Although fluid behavior along faults has often been associated with earthquakes and hydrocarbon accumulation, few observations of active, ongoing fluid discharge along the faults exist. We conducted a dense-2D multi-channel seismic (MCS) reflection survey in Uchiura Bay off Numazu, Japan, in 2018 to obtain a high-resolution image of active faults. As a result, active faults cutting the seafloor and possibly active faults cutting reflections immediately below the seafloor have been clearly imaged on MCS profiles. The spatial variation in their displacement on the seafloor has also been quantified. The seafloor displacement of the most obvious active fault, Fault A, changes by more than 2 m within a short horizontal distance of 50 m. On one of the MCS profiles, an enigmatic wavefield (EWF) is observed within the water column above the seafloor near the faults. From careful observations of the raw field data and shot records, the EWF shows three patterns that suggest fluid flow. Based on a velocity analysis of the seawater, the velocity of the EWF zone was somewhat higher than that of its surroundings. Because the seawater velocity is a function of temperature and salinity at the same depth, the effect of temperature is generally greater than that of salinity, so that the higher velocity would suggest a higher temperature. If temperature of the fluid flow is higher than that of its surroundings, the fluid would rise with its buoyancy and spread out along the sea surface. This scenario could well explain the three EWF patterns. From the appearance and non-appearance of the most obvious EWF on adjacent seismic lines, some fluid discharge might have been occurring intermittently from the seafloor near the faults in Uchiura Bay.

中文翻译：

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日本沼津附近的内浦湾高分辨率密集二维地震反射勘测观测到的可能与断层相关的流体排放

尽管沿断层的流体行为通常与地震和油气聚集有关，但很少观察到沿断层的活跃、持续的流体排放。我们于 2018 年在日本沼津附近的内浦湾进行了密集二维多道地震 (MCS) 反射勘测，以获得活动断层的高分辨率图像。因此，切割海底的活动断层和可能切割海底正下方反射的活动断层已经在 MCS 剖面上清晰地成像。它们在海底位移的空间变化也已被量化。最明显的活动断层 A 的海底位移在 50 m 的短水平距离内变化超过 2 m。在 MCS 配置文件之一上，在断层附近海底上方的水柱内观察到一个神秘的波场 (EWF)。通过仔细观察原始现场数据和射击记录，EWF 显示了三种表明流体流动的模式。根据海水的速度分析，EWF 区的速度略高于其周围环境的速度。由于海水速度是同一深度的温度和盐度的函数，温度的影响一般大于盐度的影响，因此速度越高表明温度越高。如果流体流动的温度高于其周围的温度，流体将随着其浮力上升并沿海面扩散。这种情况可以很好地解释三种 EWF 模式。