Precise Monitoring of Pore Pressure at Boreholes Around Nankai Trough Toward Early Detecting Crustal Deformation

In our recent study, we detected the pore pressure change due to the slow slip event (SSE) in March 2020 at the two borehole stations (C0002 and C0010), where the other borehole (C0006) close to the Nankai Trough seems not because of instrumental drift for the reference pressure on the seafloor to remove non-crustal deformation such as tidal and oceanic fluctuations. To overcome this problem, we use the seafloor pressure gauges of cabled network Dense Oceanfloor Network System for Earthquakes and Tsunamis (DONET) stations nearby boreholes instead of the reference by introducing time lag between them. We confirm that the time lag is explained from superposition of theoretical tide modes. By applying this method to the pore pressure during the SSE, we find pore pressure change at C0006 about 0.6 hPa. We also investigate the impact of seafloor pressure due to ocean fluctuation on the basis of ocean modeling, which suggests that the decrease of effective normal stress from the onset to the termination of the SSE is explained by Kuroshio meander and may promote updip slip migration, and that the increase of effective normal stress for the short-term ocean fluctuation may terminate the SSE as observed in the Hikurangi subduction zone.

Approbation of the Method for Examining the Performance of Seafloor Observatory Sensors Using Distant Earthquakes Records

An algorithm is presented for testing the calibration accuracy of both z-accelerometers and pressure gauges (PG) installed in seafloor observatories. The test is based on the linear relationship between the vertical acceleration component of the seafloor movement and variations of the seafloor pressure, which is a direct consequence of Newton's 2-nd law and holds valid in the frequency range of “forced oscillations.” The operability of the algorithm is demonstrated using signals registered by 28 observatories of the DONET-2 system during 4 earthquakes of magnitude Mw ~ 8 that took place in 2018-2019 at epicentral distances from 55° up to 140°.

A Thirty-Month Seafloor Test of the A-0-A Method for Calibrating Pressure Gauges

Geodetic observations in the oceans are important for understanding plate tectonics, earthquake cycles and volcanic processes. One approach to seafloor geodesy is the use of seafloor pressure gauges to sense vertical changes in the elevation of the seafloor after correcting for variations in the weight of the overlying oceans and atmosphere. A challenge of using pressure gauges is the tendency for the sensors to drift. The A-0-A method is a new approach for correcting drift. A valve is used to periodically switch, for a short time, the measured pressure from the external ocean to the inside of the instrument housing at atmospheric pressure. The internal pressure reading is compared to an accurate barometer to measure the drift which is assumed to be the same at low and high pressures. We describe a 30-months test of the A-0-A method at 900 m depth on the MARS cabled observatory in Monterey Bay using an instrument that includes two A-0-A calibrated pressure gauges and a three-component accelerometer. Prior to the calibrations, the two pressure sensors drift by 6 and 2 hPa, respectively. After the calibrations, the offsets of the corrected pressure sensors are consistent with each other to within 0.2 hPa. The drift corrected detided external pressure measurements show a 0.5 hPa/yr trend of increasing pressures during the experiment. The measurements are corrected for instrument subsidence based on the changes in tilt measured by the accelerometer, but the trend may include a component of subsidence that did not affect tilt. However, the observed trend of increasing pressure, closely matches that calculated from satellite altimetry and repeat conductivity, temperature and depth casts at a nearby location, and increasing pressures are consistent with the trend expected for the El Niño Southern Oscillation. We infer that the A-0-A drift corrections are accurate to better than one part in 105 per year. Additional long-term tests and comparisons with oceanographic observations and other methods for drift correction will be required to understand if the accuracy the A-0-A drift corrections matches the observed one part in 106 per year consistency between the two pressure sensors.

Seafloor Pressure Change Excited at the Northwest Corner of the Shikoku Basin by the Formation of the Kuroshio Large-Meander in September 2017

The Kuroshio takes a greatly southward displaced path called a large-meander (LM) path off the southern coast of Japan on interannual to decadal time scales. The transition of the current path from a non-large-meander path to an LM path is the most salient ocean current variation south of Japan. The change in pressure on the seafloor due to the formation of the LM path in September 2017 is of critical importance to understand the dynamics of the LM path and to distinguish the change due to the Kuroshio path variation from changes due to crustal deformation. Hence, we examined the seafloor pressure across the continental slope off the eastern coast of Kyushu for the period March 2014 to April 2019. The pressure and its cross-slope gradient over the continental slope shallower than 3,000 m beneath near the Kuroshio are invariable. As a mesoscale current path disturbance, called a small meander, passed over the observation stations, the pressure decreased by approximately 0.1 dbar on the continental slope deeper than 3000 m and was kept low until the end of the observation period (April 2019). The pressure decrease is consistent with the changes in sea surface height and subsurface water density and is caused by the baroclinic enhancement of the Shikoku Basin local recirculation. This seafloor pressure change implies a strengthening of the deep southwestward current, possibly as a part of a deep cyclonic circulation in the Shikoku Basin. The present study demonstrated that, in addition to altimetric sea surface height data, hydrographic data are useful to distinguish the ocean variation in seafloor pressure from the variation due to crustal deformation, and vice versa.

Keeping M-Earths habitable in the face of atmospheric loss by sequestering water in the mantle

ABSTRACT Water cycling between Earth’s mantle and surface has previously been modelled and extrapolated to rocky exoplanets, but these studies neglected the host star. M-dwarf stars are more common than Sun-like stars and at least as likely to host temperate rocky planets (M-Earths). However, M dwarfs are active throughout their lifetimes; specifically, X-ray and extreme ultraviolet (XUV) radiation during their early evolution can cause rapid atmospheric loss on orbiting planets. The increased bolometric flux reaching M-Earths leads to warmer, moister upper atmospheres, while XUV radiation can photodissociate water molecules and drive hydrogen and oxygen escape to space. Here, we present a coupled model of deep-water cycling and water loss to space on M-Earths to explore whether these planets can remain habitable despite their volatile evolution. We use a cycling parametrization accounting for the dependence of mantle degassing on seafloor pressure, the dependence of regassing on mantle temperature, and the effect of water on mantle viscosity and thermal evolution. We assume the M dwarf’s XUV radiation decreases exponentially with time, and energy-limited water loss with 30 per cent efficiency. We explore the effects of cycling and loss to space on planetary water inventories and water partitioning. Planet surfaces desiccated by loss can be rehydrated, provided there is sufficient water sequestered in the mantle to degas once loss rates diminish at later times. For a given water loss rate, the key parameter is the mantle overturn time-scale at early times: if the mantle overturn time-scale is longer than the loss time-scale, then the planet is likely to keep some of its water.