Takehi Isse

Probing South Pacific mantle plumes with ocean bottom seismographs

The seismic structure beneath the South Pacific superswell has not been well explored in spite of its significance for mantle dynamics. The region is characterized by a topographic high of more than 680 m [Adam and Bonneville, 2005]; a concentration of hot spot island chains (e.g., Society Cook-Austral, Marquesas, and Pitcairn) whose volcanic rocks have isotopic characteristics suggesting deep mantle origin; and a broad, low-velocity anomaly in the lower mantle that has been revealed by seismic tomography These observations suggest the presence of a large-scale mantle flow from the bottom of the mantle beneath the region, which is called a ‘superplume’ [McNutt, 1998[.

South Pacific hotspot swells dynamically supported by mantle flows

The dynamics of mantle plumes and the origin of their associated swells remain some of the most controversial topics in geodynamics. Here we construct a numerical model of the mantle flow beneath the French Polynesia region. Our study is based on a new regional seismic tomography model, which high resolution allows obtaining information at the scale of plumes. We find excellent correlations between the observed and the modeled dynamic swells, between the modeled flow pattern and the active volcanism and between the buoyancy fluxes obtained from our numerical model and the ones deduced from the swells morphology. These outstanding fits reveal for the first time that a direct link exists between the surface observations and mantle flows.

TIARES Project—Tomographic investigation by seafloor array experiment for the Society hotspot

We conducted geophysical observations on the French Polynesian seafloor in the Pacific Ocean from 2009 to 2010 to determine the mantle structure beneath the Society hotspot, which is a region of underlying volcanic activity responsible for forming the Society Islands. The network for Tomographic Investigation by seafloor ARray Experiment for the Society hotspot (TIARES, named after the most common flower in Tahiti) is composed of multi-sensor stations that include broadband ocean-bottom seismometers, ocean-bottom electro-magnetometers, and differential pressure gauges. The network is designed to obtain seismic and electrical conductivity structures of the mantle beneath the Society hotspot. In addition to providing data to study the mantle structure, the TIARES network recorded unprecedented data of pressure and electromagnetic (EM) signals by tsunamis associated with large earthquakes in the Pacific Ocean, including the 2010 Chilean earthquake (Mw 8.8).

New Step for Broadband Seismic Observation on the Seafloor: BBOBS-NX

Essential information for clarifying geodynamic processes is obtained by imaging the Earths interior through geophysical observations. Huge oceanic areas are important locations for conducting such observations. The broadband ocean-bottom seismometer (BBOBS) that we developed has been used since 1999 in several array observations, which gave us new information. But, the BBOBSs noise model in periods longer than 10 s indicates the high noise level in horizontal components above the new high noise model (NHNM), although the vertical one is between the NHNM and the new low noise model (NLNM). It makes it difficult to apply modern analysis methods using horizontal component waveforms even from the data of the one-year-long observation at a single station. Recently, we have developed a geophysical instrument to investigate the oceanic mantle, namely the next-generation broadband ocean-bottom seismometer (BBOBS-NX), operated by a remotely operated underwater vehicle (ROV). The BBOBS-NX provides data of much higher quality than the conventional BBOBS, because it shows a comparable noise model in horizontal components with that of land seismic stations in periods longer than 10 s. Comparison between bottom currents and horizontal particle motions of the BBOBS-NX and conventional BBOBSs clearly indicates the effective reduction of the noise due to the bottom current in this period range. A preliminary receiver function analysis also shows the advantage of the BBOBS-NX to the conventional BBOBS, even if the observation period of the former was shorter than a quarter of that of the latter.

Determination of intrinsic attenuation in the oceanic lithosphere-asthenosphere system

Determining damping of our plates For plate tectonics to operate, a weaker layer called the asthenosphere must underlie the rigid lithospheric plates. Quantifying the difference in strength comes down to how much each layer attenuates energy. Takeuchi et al. exploited an ocean-bottom seismic network and seismic energy from the 2011 Japanese Tohoku-oki earthquake to quantify the attenuation in each layer (see the Perspective by Dalton). The attenuation of energy in the asthenosphere lined up with previous estimates, but the lithospheric attenuation was roughly one-fifth as strong as that predicted by some previous models. Science, this issue p. 1593; see also p. 1536 An ocean-bottom seismic network allows estimates of the attenuation of the oceanic lithosphere and asthenosphere. We recorded P and S waves traveling through the oceanic lithosphere-asthenosphere system (LAS) using broadband ocean-bottom seismometers in the northwest Pacific, and we quantitatively separated the intrinsic (anelastic) and extrinsic (scattering) attenuation effects on seismic wave propagation to directly infer the thermomechanical properties of the oceanic LAS. The strong intrinsic attenuation in the asthenosphere obtained at higher frequency (~3 hertz) is comparable to that constrained at lower frequency (~100 seconds) by surface waves and suggests frequency-independent anelasticity, whereas the intrinsic attenuation in the lithosphere is frequency dependent. This difference in frequency dependence indicates that the strong and broad peak dissipation recently observed in the laboratory exists only in the asthenosphere and provides new insight into what distinguishes the asthenosphere from the lithosphere.

Mantle plumes beneath the South Pacific superswell revealed by finite frequency P tomography using regional seafloor and island data

We present a new tomographic image beneath the South Pacific superswell, using finite frequency P wave travel time tomography with global and regional data. The regional stations include broadband ocean-bottom seismograph stations. The tomographic image shows slow anomalies of 200-300 km in diameter beneath most hot spots in the studied region, extending continuously from the shallow upper mantle to 400 km depth. Narrow and weak slow anomalies are detected at depths of 500–1000 km, connecting the upper mantle slow anomalies with large-scale slow anomalies with lateral dimension of 1000–2000 km prevailing below 1000 km depth down to the core-mantle boundary. There are two slow anomalies around the Society hot spot at depths shallower than 400 km, which both emerge from the same slow anomaly at 500 km depth. One of them is located beneath the Society hot spot and the other underlies 500 km east of the Society hot spot, where no volcanism is observed.

Seismic Structure and Seismicity in the Southern Mariana Trough and Their Relation to Hydrothermal Activity

The Southern Mariana Trough is an active back-arc basin with hydrothermal activity. We investigated relations between the back-arc spreading system and the hydrothermal system in this area by conducting a seismic reflection/refraction survey and a three-month campaign of seismic observations using ocean bottom seismometers. From a 3D seismic velocity structure analysis, we mapped a low-velocity structure just beneath the spreading axis, a high-velocity structure with convex upward beneath an off-axis knoll, and a thickening of layer 2 (to about 3 km) over the refraction survey area compared with normal mid-ocean ridges. We found very low seismicity in the hydrothermal area and high seismicity in areas of high topographic relief that probably represent arc volcanoes. The low-velocity structure at the axis suggests that there is some magmatic activity beneath the axis in the form of sheetlike mantle upwellings. These may constitute the hydrothermal heat source at this site. The high-velocity structure with convex upward at the off-axis knoll suggests the presence of off-axis volcanism there. The very low seismicity suggests that this volcanism may have ceased, thus residual heat of this off-axis volcanism may contribute the heat for hydrothermal activity at this site. A comparison of the velocity structure with other back-arc spreading zones and mid-ocean ridges shows that the Southern Mariana Trough has a relatively thick layer 2 with lower seismic velocities, suggesting that the crust was formed by magmas with high volatile contents, consistent with upwelling mantle influenced by subduction. The very low seismicity at the hydrothermal sites indicates that there are no faults or fractures related to the hydrothermal activity. This suggests that the activity is not related to tectonic stresses there.

In Situ Characterization of the Lithosphere‐Asthenosphere System beneath NW Pacific Ocean Via Broadband Dispersion Survey With Two OBS Arrays

We conducted broadband dispersion survey by deploying two arrays of broadband ocean bottom seismometers in the northwestern Pacific Ocean at seafloor ages of 130 and 140 Ma. By combining ambient noise and teleseismic surface wave analyses, dispersion curves of Rayleigh waves were obtained at a period range of 5–100 s and then used to invert for one-dimensional isotropic and azimuthally anisotropic βV (VSV) profiles beneath each array. The obtained profiles show ~2% difference in isotropic βV in the low-velocity zone (LVZ) at a depth range of 80–150 km in spite of the small difference in seafloor ages and the horizontal distance of ~1,000 km. Forward dispersion-curve calculation for thermal models indicates that simple cooling models cannot explain the observed difference and an additional mechanism, such as sublithospheric small-scale convection, is required. In addition, the fastest azimuths of azimuthal anisotropy in the LVZ significantly deviate from the current plate motion direction. We infer that these observations are consistent with the presence of small-scale convection beneath the study area. As for azimuthal anisotropy in the Lid, the peak-to-peak intensity is 3–4% at the depth from Moho to ~40 km. The fastest direction is almost perpendicular to magnetic lineation in area A at 130 Ma and oblique to magnetic lineations in area B at 140 Ma, suggesting complex mantle flow beneath the infant Pacific Plate surrounded by three ridge axes. The intensity of azimuthal anisotropy in the LVZ is ~2%, indicating that radial anisotropy is stronger than azimuthal anisotropy therein.

Recent developments of ocean bottom seismic and electromagnetic instruments operated by ROV

Essential information to understand geodynamic processes is provided from imaging the Earths interior by using geophysical observations. Recently, we have developed two geophysical instruments to exploit oceanic mantle, the BBOBS-NX (broad-band ocean bottom seismometer of next generation) and the EFOS (Earths electric field observation system), both of them are operated by ROV (remotely operated underwater vehicle). They provide data of much higher quality than conventional instruments such as BBOBS (broad-band ocean bottom seismometer) and OBEM (ocean bottom electro-magnetometer).