Marine magnetotelluric experiment across the central Mariana subduction-arc-back arc system

我们进行了一个海洋大地电磁断面横跨中央马里亚纳俯冲弧弧回弧系统使用海底电磁磁力计（OBEMs），海底静电计（OBEs），海洋底部磁力计（OBM）提供的马里亚纳岛弧系统的导电性结构，从太平洋延伸到西马里亚纳海脊（残余弧）通过滨海槽。在KR 05 -17巡航期间，在马里亚纳俯冲弧-弧后系统中心的40个地点部署了33个OBM、7个OBM和7个OBE。从2005年12月到2006年9月的恢复期间，使用这些仪器进行了观测。在KR 06 -12巡航期间，我们在35个地点成功回收了28个OBM，7个OBM和6个OBE。由于观测延迟，我们仍在分析这些数据以估计上地幔电导率结构。但是，这些数据将提供一个全面的图像的导电性结构的中央马里亚纳俯冲弧弧后系统，因为观测点是如此密集。由此产生的电阻率结构表明，在马里亚纳海槽下约70公里深处的电导率增加，在更深的深度各向异性。马里亚纳海槽扩张轴附近的低电导率结构意味着马里亚纳海槽弧后盆地的熔融开始比典型的MORB源区更深，需要考虑海底地形对海底大地电磁数据的影响来估计可靠的电导率结构。通过数值模拟，对三种不同的海底地形影响校正方程进行了评价，并通过一系列的综合试验，确定了最合适的海底地形影响校正方法。

We carried out a marine magnetotelluric transect across the central Mariana subduction-arc-back arc system using ocean bottom electro-magnetometers (OBEMs), ocean bottom electrometers (OBEs), and ocean bottom magnetometers (OBMs) to provide the electrical conductivity structure of the Mariana island-arc system extending from the Pacific ocean to the West Mariana Ridge (remnant arc) through the Marina Trough. Deployments of 33 OBEMs, 7 OBMs, and 7 OBEs were made at 40 sites across the central Mariana subduction-arc-back arc system during KR05-17 cruise. Observations using the instruments were made from December 2005 up to the recovery in September 2006. We successfully recovered 28 OBEMs, 7 OBMs, and 6 OBEs at 35 sites during KR06-12 cruise. We are still analyzing these data to estimate the upper mantle electrical conductivity structure, because the observation was delayed. But, these data will provide a comprehensive image of the conductivity structure for the central Mariana subduction-arc-back arc system, because the observation sites are so dense.We analyzed previous 9 OBEM data obtained from the same area. The resulting electrical resistivity structure shows an increase in conductivity at depths of about 70 km beneath the Mariana Trough and anisotropy at deeper depth. The low conductivity structure near Mariana Trough spreading axis implies that the onset of melting in the Mariana Trough back-arc basin is deeper than that of the typical MORB source region.Consideration of the effect of seafloor topography on ocean bottom magnetotelluric data is needed to estimate a reliable conductivity structure. We evaluated three different correction equations to correct the effect of seafloor topography with forward modeling, and we indicate the most an appropriate correction method based on a series of synthetic tests.