Observation and measurement of natural earthquakes by fiber optic ocean bottom seismometers in the South China Sea

• Ocean-floor fiber seismometer network high-fidelity detection of natural earthquakes The Chinese mainland has perhaps the densest seismic monitoring network in the world, with thousands of stations spaced not much more than 10 km from one another. But there are virtually no permanent seismic observatories in the vast seas surrounding it, mainly for reasons of technology and cost. In view of this, plus the obvious importance of ocean seismicity observation, and the relevance and advantage of fiber optic seismic sensing technology, this work describes an ocean-bottom fiber optic Michelson interferometric accelerometer based seismic detection technology in action. In addition to an innovative design of a push-pull all-fiber sensor head for noise suppression, a high-speed and efficient demodulation algorithm is developed for fast data acquisition and processing, and a phase balance feedback technology is proposed and implemented to improve low frequency characteristics. This all-fiber ocean-bottom seismometer has a frequency band of 0.005-100 Hz, noise floor of 0.92 ng/sqrt(Hz) @1Hz, and a dynamic range of 165 dB. In late April, 2025, a small linear array of 3 nodes, each having a three-component fiber optic seismometer, connected with a 2 km single-mode communication fiber, was deployed on the ocean floor in the Wanshan sea area of Zhuhai, China as a semi-permanent, retrievable observatory. Numerous seismic and teleseismic activities have since been recorded, with magnitudes ranging from Ms 2.6 to Ms 7.3. Using the fiber optic seismic data, in conjunction and, in comparison with nearby land stations, we have been able to measure and determine the times of occurrence, the epicentral locations, and the magnitudes of these earthquakes. Moreover, the waveforms and seismic phases are as good and clear as (and in most cases better than) those from the nearby land stations. This observation run so far has demonstrated the quality and practical feasibility of the fiber optic ocean bottom seismometers, and their potential for contributing to ocean bottom observation networks, in terms hardware and experience.