洋脊俯冲对智利三联点南部孕震区尺度影响的动力学模拟

Geodynamical simulation of the effects of ridge subduction on the scale of the seismogenic zone south of Chile Triple Junction

智利三联点以北地震较多,智利三联点以南地震很少且热异常显著。为探究洋脊俯冲对由温度定义的孕震区的影响,以智利三联点区域的地质背景为基础建立二维有限元数值模型,对洋脊俯冲过程进行数值模拟,并将俯冲角度和汇聚速率等因素对孕震区的影响进行了对比。结果表明,洋脊俯冲过程中孕震区宽度减小,导致发生在智利三联点以南的地震远少于智利三联点以北。剖面附近的观测数据与数值模拟结果的对比表明,数值模拟可以大致反映智利三联点区域板块间的孕震区宽度和地表热流特征。当俯冲汇聚量相同时,板块间的汇聚速率越大,洋脊俯冲过程中孕震区则越宽且其下边界越深,海沟附近的地表热流越高。与汇聚速率相比,俯冲角度等因素对地表热流的影响较小。俯冲角度越大,洋脊俯冲过程中孕震区越窄。当数值模型包含剪切生热时,洋脊俯冲过程中孕震区宽度大约可减小至15 km,且孕震区很浅。这可导致智利三联点以南部分区域难以发生地震,出现观测不到和达-贝尼奥夫带的现象。

Earthquakes are much more frequent to the north of Chile Triple Junction than to the south,where the thermal anomaly is also more significant.To study the effects of ridge subduction on the thermally defined seismogenic zone,two-dimensional finite element models were established based on the geology of Chile Triple Junction,the process of ridge subduction was simulated,and the effects of the initial slab dip and the convergence rate on the seismogenic zone were compared.The results show that the width of the seismogenic zone decreases during the ridge subduction,inducing earthquakes to occur much less to the south than to the north of Chile Triple Junction.By comparing the observed data in the vicinity of the profiles with the numerical simulation results,we find that the numerical simulation can roughly reflect the width of the interplate seismogenic zone and the surface heat flow in the area of Chile Triple Junction.At the same convergence distance,a larger convergence rate comes with a wider seismogenic zone and the deeper the downdip limit of the seismogenic zone,the higher the surface heat flow in the vicinity of the trench.Compared with the convergence rate,factors such as slab dip have little effect on the surface heat flow.In the process of ridge subduction,larger the slab dips leads to narrower seismogenic zones.When the effect of shear heating is included in the simulation,the width of the seismogenic zone in the process of ridge subduction can shrink to about 15 km and the depth of the seismogenic zone is small.Such a narrow and shallow seismogenic zone makes it hard for earthquakes to occur and for the Wadati-Benioff plane to be observed in some areas south of Chile Triple Junction.