Scismological study of oceanic cruts in the mantle transition zone

美国西部地震台阵记录的14次马里亚纳深源地震（19°N附近）的短周期波形资料显示，在直达P波（简称P波或P波）到达后约30 s内出现两个异常波包：一个在13 s左右（称为X1相），另一个在29 s左右（称为X2相）。我们进行阵列分析，以定位这些阶段的来源。在第一步中，我们测量到达时间，慢度，后方位角，和这些阶段相对于P的振幅。X1和X2阶段的振幅相互关联的个别事件，并改变超过一个数量级的事件之间的集群在一个区域跨越小于50公里。振幅相关性表明这两个波具有相似的起源，并且大的振幅变化消除了作为相位起源的接收器侧混响。X1相位的慢度和到达方位角与整个阵列上的P波的慢度和到达方位角没有区别。 ...更多信息 y，并且具有几乎恒定的延迟时间，而与焦深（从567到605 km）无关。在第二步中，我们计算了焦点附近P-to-P和S-to-P单次散射情况下的复合语义系数。X1和X2相最好解释为震源区北部最上层下地幔中产生的S-到-P散射波，基于三个观测：（1）最高的合成语义值，（2）在500和600 km深度附近的两个事件组之间相互一致的散射体位置，以及（3）所需的合理数量的弹性属性异常。在19.8°±0.3°N，145.7°±0.3°E，710±30 km处测定了X_1相位散射体。另一方面，X2相的散射体似乎分裂成两个物体：一个位于20.4°±0.4° N，145.4 °±1.0°E，900±80 km，另一个位于20.6°±0.4°N，147.4°±0.5°E，860±40 km。虽然X1和X2相的散射体的几何形状不受约束，但水平或近水平的不连续性是不可能的。与这些异质物体相关的弹性性质的变化可能发生在几公里内，根据它们转换短周期波的高效率。因此，它们可能代表主要元素组成的急剧化学变化。这些物体位于下地幔顶部增厚的高速异常区内，这是由以前的地震研究确定的。对这些天体的一个合理的构造解释是，它们是前洋壳的碎片，被夹带在太平洋板块中，撞击在更粘稠的下地幔上。少

Short-period waveform data recorded at the western US seismic array for 14 Mariana deep earthquakes near 19°N show two anomalous wave packets within about 30 s after the arrival of the direct P-wave (simply called ‘P' or ‘P-wave') : one at about 13 s (called ‘X1 phase') and the other around 29 s (called ‘X2 phase'). We perform array analyses to locate the sources of these phases. In the first step, we measure arrival time, slowness, back-azimuth, and amplitude of these phases relative to P. The amplitudes of the X1 and X2 phases correlate with each other for the individual events, and vary more than an order of magnitude among the events which cluster in a region spanning less than 50 km. The amplitude correlation suggests that the two waves have a similar origin, and the large amplitude variation eliminates receiver side reverberation as the origin of the phases. The X1 phase has a slowness and an arrival azimuth which are not distinguished from those of P-waves across the entire arra … More y, and has nearly constant delay times regardless of the focal depths (from 567 to 605 km). The X2 phase arrives as a more emergent wave packet with slowness and azimuth different from those of P. In the second step, we compute composite semblance coefficients for the cases of P-to-P and S-to-P single scattering near the foci. The X1 and X2 phases are best interpreted as S-to-P scattered waves generated in the uppermost lower mantle north of the focal region based on three observations : (1)highest composite semblance values, (2)scatterer locations mutually consistent between the two event groups near the depths of 500 and 600 km, and (3)a reasonable amount of elastic property anomalies required. The X1 phase scatterer is determined at 19.8°±0.3°N, 145.7°±0.3°E, 710±30 km. On the other hand, the scatterer of the X2 phase appears to split into two objects : one at 20.4°±0.4°N,145.4°±1.0°E, 900±80 km, and the other at 20.6°±0.4°N, 147.4°±0.5°E, 860±40 km. Although the geometries of the scatterers of X1 and X2 phases are not constrained, horizontal or subhorizontal discontinuities are unlikely. The changes in elastic properties associated with these heterogeneous objects probably occur within several kilometers, according to their high efficiency at converting short-period waves. They are thus likely to represent sharp chemical variations in major element composition. These objects are located within a thickened high-velocity anomaly at the top of the lower mantle, which has been determined by previous seismic studies. A plausible tectonic interpretation of these objects is that they are fragments of former oceanic crust which are entrained in the Pacific slab impinging on the more viscous lower mantle. Less