KNIPAS - Knipovich Ridge passive seismic experimentStudying active mid-ocean ridge spreading processes and lithospheric structure on segment scale

KNIPAS是有史以来在大洋中脊(MOR)进行的最大被动地震实验。沿着Knipovich Ridge长达160公里的一段，28台海底地震仪(OBS)在一年的时间里记录了地震。莫尔斯的地震使人们得以洞察活跃的扩散过程，但陆地上的地震仪或水声阵列只能捕捉到最大的地震，其震中位置和震源深度仍然不确定。小规模的OBS研究以较小的震级提供了更详细的传播过程图像。最近的OBS研究表明，Knipovich Ridge所属的超低扩展MOR表现出明显不同于较快扩展脊的地震活动模式，从而显示出不同的扩展机制。超低莫尔分布广泛的火山在地震上可能非常活跃。相比之下，非岩浆山脊段只会产生微弱的地震，并显示出厚厚的岩石圈，其上地幔中有无地震的区域，这可能表明蛇纹岩作用的深度比以前所知的要深得多。然而，尚不清楚这些在当地研究的扩散过程在整个脊段的尺度上有多大的相关性。KNIPAS缩小了小规模OBS和远程地震研究之间的差距。通过预期的9000次1-3级井位地震，我们将研究岩石圈的热状况、厚度和结构以及沿整个山脊段的普遍变形模式。我们将调查岩石圈底部的地形是否足以保证熔体沿着整段向上流向火山中心，那里的岩浆产量远远超过当地的熔体产量。此外，我们将研究火山之间岩浆区的扩散是如何进行的，以及上地幔潜在的深层次蛇纹岩作用对变形模式的影响程度。此外，目前尚不清楚上地幔的组成和物理性质是否存在差异，这可能支配着岩浆和岩浆脊段中超低扩展MOR的长期分段。由于其空间范围很大，以及预期的地壳和岩石圈厚度和结构的显著差异，KNIPAS进一步形成了一个理想的数据集，用于测试和优化最近才引入海洋地震学的被动地震方法。将利用接收函数和远震地震和环境噪声的面波反演来研究沿山脊的上地幔的地震速度和结构的变化。我们计划进一步开发这些方法，用于OBS网络的常规应用，并严格检查它们的分辨率潜力。

KNIPAS belongs to the largest passive seismic experiments that have ever been conducted at a mid-ocean ridge (MOR). Along a 160 km long section of Knipovich Ridge, 28 Ocean Bottom Seismometers (OBS) recorded earthquakes over a period of one year. Earthquakes at MORs give insight into the active spreading processes but only the largest earthquakes can be captured by seismometers on land or by hydroacoustic arrays and their epicentre locations and source depths remain uncertain. Small-scale OBS studies give a more detailed image of spreading processes at smaller magnitudes. Recent OBS studies showed that ultraslow spreading MOR, to which Knipovich Ridge belongs, display distinctively different seismicity pattern than faster spreading ridges and therefore different spreading mechanisms. The widely spaced volcanoes of ultraslow MOR may seismically be very active. Amagmatic ridge sections, in contrast, produce only weak earthquakes and show a thick lithosphere with aseismic regions in the upper mantle that may point to serpentinisation reaching far deeper than previously known. It remains however unknown how relevant these locally studied spreading processes are on the scale of entire ridge segments.KNIPAS closes the gap between small-scale OBS and teleseismic studies. With an expected 9000 well-located earthquakes of magnitudes M 1-3 we will examine the thermal regime of the lithosphere, its thickness and structure and the prevalent deformation mode along an entire ridge segment. We will investigate if the topography of the base of the lithosphere is sufficient to warrant an upward flow of melts along an entire segment towards the volcanic centres where magma output exceeds by far the local melt production. In addition, we will examine how spreading works in the magmatic regions between the volcanoes and to what extent a potentially deep-reaching serpentinisation of the upper mantle influences the deformation mode. Furthermore it is currently not known if there are variations in the composition and physical properties of the upper mantle that may govern the long-lived segmentation of ultraslow spreading MOR in magmatic and amagmatic ridge sections.Due to its large spatial extent and the expected pronounced differences in crustal and lithospheric thickness and structure, KNIPAS furthermore forms an ideal data set to test and optimize passive seismic methods that only recently have been introduced in marine seismology. Receiver functions and the inversion of surface waves from teleseismic earthquakes and ambient noise will be used to study variations in seismic velocity and structure of the upper mantle along the ridge. We plan to further develop these methods for routine application to OBS networks and critically examine their resolution potential.