We present 3-D Vp and Vp/Vs tomographic images for depths down to 22 km along with precise earthquake locations beneath the Sea of Marmara using the seismological data sets collected during the Ocean Bottom Seismometer (OBS) experiments conducted in 2001 and 2006. The unique data set collected by 52 OBSs and augmented by 14 land stations include 3852 P and 2643 S arrival times from 434 earthquakes and 4744 P arrival times from 557 air-gun shot records. From the tomographic depth sections and relocated seismicity, we identify two shear zones which are marked by narrow-width (∼10 km), low Vp (4.3–5.3 km s^–1) and high Vp/Vs ratio (∼2.0) anomalies coinciding with the surface trace of the Main Marmara Fault (MMF) and extending near vertically from 8 km depth below the basins into the deeper part of the crust. We consider these shear zones to be the strike-slip deformation signature of the MMF at depth. The western shear zone imaged lies between the Western High and the eastern end of the Central Basin including the 60-km-long segment of the MMF and contains higher seismicity and is visible in the 8–22 km depth range. The eastern shear zone, coinciding with the NW–SE trending surface trace of the MMF, is imaged beneath the Çınarcık Basin with lower seismicity and tomographic resolution. Furthermore, these shear zones may contain fractures filled with deep crustal fluids allowing partial aseismic creep. Within these shear zones, we also identify small-scale, high Vp (6 km s^–1) anomalies with low seismicity indicative of strong patches of the fault at depth that can be considered as asperities. Between these shear zones, our tomographic images clearly show a central zone that has homogenously high Vp (∼6.0–6.5 km s^–1) and low Vp/Vs ratio (∼1.7) over a large area including the 50-km-long central segment of the MMF lying between the eastern end of the Central Basin and western end of the Çınarcık Basin. Thus, we infer that the central MMF is in a locked state based on its high shear strength, low seismicity and a lack of detectable geodetic slip. Furthermore, a large earthquake might not nucleate on the central MMF since it may be strongly locked at the current level of stress loading, and thus it may behave as a seismic barrier or anti-asperity. On the other hand, the central MMF is a good candidate for a supershear rupture because it has nearly uniform elastic properties and low background seismicity indicating the homogeneity of friction and pre-stress across the fault, which are presented as the main characteristics of supershear fault ruptures. From the seismic hazard perspective, we speculate that a large earthquake may be expected to nucleate within either of the imaged shear zones with the possibility of the central MMF acting as a seismic barrier to stop or to join the rupture in a supershear mode. The imaged features have important implications for various aspects of seismic hazard and crustal dynamics in the Sea of Marmara region.

中文翻译：

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马尔马拉海下方的三维地震层析成像：马尔马拉主断层锁定和蠕动部分的证据

我们使用在2001年和2006年进行的海底地震仪（OBS）实验中收集的地震数据集，提供了3D Vp和Vp / Vs断层图像，以及深至22 km的马尔马拉海下方的精确地震位置。 52个OBS收集并增加了14个陆地站的独特数据集包括来自434次地震的3852 P和2643 S到达时间以及来自557气枪射击记录的4744 P到达时间。从断层的深度剖面和重新定位的地震活动，我们确定了两个剪切带，它们以窄幅（〜10 km ），低Vp（4.3–5.3 km s ^–1）和高Vp为标志/ Vs比率（〜2.0）异常与马尔马拉主断层（MMF）的表面轨迹相吻合，并且从盆地下方的8 km深度垂直延伸到地壳的较深部分。我们认为这些剪切带是MMF在深处的走滑变形特征。成像的西部剪切带位于西部高地和中央盆地的东端之间，包括MMF的60公里长，并具有较高的地震活动性，并且在8-22 km的深度范围内可见。与MMF的NW–SE趋势表面轨迹相吻合的东部剪切带以较低的地震活动性和层析成像分辨率在Çınarcık盆地下方成像。此外，这些剪切带可能包含充满深层地壳流体的裂缝，从而允许部分地震蠕变。在这些剪切带内，我们还确定了小范围，高具有低地震活动性的Vp（6 km s ^–1）异常表明在深处的断层有很强的斑点，可以认为是粗糙的。在这些剪切带之间，我们的断层扫描图像清楚地显示出一个中心带，该中心带具有均一的高Vp（〜6.0–6.5 km s ^–1）和低Vp / Vs在包括中部盆地东端和Çınarcık盆地西端之间的MMF的50 km长的中段在内的大范围内，该比值约为1.7％。因此，我们基于中央MMF的高剪切强度，低地震活动性和缺乏可检测的大地滑移来推断其处于锁定状态。此外，大地震可能不会在中央MMF上成核，因为它可能被牢固地锁定在当前的应力负荷水平，因此可能会起到地震屏障或反作用的作用。另一方面，中央MMF是超剪切断裂的理想候选者，因为它具有近乎均匀的弹性特性和较低的背景地震强度，表明整个断层的摩擦力和预应力是均匀的，这被认为是超剪切断裂的主要特征。破裂。从地震危险的角度出发，我们推测可能会在成像剪切带中的任何一个区域内发生大地震，而中央MMF可能会作为地震屏障以超剪切模式停止或加入破裂。成像的特征对于马尔马拉海地区地震灾害和地壳动力学的各个方面都具有重要意义。