Sagami Bay is an active tectonic area in Japan. In 1993, a real-time deep sea floor observatory was deployed at 1,175 m depth about 7 km off Hatsushima Island, Sagami Bay to monitor seismic activities and other geophysical phenomena. Video cameras monitored biological activities associated with tectonic activities. The observation system was renovated completely in 2000. An ocean bottom electromagnetic meter (OBEM), an ocean bottom differential pressure gauge (DPG) system, and an ocean bottom gravity meter (OBG) were installed January 2005; operations began in February of that year. An earthquake (M5.4) in April 2006, generated a submarine landslide that reached the Hatsushima Observatory, moving some sensors. The video camera took movies of mudflows; OBEM and other sensors detected distinctive changes occurring with the mudflow. Although the DPG and OBG were recovered in January 2008, the OBEM continues to obtain data.

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Coupling an intense electric field of terahertz (THz) pulse to a scanning tunneling microscope (STM) has opened new avenues for conducting ultrafast electron manipulation and tracking quantum dynamics at the nanometer spatial resolution. Here we combined the THz-field-driven STM with a photon detection system and demonstrated nanospectroscopic investigations of STM-induced luminescence triggered by THzfield-driven electrons. Owing to the abundant spectral information on photons, we were able to separately measure and characterize luminescence from a localized plasmon excited by THz-field-and DC-field-driven inelastic electron tunneling. We revealed that the plasmon excitation by THz-field-driven electrons in our experiment occurred at an extremely high voltage and current region due to the use of intense THz pulses, which would provide a unique experimental platform for exploring the light−matter interactions in a plasmonic nanocavity. Our method also paves the way for investigating quantum energy dynamics accompanying quantum conversions with subpicosecond time and atomic-scale spatial resolution.