A Tomographic Study Of Seismic Velocity,Anisotropy And Attenuation In Subduction Zones

Subduction zone is the main place where material and energy exchange take place between the earth surface and deep parts.Slab dehydration and mantle wedge conversion exist in subduction zone,which cause obvious temperature changes and generate volcanic and non-volcanic low-frequency earthquakes.It's very important to study structural character and evolution process of the subduction zone so that we can understand the geometry,lithosphere age,physical property of the subducting slab and the generation mechanism of the low-frequency earthquake and the megathrust earthquake.In the past four decades,many seismic tomography studies have been conducted in subduction zones,which provide important information for the slab geometry,the physical-chemistry property of the mantle wedge,and the origion of the arc-volcano.Previous studies show that the subducting slab has a low temperature and a high seismic velocity.Slab dehydration takes place and releases fluids into hot mantle when the cold slab subducted,reducing the melting point of the mantle wedge and generating the arcvolcano.The resulted fluids ascend to the crust.When the fluids enter an active fault in the crust,pore pressure and fault zone friction will decrease,which can trigger large crustal earthquakes.In recent years,seismic anisotropy tomographic studies and seismic attenuation tomographic studies have also provided important informations for us to understand subduction dynamics.The slab geometry,physical property of the subducting slab and the seismotectonics and seismogenesis vary with subduction periods.Seismic tomographic studies in subduction zones at different subduction periods can promote our understanding of the subduction pattern with different geodynamic.However,there is few seismic studies to compare the characters of subduction zones at different subduction period.To improve our understanding of the subduction pattern and seismogenesis at different subduction periods,in this study we study high-resolution 3-D structures of the Western Junggar subduction zone(a fossil subduction zone),the Alps subtuction zone(continental deep subduction zone)and the Jappan subduction zone using seismic velocity tomography,seismic anisotropy tomography and seismic attenuation tomography.We obtained the following results:(1)A high-resolution 3-D P-wave velocity model of the upper mantle beneath the Western Junggar is determined by inverting a large number of relative travel-time residuals of teleseismic events.The NE-SW striking high-V anomaly at depths of 50-200 km beneath the western Junggar basin dips toward the northwest,which represents a fossil oceanic slab.The low-V anomaly may reflect an intraoceanic arc-related terrain that has experienced strong metasomatism by hot upwelling asthenosphere in the late Paleozoic,resulting in adakitic intrusions and concentration of porphyry Cu-Au deposits in the Darbut belt.(2)The first tomographic images of P wave azimuthal and radial anisotropies in the crust and upper mantle beneath the Alps are determined by joint inversions of arrival time data of local earthquakes and teleseismic events.Our results show the south dipping European plate with a high-velocity(high-V)anomaly beneath the western central Alps and the north dipping Adriatic plate with a high-V anomaly beneath the Eastern Alps,indicating that the subduction polarity changes along the strike of the Alps.The results of anisotropic tomography provide important new information on the complex mantle structure and dynamics of the Alps and adjacent regions.(3)Seismic velocity and attenuation tomographic images of P and S wave beneath the Hokkaido island are determined.Seismic attenuation model show that the dipping Northeastern(NE)Japan arc(high Q)is descending beneath the Kuril arc.Low-Q anomalies are also revealed in the forearc crust and upper-mantle wedge,which reflect the existence of fluids from dehydration of the subducting Pacific slab.When the fluids enter the active faults in the overlying plate,pore pressure will increase and fault zone friction will decrease,which can trigger a large earthquake.For the active ocean-continental subduction zone,the subducting slab is imaged as a high seismic velocity and a low seismic attenuation anomaly,it has a clear upper boundary.The seismic anisotropy model shows different patterns in the subducting slab and mantle wedge.For the continental deep subduction zone,the subducting slab has a clear geometry,however,the slab may fold at deep mantle when losing driven force.The seismic anisotropy pattern in the cold slab and hot mantle do not show a significant change.For a fossil subduction zone,the fossil slab can keep a high-V anomaly,however,it do not has a clear geometry.The results of this study play a key role for us to understand the slab geometry,subduction geodynamics and seismogenesis of subduction zone at different subduction periods.