XF, et al., 2010), which is demonstrated by the simulation prediction.

Fig.4.The maximum principal compressional stress field of 0.5°×0.5° in the Bohai Sea based on the simulation results.

2.5.Anomaly characteristics of regional Bouguer gravity

Regional Bouguer gravity anomalies are caused by changes in crustal thickness and crustal material inhomogeneity.The Bouguer anomaly isoline has a high density and zonal occurrence, which mostly indicate the deep faults.The lack of gradient zone indicates that deep crustal faults are not developed.Therefore, gravity gradient zone are closely related to deep faults and seismic zones.The highvalue gravity anomaly gradient zone is the crust modern active area, which has poor regional stability.

The Bouguer gravity data is obtained by 20 km×20 km Bouguer grid data in mainland China, and the Bouguer gravity anomaly data in the sea area is obtained by Bouguer correction using seawater depth data published by Scripps Research Institute.According to the Bouguer gravity color contour map (Fig.5), the Bouguer gravity anomaly in Bohai Strait is divided into east and west anomalous areas by the NE gravity anomaly gradient zone at the Yingkou-Weifang fault.The region has a low gravity anomaly zone in the west.

Fig.5.Color contour map of Bouguer gravity anomaly.

2.6.Geological disasters in shallow seabed

The phenomenon of geological disasters in the shallow seabed seriously affects the safety of marine engineering construction.Therefore the influence of this factor on regional crustal stability should be fully considered in the geological.The Bohai Strait is a channel for material exchange between the Yellow Sea and the Bohai Sea.The geological environment is complex and there are many geological disaster factors.Combined with previous research results(Chen XH, et al., 2014), the distribution of major disaster geology in the Bohai Strait was delineated by a fine interpretation of high-resolution sub-bottom profiles and sidescan sonar data (Fig.6).In the Bohai Strait, since the Late Pleistocene in the Bohai Strait, there are many types of geological disaster factors, such as tidal sands, tidal scour trough, eroded ridges, trenches, erosion depression, reefs,shallow gas, buried channel, active fault, irregular bedrock,etc.(Table 2 and Fig.7).

Fig.6.Seabed geological disaster map in the Bohai Strait.

Fig.7.Seismic interpretation profile of the Bohai Strait (Dazhushan-Weihai North Fault).

Table 2.Classification of geological disaster in Bohai Strait.

3.Comprehensive evaluation of regional geological stability

3.1.Divisions of evaluation area

The division of the to-be-evaluated area is the basis for the evaluation of reginal crustal stability.As the key area for the evaluation of crustal stability is the Bohai Strait Trans-straits Passage area, the evaluation area is smaller than the study area, which ranges from 120°E to 122°E, and 37°E to 39°E.On the premise of the regional crustal stability evaluation range and scale, the study area is divided into 1 km×1 km grids, and finally divided into 90577 calculation units, which uses WGS84 coordinate system and Mercator projection with a central meridian longitude of 121°E and a non-deformed latitude of 38°24’N.

3.2.Quantification of evaluation index

The geological elements of evaluation indexes should be fully expressed.Some of them are descriptive and cannot be quantified by absolute values.Therefore semi-quantitative methods can only be used to reflect their differences in different sections.In accordance with the above guiding ideology, the authors take the evaluation of regional crustal stability in the Trans-strait Passage as a goal to quantify various indicators affecting regional crustal stability.The quantification of evaluation indicators and the final evaluation analysis were completed on ArcGIS.During the quantification process, four indexes can be directly quantified (tectonic geostress field, Bouguer gravity anomaly value and the size of current crustal deformation), contour lines are drawn first and then the values are assigned by equally spaced scores of 0-10.For indicators that cannot be directly quantified, the scores of different levels are assigned referring to the existing research results.

The quantification of the fault activity age is based on the study of active faults.The newer age indicates that the activity is stronger, which can be considered together with the rate of the fault activity.There are 99 faults with different scales in Bohai Strait.Through the correlation of the relevant data collected and the seismic profile acquired by this project, the activity age of each fault can be basically determined.According to the activity age of the fault, it is divided into five grades, which are assigned respectively: (1) Holocene fault, assigned 10 points; (2) Late Pleistocene fault, assigned 8 points; (3) Early-middle Pleistocene fault, assigned 5-7; (4) The former Quaternary fault, assigned 3 points.(5) No active break, assigned 0 points.

For the quantification of seismic evaluation indicators,scores are assigned according to different magnitude levels of potential hypocenter areas (hereinafter referred to as ms):(1) ms≥8, assigned 10 points; (2) ms=7.5, assigned 8 points;(3) ms=7, assigned 6 points; (4) ms=6.5, assigned 4 points;(5) ms=6, assigned 2 points; ms≤5, assigned 0 points.

For the quantification of shallow seabed geological disasters and the dangerous level of different types of geological disasters (Ye YC, 2012), the following principles are applied:

Sand ridges and shallow gas affected areas, as relatively direct geological factors of seabed disasters, are assigned 10 points; the scouring trough affected area, is assigned 8 points;the seabed eroded ridge affected area, is assigned 6 points;magmatic rock and channel buried affected area, as potential disaster geology, is assigned 4 points; the area without disaster geological influence, is assigned 0 point.

3.3.Weight distribution of evaluation index and calculation method of stability index

There are many methods to determine the magnitude or importance (weight) of each evaluation factor in the regional crustal stability, such as the expert direct experience method,survey statistical method, mathematical statistics method,analytic hierarchy process, etc.According to the weight distribution scheme of the quantitative evaluation index of the crustal stability recommended by predecessors (Sun Y, et al 1997), and the data collected in the study area, the weights of the selected six evaluation indexes are assigned as follows:

(1) The weight of the fault and its activity is 0.2;

(2) The weight of seismic activity is 0.2;

(3) The weight of the current stress concentration is 0.15;

(4) The weight of the crustal deformation rate is 0.15;

(5) The weight of the gravity anomaly gradient is 0.15;

(6) The weight of shallow seabed geological disasters is 0.15.

Based on the above weight distribution scheme, the score of each single element of each calculation unit was calculated in ArcGIS, and then the stability index of each unit was calculated according to the above weight value, that is crustal stability index = fault × 0.2 + earthquake × 0.2 + geostress ×0.15 + vertical crustal activity rate × 0.15 + gravity anomaly gradient characteristic × 0.15 + geological disaster × 0.15.

3.4.Regional geological stability evaluation based on GIS

As can be seen from the above evaluation indexes of geological stability in the study area, the data types involved are complex, it is necessary to integrate these data into a unified spatial database management for conducting comprehensive research and data utilization.

In order to facilitate the classification by using the stability index, the calculated results were normalized according to equation (1).Its purpose is to make the evaluation results of all factors within the interval [0, 10].Its calculation formula is as follows:

Where:Yi,j-the numerical value of the i evaluation unit of the evaluation elementjafter normalization;Xi,j-the numerical value of theievaluation unit of the evaluation elementj; minXi,j-the minimum value of the evaluation elementj; maxXi,j-the maximum value of the evaluation elementj.

By ArcGIS spatial analysis function, the corresponding vector data of each index according to the above crust stability evaluation method is used to calculate the buffer analysis, overlay analysis and topology processing operation,set up the evaluation index and the corresponding attribute database, generate a comprehensive partition map and a research area evaluation partition map of all evaluation indexes.

According to the stability index and grading standard of the comprehensive evaluation of each unit, the results of regional crustal stability evaluation of the Bohai Strait Transstraits Passage are obtained (Fig.8), which can be divided into four levels: stable, relatively stable, relatively unstable and unstable.The stable area is about 21232 km2, accounting for 23.5% of the area; 22183 km2of the relatively stable area,accounts for 24.5%; 22826 km2of the relatively unstable area,accounts for 25.3% of the area; 23997 km2of the unstable area accounts for 26.7%.In general, the east of the Bohai Strait is the north Yellow Sea Basin with the best regional crustal stability, while the western Bohai Basin region and the southern strait region have poor crustal stability.

Fig.8.Evaluation map of regional geological crust stability.

4.Conclusions

(i) We compiled the distribution map of the main active fault in the Bohai Strait Trans-straits Passage area, based on the collected high resolution seismic profile.The crustal stability of the northern Yellow Sea Basin on the east of the Bohai Strait is the best, and the crustal stability of the western Bohai Basin and the southern Strait area is poor.The relatively unstable area is greatly affected by the active fault zone in the area, is concentrated in the NE Zhangjiakou-Penglai fault zone, the NNE Yingkou-Weifang fault zone and the NW Miaoxibei- Huanghekou fault zone.

(ii) In the southern Bohai Strait, there are frequent small and moderate earthquakes, but there are no earthquake records of magnitude 7 or above in history.The geological factors such as tidal sands and shallow gas are more concentrated in the northern and central parts of the strait.Both of the earthquakes and the geological factors can adversely impact on the construction of the Trans-straits Passage, which should draw sufficient attention.In the possible selection site of the Bohai Strait Trans-straits Passage Project, more detailed engineering geological stability investigations and research work is needed.

Acknowledgement

This work was supported by the National Natural Science Foundation of China (41506119, 41276060), the Geological Investigation Project of China Geological Survey(201200504, DD20191003).Thanks Prof.Xu Jie, Zhang Lifang and other anonymous reviewers for their comments and suggestions, which significantly improved the quality of the manuscript.