Seismic data telemetry system and precise hypocenter location for distributed acoustic sensing observation using seafloor cable off Sanriku, Japan

A seismic observation using Distributed Acoustic Sensing (DAS) using seafloor cable can provide spatially high-density data for a long distance in marine areas. A seafloor seismic and tsunami observation system using an optical fiber cable off Sanriku, northeastern Japan was deployed in 1996. Short-term DAS measurements were sporadically repeated since February 2019 using spare fibers of the Sanriku system (Shinohara et al., 2022). A total measurement length is approximately 100 km.  It has been concluded that measurement with a sampling frequency of 100 Hz, a ping rate of 500 Hz, gauge length of 100 m, and a spatial interval of 10 m is adequate for earthquake and tsunami observation.  From March 2025, we started a continuous DAS observation to observe seismic activity. When the continuous DAS observation was commenced, we developed quasi real time data transmission system through the internet. Because a DAS measurement generates a huge mount of data per unit time and capacity of internet is limited, decimation for spatial direction is adopted. In addition, data format is converted from HDF5 to conventional seismic data exchange format in Japan (win format). An interrogator generates a HDF5 file every 30 seconds. After the file generation, the telemetry system reads the HDF5 file, and decimates data for spatial domain. Then, the data format is changed to the win format and the data are sent to the internet. In other words, data transmission is delayed for a slightly greater than 30 seconds. Data with the win format can be applied to various seismic data processing which has been developed before. To locate a hypocenter using DAS data, seismic phases in DAS data must be identified. To evaluate performance of hypocenter location using DAS records, arrival times of P- and S-waves were picked up on the computer display for local earthquakes. Every 100 channel records on DAS data and data from surrounding ordinary seismic stations were used. Location program with absolute travel times and one-dimensional P-wave velocity structure was applied. Results of location of earthquakes were evaluated by mainly using location errors. Errors of the location with DAS data were smaller than those of the location without the DAS data. Increase of arrival data for DAS records seems to be efficient to improve a resolution. However, picking up signals for all channels (seismic station) manually are costly due to a large number of channels. To expand the location method, an improved automatic pick-up program using evaluation function from conventional seismic network data by seismometers for DAS data (Horiuchi et al., 2025) was applied to the DAS data obtained by the Sanriku system. As a result, arrivals time of P, S and converted PS waves can be precisely identified with high resolution. We have a plan to locate earthquakes using all DAS channels (seismic stations)  and surrounding ordinary marine and land seismic stations.