| Izu-Bonin-Mariana(IBM)subduction zone is an intraoceanic convergent margin between the overriding Philippine Sea plate and the subducting Pacific plate.Strong tectonic activity,magmatic activity and regional metamorphism are detected in the subduction zone,and seismicity distributed in heterogeneity.To lay a foundation for better understanding the subduction dynamics process and key characteristics,it is necessary to study the seismotectonic characteristics,assess the sesmic risk and analyze the main mechanism causing the uneven distribution of earthquakes in IBM subduction zone.A series of dynamic process,including trench retreating,slab deflection,mineral dehydration,mantle wedge melting and volcanic activity,could be induced when the cooler ocean plates inserted into the warmer mantle,which would cause local stress concentration.In the absence of observed stress field data,seismic b value can be used as a tool to describe stress.In order to characterize the stress distribution and its causes in IBM subduction zone,this paper combined seismic b-value and thermal model to study seismotectonic characteristics with related mechanism.The main work includes:(1)Based on systematic analysis of the geological background,including the features of slab morphology,submarine topography,back-arc spreading and the location of great earthquakes,the IBM subduction zone is divided into five segments,including Izu,Bonin,northern Mariana,central Mariana and southern Mariana,respectively.(2)Method and numerical calculation: firstly,the spatial distribution of b-values in each segment with its uncertianty were calculated and analyzed based on earthquake catalog;secondly,the geological models were constructed based on geological and geophysical information,and the temperature distribution of five segments were simulated using finite element method with appropriate parameters.(3)Seismotectonic characteristcs: firstly,the seismotectonic characteristics in each segment was studied based on combining the distribution in b-values and temperature with the results of petrological experiments;secondly,regional changes in the b-value distribution before and after large earthquakes were analyzed to study the influence of the great earthquake on local seismogenic process,for discussing the potential of great earthquakes in the future;finally,the main mechanisms of earthquake heterogeneity were analyzed based on the commonness and differences of seismotectonic characteristcs in each segment.Based on the analysis and discussion of the seismotectonic characteristics in different regions and the results of previous research,the main mechanisms for earthquake in heterogeneity were concluded,including tension and compression,dehydration,mantle wedge melting and volcanic activities.(1)Tension and compression: including tension stress caused by back-arc spreading,negative buoyancy,tensile stress by slab bending,and compression caused by slab bending and convergence between plates.The difference of stress in different regions and depths led to earthquake distribution in heterogeneity.(2)Dehydration: mineral dehydrated as temperature rised in the process of subduction,and serpentinization occurred at the mantle wedge to changed the friction coefficient between plates,which affected the degree of coupling.Earthquake at intermediate depth occurred due to different minerals dehydration under different temperature-pressure conditions.Because hydration of the Pacific plate in different regions were different,the degree of serpentinization at the bottom of the mantle wedge is different.Bonin,central Mariana and southern Mariana were weaker coupling than Izu and northern Mariana.(3)Mantle wedge melting: The embrittlement/ductile fracture were caused by the different temperature-pressure conditions of the fluid entering the mantle wedge,which affected the earthquake distribution in the mantle,mainly distributed in Bonin and central Mariana.(4)Volcanic activity: the lithosphere was weakened under the volcano,and the earthquake distribution were affected by fault development.High temperature environment weakened the crust,and continuous influx of magma eventually erupted causing many small faults,which triggered a series of small earthquakes and formed low stress areas.The low stress zone under the volcano was mainly distributed in the central Mariana. |
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