西北太平洋和汤加俯冲地区深震特殊聚簇的特性及成因探究

对比研究了具有不同热参数、不同俯冲形态的西北太平洋俯冲地区和汤加俯冲地区的深震特殊聚簇的地震学特性和成因.利用单键群方法探测到两个特殊的深震聚簇G1N和G1T.聚簇G1N位于地震空区下方，具有极低的b值（~0.54），完全不同于具有高b值（~1.04）的汤加俯冲地区聚簇G1T.通过对聚簇地区板块形态、地震主应力轴、地震深度分布特征的分析，以及和汤加典型的板片折曲处地震活动性的对比，我们认为深震聚簇G1N附近的板块表现出板片折曲的特征，板块俯冲到地幔过渡带底部受到下地幔的黏性阻力，板片局部向上凸起发生折曲，产生局部的拉张应力，叠加在俯冲造成的压缩构造背景上，应力状态发生改变，从而影响该深震聚簇的地震活动性.汤加地区G1T聚簇深震的成因则完全不同，没有体现出板片折曲、应力变化的特征；相反，这些深震发生在较冷的Vitiza-Fiji俯冲板块上，该板块在5~8 Ma年前先行俯冲到G1T区域并与Tonga板块发生拆离，G1T聚簇深震就发生在这些温度依然很低、滞留于~500 km深度处的高速板片残留体上.

Characteristics and origin of deep earthquake clusters beneath Northwest Pacific and Tonga subduction zone

We comparatively study the characteristics of isolated clusters of deep earthquakes in the Northwest Pacific subduction zone and Tonga region, which have different thermal parameters and subduction morphology in the earth's interior. By using Single Link Cluster method, we identify two special deep earthquake clusters G1N and G1T. The cluster G1N in NWP is located below a seismic gap, and has a low b value (~0.54), totally different from that of G1T in Tonga with a rather high b value (~1.04). Based on detailed analysis of slab morphology, seismic principal stress axes, depth distribution of deep earthquakes, as well as a comparison of seismicity around hinge zones detected in Tonga, we propose that cluster G1N is controlled by slab buckling, which might be the consequence of the resistance to subduction caused by the viscosity jump across the upper-lower mantle boundary. With the formation of convex-up folds in the slab, a local tensional stress is generated, and superimposed on the background of a compressive tectonic stress field caused by subduction, which changes the seismic activity of G1N. The feature of G1T in Tonga, however, does not show any characteristics of slab buckling. Instead, the isolated cluster belongs to a kind of outboard earthquakes, occurring on the cold but ancient Vitiza-Fiji subduction plate. This plate system had subducted to the region around G1T 5~8 Ma years ago, residing above the depth~500 km, and separated from the ongoing active Tonga plate.