Takehi Isse

The University of Tokyo | Todai · Earthquake Research Institute

The oldest oceanic basin (160–180 Ma) in the western Pacific is the birthplace of the Pacific Plate and is thus essential for understanding the formation and evolution of the oceanic plate. However, the upper mantle structure beneath the region has not been thoroughly investigated because of the remoteness and difficulties of long‐term in situ seis...

We constrain olivine fabrics in the oceanic lithosphere using active and passive seismic observations of Pn azimuthal anisotropy. We first analyze active‐source data derived from a broadband ocean bottom seismometer array that was deployed in the Northwestern Pacific. We infer the azimuthal dependence of the Pn velocity, including the 2ϕ and 4ϕ ter...

We analyze seismic records collected at the oldest (170–180 Ma) Pacific seafloor using broadband dispersion array analysis. Using ambient noise and teleseismic waveforms, we measure Rayleigh‐wave phase velocities in a period range of 5–200 s that are inverted for array‐average one‐dimensional isotropic and azimuthally anisotropic shear‐wave velocit...

We characterize the sources of background noise recorded by an ocean-bottom seismometer array deployed on the oldest oceanic lithosphere of the western Pacific plate. This igneous oceanic crust is blanketed up to 1000 m cover of sediments, making it a unique place to investigate correlations between sediment thickness and background noise levels in...

We modified existing methods of noise reduction to data recorded by Japanese broadband ocean-bottom seismometers (BBOBSs) deployed on the western Pacific seafloor conducted as the Oldest-1 Array observation. We removed the tilt and compliance noise that are dominant in low-frequency seismograms. We estimated the noise transfer function from the hor...

Inversion for seismological structures of the oceanic lithosphere-asthenosphere system is important to understand the mechanisms of plate tectonics. Previous models of the oceanic upper mantle have been primarily obtained via global tomography using surface waveforms. However, besides scarcity of waveform data in the oceanic regions, difficulties i...

We determined the depths of the mantle discontinuities with a receiver function method using broadband data recorded on the seafloor and islands of the Ontong Java plateau (OJP) and its vicinity. The 410-km discontinuity is broadly elevated by 10–20 km beneath the OJP. The elevation is observed in the low-velocity region above a stagnant paleo-Paci...

This model is Shear wave velocity model (PAC-age ) at depths between 50 and 250 km. netcdf grid file made by GMT4.5 Voigt average shear wave velocity with 8-degree (Western Pacific) or 15-degree (Whole Pacific) spatial filter (Vs=sqrt(2/3*Vsv^2+1/3*Vsh^2) ) : PAC-age-vsiso{depth}-F{8,15}.grd Radial anisotropy of shear waves with 10-degree (Wester...

We investigated the seismological structure beneath the equatorial Melanesian region, where is tectonically unique because an immense oceanic plateau, a volcanic chain and subduction zones meet. We conducted a multi-frequency P-wave tomography using data collected from an approximately 2-year-long seismic experiment around the Ontong Java Plateau (...

The Ontong Java Plateau in the western Pacific Ocean is the world’s largest oceanic plateau. It was formed 122 million years ago by a massive volcanic event that significantly affected Earth’s environment. The cause of the magmatic event remains controversial because the upper mantle structure beneath the plateau is poorly known. Here we use passiv...

This model (OJP-S-Isse) is Shear wave velocity model around the Ontong Java Plateau at depths between 20 and 300 km.

Plain Language Summary Spatially localized sources that persistently generate long‐period seismic waves at periods longer than 10 s are reported worldwide. For some sources, the excitation mechanisms have been well‐investigated. A volcano in Japan generates such waves that travel more than thousands of kilometers. For others, however, the excitatio...

We performed waveform inversion of the P waveforms recorded by our BroadBand Ocean Bottom Seismometers (BBOBSs) deployed in the Northwestern Pacific. Consequently, the depth profile of the P velocity of the oceanic upper mantle, which has not been well resolved by previous surface wave or receiver function analyses, was revealed. We considered the...

Abstract The Ontong Java Plateau (OJP) in the southwest Pacific is the largest oceanic large igneous provinces (LIP) on Earth. Detailed seismic structure of the plateau has not been understood well because of sparse seismic stations. We investigated seismic attenuation of the mantle beneath the plateau by analyzing data from temporary seismic stati...

The Early Cretaceous Ontong Java Plateau (OJP) in the southwestern Pacific Ocean is the largest oceanic plateau by volume on Earth, and a broad range of observations has been conducted to reveal its formation and evolution. However, because seafloor seismic observations of the OJP and surrounding areas have been insufficient so far, such experiment...

Using broadband seismic waveforms recorded on the seafloor by more than 200 broadband ocean bottom seismometers, as well as those on land, we measured the phase speed dispersions of Love and Rayleigh waves up to the 4th higher mode to determine the three-dimensional radially anisotropic shear-wave speed structure in the upper mantle beneath the Pac...

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...

We conducted geophysical observations on the Ontong Java Plateau (OJP) and its vicinity from late 2014 to early 2017 to determine the underlying crust and upper mantle structure beneath the OJP. Most of the OJP was emplaced in the present South Pacific region at 122 Ma by massive volcanism, but the origin of this volcanism are still debated. Previo...

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-...

We conducted a joint electromagnetic (EM) and seismic experiment to reveal the mantle structure beneath a normal seafloor at 130–145 Ma in the northwestern Pacific, where the seafloor is relatively flat and the underlying mantle is expected to be normal (free from tectonic perturbations). In the experiment, we deployed state-of-the-art instruments...

We present a new tomographic image beneath the South Pacific superswell, using finite frequency P-wave traveltime 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 hotspots in the studied region, exten...

We analyzed seismic ambient noise and teleseismic waveforms of nine broadband ocean bottom seismometers deployed at a 60 Ma seafloor in the southeastward of Tahiti island, the South Pacific, by the TIARES project. We first obtained one-dimensional shear-wave velocity model beneath the array from average phase velocities of Rayleigh waves at a broad...

We determined the three-dimensional shear wave velocity structure beneath the South Pacific superswell down to a depth of 200 km by analyzing fundamental Rayleigh wave records from permanent and temporary land-based and seafloor seismometers in the Pacific Ocean. Data from the Tomographic Investigation by seafloor ARray Experiment for the Society h...

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 veloci...

Ocean bottom seismometer records seismograms using a clock in a recorder. The recorder has a very precise clock although a few seconds of time shift occurs during a one-year seafloor observation. Usually, we measure time difference between a recorder's clock and a GPS clock before and after an observation and calibrate recordings. Recently, we...

[1] We have analyzed broadband surface wave data from ocean bottom seismometers deployed in the Shikoku Basin in the northeastern Philippine Sea to determine the radially anisotropic uppermost mantle structure beneath this oceanic basin. We first applied noise correlation method to continuous microseismic records to obtain phase velocities for fund...

Essential information for clarifying geodynamic processes is obtained by imaging the Earth's 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 ne...

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 So...

Since 1999, we had developed the broadband ocean bottom seismometer (BBOBS) and its new generation model (BBOBS-NX), and performed several practical observations with them in these ten years to create a category of the ocean floor broadband seismology. Now, the BBOBS data is proved to be acceptable for broadband seismic analyses. In these studies,...

Recently, low-frequency earthquakes and slow slip events are recognized in deep region of the plate boundary between the landward plate and the subducting Philippine plate below the southwestern Japan [e.g., Obara, 2002; Kawasaki, 2004]. The very low frequency earthquakes (VLFEs) occurring close to the Nankai Trough are also reported by using the b...

Essential information to understand geodynamic processes is provided from imaging the Earth's 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 (Earth's electric field observation system), b...

Recently, low-frequency earthquakes and slow slip events are recognized in deep region of the plate boundary between the landward plate and the subducting Philippine plate below the southwestern Japan [e.g., Obara, 2002; Kawasaki, 2004]. The very low frequency earthquakes (VLFEs) within the accretionary prism close to the Nankai Trough are also rep...

The radial anisotropy within the oceanic lithosphere appears weaker than that in the asthenosphere, and various origins for this difference are proposed, such as high shear (Nettles and Dziewonski, 2008) or thin melt layers (Kawakatsu et al., 2009) in the asthenosphere. Tomography studies using surface waves, however, usually analyze periods longer...

Since 1999, we have already developed the mobile broadband ocean bottom seismometer (BBOBS), and many practical observations in the northwestern Pacific Ocean and French Polynesian Sea have been conducted. But, through evaluations of these seismic data, the noise level of horizontal components in long periods (30s~), those are important in modern d...

Seismic anisotropy can provide fundamental information on past and present-day deformation processes in the upper mantle. Using Rayleigh and Love waves recorded by land and seafloor broadband seismometers, we analyzed the isotropic and anisotropic shear-wave velocity structures in the northern Philippine Sea region. We found that the fast direction...

The dynamics of mantle plumes and the origin of their associated swells remain some of the most controversial topics in geodynamics. According to the plume theory, originally proposed by Morgan, the hotspot volcanoes are created by jets of hot material (plumes) rising from the deep mantle. With later studies, troubling inconsistencies began to emer...

We have developed a visualization system for multidisciplinary geoscience data, which visualizes seismic tomographic models, geochemical datasets of rocks, and geomagnetic field models by exploiting Google Earth technologies. As Google Earth supports the ad hoc language, Keyhole Markup Language (KML), we have developed software packages to convert...

The South Pacific region is characterized by a broadly elevated seafloor known as the South Pacific superswell. This region has a concentration of midplate volcanoes that experienced massive eruptions in the mid-Cretaceous period (90-120 Ma). These characteristics suggest the presence of a large-scale mantle plume beneath the South Pacific, called...

We have determined the three-dimensional shear wave speed structure of the upper mantle in and around the Philippine Sea region using seismograms recorded by dense land-based and long-term broadband ocean bottom seismographic stations. We used a surface wave tomography technique in which multimode phase speeds are measured and inverted for a 3-D sh...

Since 1999, we have already developed the mobile broadband ocean bottom seismometer (BBOBS), and many practical observations in the northwestern Pacific Ocean and French Polynesian Sea have been conducted. But, through the evaluation of the seismic data, the noise level of horizontal components in long periods, those are important in data analyses,...

To investigate the stagnant slab beneath the northern Philippine Sea, we had conducted a three-year array observation from 2005 until 2008 by using broadband ocean bottom seismometers (BBOBSs) and ocean bottom electro-magnetometers (OBEMs). It is a key part of the "Stagnant Slab Project" started in 2004 for 5 years as a cross disciplinary project,...

Situated on the South Pacific Superswell, French Polynesia is a region characterized by numerous geophysical anomalies among which a high volcanism concentration. Seven hotspots are required to explain the observed chains, volcanism ages and geochemical trends. Many open questions still remain on the origin of these hotspot chains: are they created...

We determined three-dimensional shear wave speed structure beneath the South Pacific superswell down to a depth of 200 km by analyzing Rayleigh wave records from broadband ocean bottom seismograph stations and island stations in the Pacific Ocean. The ocean bottom stations were deployed from 2003 to 2005 on the seafloor in the French Polynesian reg...

We obtained three-dimensional (3-D) shear wave speed structure beneath the Philippine Sea and the surrounding region from seismograms recorded by land-based and long-term broadband ocean bottom seismographic stations. The ocean bottom data gave us a better station coverage to obtain a higher spatial resolution (about 300-400 km) in the Philippine S...

Previous seismic tomography studies show a broad low velocity anomaly in the lower mantle, so-called superplume, beneath the South Pacific and there are hotspot chains and large scale topographic high at surface of this region. However, the resolution of seismic tomography is poor, especially in the upper mantle, because of limited spatial distribu...

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 roc...

The Philippine sea is a marginal basin in large part opened through the two episodes of back-arc spreading. The evolution history of the Philippine Sea plate should be reflected in the upper mantle structure. However, the spatial resolution achieved by previous studies is not good enough to discuss seismological structures in terms of the plate tec...

SUMMARY We obtained phase velocities of fundamental Rayleigh waves in the northwestern Pacific ocean and the northern Philippine sea from seismograms retrieved from a long-term broad-band ocean bottom seismometer (LT-OBS) array deployed in 1999-2000 and from those recorded in Japan, Guam and Ponape islands. This array observation was one of the fir...

The inner-core anisotropy of P-wave velocity has been suggested since 1986. The detail of the anisotropic structure is now under discussion. We have studied a regional structure in the inner core beneath Australia to reveal the detail of the anisotropy using the residuals of the differential traveltimes of PKP (BC) minus PKP (DF). We have collected...

To estimate Qscs, previous studies used the spectral ratios of multiple ScS phases calculated for specific time windows. They assumed that the spectral ratios had linear relation with frequency. However, the spectral ratios very often did not seem to be linear, and the error bars were very large for frequencies higher than about 0.05 Hz. We have pe...