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

人们对缓慢扩张的山脊上的岩浆管道系统知之甚少，尤其是与对快速扩张的山脊上的岩浆结构和过程的了解相比。岩浆是如何储存在地壳中并通过地壳输送的，以及这个岩浆系统如何与断层作用相互作用，这是地壳在缓慢扩张的山脊上吸积的两个关键但未知的方面。最近在北纬35度的大西洋中部山脊进行的地震层析成像实验显示，在一个神奇而坚固的扩张段的内谷下方，低地震P波速度的分布非常有趣。由于较浅的低速区位于海底火山地貌的正下方，断层图像可能表明存在与地壳岩浆管道系统有关的高温和部分熔融。然而，低速区可能是由于孔隙度的增加，这是一种由观察到的随深度增加的Vp/Vs增加而支持的另一种解释。同一地段中心的微震震中在谷底内侧较窄的深度范围(3-4公里)内呈轴向趋势。震源机制大多与伸展断裂一致，但也存在走滑等异常机制。这些震源似乎与西部山谷边界的断层有关。同时，地震的位置接近低速异常性质的变化，低速异常的性质从4公里以下的中心位置的管状物体转变为沿轴线的上地壳异常低速区域。

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.