鄂霍次克微板块东部俯冲带区域地震b值及应力场特征

古登堡-里克特震级-频度关系式中的b值与剪切应力(或偏应力)大小被认为存在着负相关的关系,因此b值常被用作估算区域应力大小的指标.本文利用1970-2018年鄂霍次克微板块东部俯冲带区域的地震目录,使用最大似然法对该区域的b值进行空间扫描,得到了该区域沿海沟走向不同区域及不同深度的b值分布,进而调查与分析其应力状态及地球动力学特征.结果显示不同俯冲区域的b值分布具有4个共同特征:1)地壳范围内的高b值特征,表明其剪切应力较低;2)俯冲板片与上覆板块耦合强烈的区域b值较低,表明该位置剪切应力较高;3)弧前区域b值较高,表明其剪切应力较低;4)海沟东侧的太平洋板块与软流层接触的区域b值较高,表明该位置剪切应力较低.上述这些b值分布结果及其剪切应力分布,是能够与俯冲带的地球动力学结构与特征相关联的.我们也发现在水平方向上,在浅部区域(0~40km深度范围),勘察加地区的剪切应力相对于北日本地区和千岛地区更低;在更深的区域(40~80km深度范围),千岛地区弧前区域的剪切应力较低.由本文俯冲带区域力平衡估算得到的俯冲接触面上的剪应力大小能够解释鄂霍次克微板块东部俯冲带不同接触界面上的b值大小及其分布差异.本文得到的日本海沟附近区域平均b值在大地震前后的时间变化揭示了该区域震间、震前和震后的应力演化过程.

Seismic b-value and stress field characteristics in the eastern subduction zone of Okhotsk micro-plate

The b value in the Gutenberg-Richter relation is believed to be negatively correlated with the magnitude of shear stress (or deviatoric stress). Hence b value is often used as an index to estimate the level of regional stress. Based on the seismic catalog data in the eastern subduction zone of Okhotsk micro-plate from 1970 to 2018, we used the maximum likelihood method to calculate the regional b values, and obtained the distributions of b values in different regions along the trench strike and different depths, We used the b-value map to investigate and analyze the regional stress states and geodynamic characteristics. The results show four common characteristics:1) a higher b value exists in the crust indicating a lower shear stress here; 2) the locations where the subducting and overriding plates are strongly coupled have a lower b value, indicating a higher shear stress here; 3) the b value is higher in the forearc, reflecting the lower shear stress in this region; 4) on the east side of the trench, the contacting regions between the lithosphere of Pacific plate and its underlying asthenosphere have a higher b-value, suggesting a lower shear stress at these locations. The distribution of shear stress estimated from the distribution of b value in the subduction zone can be explained by the regional geodynamic structures and characteristics. We also find that in the horizontal direction, the shear stress in Kamchatka region is smaller than that in the northern Japan region and the Kuril region within the shallow region (0~40 km); in the deeper region (40~80 km), the shear stress in the forearc of the Kuril region is relatively small. The shear stress on the subduction contact surface calculated from the force quilibrium in this study can explain the b-value magnitude and its distribution at the contact surface in different regions of the eastern subduction zone of the Okhotsk micro-plate. The temporal variation of our calculated average b values near the Japan Trench before and after the 2011 M9.1 Tohoku-Oki earthquake can explain the stress evolution process during interseismic, preseismic and postseismic phases.