Seismic Attenuation Structure and the Fine-Scale Structure of a Seismic Zone at the Mid-Atlantic Ridge, 35 degrees N

人们对缓慢扩张脊的岩浆管道系统了解甚少，特别是与我们对快速扩张脊的构造和过程的了解相比。岩浆是如何在地壳中储存和运输的，以及这个岩浆系统是如何与断层作用相互作用的，这是缓慢扩张脊的地壳增生的两个关键但未知的方面。最近在北纬35度的大西洋中脊进行的地震层析成像实验显示，在岩浆活跃的扩张段的内谷下方，有一个有趣的低地震P波速度分布。由于较浅的低速区直接位于海底火山特征之下，层析成像可能表明存在与地壳岩浆管道系统相关的高温和部分熔融。然而，低速区可能是由于孔隙度增加，另一种解释得到了随深度增加的Vp/Vs的支持。同一段中心的微地震震源在内谷底下的一个狭窄深度范围内（3-4公里）呈沿轴趋势。大部分震源机制与伸展断裂一致，但也存在走滑等异常机制。震源似乎与西部的山谷边界断层有关。同时，这些地震位于低速异常特征的变化点附近，从4 km以下的中心管状体向上地壳上沿轴的异常低速区转变。

The magma plumbing system at slow-spreading ridges is poorly understood, especially in comparison with out understanding of ridge structure and processes at faster spreading ridges. How magma is stored in and transported through the crust, and how this magmatic system intereacts with faulting, are two crucial, but unknown, aspects of crustal accretion at slow-spreading ridges. Recent seismic tomography experiments at the Mid-Atlantic Ridge at 35 degrees N have imaged an intriguing distribution of low seismic P wave velocities beneath the inner valley of a magmatically-robust spreading segment. As the shallower low-velocity regions lie directly beneath seafloor volcanic features, the tomographic images may indicate the presence of high temperatures and partial melt associated with a crustal magma plumbing system. However, the low-velocity regions may instead be due to increased porosity, an alternative interpretation that is supported by an observed increase in Vp/Vs with depth. Microearthquake hypocenters at the center of the same segment devine an along-axis trend within a narrow depth range (3-4 km) beneath the inner valley floor. Most of the focal mechanisms are consistent with extensional faulting, but strike-slip and other anomalous mechanisms are also present. The hypocenters appear to be associated with a western valley-bounding fault. At the same time, the earthquakes are located near a change in character of the low-velocity anomalies from a centrally-located, pipe-like body below 4 km depth to an along-axis region of anomalously low velocities in the upper crust.