Seismic scatterers in the mid-lower mantle beneath Tonga-Fiji
Seismic scatterers in the mid-lower mantle beneath Tonga-Fiji
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DOI：
10.1016/j.pepi.2017.09.007
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发表时间：
2018-01-01
影响因子：
2.3
通讯作者：
Kaneshima, Satoshi
中科院分区：
文献类型：
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作者：
Kaneshima, Satoshi
本文分析了汤加-斐济地区的深、中深度地震，揭示了中-下地幔散射物的分布。通过阵列处理美国、阿拉斯加和日本区域地震台记录的波形数据，我们研究了汤加-斐济中下地幔中千米尺度化学性质不同的物体产生的P尾波S-to-P散射波。加上笔者之前报道的10个散射体，在900公里深度以下已探测到23个中-下地幔散射体，而1900公里以下的散射体尚未被发现。强烈的中-下地幔s - p散射最常发生在1400 ~ 1600 km深度范围内的散射体上。在1600 km深度以下，散射体数量减少，越深的物体散射体越弱。散射分布可能反映了玄武岩相对于周围地幔岩石的弹性异常随深度的衰减，这是矿物物理学预测的。在较窄的深度范围内，强s - p散射波的主要出现可能反映了玄武岩中辉长石铁弹性转变导致的刚性显著降低。与中地幔散射体相关的非常大的信号只在整个地震阵对的一小部分被观测到。这种对大散射信号的罕见观测，加上每个散射体在散射强度上的相当大的事件间差异，表明强烈的到达近似代表了具有平面几何形状的物体上的射线理论s - p转换波。强散射体的平面部分往往会陡降，其大小可超过100公里。对于一些强散射体，显示清晰散射波的接收器范围在地震阵列对之间有很大差异。在不同的阵列上也观察到一些散射体，这些阵列对散射体具有明显不同的入射波方向。此外，在一些散射体上观测到微弱但相干的p - p散射波和s - p散射波。这些观测结果表明，局部平面散射体也具有大量的地形。
We analyze deep and intermediate-depth earthquakes at the Tonga-Fiji region in order to reveal the distribution of scattering objects in the mid-lower mantle. By array processing waveform data recorded at regional seismograph stations in the US, Alaska, and Japan, we investigate S-to-P scattering waves in the P coda, which arise from kilometer-scale chemically distinct objects in the mid-lower mantle beneath Tonga-Fiji.With ten scatterers previously reported by the author included, twenty-three mid-lower mantle scatterers have been detected below 900 km depth, while scatterers deeper than 1900 km have not been identified. Strong mid-lower mantle S-to-P scattering most frequently occurs at the scatterers located within a depth range between 1400 km and 1600 km. The number of scatterers decreases below 1600 km depth, and the deeper objects tend to be weaker. The scatterer distribution may reflect diminishing elastic anomalies of basaltic rocks with depth relative to the surrounding mantle rocks, which mineral physics has predicted to occur. The predominant occurrence of strong S-to-P scattering waves within a narrow depth range may reflect significant reduction of rigidity due to the ferro-elastic transformation of stishovite in basaltic rocks.Very large signals associated with mid-mantle scatterers are observed only for a small portion of the entire earthquake-array pairs. Such infrequent observations of large scattering signals, combined with quite large event-to-event differences in the scattering intensity for each scatterer, suggest both that the strong arrivals approximately represent ray theoretical S-to-P converted waves at objects with a plane geometry. The plane portions of the strong scatterers may often dip steeply, with the size exceeding 100 km. For a few strong scatterers, the range of receivers showing clear scattered waves varies substantially from earthquake-array pair to pair. Some of the scatterers are also observed at different arrays that have significantly different directions of incident waves to the scatterers. Furthermore, weak but coherent P-to-P scattered waves as well as S-to-P waves are observed for a few of the scatterers. These observations indicate that the locally plane scatterers also possess substantial topography.