Journal Of Geophysical Research: Solid Earth, Jun 1, 2023
We constrain olivine fabrics in the oceanic lithosphere using active and passive seismic observat... more 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 terms of the sinusoidal functions, where ϕ is the back azimuth. We observe a skewed azimuthal dependence of the Pn velocity, with a large peak‐to‐peak amplitude of about 12%. Similar features are observed using an independent data set of teleseismic Pn waves. We constrain the direction of the crystallographic axes of olivine to explain the observed azimuthal dependence and identify A‐type olivine with a slightly dipping a‐axis and slightly tilting b‐axis being compatible with our observations. In contrast, we find that D‐type olivine with any direction of crystallographic axes cannot explain our observations. Secondary deformation and recrystallization in the older Pacific may be responsible for this strong and complex seismic anisotropy in the study region.
The Miyake-jima volcano abruptly erupted on July 8, 2000 after 17 years of quiet and gave birth t... more The Miyake-jima volcano abruptly erupted on July 8, 2000 after 17 years of quiet and gave birth to a crater, 1 km in diameter and 250 m deep. This expected unrest was monitored during the years 1995–2000 by electromagnetic methods including DC resistivity measurements and self-potential (SP) surveys. Beneath the 2500 yr old Hatcho-Taira summit caldera audio-magnetotelluric soundings made in
In the Western Pacific, there are unique geologic/tectonic features such as oldest oceanic crust ... more In the Western Pacific, there are unique geologic/tectonic features such as oldest oceanic crust in the Pacific, seamount chains derived from the Caroline hotspot, the Ontong-Java plateau which is the largest large igneous province, and complex plate boundaries between major and microplates. Investigation of isotropic and anisotropic velocity structure for this region is essential to our understanding of those unique features aforementioned. We estimate an S-wave isotropic and radially anisotropic velocity model beneath the Western Pacific by applying partitioned waveform inversion to three-component seismograms collected from the Incorporated Research Institutions for Seismology Data Managing Center, the Oldest-1 Array deployed in 2018-2019 by a joint Korean-Japan research team, and the Ocean Hemisphere network Project (OJP, NM, and SSP networks). We nonlinearly invert three-component waveforms from 17,038 raypaths (Mw > 5.5) with a 3-D reference model consisting of Crust1.0 and AK135 and resulting constraints are used for iterative least-squares inversion to build an S-velocity model. Our isotropic Vs model shows low-Vs anomalies at ~40 km depth beneath the Ontong-Java Plateau indicating a thick crust, at ~200 km depth beneath the Woodlark spreading center and Caroline seamount chain and at ~600 km depth beneath the center of the Eauripik rise. High-Vs anomalies are observed beneath the center of the Ontong-Java Plateau at 40-150 km depth and at ~50 km depth beneath the West Philippine basin, the Parece-Vela basin, and the Caroline basin. Overall positive radial anisotropy anomalies are observed in the Western Pacific, but the contrast of anisotropy was found in the Pacific plate, Philippine Sea plate, and Caroline plate at ~50 km depth. Negative radial anisotropy anomalies found in the Perace-Vela basin at ~30 km depth, and strong positive anisotropy anomalies are observed at the northern boundary of the Ontong-Java Plateau and beneath the Sorol trough and Caroline seamount chain.
We have conducted a magnetotelluric (MT) survey across the central Mariana area (around 18°N) to ... more We have conducted a magnetotelluric (MT) survey across the central Mariana area (around 18°N) to provide a comprehensive electrical resistivity image of the mantle beneath the Mariana subduction zone, fore-arc, arc, and back-arc system, at a depths down to the transition zone. Our transect will address issues of hydration of the mantle wedge and subsequent melting, the origin of arc
The next generation type of broadband ocean bottom seismometer (BBOBS-NX) has been tested since 2... more The next generation type of broadband ocean bottom seismometer (BBOBS-NX) has been tested since 2008 and the brief report of the experimental study was already presented in the 2008 and 2010 AGU fall meeting (S43D-1918 and S13C-2038, respectively). This BBOBS-NX is currently designed as that is operated with the ROV. Because the averaged noise level (i.e., the noise model of the IRIS standard) of the BBOBS-NX is below the new high noise model for all three components in periods longer than 20 s, which is more than 20 dB of noise reduction in horizontal components compared to the conventional BBOBS, the data obtained is suitable for analyses using horizontal component waveforms, such as the receiver function analysis. From a rough estimation, one year observation by the BBOBS-NX is long enough to get a reliable receiver function at a single station, although three years long observation was required for the same result by using the conventional BBOBS. With latest instrumental preparations of the BBOBS-NX and a new earth electric field observation system (EFOS), we have started the normal oceanic mantle (NOMAN) project since 2010 by using with several BBOBS and ocean bottom electro-magnetometer (OBEM) around the Shatsky Rise in the northwestern Pacific. A small observation array consisting 5 stations (NM01–05) were deployed in June 2010 as the pilot study. Two stations (NM02 and 03) have the BBOBS-NX, and they were settled for one year long observation. A part of the main study was started in November 2011 but only for BBOBS and OBEM at the southeastern area of the Shatsky Rise due to delayed ship schedule by the 2011 off Tohoku earthquake. So that, the recovery of these two BBOBS-NX and new 6 deployments are planned to be done in August–September 2012 during the cruise of R/V KAIREI (KR12-14), when we started the main observation of this project. Finally, we performed the two recoveries and 5 deployments for the BBOBS-NX, and also done one recovery and 3 settlements with 3 km long cables for the EFOS. These instruments are planned to be recovered after two years long observation in 2014. This poster presentation is a preliminary report of the first practical BBOBS-NX observation.
Physics of the Earth and Planetary Interiors, May 1, 2004
Electric field observations by using 1000km scale submarine cables have been performed since earl... more Electric field observations by using 1000km scale submarine cables have been performed since early 1990s. One of the main purposes of the observations is to obtain observational constraints on the dynamics of Earth’s core such as the strength and the distribution of the toroidal magnetic field and its variation at the core mantle boundary. Several constraints have been obtained until
Physics of the Earth and Planetary Interiors, Nov 1, 2010
ABSTRACT We performed a three-year seafloor electromagnetic survey in the Philippine Sea, includi... more ABSTRACT We performed a three-year seafloor electromagnetic survey in the Philippine Sea, including the western edge of the Pacific Ocean, to image electrical features of a deep mantle slab stagnating in the transition zone and the surrounding mantle in three dimensions (3-D). The project iterated one-year deployment of ocean bottom electromagnetometers (OBEMs) using a total of 37 instruments installed at 18 sites. The data obtained have been analyzed in the order of their recovery based on a magnetotelluric (MT) method. In this study, we attempt to obtain a one-dimensional (1-D) electrical conductivity model beneath the Philippine Sea and the Pacific region separately that can be used as a reference model in the first step toward the 3-D analysis. The resultant 1-D models show three main features: (1) The conductivity in the shallower 200 km of the upper mantle depths of the two regions contrasts sharply, which is qualitatively consistent with the large difference in lithospheric age. (2) The conductivity at 200–300 km depth in both regions is more or less the same at approximately 0.3 S m−1. (3) The conductivity just below 400 km depth is higher for the Philippine Sea mantle than for the Pacific mantle. The conductivity structure can be interpreted in terms of the thermal structure, mantle hydration, and existence of partial melt using experimental results for the conductivity of mantle minerals. If the conductivity is interpreted simply as the effect of temperature, the mantle beneath the Philippine Sea could be hotter than the dry solidus of mantle peridotite and thus partially molten. However, beneath the Pacific region, the present analysis suggests that the partial melting is not required under the assumed peridotitic composition even if we consider mantle hydration.
ABSTRACT We have run a seafloor electromagnetic survey project in the Philippine Sea in order to ... more ABSTRACT We have run a seafloor electromagnetic survey project in the Philippine Sea in order to image the deep mantle slab stagnating in the transition zone and surrounding mantle in three dimensions. Seafloor observations at every 500 km or so is necessary to resolve the geometry of the slab because existing data sets are based on the observations by land geomagnetic stations and submarine cables, which are distributed coarsely and unevenly. Although it is difficult to establish a bunch of nearly permanent observation stations at seafloor, iterative maneuver observations using ocean bottom electromagnetometers (OBEMs) can acquire the data required to probe down to the mantle transition zone. The project iterates one-year-long deployment three times. Earthquake Research Institute, University of Tokyo and Institute for Research on Earth Evolution (IFREE), Japan Agency for Marine-Earth Science and Technology (JAMSTEC) have resourced the project with the OBEMs. In the first phase, we deployed 11 OBEMs in October, 2005 and recovered all of them successfully in November, 2006. R/V Kairei of JAMSTEC was utilized for both cruises. In the second cruise, we deployed another 12 OBEMs and started the second phase. The recovery of the OBEMs and the third initiation will be done in November, 2007. The quality of the first phase data is quite good except for the electric field at one site. The MT responses are estimated at 10 sites in the period range of from about 300 to 60,000 seconds. The geomagnetic transfer functions are also estimated at all the 11 sites in the range of about 300 to 1,000,000 seconds. The responses at the period longer than 100,000 seconds are somewhat inaccurate and the improvement is expected by further data collection in the second and third phases. These responses will be analyzed together with the responses which were obtained by past experiments in the Philippine Sea. The features of the responses may be classified by basins composing the Philippine Sea plate. Some of these features are explained by land-ocean distribution and seafloor topography. Three-dimensional forward modelling study is now on going to examine effects of the surface heterogeneities and some simple patterns of sub-surface heterogeneities. These results and preliminary report on the second phase will be presented in the meeting.
Journal Of Geophysical Research: Solid Earth, Jun 1, 2023
We constrain olivine fabrics in the oceanic lithosphere using active and passive seismic observat... more 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 terms of the sinusoidal functions, where ϕ is the back azimuth. We observe a skewed azimuthal dependence of the Pn velocity, with a large peak‐to‐peak amplitude of about 12%. Similar features are observed using an independent data set of teleseismic Pn waves. We constrain the direction of the crystallographic axes of olivine to explain the observed azimuthal dependence and identify A‐type olivine with a slightly dipping a‐axis and slightly tilting b‐axis being compatible with our observations. In contrast, we find that D‐type olivine with any direction of crystallographic axes cannot explain our observations. Secondary deformation and recrystallization in the older Pacific may be responsible for this strong and complex seismic anisotropy in the study region.
The Miyake-jima volcano abruptly erupted on July 8, 2000 after 17 years of quiet and gave birth t... more The Miyake-jima volcano abruptly erupted on July 8, 2000 after 17 years of quiet and gave birth to a crater, 1 km in diameter and 250 m deep. This expected unrest was monitored during the years 1995–2000 by electromagnetic methods including DC resistivity measurements and self-potential (SP) surveys. Beneath the 2500 yr old Hatcho-Taira summit caldera audio-magnetotelluric soundings made in
In the Western Pacific, there are unique geologic/tectonic features such as oldest oceanic crust ... more In the Western Pacific, there are unique geologic/tectonic features such as oldest oceanic crust in the Pacific, seamount chains derived from the Caroline hotspot, the Ontong-Java plateau which is the largest large igneous province, and complex plate boundaries between major and microplates. Investigation of isotropic and anisotropic velocity structure for this region is essential to our understanding of those unique features aforementioned. We estimate an S-wave isotropic and radially anisotropic velocity model beneath the Western Pacific by applying partitioned waveform inversion to three-component seismograms collected from the Incorporated Research Institutions for Seismology Data Managing Center, the Oldest-1 Array deployed in 2018-2019 by a joint Korean-Japan research team, and the Ocean Hemisphere network Project (OJP, NM, and SSP networks). We nonlinearly invert three-component waveforms from 17,038 raypaths (Mw > 5.5) with a 3-D reference model consisting of Crust1.0 and AK135 and resulting constraints are used for iterative least-squares inversion to build an S-velocity model. Our isotropic Vs model shows low-Vs anomalies at ~40 km depth beneath the Ontong-Java Plateau indicating a thick crust, at ~200 km depth beneath the Woodlark spreading center and Caroline seamount chain and at ~600 km depth beneath the center of the Eauripik rise. High-Vs anomalies are observed beneath the center of the Ontong-Java Plateau at 40-150 km depth and at ~50 km depth beneath the West Philippine basin, the Parece-Vela basin, and the Caroline basin. Overall positive radial anisotropy anomalies are observed in the Western Pacific, but the contrast of anisotropy was found in the Pacific plate, Philippine Sea plate, and Caroline plate at ~50 km depth. Negative radial anisotropy anomalies found in the Perace-Vela basin at ~30 km depth, and strong positive anisotropy anomalies are observed at the northern boundary of the Ontong-Java Plateau and beneath the Sorol trough and Caroline seamount chain.
We have conducted a magnetotelluric (MT) survey across the central Mariana area (around 18°N) to ... more We have conducted a magnetotelluric (MT) survey across the central Mariana area (around 18°N) to provide a comprehensive electrical resistivity image of the mantle beneath the Mariana subduction zone, fore-arc, arc, and back-arc system, at a depths down to the transition zone. Our transect will address issues of hydration of the mantle wedge and subsequent melting, the origin of arc
The next generation type of broadband ocean bottom seismometer (BBOBS-NX) has been tested since 2... more The next generation type of broadband ocean bottom seismometer (BBOBS-NX) has been tested since 2008 and the brief report of the experimental study was already presented in the 2008 and 2010 AGU fall meeting (S43D-1918 and S13C-2038, respectively). This BBOBS-NX is currently designed as that is operated with the ROV. Because the averaged noise level (i.e., the noise model of the IRIS standard) of the BBOBS-NX is below the new high noise model for all three components in periods longer than 20 s, which is more than 20 dB of noise reduction in horizontal components compared to the conventional BBOBS, the data obtained is suitable for analyses using horizontal component waveforms, such as the receiver function analysis. From a rough estimation, one year observation by the BBOBS-NX is long enough to get a reliable receiver function at a single station, although three years long observation was required for the same result by using the conventional BBOBS. With latest instrumental preparations of the BBOBS-NX and a new earth electric field observation system (EFOS), we have started the normal oceanic mantle (NOMAN) project since 2010 by using with several BBOBS and ocean bottom electro-magnetometer (OBEM) around the Shatsky Rise in the northwestern Pacific. A small observation array consisting 5 stations (NM01–05) were deployed in June 2010 as the pilot study. Two stations (NM02 and 03) have the BBOBS-NX, and they were settled for one year long observation. A part of the main study was started in November 2011 but only for BBOBS and OBEM at the southeastern area of the Shatsky Rise due to delayed ship schedule by the 2011 off Tohoku earthquake. So that, the recovery of these two BBOBS-NX and new 6 deployments are planned to be done in August–September 2012 during the cruise of R/V KAIREI (KR12-14), when we started the main observation of this project. Finally, we performed the two recoveries and 5 deployments for the BBOBS-NX, and also done one recovery and 3 settlements with 3 km long cables for the EFOS. These instruments are planned to be recovered after two years long observation in 2014. This poster presentation is a preliminary report of the first practical BBOBS-NX observation.
Physics of the Earth and Planetary Interiors, May 1, 2004
Electric field observations by using 1000km scale submarine cables have been performed since earl... more Electric field observations by using 1000km scale submarine cables have been performed since early 1990s. One of the main purposes of the observations is to obtain observational constraints on the dynamics of Earth’s core such as the strength and the distribution of the toroidal magnetic field and its variation at the core mantle boundary. Several constraints have been obtained until
Physics of the Earth and Planetary Interiors, Nov 1, 2010
ABSTRACT We performed a three-year seafloor electromagnetic survey in the Philippine Sea, includi... more ABSTRACT We performed a three-year seafloor electromagnetic survey in the Philippine Sea, including the western edge of the Pacific Ocean, to image electrical features of a deep mantle slab stagnating in the transition zone and the surrounding mantle in three dimensions (3-D). The project iterated one-year deployment of ocean bottom electromagnetometers (OBEMs) using a total of 37 instruments installed at 18 sites. The data obtained have been analyzed in the order of their recovery based on a magnetotelluric (MT) method. In this study, we attempt to obtain a one-dimensional (1-D) electrical conductivity model beneath the Philippine Sea and the Pacific region separately that can be used as a reference model in the first step toward the 3-D analysis. The resultant 1-D models show three main features: (1) The conductivity in the shallower 200 km of the upper mantle depths of the two regions contrasts sharply, which is qualitatively consistent with the large difference in lithospheric age. (2) The conductivity at 200–300 km depth in both regions is more or less the same at approximately 0.3 S m−1. (3) The conductivity just below 400 km depth is higher for the Philippine Sea mantle than for the Pacific mantle. The conductivity structure can be interpreted in terms of the thermal structure, mantle hydration, and existence of partial melt using experimental results for the conductivity of mantle minerals. If the conductivity is interpreted simply as the effect of temperature, the mantle beneath the Philippine Sea could be hotter than the dry solidus of mantle peridotite and thus partially molten. However, beneath the Pacific region, the present analysis suggests that the partial melting is not required under the assumed peridotitic composition even if we consider mantle hydration.
ABSTRACT We have run a seafloor electromagnetic survey project in the Philippine Sea in order to ... more ABSTRACT We have run a seafloor electromagnetic survey project in the Philippine Sea in order to image the deep mantle slab stagnating in the transition zone and surrounding mantle in three dimensions. Seafloor observations at every 500 km or so is necessary to resolve the geometry of the slab because existing data sets are based on the observations by land geomagnetic stations and submarine cables, which are distributed coarsely and unevenly. Although it is difficult to establish a bunch of nearly permanent observation stations at seafloor, iterative maneuver observations using ocean bottom electromagnetometers (OBEMs) can acquire the data required to probe down to the mantle transition zone. The project iterates one-year-long deployment three times. Earthquake Research Institute, University of Tokyo and Institute for Research on Earth Evolution (IFREE), Japan Agency for Marine-Earth Science and Technology (JAMSTEC) have resourced the project with the OBEMs. In the first phase, we deployed 11 OBEMs in October, 2005 and recovered all of them successfully in November, 2006. R/V Kairei of JAMSTEC was utilized for both cruises. In the second cruise, we deployed another 12 OBEMs and started the second phase. The recovery of the OBEMs and the third initiation will be done in November, 2007. The quality of the first phase data is quite good except for the electric field at one site. The MT responses are estimated at 10 sites in the period range of from about 300 to 60,000 seconds. The geomagnetic transfer functions are also estimated at all the 11 sites in the range of about 300 to 1,000,000 seconds. The responses at the period longer than 100,000 seconds are somewhat inaccurate and the improvement is expected by further data collection in the second and third phases. These responses will be analyzed together with the responses which were obtained by past experiments in the Philippine Sea. The features of the responses may be classified by basins composing the Philippine Sea plate. Some of these features are explained by land-ocean distribution and seafloor topography. Three-dimensional forward modelling study is now on going to examine effects of the surface heterogeneities and some simple patterns of sub-surface heterogeneities. These results and preliminary report on the second phase will be presented in the meeting.
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