Study On Seismic Activities In Seismic Zoning Of China's Seas And Adjacent

With the rapid development of the marine economy,it is necessary to compile marine seismic zoning maps to inform the engineering and construction of marine projects.One of the core contents of a marine seismic zoning map is the characterization of the region's seismicity and the determination of the relevant seismicity parameters.However,due to the special location of the sea area,the quality of the data is limited by the density of the network.Compared to the high station density typically found in seismically active areas on land,the basic seismic activity data of the marine area is insufficient,the seismic data are scattered,sources and magnitude scale are diversified.The characteristics of seismic activity in the sea area are both intraplate and interplate earthquake,and the two types of earthquakes are different in nature,intensity,source depth,seismic activity rule and mechanism,resulting in the relative lack of studies on seismic activity in the sea area.At present,there are some problems in the study of seismicity in China's marine seismic zoning,such as the lack of a unified seismic catalog and its integrity analysis,not suitable for sea area in the magnitude of conversion method and empirical formula,there is a lack of in-depth study on marine seismicity,as well as seismicity models and seismicity parameters for different source depths(including subduction zones)in the sea area,which are based on the characteristics of seismic activity in the sea area.In this study,we built a unified seismic catalog and conducted an integrity analysis of the seismic that cover China's seas,which provides essential reference data for future studies that will investigate the tectonically active structures,potential source areas areas at shallow and the "three-dimensional" potential source areas at middle and deep depths,and seismicity trends in the China's oceanic areas.Once we established the catalog,we analyzed the seismicity and determined the relevant seismicity parameters for the recorded earthquakes.These baseline data are vital for the creation and refinement of seismic zoning maps for China's seas.The contents of our study include the following.(1)The unified earthquake catalogue of China's sea and adjacent areas has been compiled,which fills the gap in the compilation of the earthquake catalogue of China's sea and adjacent areas.We established a catalog that contains information on both destructive earthquakes(M? 4.7)and medium-and small-magnitude earthquakes(2.0 ? M<4.7)that occurred in China's seas.It provides important basic data for the future compilation of seismic zoning maps in China's marine regions and the refinement of the Chinese marine seismic catalog,and further improves the technical method of seismic catalogue compilation in China.(2)The method of magnitude conversion suitable for sea area is proposed and the corresponding formula of magnitude conversion is established.After systematically analyzing the differences between the surface wave magnitudes and the moment magnitudes measured by GCMT and NIED,we compared those magnitudes with those of earthquakes measured on land.We also examined the magnitude differences between the earthquakes recorded on the China Seismic Network,the Taiwan Seismic Network,and the Philippines Seismic Network over a range of depths,time periods,and geographical regions.Then,we used orthogonal regression and linear least squares regression to obtain formulas that convert the surface magnitudes from GCMT and NIED into moment magnitudes.Furthermore,we established magnitude conversion equations for the earthquake magnitudes recorded in the China Seismic Network,the ML earthquake magnitudes recorded in the Taiwan Seismic Network,and the MS earthquake magnitudes recorded in the Philippine Seismic Network.These conversion relationships are distinctly different from previously established conversation equations that only apply to shallow continental earthquakes.In addition to forming the baseline for future marine earthquake magnitude assessments,this information will facilitate the refinement of the seismic network gauge function,the correction of magnitude deviations,and the reconciliation of disparate seismic data from the same earthquake in different seismic networks.(3)The spatial and temporal distribution characteristics of the Magnitude of completeness and the completeness of seismic data in in China's sea and adjacent areas are given.We evaluated the earthquake-monitoring capabilities and the accuracy of the earthquake epicentral locations of the different seismic networks in various sea regions and time periods.By identifying the areas with poor seismic monitoring and inaccurate earthquake locations,we can improve both the results of the integrity analysis and the marine seismic monitoring networks in China's coastal and offshore areas.After removing the foreshocks and aftershocks from the earthquake catalog,we Obtained the beginning years of each magnitude interval of seismic zone and determined the spatial and temporal distribution of Mc values over a range of different source depths using the accumulative frequency method and the entire-magnitude-range method(EMR).(4)Based on the completeness of the seismic data and the characteristic seismic activity,we developed seismicity models and established the corresponding seismicity parameters for different regions in China's marine regions.We studied the spatial and temporal,intensity and frequency distribution distribution of the seismic activity both within plates and at plate boundaries such as subduction zones,and studied the relationship between seismic activity and tectonic structure.After evaluating the applicability of applying the least squares(LS)method and the maximum likelihood(MLE)method in calculating the b value of China's sea area and adjacent area,we adopted the multi-scheme method to determine the seismicity parameters,the b-values,and the V4 values of those seismicity parameters in China's marine seismic zones.The least squares method we applied to the data is different from the least squares method typically applied to earthquakes that occur on land;this study represents the first successful attempt to determine the seismicity parameters in multiple seismic zones located in China's marine areas,in view of the seismic data integrity and the characteristics of seismic activity.This paper analyzes the distribution characteristics of focal depth and seismic activity in subduction zone,and provides a basis for dividing the shallow potential source area and the "stereoscopic" potential source area in the subduction zone at plate boundary and determining the seismicity parameters.The distribution pattern of subduction action in the subduction zone is understood,and the b value of the deep source region in the subduction zone is studied in detail.Analysis of subduction zone b value variation characteristics with depth in the different area,and the distribution characteristics of b values variation with depth in different sections of the subduction zone.The research results of this paper are directly used in the trial compilation of marine seismic zoning map,and also provide the basic data and technical support necessary for long-term earthquake prediction,seismic safety evaluations,seismic zoning,the engineering and construction of various ocean projects,and the improvement of the seismic disaster prevention system in China.The map of the different b values can be used to identify high-stress areas of a given plate boundary that may break in the future under different regional tectonic backgrounds,which provides valuable insights into the different mechanisms and processes that result in deep seismicity.