![]() At lower frequencies, Ajo-Franklin et al. ![]() ( 2018) have demonstrated the similarity between DAS and seismograph strain measurements, and confirmed that the observed signal is axial strain along a fiber-optic cable in the ordinary frequency range for seismic observations (i.e., several Hertz). It has been reported that the noise levels of the strain measurements are sufficiently small to detect natural earthquakes (Lindsey et al. We can simultaneously measure the strain at many locations along the cable (meter-scale intervals) at a high-sampling rate (~ 1000 samples per second (sps)) by simply connecting an interrogator, which is a laser transmitter/receiver system, to one end of the cable. It is a relatively inexpensive way to obtain spatially dense and temporally continuous measurements, when we can use fiber-optic cables that have already been installed for communication purposes. Although DAS was originally developed for the oil and gas industry, it has since been utilized to address a wide range of seismological research topics (e.g., Zhan 2020). These events may reflect oceanic internal surface waves and deep-ocean water mixing processes that are the result of ocean current–tidal interactions along an irregular seafloor boundary.ĭistributed acoustic sensing (DAS) is a new geophysical imaging technique that measures the ground strain (or strain rate) along a fiber-optic cable, using variations in the phase-coherent Rayleigh backscattering that are generated by a sequence of laser pulses (e.g.,Hartog 2017 Miah and Potter 2017 Lu et al. Temperature appears to exert a greater control on the DAS signal than real strain in the quasi-static, sub-seismic range, where we can regard our DAS record as distributed temperature sensing (DTS) record, and detected many rapid temperature change events migrating along the cable: a small number of large migration events (up to 10 km in 6 h) associated with rapid temperature decreases, and many small-scale events (both rising and falling temperatures). Many earthquakes were observed during the 5-day observation period, with the minimum and maximum detectable events being a local M1.1 event 30–50 km from the cable and a teleseismic Mw7.7 event that occurred in Cuba, respectively. Nevertheless, the noise levels at the well-coupled channels of DAS are almost comparable to those observed at nearby permanent ocean-bottom seismometers, suggesting that the cable has the ability to detect nearby micro earthquakes and even tectonic tremors. The observed signal amplitude varies widely among the DAS channels, even over short distances of only ~ 100 m, which is likely attributed to the differences in cable-seafloor coupling due to complex bathymetry along the cable route. Here we present the results of DAS observations from a submarine cable offshore Cape Muroto, Nankai subduction zone, western Japan. Distributed acoustic sensing (DAS) is a new method that measures the strain change along a fiber-optic cable and has emerged as a promising geophysical application across a wide range of research and monitoring.
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