Deep Structure,Deformation And Seismogenic Condition In Subduction Zones

Subduction zones are one type of active convergent plate boundaries where abundant and different earthquake phenomena take place.Slow earthquakes and three kinds of ordinary earthquakes are found,i.e.,the intraslab earthquake,the interplate earthquake,and the overriding plate earthquake.However,how plate subduction affects earthquake generation and what's the common seismogenic condition of these different earthquakes are still poorly understood.To have a systematical further knowledge of different kinds of subduction earthquakes,I take slow earthquakes in Alaska and three normal earthquakes as examples and apply seismic anisotropy tomography to study their deep deformation background and seismogenic conditions,i.e.,the 2018 Alaska Anchorage intraslab earthquake(M 7.1),the 1964 Alaska megathrust earthquake(Mw 9.2),and the 2018 Hokkaido Eastern Iburi shallow inland earthquake(M 6.7).Firstly,to have a good knowledge of deep deformation of the subduction zone,I obtained the high-resolution 3-D deep azimuthal and radial anisotropic models of Alaska.The depth-varying anisotropy reveals that flat subduction of the Yakutat/Pacific plate plays an important role in interplate and intraplate deformation.Secondly,to study the intraslab earthquake,I obtained the high-resolution 3-D seismic structural models(Vp,Vs and Poisson's ratio)of the source zone of the 2018 Anchorage earthquake.The results show that the mainshock occurred in a strong dehydration zone,which is characterized by a higher Poisson's ratio than the normal slab.Fast velocity directions(FVDs)of Vp azimuthal anisotropy are well consistent with east-west oriented extensional axes of background intraslab focal mechanisms,suggesting that the intraslab earthquakes including the Anchorage earthquake are related to not only slab bending but also preexisting fossil fabrics.Thirdly,to investigate the interplate earthquake,I analyzed the structural heterogeneity of the source zone of the 1964 Alaska megathrust earthquake.The results show that the initial rupture of this huge earthquake took place beneath the overriding crust with a high-Vp,high-Vs and high Poisson's ratio feature,whereas large coseismic slips occurred beneath a low-Vs and high Poisson's ratio zone.It indicates that this megathrust earthquake was controlled by structural heterogeneities of the overlying plate with fluid contributions.Fourthly,to study the overriding plate earthquake,I obtained the high-resolution seismic velocity and Vp anisotropic models of the source zone of the 2018 Hokkaido Eastern Iburi earthquake.The tomographic results reveal that the mainshock and its main aftershocks occurred at an edge of high-Vp brittle seismogenic zone in the uppermost mantle.A low-Vs and high Poisson's ratio anomaly is imaged in and beneath the source zone and extends to the upper surface of the subducting Pacific plate,which most likely reflects slab dehydration and releasing fluids ascending in the mantle wedge.The fluids entered the seismogenic fault and might trigger the Eastern Iburi earthquake.In addition,predominantly east-west FVDs of Vp azimuthal anisotropy are revealed in the uppermost mantle,probably indicating a crust-mantle shear induced by mantle convection due to the arc-arc collision.Under an east-west compressional stress regime,strain accumulation took place in the seismogenic fault and so resulted in the Eastern Iburi earthquake.Finally,the tomographic results reveal the close relationship between slow earthquakes and subduction dehydration.In the Upper and Lower Cook Inlet,two areas with long?term slow slip events predominantly exhibit a low Vs and a high Poisson's ratio in the lowermost portion of the crust and the cold nose of the mantle wedge,whereas a low Poisson's ratio zone is revealed between them.These features suggest that their segmentation is likely induced by localized slab?releasing fluids,due to along-strike permeability variations of the megathrust zone.In conclusion,the high-resolution 3-D seismic velocity and Vp anisotropic models indicate that subduction and its related processes control mantle convection,tectonic deformation and earthquake generation.The present study reveals a key role of slab dehydration and its releasing fluids in generations of various earthquake phenomena in subduction zones.In addition,seismic anisotropy tomography acts as a powerful tool for deepening understanding of tectonic deformation in subduction zones and provides important insight into seismogenic condition there.