Detection of hydroacoustic signals on a fiber-optic submarine cable.

A ship-based seismic survey was conducted close to a fiber-optic submarine cable, and 50 km-long distributed acoustic sensing (DAS) recordings with air-gun shots were obtained for the first time. We examine the acquired DAS dataset together with the co-located hydrophones to investigate the detection capability of underwater acoustic (hydroacoustic) signals. Here, we show the hydroacoustic signals identified by the DAS measurement characterizing in frequency-time space. The DAS measurement can be sensitive for hydroacoustic signals in a frequency range from [Formula: see text] to a few tens of Hz which is similar to the hydrophones. The observed phases of hydroacoustic signals are coherent within a few kilometers along the submarine cable, suggesting the DAS is suitable for applying correlation analysis using hydroacoustic signals. Although our study suggests that virtual sensor's self-noise of the present DAS measurement is relatively high compared to the conventional in-situ hydroacoustic sensors above a few Hz, the DAS identifies the ocean microseismic background noise along the entire submarine cable except for some cable sections de-coupled from the seafloor.

Experimental evidence characterizing pressure fluctuations at the seafloor-water interface induced by an earthquake.

An unusual combination of a laboratory experiment and in situ measurement of pressure fluctuations during an earthquake allows us to resolve some uncertainties in bottom pressure recorders (BPRs). In situ BPRs are usually contaminated by seismic waves during earthquakes; thus uncertainty still remains in the data obtained from BPRs. We examine in situ BPR data together with pressure variations produced by a dead weight (a pressure standard) in a laboratory experiment during an earthquake. The features recorded by the in situ BPRs are analysed as part of the overall experiment. We demonstrated that a 10-kg dead weight on a piston-cylinder across an area of 10 mm2 is capable of reproducing pressure fluctuations at a depth of 1000 m in the water column. The experiment also indicates that the internal mechanics of BPRs are isolated from incident seismic waves, suggesting that BPRs measure true in situ pressures without instrumentally induced disturbances. This constitutes the first instance in which pressure fluctuations recorded by in situ BPRs during an earthquake were reproduced using a pressure standard in the laboratory.

Shallow very-low-frequency earthquakes accompany slow slip events in the Nankai subduction zone.

Recent studies of slow earthquakes along plate boundaries have shown that tectonic tremor, low-frequency earthquakes, very-low-frequency events (VLFEs), and slow-slip events (SSEs) often accompany each other and appear to share common source faults. However, the source processes of slow events occurring in the shallow part of plate boundaries are not well known because seismic observations have been limited to land-based stations, which offer poor resolution beneath offshore plate boundaries. Here we use data obtained from seafloor observation networks in the Nankai trough, southwest of Japan, to investigate shallow VLFEs in detail. Coincident with the VLFE activity, signals indicative of shallow SSEs were detected by geodetic observations at seafloor borehole observatories in the same region. We find that the shallow VLFEs and SSEs share common source regions and almost identical time histories of moment release. We conclude that these slow events arise from the same fault slip and that VLFEs represent relatively high-frequency fluctuations of slip during SSEs.

Sporadic low-velocity volumes spatially correlate with shallow very low frequency earthquake clusters.

A low-velocity zone (LVZ) has been detected by seismic exploration surveys within the Nankai accretionary prism toe off the Kii Peninsula, southwestern Japan, and is considered to be a mechanically weak volume at depth. Such mechanical heterogeneities potentially influence seismic and tsunamigenic slips on megathrust earthquakes in the subduction zone. However, the spatial distribution of the LVZ along the trough-parallel direction is still elusive. Here we show sporadic LVZs in the prism toe from one-dimensional shear wave velocity (Vs) profiles obtained at 49 cabled ocean bottom stations, which were estimated by a nonlinear inversion technique, simulated annealing, using the displacement-pressure ratios of the Rayleigh wave. The estimated distribution of LVZs along the trough widely correlates with the epicentres of shallow very low frequency earthquakes (sVLFEs), which suggests that sVLFEs are activated in the sporadically distributed low-velocity and mechanically weak volumes where fluids significantly reduce the shear strength of faults.

Recurring and triggered slow-slip events near the trench at the Nankai Trough subduction megathrust.

The discovery of slow earthquakes has revolutionized the field of earthquake seismology. Defining the locations of these events and the conditions that favor their occurrence provides important insights into the slip behavior of tectonic faults. We report on a family of recurring slow-slip events (SSEs) on the plate interface immediately seaward of repeated historical moment magnitude (Mw) 8 earthquake rupture areas offshore of Japan. The SSEs continue for days to several weeks, include both spontaneous and triggered slip, recur every 8 to 15 months, and are accompanied by swarms of low-frequency tremors. We can explain the SSEs with 1 to 4 centimeters of slip along the megathrust, centered 25 to 35 kilometers (km) from the trench (4 to 10 km depth). The SSEs accommodate 30 to 55% of the plate motion, indicating frequent release of accumulated strain near the trench.

Seismic evidence for sharp lithosphere-asthenosphere boundaries of oceanic plates.

The mobility of the lithosphere over a weaker asthenosphere constitutes the essential element of plate tectonics, and thus the understanding of the processes at the lithosphere-asthenosphere boundary (LAB) is fundamental to understand how our planet works. It is especially so for oceanic plates because their relatively simple creation and evolution should enable easy elucidation of the LAB. Data from borehole broadband ocean bottom seismometers show that the LAB beneath the Pacific and Philippine Sea plates is sharp and age-dependent. The observed large shear wave velocity reduction at the LAB requires a partially molten asthenosphere consisting of horizontal melt-rich layers embedded in meltless mantle, which accounts for the large viscosity contrast at the LAB that facilitates horizontal plate motions.

Trial of multidisciplinary observation at an expandable sub-marine cabled station "off-hatsushima island observatory" in sagami bay, Japan.

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.