地球内部ダイナミクス領域

IFREEセミナーのお知らせ

セミナー履歴 (IFREE HQ)

2012

●IFREE HQ セミナーのお知らせ
日 時:2012年11月26日 16:00-17:30
場 所:本館5F国際交流施設
発表者:馬場聖至

タイトル:
Where is the petit-spot magma come from? - Insight from electrical conductivity

概要:
Petit-spot is young volcanic activity on very old oceanic plate characterized as a clump of small knolls which erupted strong to moderate alkaline basalt. This volcanic field is associated with neither any plate boundaries nor hot spots. To elucidate the magma generation process of this new-type volcanic activity, a collaborative study of various geophysical and geochemical approaches has been carried out. In this seminar, we introduce some results obtained from seafloor magnetotelluric (MT) surveys around the petit-spot fields in the northwestern Pacific.

We carried out seafloor MT surveys using ocean bottom electromagnetometers (OBEMs) in May - August, 2005 and in May, 2007 - August, 2008. Total nine OBEMs were deployed and seven of those were successfully recovered with good quality data. The data collected at two other sites in July, 2003 - November, 2004 were also compiled in this study.

We first estimated a one-dimensional (1-D) electrical conductivity structure model which explains the data of all sites averagely based on iterative topographic effect correction and inversion approach. The estimated 1-D conductivity model shows thick (~150 km) and resistive (<0.01 S/m) layer and deeper more conductive (0.01-0.1 S/m) layer, which are usually interpreted as lithospheric and asthenospheric mantle, respectively. Then, we carried out 3-D inversion analysis using the 1-D model as the initial and prior model. The obtained 3-D model shows two distinct features. 1) The lithospheric mantle beneath the petit-spot field at 37.5N, 149.8E (Yukawa knolls) is relatively more conductive than surrounding area. The conductivity is about 0.003 S/m at about 70 km depth. This feature is depicted as thinned resistive layer in the vertical section. 2) High conductivity (about 0.1 S/m) layer at around 200 km depth is not isolated beneath the petit-spot field but rather distribute widely beneath the survey area except for the area to the WNW direction of the Yukawa knolls where other two petit-spot fields were discovered. Checker board tests and a foward modeling test support that these features are well resolved by the data.

The above features make us to speculate that the asthenosperic mantle is partially molten and the melt is extracted to the lithosphere (and partly to the seafloor) by the petit-spot activity. The electrical conductivity at 200 km depth can be explained by small fraction of hydrous and carbonated silicate melt on temperature above the solidus of peridotite including H2O and CO2. However, further discussion is necessary for the reality of the temperature and the amounts of H2O and CO2 in the mantle.

●IFREE HQ セミナーのお知らせ
日 時:2012年11月14日 16:00-17:00
場 所:本館1Fセミナー室
発表者:北村有迅

Title:
Geology of the shallow subduction plate boundary

Abstract:
Geological study on subduction plate boundary process has been limited for its intrinsic nature that the material involved is going to subduct. Seismology has revealed quite a few facts with using indirect observation methods such as seismic wave analysis or crustal imaging. However, direct information of seismogenic process, a major plate boundary process, is still unclear. Furthermore, recently observed slow earthquakes may take large part of the whole process in terms of releasing energy because it takes longer duration than the ordinary earthquakes. Indirect observation cannot resolve such small-scaled phenomena. Here we introduce geological setting of the plate boundary rocks and discuss their dynamic role in the plate boundary deformation. We hypothesize a "thick plate boundary" where the deformation is partitioned into "fast/episodic" and "slow/continuous" deformation within.

●IFREE HQ セミナーのお知らせ
日 時:2012年10月3日 16:00-17:30
場 所:本館5F国際交流施設
発表者:Teh-Ru Alex Song

タイトル:
Subduction of Oceanic Asthenosphere: evidence from seismic anisotropy.

概要:
The oceanic asthenosphere is characterized as a low viscosity channel down to 200-300 km depth separating the cold lithosphere from above, and it is intimately linked to a layer of low seismic velocity and prominent seismic anisotropy observed globally beneath ocean basins. While subduction of tectonic plates in convergent margins is well recognized, the fate of oceanic asthenosphere remains enigmatic. We demonstrate that subduction of the oceanic asthenosphere characterized by weak azimuthal anisotropy and strong radial anisotropy explains the essence of sub-slab shear-wave splitting patterns, where the fast splitting direction changes from predominantly trench-parallel (or sub-parallel) under relatively steep subduction zones to frequently trench-normal under shallow subduction zones. To explain the observed splitting time, the thickness of the subducted asthenosphere is estimated to be 100±50 km.

To further validate this scenario, we examine SKS splitting patterns observed across the fore-arc central Alaska. Here the fast splitting direction varies from plate motion sub-parallel near the trench to mostly trench-parallel beyond the 100km slab-isodepth contour, while being strongly variable in between. After taking into account the rotation of anisotropy symmetry in the oceanic asthenosphere with respect to the local plate motion obliquity and down-dip variations in the slab dip, we reproduce a general 90-degree switch in fast splitting direction as well as the back azimuth dependent splitting pattern across the entire fore-arc. The current validation further augments the idea that, apart from anisotropy in the mantle wedge and the subducting slab, subduction of the oceanic asthenosphere is likely to be the dominant source of seismic anisotropy in central Alaska and potentially in many subduction zones. Furthermore, this result also provides alternative views on seismic anisotropy in the mantle wedge and on the length scale in which the 3D mantle flow may be important.

●IFREE HQ セミナーのお知らせ
日 時:2012年9月19日 16:00-17:30
場 所:本館1Fセミナー室
発表者:熊谷英憲

タイトル:
A real-time gas monitoring system on D/V Chikyu: today and prospect in future ultra deep drillings.
(「ちきゅう」設置リアルタイムガスモニタリング設備の現状と超深度掘削への展望)

概要:
As a real-time mud-logging system, a gas monitoring apparatus is now on-service on D/V Chikyu. It consists of degasser, gas transfer line and analyzers; Gas chromatogram, quadrupole mass spectrometer are standard analyzer. A Rn analyzer is also available as a third party tool owned by IFREE. Here, I report the present status of the gas monitoring system on D/V Chikyu, and preliminary data recently obtained; significant for example, Rn were detected in Exp. 906.

●IFREE HQ セミナーのお知らせ
日 時:2012年9月6日 14:30-16:00
場 所:本館1F大講義室
発表者:後藤忠則(京都大学)
Content: Current status and Future of Geophysical Exploration of Submarine Hydrothermal Ore.

●IFREE HQ セミナーのお知らせ
日時:2012年8月22日 (水) 16:30-18:00
場所:横須賀本部 本館1階セミナー室
発表者:Dr. Andrea Tommasi
(CNRS Senior Research Scientist,Geosciences Montpellier,Universite Montpellier II)

タイトル:
Deformation and anisotropies in the mantle

概要:
The propagation of seismic waves, the conduction of heat and electricity, and the viscoplastic deformation in an olivine crystal have a strong directional dependence. As a consequence, deformation in the upper mantle results in strong olivine crystal preferred orientation, which allow the transfer of these anisotropies to much larger scales, ranging from a few to hundreds of kilometers. In active plate boundaries and at the lithosphere-asthenosphere boundary, these anisotropies may be further enhanced by feedbacks between deformation and melt distribution. The association of observations in natural systems and multi-scale numerical models allow constraining the interpretation of seismic and magnetotelluric anisotropy data in terms of flow patterns in the upper mantle and the role of large-scale thermal and viscoplastic anisotropies on the reactivation of large-scale lithospheric structures in plate tectonics.

●IFREE HQ セミナーのお知らせ
日 時:2012年8月1日 16:00-17:00
場 所:本館5F国際交流施設
発表者:金松敏也

タイトル:
Mass Transport Deposits in Nankai Trough, IODP Expedition 333: Rheology of submarine landslides inferred from rock-magnetic properties

概要:
IODP Expedition 333 was conducted for two projects: NanTroSEIZE and NanTroSLIDE (Nankai Trough Submarine Landslide History). A drilling target for NanTroSLIDE was a stacked mass transport deposits (MTDs) recognized in 3-D seismic data in the slope basin of the mega-splay fault, offshore Kii Peninsula. The scientific objectives are to establish a mass-movement event stratigraphy and analyze their rheological properties to constrain sliding mechanisms. Six MTD units with undeformed hemipelagic intercalations were recovered from Hole C0018. Various deformation structures, which were formed during landsliding were found in the sequence. Onboard stratigraphies reveal that all MTDs were formed after 1Ma. Below MTDs, a sequence of sandy turbidites interbedded with silty clay was recovered. Anisotropy of magnetic susceptibility (AMS) of samples were measured to characterize their magnetic fabrics, which can be indicators of their deformations during landsliding. Oblate magnetic fabrics indicating simple vertical compaction were found in the undeformed sequences. On the other side, MTD1-MTD5 are characterized by prolate type in the magnetic fabric category, suggesting soft sediment lateral compactions. A magnetic fabric of MTD6 reveals a stronger deformation than MTD1-5. Reoriented lateral compacting directions of MTDs are mostly in NW-SE. Below the MTD sequence, oriented magnetic fabrics indicate NE-SW lineations, probably induced by turbidity currents along a paleo-trench system before 1Ma. A mass transport deposit (MTD6) could be caused from a slope collapse in the slope close to the paleo-trench with the stronger deformation around 1Ma. Subsequent MTD1-5 were originated from the softer surface sediment collapses. Such change in MTD deformation style might be related to the change in the tectonic setting of drilling site governed by the outer ridge imbricating and uplifting along geological time.

●IFREE HQ セミナーのお知らせ
日 時:2012年7月18日 16:00-17:00
場 所:本館1Fセミナー室
発表者:望月将志(東京大学)

タイトル:
Dynamic role of the rheological contrast between cratonic and oceanic lithospheres in the longevity of cratonic lithosphere

概要:
In the numerical modeling of mantle convection, it is still difficult to find the conditions which allow both stable cratonic lithosphere and plate tectonics. The three-dimensional numerical model presented herein makes it possible to model the cratonic lithosphere that survives for a sufficiently long geological timescale. An important factor in the longevity of cratonic lithosphere is the localized rheological (viscosity) contrast between the cratonic and oceanic lithospheres, i.e., the presence of a weak (low-viscosity) continental margin (WCM), such as tectonically mobile (orogenic) belts, that surrounds the lateral side of cratonic lithosphere. The WCM protects the cratonic lithosphere from being stretched by the surrounding convection force. In addition to the presence of a WCM, the higher viscosity of the cratonic lithosphere itself effectively contributes to the stability of the cratonic lithosphere, as suggested by the previous numerical modeling. However, the results of the present study suggest that the WCM plays a primary role in the longevity of cratonic lithosphere, even if the viscosity contrast between the cratonic and oceanic lithospheres is quite high, 10^3, and the high-viscosity of cratonic lithosphere may play a secondary role in the longevity of cratonic lithosphere. The combination of the presence of a WCM and the high-viscosity of cratonic lithosphere may realize the longevity of cratonic lithosphere that survives for over two billion years.

●IFREE HQ セミナーのお知らせ
日 時:2012年7月4日 16:00-17:00
場 所:本館1Fセミナー室
発表者:大林政行

タイトル:
P-wave tomography of Northeast China observed with NECESSArray; a implication for volcanoes and subducting slabs

概要:
A passive broadband seismic experiment, NorthEast China Extended SeiSmic Array (NECESSArray) has been deployed since 2009 for two years. Northeastern China is a very interesting region because slabs subducting from the south Kuril and Japan trenches are stagnant in the mantle transition zone and extends to northeastern China, and above the stagnant slabs, Sino-Korea craton and unusual volcanism in the continent exist. The relationships between the deep slabs and shallow structures are important clues to understand the tectonic features. P-wave travel-time picks of the NECESSArray stations were made interactively, while the teleseismic arrival time residuals were extracted using the adaptive stacking method. We picked more than 13,000 event-station pairs. Relative travel-times of P-wave between different stations were measured as a function of frequency using deep events of which P-waves separate in time from depth phases and very shallow events of which P-waves and depth phases are completely coincide. We found strong dispersive effect that is not predicted by our previous three dimensional (3D) P-wave model. We will combine the picked travel times and the frequency depended relative travel times to image a 3D P-wave heterogeneities of the northeastern China.
The result shows slow anomalies below the Cenozoic volcanoes around the Songliao basin. The slow anomalies can be traced down to 200km depth. On the other hand fast anomalies are observe below the Songliao basin in the uppermost mantle up to a few hundred km depth. In the mantle transition zone, no extended flatten slab is observed and it look like a hole of the stagnant slab. These observations is inconsistent with the "Big Mantle Wedge" hypothesis. To the south of this region, the very extended stagnant slab is observed and it seems to be buckling.

●IFREE HQ セミナーのお知らせ
日 時:2012年6月20日 16:00-17:00
場 所:本館1Fセミナー室
発表者:柳澤孝寿

タイトル:
Thermal convection in liquid metals under a uniform magnetic field: spontaneous reversals of the flow direction

概要:
The study on the nature of thermal convection in liquid metals is essential for the dynamics of the Earth's outer core, and the behavior of flow under a magnetic field is very important. We are studying the flow in liquid metals both by laboratory experiments and numerical simulations.
In highly conductive (low Prandtl number (Pr)) fluids like liquid metals, theoretical studies propose following features. (1) Two-dimensional steady roll structure emerging at the onset of convective flow easily becomes time-dependent just above the critical Rayleigh number (Ra) at the condition without external magnetic field, and producing oscillatory instability such as "traveling-wave convection" in the direction of the roll axis. (2) Under a horizontal magnetic field, the axis of the roll structure at the onset of convection is forced to align in the direction of the magnetic field, and the Ra for transition to time-dependent flow regime is increased. (3) At much higher Ra where turbulence is developed without magnetic field, suppression of turbulence and formation of anisotropic flow structure are expected under a magnetic filed. These features are confirmed in our laboratory experiments by visualizing flow patterns, and we found very interesting phenomena.
The vessel we used has a square geometry with aspect ratio five, and a uniform horizontal magnetic field is applied to the whole vessel. Flow patterns were visualized by ultrasonic velocity measurements, and time variations of convective flow structure were clearly observed. We found five flow regimes depending on Ra and the intensity of applied magnetic field B; those are, (1) isotropic large-scale cell pattern, (2) anisotropic cell with larger flow velocity perpendicular B, (3) short-period oscillatory behavior of rolls aligned B, (4) long-period oscillatory behavior of rolls with "random reversals of the flow direction", and (5) steady 2-dimensional rolls. We analyzed details of the 3-dimensional structure by mapping the velocity field, and considered the cause of these variations. The observed phenomena in these experiments are reversals of flow direction, but these features are analogous to the reversals of the geomagnetic field. The study of this flow reversal can provide a key for the mechanism of geomagnetic field reversals.

●IFREE HQ セミナーのお知らせ
日 時:2012年5月9日 16:00-17:00
場 所:本館1Fセミナー室
発表者:森重学

タイトル:
Mantle flow and deformation of subducting slabs around Japan

概要:
We will talk about two topics concerning mantle flow and deformation of subducting slabs around Japan based on the numerical simulation of mantle convection.

The first topic is about the structure in the mantle wedge beneath the northeast Japan. As an explanation of the spatial pattern of "hot finger" (Tamura et al., 2002) observed in this region, Honda and Yoshida (2005) suggest the existence of small-scale convection there. We estimated the P-wave anisotropy expected from small-scale convection base on a theory of LPO (lattice-preferred orientation) development and found that the signals of small-scale convection can be seen more clearly in vertical cross-section than in horizontal cross-section, although P-wave azimuthal anisotropy is obtained only in horizontal cross-section at the present stage (e.g., Ishise and Oda, 2005) in this region. It suggests the importance of determining the seismic anisotropy including vertical direction to reveal the structure in the mantle wedge.

The second topic is about the junction of Japan and Kurile arcs. We show that we can capture some characteristic features observed in this region by considering the shape of the trench. First, 3D flow in the mantle wedge which may be consistent with the observation of seismic anisotropy (Nakajima and Hasegawa, 2006) arises where the oblique subduction occurs. Second, the angle of subduction shows some variation along the strike of the trench, which qualitatively agrees with the observation (e.g., Gudmundsson and Sambridge, 1998). This may be explained by the torque balance acting on subducting slabs for the oblique subduction case. Third, the difference of the behavior concerning slab stagnation between Japan and Kurile arcs (e.g., Fukao et al., 2001) can be explained by the shape of the trench.

●IFREE HQ セミナーのお知らせ
日 時:2012年4月18日 16:00-17:00
場 所:本館3Fセミナー室
発表者:宮腰剛広

タイトル:
Geomagnetic field variation caused by length-of-day variation

概要:
The length-of-day variation is observed in the Earth. It probably affects outer core convection and geomagnetic field. Hamano (1992) pointed out relation between geomagnetic field variation and length-of-day variation associated with Milankovitch cycle. However, there are no geodynamo models which include length-of-day variation effect. In this paper we show magnetohydrodynamic geodynamo model which includes that effect. The Yin-Yang dynamo model (Kageyama and Sato 2004) was developed to include this effect. Two new elements are added in the model. (1) The rotational speed in the Coriolis force changes as time passes. (2) A new term, cross product of position vector and time differential of rotational speed, is added in a momentum equation. We take the Ekman number and Rayleigh number are 1.9E-5 and 1.5E8, respectively. Prandtl and magnetic Prandtl numbers are both unity. The rotational speed variation is given as a sine function of time. The amplitude of rotational speed variation is two percent. The period is the same as the magnetic diffusion time. (It corresponds to twenty thousand years in the Earth. This also corresponds to one of the Milankovitch cycle.) Numerical simulation results show the length-of-day variation causes variation of magnetic energy in the outer core, kinetic energy of convection, and magnetic dipole moment. The amplitude of magnetic energy and dipole moment variation is about thirty percent, which is much larger than the amplitude of the rotational speed variation. The phase of magnetic energy variation is out of the phase with the rotational speed variation. The difference is pi over two. In addition to the magnetic energy, quantity of heat generated by ohmic dissipation and heat flux at core-mantle boundary also oscillate. The detail mechanism of geomagnetic field variation as well as the result of the shorter period case will be discussed in this paper.

●IFREE HQ セミナーのお知らせ
日 時:2012年3月14日 16:00-17:00
場 所:本館1Fセミナー室
発表者:望月将志(東京大学)

タイトル:
Utilization of acoustic video camera for underwater imaging

概要:
DIDSON (Dual-Frequency IDentification SONar) is an acoustic lens-based sonar. It has sufficiently high resolution and rapid refresh rate that it can substitute for optical system in turbid or dark water where optical systems fail.
Institute of Industrial Science, University of Tokyo ( IIS ) has understood DIDSON's superior performance and tried to find new method for utilization of it. The observation systems that IIS has ever developed based on DIDSON are waterside surveillance system, automatic measurement system for fish length, automatic system for fish counting, diagnosis system for deterioration of underwater structure and so on.
We have had more and more opportunities for DIDSON to be equipped on the JAMSTEC's vehicles over the several past years.
In the seminar, I will introduce the overview of DIDSON and examples of its applied studies including the collaborative works with JAMSTEC.

●IFREE HQ セミナーのお知らせ
日 時:2012年3月7日 16:00-17:00
場 所:本館1Fセミナー室
発表者:志藤あずさ

タイトル:
Seismic structure of the oceanic lithosphere inferred from guided wave

概要:
Seismological observation using Broad-Band Ocean Bottom Seismometers (BBOBSs) was conducted in the northwestern Pacific from 2007 to 2008 and from 2010 to 2011. In the BBOBS data, unusual phase generated by events in the subducting Pacific plate were recorded universally. The phase shows some prominent features; large negative travel time anomaly (~ 10 %), high-frequency content (> 10 Hz), and long coda for both P and S waves. These features indicate that the unusual phase is guided wave traveling mainly in the oceanic lithosphere. The guided wave was previously called as Po/So waves and was studied actively from the 1970s to 1990s. It is now generally believed that these phases travel efficiently through the oceanic lithosphere. However, the mechanism of generation and propagation of the phases are still controversial.

We conduct travel time analysis of the guided wave. Both the guided wave and the direct wave are contained in each record and the guided wave is observed as a later phase in most case. We pick the onsets of the guided wave and the direct wave manually in high-pass filtered record with the corner frequencies of 10 Hz and 0.5 Hz, respectively. The travel time interval of the two phases changes systematically depending on the focal depth, the epicentral distance, and the azimuth. The apparent velocity of the guided P-wave varies from 7.8 to 8.6 km/s depending on the azimuth. The azimuth of the maximum velocity is north-south direction. These results are consistent with the previous explosion experimental studies and indicate that the guided wave travel horizontally in the oceanic lithosphere.

●IFREE HQ セミナーのお知らせ
日 時:2012年2月15日 16:00-17:00
場 所:本館3Fセミナー室
発表者:斎藤実篤

タイトル:
Present-day stress state in the Costa Rica subduction zone:
Preliminary estimates from logging-while-drilling, IODP Expedition 334
(孔内検層から推定されるコスタリカ沈み込み帯の応力状態:IODP第334次研究航海の成果)

概要:
Observation of stress state during inter-seismic period is an essential component to constrain hypotheses of stress change in seismic cycles. Present-day stress orientation has been typically determined using borehole breakouts imaged by downhole logging. Particularly for subduction margins, breakouts have only become possible to obtain since we adopted state-of-the-art borehole imaging technologies using logging-while-drilling (LWD) in the recent decade. However, no stress dataset has been collected from erosional subduction margins. IODP Expedition 334 is the first step in the Costa Rica Seismogenesis Project (CRISP), designed to understand the processes that control nucleation and seismic rupture of large earthquakes at erosional subduction zones. During the expedition two sites were drilled with LWD at Site U1378 in the mid-slope and Site U1379 in the shelf to obtain a comprehensive suite of geophysical logs to document difference in stress state between erosional and locked seismogenic zones in the Costa Rica margin. Borehole images clearly display vertical bands of large borehole radius at both two sites, interpreted as borehole breakouts caused by differences in the principal horizontal stresses. Based on the average azimuth of the breakouts, the maximum horizontal stress (SHmax) is oriented E-W to ENE-WSW in the mid-slope and NW-SE to NNW-SSE in the shelf, respectively. SHmax orientations combined with anelastic strain recovery results suggest that normal fault regime only in the shelf sediments and strike-slip regime in the shelf basement and the mid-slope. Although previous studies proposed extensional state in erosional seismogenic zone during inter-seismic period (Wang et al. 2010; von Huene et al., 2004), our first observations indicate compressional state in the Costa Rica margin. Further studies are on going to determine stress magnitude to be estimated from width of breakouts and uniaxial compressional strength measurements on the cores.

●IFREE HQ セミナーのお知らせ
日 時:2012年2月8日 16:00-17:00
場 所:本館1Fセミナー室
発表者:市原寛

タイトル: 
2-D Electrical resistivity structure around the seismo-genic zone of the 2011 Tohoku earthquake (M9.0)

概要:
Electrical resistivity in the crust and upper mantle depends on the pore-fluid distribution, salinity, and connectivity of fluid-filled rock pores. Thus imaging of resistivity distribution gives us fundamental information about fluid and lithological feature of subduction zones. In this study, we discuss 2-D resistivity distribution around the rupture zone of the 2011 Tohoku earthquake (M 9.0) based on the marine magnetotelluric (MT) surveys carried out between 2009 and 2011. The EM data were collected at 13 sites by the research cruses named KR09-16, KR10-12, NT11-08, NT11-13, and YK11-E06. Good quality MT impedances were obtained form these OBEM data based on the BIRRP program (Chave and Thomson, 2004). However, the impedances are severely distorted by 2D/3D bathymetric effect (e.g. Key and Constable 2011). Thus we conducted 2-D inversion that can include 2-D bathymetric effect after the 3-D bathymetric correction.
The inverted resistivity model entirely consists of conductive surface layer (< 3 ohm-m) and subsequence resistive area (> 300 ohm-m). While the main rupture zone shows high resistivity (> 3000 ohm-m), resistivity near the trench and the deeper area along the plate boundary is relatively low. The former and the later conductive areas may represent accretionary prism and dehydrated zone, respectively. Thus the main rupture zone seems to be located on an isolated resistive area along the plate boundary. The resistivity in the fore area of NE Japan arc (> 300 ohm-m) is much higher than that of the Nankai Trough (about 10 ohm-m) (Kasaya et al., 2005). It implies that large amount of aqueous fluid is transported into the NE Japan arc, which may affect volcanic activities and mantle convections.

●IFREE HQ セミナーのお知らせ
日 時:2012年1月25日 16:00-17:00
場 所:本館3Fセミナー室
発表者:田中聡

Title;
P-wave velocity structure in the lowermost 600 km of the mantle beneath western Pacific inferred from travel times and amplitudes observed with NECESSArray

Abstract;
NECESSArray (NorthEast China Extended SeiSmic Array) is a large-scale broadband seismic array deployed in northeastern China. Although the main objective of NECESSArray is to investigate the upper mantle structure beneath the northeastern China, it is also a powerful tool as a large aperture array to study deep Earth.
Here, we examine P-wave travel times observed with NECESSArray to determine P-wave velocity structure in the lower mantle beneath Western Pacific. Relative travel times with respect to those predicted by PREM are measured on short period seismograms from 15 earthquakes occurred in Tonga, Fiji, and Kermadec regions since Sep. 2009 to April 2010, so far, by using adaptive stacking method [Rawlinson and Kennett, 2004]. The residuals are defined as fluctuations with respect to an average of the whole array for each event. Station correction is defined as a median value of the residuals at each station. After applying the station corrections, ellipticity corrections, and distance corrections for surface focus, we synthesize all the residuals and finally obtain a characteristic residual variation as a function of epicentral distance from 80 to 95 degrees. The travel time residuals show an inverted V-pattern with the maximum delay of approximately 0.3 s at 87 degrees compared from a reference level at 80 and 95 degrees. To simply interpret this pattern through Herglotz-Wiechert inversion, we solve the velocity structure in the lowermost 600 km of the mantle and assume that above the inferred region is identical to PREM. The result indicates that the difference of the P-wave velocities from those of PREM gradually increase with depth, and reach the maximum velocity reduction of 0.20% at 300 km above the CMB and quickly increase.
P-wave amplitudes are used as supplementary data. Station corrections for amplitude are inferred from 6 deep Fiji earthquakes in the distance range 75° to 90°. Their focal mechanisms are corrected with the Global CMT solutions and theoretical amplitude variations due to elastic and anelastic structures with the reflectivity method are considered. The corrected amplitude that are sensitive to the velocity structure in the lowermost about 100 km of the mantle are obtained from 3 earthquakes occurred in Kermadec islands (their latitudes vary from 29.2 S to 31.6S) in the distance range from 86° to 96°. Although they are closely located each other, the data from the southernmost event indicate significantly rapid amplitude decay, and those from the northernmost event indicate moderate amplitude decay with a bending point at distance of about 90° in the amplitude variation as a function of an epicentral distance, those from the middle event show a large scatter. This observation suggests that a rapid horizontal change of the D" structure exists in the southwestern edge of the examined region.

●IFREE HQ セミナーのお知らせ
日 時:2012年1月11日 16:00-17:00
場 所:本館1Fセミナー室
発表者:多田訓子

タイトル:
Approximate treatment of seafloor topographic effects in 3-D marine magnetotelluric inversion and method of applying to real data

概要:
Magnetotelluric (MT) method, which reveals electrical conductivity structures, is one of the useful tools for understanding inside of the Earth. The electrical conductivity of the mantle minerals strongly depends on temperature, composition (including the degree of mantle hydration), and the fraction and connectivity of melt, all of which are important parameters in understanding the mantle dynamics. Therefore, electrical conductivity structures of upper mantle and transition zone play an important role for understanding mantle dynamics by collaborating with seismic tomography. However, there is no available 3-D inversion code for marine MT data obtained by ocean bottom electromagnetometers (OBEMs), so that 3-D resistivity structures beneath the seafloor have not been estimated so far. Thus, we developed a 3-D marine MT inversion code so that it can treat complex seafloor topography and seafloor data. This code only treats long-wave length topographies which are longer than computational grid. It allows us to save computational burden and enable the inversion in practical use. We also checked the effect of short-wave length topographies using real topography of the Pacific Ocean. Correction of short-wave length topographies for an input data improves recovery of a true resistivity structure. It indicates that we should correct short-wavelength topographic effects for observed data and input the corrected data into the inversion code when we estimate a resistivity structure beneath the seafloor.