Normal Oceanic Mantle WS Program & Abstract	

Interna'onal	
  Workshop	
  on	
  	
  
	
  	
  	
  	
  	
  	
  	
  	
  	
  	
  	
  	
  	
  	
  	
  	
  	
  	
  	
  	
  Normal	
  Oceanic	
  Mantle	

March	
  4-­‐5,	
  2013	  Earthquake	
  Research	
  Ins'tute	
  University	
  of	
  Tokyo	Contents
Program..................................................1

Session 1: Projects #1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
Session 2: Projects #2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
Session 3: Mangle imaging . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
Session 4: Seismic Anisotropy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
Session 5: Physical properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
Session 6: Future . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
Poster. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30

International Workshop on Normal Oceanic Mantle
March 4-5, 2013
At meeting room-A, Earthquake Research Institute, University of Tokyo
March 4
Registration (09:30-10:00)
Session 1: Projects #1 (10:00-12:15)

Chair: Nozomu Takeuchi (ERI)

Utada, H. (ERI), Kawakatsu, H., Shiobara, H., Baba, K., Isse, T., Suetsugu D.,
and NOMan Project team
NOMan Project 1: Overview
Kawakatsu, H. (ERI)NOManProject
2:
Towardcomprehensive
understanding
of
the	
 
lithosphere/asthenosphere system of the normal oceanic mantle	  via in-situ ocean
bottom geophysical observations(Break)Shiobara, H. (ERI), Sugioka, H., Isse, T., Ito, A., Shinohara M., and Kanazawa, T.
NOMan Project 3: BBOBS-NX: practical observation tool for broadband seismology
at the seafloor
Forsyth, D.W. (Brown), Shintaku, N., Booth, C., Takeo, A., and Weeraratne, D.
The PLATE Project in the Western PacificSession 2: Projects #2 (13:30-14:30)

Chair: Hisayoshi Shimizu (ERI)

Gaherty, J.B. (Lamont), Lizarralde, D., Collins, J.A., Evans, R., and Hirth, G.
Lithospheric structure of the central Pacific:

early returns from the NoMelt

Experiment
Tarits, P. (UBO)
ELECTROLITH: Volatile Cycling and Electrical Conductivity in the Earth's Mantle:
The Carbonatite connection1

Session3: Mantle imaging (14:30-17:00)

Chair: Mamoru Kato (Kyoto)

Shito, A. (IFREE), Suetsugu, D., Furumura, T., and NOMan Project Team
Small-scale heterogeneities in the oceanic lithosphere inferred from guided waves(Break)Isse, T. (ERI), Shiobara, H., Sugioka, H., Ito, A., Suetsugu, D.
Three-dimensional shear wave structures of the upper mantle from seafloor
observations in the western Pacific Ocean
Baba, K. (ERI), Tada, N., Utada, H., Shimizu, H.
Preliminary result on electrical conductivity imaging of normal oceanic mantle
Takeo, A. (ERI) and NOMan Project team
Uppermost mantle structure beneath oceanic regions from broadband array analysis
of surface waves recorded by ocean bottom seismometers

Banquette at Hotel Edmont (19:00-21:00)

2

March 5
Session 4: Seismic anisotropy (09:30-12:15)

Chair: Kazunori Yoshizawa (Hokkaido)

Ekström, G. (Lamont)
Oceanic mantle structure from global tomography
Becker, T.W. (USC)
Seismic anisotropy below oceanic plates and large-scale mantle convection
Michibayashi, K. (Shizuoka)
Possible lateral variation of seismic anisotropies in the oceanic lithosphere due to an
active mantle flow in the mid-ocean ridge: some insights from the Oman Ophiolite(Break)Song, TR. (IFREE) and Kawakatsu, H.
Subduction of oceanic asthenosphere: Evidence from sub-slab seismic anisotropy
Mainprice, D (Montpellier)
A database of crystal preferred orientation of olivine in upper mantle rocks – thedirect mantle sample

Poster Session (13:30-14:30)
Session 5: Physical properties (14:30-16:00)

Chair: Takehiko Hiraga (ERI)

Hirschmann, M. (Minnesota)
Volatile-rich partial melt at the base of the oceanic lithosphere and in theasthenosphere
Takei Y. (ERI), Karasawa F., and Suzuki A.
Experimental study on anelasticity of polycrystalline material for seismologicalapplication
Utada, H. (ERI) and Baba, K.
Estimating electrical conductivity of melt from observation(Break)Session 6: Future (16:30-17:30)

Chair: Daisuke Suetsugu (IFREE)

Singh, S. (IPGP)
Imaging of Lithosphere Asthenosphere Boundary Across Atlantic Ocean: A Dream

Discussions (17:00-)

3

Posters
Evans, R. (WHOI)
An extensive melt layer beneath the oceanic lithosphere asthenosphere boundary
discovered by magnetotelluric data
Schardong, L. (ERI)
Toward a high resolution tomographic imaging of the lowermost mantle
Morishige, M. (IFREE)
Flow and thermal structure around the junction of Japan-Kurile arcs
Tada, N. (IREE)
3-D electrical conductivity structure beneath the Philippine Sea Plate
Takeuchi, N. (ERI)
Absence of the stagnant slab beneath northeast China constrained by a seismic
station in Mongolia
Shimizu, H. (ERI)Electrical voltage measured by long baseline submarine cables to probe the Earth's
deep interior
Honda, S. (ERI)
Importance of hot anomaly adjacent to the subducting slab
Zhang, L. (ERI)Three-dimensional simulation of the EM fields induced by 2011 Tohoku tsunami
Nishida, K. (ERI)
Global propagation of body waves revealed by cross-correlation analysis of seismic
hum
Obayashi, M. (IFREE)
P-wave tomography using BBOBS data4

Session 1: Projects #1

The NOMan Project: Overview
Hisashi Utada1, Hitoshi Kawakatsu1, Hajime Shiobara1, Kiyoshi Baba1, Takehi Isse1, Daisuke
Suetsugu2, and the NOMan Project team1

Earthquake Research Institute, University of Tokyo, Japan,
2
Japan Agency for Marine-Earth Science and Technology

The oceanic mantle is an important region to understand the Earth system, because
more than 2/3 of the surface is covered by oceanic area. The NOMan project focuses on the‘normal oceanic mantle’ in particular. Here ‘the normal oceanic mantle’ can be defined as theoceanic mantle without disturbance by vertical motion of mantle flow such as mid oceanic
ridges, plate subductions, or hotspots (Fig. 1). We try to solve a couple of most fundamental
questions concerning the normal oceanic mantle.
First question is on the cause of the asthenosphere, which is a lubricating layer below
oceanic plate (lithosphere). Plate tectonics is based on a concept that a rigid lithosphere moves
over a weaker asthenosphere, and thus the precise knowledge of its lubrication mechanism isfundamental to understand how our planet works.
The presence of water is one of the properties characterizing the planet Earth.
Second question is the amount of water within the Earth, particularly in the mantle transitionzone (MTZ). Because the MTZ minerals have very high water solubility that that of upper orlower mantle minerals, accurate estimation of water content in the MTZ is essential tounderstand the Earth’s total water budget. The question may never be fully solved without theknowledge for the “normal oceanic mantle” that occupies the largest part of the entire mantle.
The NOMan project is carried out for 5 years from 2010. It aims to solve these twoquestions from observational approach in two target areas (A and B) in the northwestern
Pacific Ocean (Fig. 2) where the mantle supposed to be normal. We plant to carry out
long-term observation for one to three years at each of observation sites shown in Fig. 2,
deploying state-of-the-art ocean bottom geophysical (seismic and electromagnetic) instruments
installed by using ROV, as well as free-fall type instruments, which were all originallydeveloped by our group.
In 2010, we started a ‘pilot experiment’ by deploying five sites in the area A.
Although we originally planned to start a large array from the summer of 2011, major
deployment was postponed by nearly one year due to the occurrence of the 2011 Tohoku
Earthquake and Tsunami. In the fall of 2011, we have deployed only free-fall type instrumentsin the area B. Finally in the summer of 2012, we successfully deployed all stations in the area
A for long-term observation. After replacement of several stations, all instruments will berecovered in the summer of 2012.
This presentation will give an overview of the NOMan Project, including researchhistory of our group, motivation, progress, and perspective of the project.

5Session 1: Projects #1

Fig. 1. The target (in the white circle) of the NOMan project, where the oceanic plate is moving
horizontally. We try to have a clear image of the normal oceanic mantle from the LAB down to MTZ
by long-term seafloor geophysical observation.

Area A

Area B

Fig. 2. Current site distribution in the NOMan study areas A and B.

6

Session 1: Projects #1

Toward comprehensive understanding of
the lithosphere/asthenosphere system of the normal oceanic mantle
via in-situ ocean bottom geophysical observations
Hitoshi Kawakatsu1
1

Earthquake Research Institute, University of Tokyo, Japan

Plate tectonics started as a theory for the oceanic basin by investigating its shallowest
part, leaving the deeper part of the lithosphere or lithosphere/asthenosphere system behind.
Geophysical exploration in the ocean in the past several decades have focused on tectonically
active areas, such as subduction zones, hot spots, mid-oceanic ridges, etc. Although theseattempts have successfully elucidated the active part of the earth’s processes, the importanceof apparently quiet normal oceanic areas, where the structure below may offer a “textbookview” of the deep mantle, might have been underestimated. While which part of the ocean isreally normal might be another interesting and difficult question to answer, a key for the
breakthrough likely resides beneath there. There are few multi-disciplinary ocean bottom
geophysical observations with state-of-the-art equipments have been undertaken in the“normal” Pacific ocean with which we hope to share the current knowledge and exchange
ideas about future planning at this workshop.
In this summary presentation, I will review some of the recent progress made in theseismological literatures in the context of Normal Mantle Project, especially focusing on thelithosphere/asthenosphere system, and also discuss some of the issues that we hope to resolvein the project that might be also topics of the workshop.
The main issues to be discussed include:(1) what is the nature and origin of the LID, G-discontinuity, and LVZ?
(2) what is the nature and origin of seismic anisotropy in the oceanic region?
(3) how the in-situ ocean bottom observation may help solving these issues?

(left) High school textbook view of plate tectonics in the ocean.
(right) Schematic diagram showing the relationship of LID, LVZ, G and LAB.

7

Session 1: Projects #1

BBOBS-NX: practical observation tool for
broadband seismology at the seafloor
Hajime Shiobara1, Hiroko Sugioka2, Takehi Isse1, Aki Ito2,
Masanao Shinohara1 and Toshihiko Kanazawa1
1

2

Earthquake Research Institute, University of Tokyo, Japan
IFREE, Japan Agency for Marine-Earth Science and Technology, Japan

Since 1999, we have already developed the mobile broadband ocean bottom
seismometer (BBOBS, Fig. 1), and many practical observations have been conducted. But,
the noise level of BBOBS's horizontal components in long periods is rather high in averageand its variation in time is also large. The reason of this high noise level is assumed as thesmall tilt variation of the large housing sphere that contains the broadband sensor, recorderand batteries, due to the bottom current. To clear this problem, one idea of observation
without the tilt variation is the use of small and low profile broadband sensor that enables topenetrate into the sediment, that is apart from the large housing. This next generation type of
BBOBS (BBOBS-NX, Fig. 2) has been tested since 2008. The observation procedure of the
BBOBS-NX is as following; (1) free-fall the BBOBS-NX to the seafloor aiming enoughpenetration of the sensor unit (CMG-3T special, 0.02–360 s) with the recording unit (ø650 Ti
sphere housing) attached above it, (2) detach and move the recording unit from the sensor unit
by using the ROV to start the observation, (3) pull-up the recording unit and the sensor unit
connected with the umbilical rope to recover the whole BBOBS-NX by the ROV.
The averaged noise level of the BBOBS-NX is below the new high noise model(NHNM) for all three components in periods longer than 20 s, which is more than 20 dB ofnoise reduction in horizontal components compared to the BBOBS (Fig. 3). So, theBBOBS-NX's data is suitable for analyses using horizontal component waveforms, such as
the receiver function analysis (Fig. 4). 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 at
least three years long observation was required for same quality by the BBOBS. This highperformance in the broadband seismic observation at the seafloor is the key of this normal
oceanic mantle project to research the nature of the LAB and the water content in the MTZ.
Now, six BBOBS-NXs are deployed around the Shatsky Rise with other instruments.

Fig. 1 The BBOBS on the deck (left) and on the seafloor (right).

8Session 1: Projects #1

Sensor unit

Recording unit

Cable & rope

Fig. 2 The BBOBS-NX soon to be launched (left) and in observation at the seafloor (right).
The ROV is required to detach from the sensor unit and move the recording unit (orange
colored Ti sphere housing) and also to recover the whole BBOBS-NX.

Fig. 3 The noise models of the BBOBS (1 year data, gray scale) and the BBOBS-NX (2months data, color) at the same station in the Philippine Sea. Horizontal's noise levels (H1 andH2) of the BBOBS-NX are lower than the NHNM in the long period range (20 s –).

Fig. 4 Examples of receiver function analysis by using the data of the BBOBS (TOK08) andthe BBOBS-NX (NX2). Only the NX2's result shows signals corresponding with the MTZ.

9

Session 1: Projects #1

	  The PLATE Project in the Western PacificDonald W. Forsyth1, Natsumi Shintaku1, Catherine Booth1, Akiko Takeo2,
Dayanthie S. Weeraratne3

1

Dept of Geological Sciences, Brown University, Providence, RI, USA. 2Earthquake Research
3
Institute, University of Tokyo, Japan, Calif. St. University, Northridge, CA, USA.

	  The Pacific Lithosphere Anisotropy and Thickness Experiment (PLATE) is designed
to address the evolution of old, normal, oceanic lithosphere and asthenosphere. On each limb
of a magnetic bight south of the Shatsky Rise in the western Pacific, we deployed a small
array of 8 ocean bottom seismographs for approximately one year. The magnetic bightdefines a right angle bend in the magnetic anomalies formed near a ridge-ridge-ridge triple
junction 150 to 160 mybp, Much of the seafloor in this area is undisturbed by subsequentvolcanism, with abyssal hill topography still present. This is the oldest undisturbed seafloor
in the Pacific with identifiable magnetic anomalies.
There were two primary goals: determine the structure of old lithosphere beneathundisturbed seafloor with high lateral resolution to avoid contamination with neighboring
disturbed seafloor; and to detect the depth that anisotropy changes from being frozen into thelithosphere to where it is dynamically created by motion between the plate and the deeper,
underlying mantle. Despite a number of instrument problems, we are making progress
towards those goals with PLATE. We have measured: azimuthal anisotropy using Pn phasesfrom earthquakes in subduction zones in the western Pacific; azimuthal anisotropy of shortperiod Rayleigh waves from ambient noise analysis; and P wave attenuation from deep
earthquakes in the Marianas and Izu-Bonin subduction zones.
Key observations will be the dispersion and attenuation of Rayleigh waves at long
periods. We are working on extending the useful period range by removing noise generated
by long-period water (gravity) waves, using the coherence of vertical displacement withpressure recorded on differential pressure gauges (DPG).
Finally, a somewhat similar experiment is suggested for normal seafloor 30-40 Ma
old to test a new hypothesis for the origin of the relatively sharp lithosphere-asthenosphereboundary (LAB). Key observations will be attenuation and shear wave velocity within thelithosphere and the depth of the LAB.
	
  

10

Session 1: Projects #1

Pn anisotropy on the western and eastern limbs of the magnetic bight. In both cases, thefastest direction is approximately in the direction of fossil seafloor spreading.
	
  

11

	
  

Session 2: Projects #2

Lithospheric structure of the central Pacific: early returns from theNoMelt experimentJ. B. Gaherty1, D. Lizarralde2, J.A. Collins2, R. Evans2, and G. Hirth3
1Lamont-Doherty Earth Observatory of Columbia University
2
Woods Hole Oceanographic Institution
3
Brown University

Recent advances in laboratory measurements and theoretical models of the seismic
properties of mantle rocks predict seismic velocity profiles for mature oceanic
lithosphere that are fundamentally inconsistent with the best observations of seismic
velocities in two ways. First, the theoretical models consistently display negative
velocity gradients with depth in the lithosphere, while observed velocity profiles havepositive gradients. Second, the transition from the high-velocity lithosphere to the
low-velocity asthenosphere (the LAB) is too sharp, and too shallow, to be controlled
entirely by temperature. These inconsistencies suggest that non-thermal factors suchas bulk composition, mineral fabric, grain size, and dehydration play important roles
in controlling the formation of the lithosphere, and thus the underlying LAB. There islittle consensus on which of these factors are dominant, in part because observationsof detailed lithosphere structure are limited.
In 2011-2012, we conducted the NoMelt experiment on ~70 Ma Pacific lithospherebetween the Clarion and Clipperton fracture zones. The experiment consists of a
600x400 km array of broad-band (BB) and short-period (SP) ocean bottom
seismometers (OBS) and magnetotelluric (MT) instruments. The BB OBS array willmeasure anisotropic velocity and anelasticity structure based on surface waves andmap the LAB based on receiver functions. The MT instruments will reveal melt andvolatiles. The BB OBS and MT arrays were recovered in January 2013, and analysis
is just commencing. Observations from the SP OBS array, presented here, constrain
the velocity of the uppermost mantle to a depth of ~30 km below the Moho based onseismic refraction arrivals (Pn) sourced from an airgun array firing shots at 500-m
intervals. We present results from the primary active-source line, which extends 600
km along the center of the segment in the flow-line direction and was instrumented
with 31 SP OBS spaced 20 km apart. The instruments record Pn arrivals to distances
as great as 500 km. The apparent velocity of Pn arrivals is ~8.5 km/s over most of thetransect, but apparent velocities as high as 9.0 km/s are observed for Pn propagating
westward over the western end of the transect. The unusually high horizontal phasevelocity may indicate dipping structure within the uppermost mantle beneath a portionof the transect. Sub-Moho structure is also indicated by wide-angle reflections thatapproach the Pn branch near 185 km and 400 km range. A very preliminary analysisof the data suggest that these wide- angle reflections arise from discontinuities at ~1217 and ~30 km below the seafloor. We expect that a complete analysis of this dataset
will reveal uppermost lithospheric structure in unprecedented detail.

12

Session 2: Projects #2

ELECTROLITH: Volatile Cycling and Electrical Conductivity in the
Earth’s Mantle: The Carbonatite connection
Pascal Tarits1,2
1

Institute for Marine Sciences, University of Western Brittany, France,
2
Earthquake Research Institute, University of Tokyo, Japan,

The purpose of the ELECTROLITH project is to address the topic of fluids andvolatiles in the Earth mantle. We hypothesize that volatiles are stored and transported bycarbonatite melts and westudy this process using a multidisciplinary approach.
ELECTROLITH brings together specialists of mantle fluids with complementary
methodological approaches including experimental petrology, experimental physics,
theoretical physics and field geophysics. The overall aim consists in completing a relatively
well established view of the geochemical role of carbonatite fluids in the deep carbon cycleby physical quantities that will allow us to evaluate the ability of carbonatite to transport othervolatiles than carbon, to provide constraints on the impact of carbonatites on physicalproperties of mantle rocks to provide geophysical images allowing the localization ofcarbonatite fluids and to present a joined perspective on mantle geophysical anomalies andcarbonatite transfers. To achieve these goals, we are collecting new data on physicalquantities that will:- Define the atomic structure and thermodynamic properties of multi-component carbonatite
fluids using high pressure high temperature experimentation, spectroscopy and molecular
dynamic simulations- Determine the ability of carbonatites to transfer volatiles such as water and noble gases in
addition to carbon.
- Characterize the effect of carbonatite fluids on the electrical conductivity and viscosity ofmantle rocks by laboratory measurements- Determine the mechanisms and velocity of carbonatite fluid transfers in the mantle and theirrelationships with stress distribution- Trace carbonatite flows through the mantle using electrical conductivity as derived fromdeep
The first series of studies are devoted to help improve the picture of carbonatite
physical properties. One task is experimental and involves high pressure high temperaturesynthesis and spectroscopic examination of carbonatites, to elucidate their structure andsolubility laws for volatiles. The second task, strongly complementary to the first one, isnumerical and involves molecular dynamic calculations on carbonatite melts. Most of the dataobtained in the experimental task will be simulated by MD, but MD methods can also provide
many other properties such as electrical conductivity, viscosity, and several thermodynamic
parameters. Those calculations serve as theoretical guidance for the interpretation ofexperimental data and the consistency between both set of data will be tested. This allows us

13Session 2: Projects #2

to provide an internally consistent set of data on the physical properties of those melts.
Furthermore MD will be extremely useful to extrapolate some of the carbonatite properties
toward pressure and temperature conditions where experimental measurements are impossible.
Other studies address the physical properties of mantle rocks containing carbonatite melts, onelectrical and viscous properties of carbonatite-bearing peridotite and on the transfer ofcarbonatite through grain boundaries. A good connection of the film together with exceptionaltransport properties of carbonatite melts implies that impregnation rates should be extremely
fast. Then, how deformation affects impregnation is investigated. Finally, the studies arecompleted by investigations on geophysical sourcing of carbonatite melts in the mantle.
Given the overall general picture wanted by ELECTROLITH, there is a need of large-scaleprobing of mantle fluids to complete and extend the above part. The purpose here is tocomplete the carbonatite model by performing in the EAR and exploiting in Kerguelen
conductivity measurements of the mantle. Those two sites offer a view of two contrasted
geodynamic settings: Continental break-up in the African rift, and a hotspot (considered bysome authors as large igneous province) with a possible plume activity at Kerguelen.

14Session 3: Mantle imaging

Small-scale heterogeneities in the oceanic lithosphere inferred from guided
waves
Azusa Shito1, Daisuke Suetsugu1, Takashi Furumura2, and NOMan Project Team
1

Japan Agency for Marine-Earth Science and Technology, Yokosuka, Japan
2

Earthquake Research Institute, University of Tokyo, Tokyo, Japan

Characteristic seismic waves are observed by seismological experiment using Broad-Band
Ocean Bottom Seismometers (BBOBSs) conducted in the northwestern Pacific from 2007 to2008 and 2010 to 2011. The seismic waves have low frequency onset (< 1 Hz) followed byhigh frequency later phases (2.5-10 Hz). The high frequency later phases have large amplitude
and long duration for both P and S waves. The seismic waves are observed commonly at the
BBOBS array from events in the subducting Pacific plate. To investigate generation andpropagation mechanisms of the seismic wave will help us to understand the seismic structure
and the origin of the oceanic lithosphere.
We apply the method proposed by Furumura and Kennett [2005] to reproduce theseismograms recorded by the BBOBS array. We conduct 2-D numerical simulation of seismic
wave propagation up to 5 Hz by using finite difference method. The 2-D model area covers
1600 km in horizontal distance and 400 km in depth with a uniform grid interval of 0.04 km.
The parallel simulation is conducted by using supercomputer system at JAMSTEC. Synthetic
waveforms from the earthquakes in the Pacific plate at the depths of 205 km and 15 km arecomputed for a variety of oceanic lithosphere models. We satisfactorily reproduce thecharacteristic seismograms having low frequency first arrivals and high frequency later
phases. The resultant preferred model includes small-scale random heterogeneity both insubducting and horizontal part of the oceanic lithosphere. The small-scale random
heterogeneity has elongated scatters described by von Karman function with correlation
length of 10 km in stretcher direction and 0.5 km in thickness. The standard deviation of theseismic wave velocity fluctuation from the background model is 2 %.

15

Session 3: Mantle imaging

Three-dimensional shear wave structures of the upper mantle from seafloor
observations in the western Pacific Ocean
Takehi Isse1, Hajime Shiobara1, Hiroko Sugioka2, Aki Ito2, Daisuke Suetsugu2
1

2

Earthquake Research Institute, University of Tokyo, Japan
Institute for Research on Earth Evolution, Japan Agency for Marine-Earth Science and Technology,
Japan

Since 1990s, seafloor observations by using newly developed long-term broadband
ocean seismometers (BBOBSs) have been operated in the northwestern and center part of the
Pacific Ocean. Most of them were operated in the Philippine Sea region.
The Philippine Sea, located in the northwestern part of the Pacific Ocean, consistsof several small basins, ridges and troughs with various seafloor ages. The age of the seafloor
in the West Philippine Basin is 35–50 Ma, whereas that in the Shikoku Basin and theParece-Vela Basin is 15–30 Ma. Seafloor spreading within the Mariana Trough started atabout 6 Ma. These ages have been explained in terms of two episodes of back-arc spreading.
The older parts of the Pacific plate (seafloor ages of around 150 Ma) are subducting beneaththe Philippine Sea plate along the Izu–Bonin–Mariana island arc at the eastern boundary of
the Philippine Sea plate. The northern margin of the Philippine Sea plate is subducting
beneath the Eurasian plate. The upper mantle structure beneath the Philippine Sea plate,which may reflect the complex evolutionary history described above, as well as lateralvariations caused by modern-day subduction, has been extensively studied using surface
waves and P- and S-wave travel times. However, a lack of both seismic stations andearthquakes in the central Philippine Sea has limited the spatial resolution of the upper mantle
in this region in previous models, especially in the southern part of the Philippine Sea, making
it difficult to reduce artifacts caused by structures located outside the Philippine Sea.
We have determined the three-dimensional shear wave speed structure of the upper
mantle in the northwestern Pacific Ocean using seismograms recorded by dense land-based
seismic stations and BBOBS stations [1, 2]. We used a surface wave tomography technique inwhich multimode phase speed of the Rayleigh wave are measured and inverted for a 3-D
shear wave speed structure by incorporating the effects of finite frequency and ray bending.
BBOBS data provide us with improved spatial resolution (~300 km) in the Philippine Sea
region. In the upper 120km, the shear wave speed structure is well correlated with the seafloor
age of the provinces. At depths greater than 160 km, fast anomalies of the subducting PacificPlate are clearly defined. Along the Izu-Bonin-Mariana arc, we have detected three separate
slow speed anomalies in the mantle wedge at depths shallower than 100 km beneath the reararc. Each anomaly has a width of ~500 km [2].
In recent study, we analyze the azimuthal and radial anisotropic structures of thenorthern Philippine Sea region and show their relationships with plate motion anddeformation processes using the phase velocity dispersion curves of fundamental modes ofLove and Rayleigh waves [3].
We found that the fast directions of azimuthal anisotropy are parallel to the16Session 3: Mantle imaging

directions of ancient seafloor spreading in the lithosphere of the Shikoku and West Philippine
Basins and Pacific Ocean, whereas they are parallel to the direction of the present-day
absolute plate motion (APM) in the asthenosphere of the Shikoku Basin, and oblique to thedirection of the APM in the Pacific Ocean (by ~30º) and in the northern part of the West
Philippine Basin (by ~55º). In the subduction zones around the Philippine Sea plate, the fast
direction of azimuthal anisotropy is trench-parallel in the Ryukyu arc, and oriented NW–SE in
the Izu–Ogasawara island arc. The Philippine Sea plate, which is a single plate, shows verylarge lateral variations in azimuthal and radial anisotropies compared with the Pacific plate.
In these previous studies, we have focused on the structure around the subduction
zone. In NOMan project, we have focused on the structure of the normal oceanic mantle. We
started a one-year-long pilot observation by deploying five BBOBSs in 2010 and four of themwere recovered. We obtained a new upper mantle seismic structure from surface wave using
data of previous studies and of the pilot observation. The preliminary results suggest that pilot
observation area, where the upper mantle structure supposed to be normal, is lessheterogeneous compare to the surrounding area. We will show these preliminary results inthis presentation.
References[1] Isse, T., Yoshizawa, K., Shiobara, H., Shinohara, M., Nakahigashi, K., Mochizuki, K.,
Sugioka, H., Suetsugu, D., Oki, S., Kanazawa, T., Suyehiro, K., Fukao, Y., 2006.
Three-dimensional shear wave structure beneath the Philippine Sea from land and ocean
bottombroadbandseismograms.
J.
Geophys.
Res.,
111:
B06310,
doi:06310.01029/02005JB003750.
[2]Isse, T., Shiobara, H., Tamura, Y., Suetsugu, D., Yoshizawa, K., Sugioka, H., Ito, A.,
Kanazawa, T., Shinohara, M., Mochizuki, K., Araki, E., Nakahigashi, K., Kawakatsu, H.,
Shito, A., Fukao, Y., Ishizuka, O., Gill, J. B., 2009. Seismic structure of the upper mantle
beneath the Philippine Sea from seafloor and land observation: Implications for mantle
convection and magma genesis in the Izu-Bonin-Mariana subduction zone. Earth Planet. Sci.
Lett., 278: 107-119.
[3] T. Isse, H. Shiobara, J.-P. Montagner, H. Sugioka, A. Ito, A. Shito, T. Kanazawa, K.
Yoshizawa, 2010. Anisotropic structures of the upper mantle beneath the northern Philippine
Sea region from Rayleigh and Love wave tomography, Phys. of the Earth and Planet. Intr.,
183, 33-43, doi: 10.1016/j.pepi.2010.04.006.

17

Session 3: Mantle imaging

Preliminary result on electrical conductivity imaging of normal oceanic
mantleKiyoshi Baba1, Noriko Tada2, Hisashi Utada1, and Hisayoshi Shimizu1
1

2

Earthquake Research Institute, University of Tokyo, Japan
Instiute for Research on Earth Evolution, Japan Agency for Marine-Earth Science and Technology,
Japan

In the Normal Oceanic Mantle (NOMan) project, seafloor electromagnetic surveys havebeen conducted to image the electrical conductivity of the mantle beneath the two areas in thenorthwestern Pacific Ocean; northwest (area A) and southeast (area B) of Shatsky Rise. The
observation started in June 2010 and is still going on replacing instruments. In thispresentation, we introduce preliminary result for the estimation of electrical conductivity
structure beneath the area A based on the data acquired in the pilot survey during June 2010
and August 2012.
The raw time series obtained by ocean bottom electromagnetometers (OBEMs) at four sites
were processed to obtain magnetotelluric (MT) response at each site. We then estimated
one-dimensional (1-D) conductivity structure model which explains the data of all sites
averagely, correcting the topographic effect on the observed MT responses, as we applied tothe previous studies for the Philippine Sea and off Bonin Trench in the Pacific Ocean (Baba etal., 2010), and the petit-spot area between Japan Trench and the area A of the NOMan project(Baba et al., in prep.).
The obtained 1-D model shows the resistive upper layer and underling conductive zone, as
recognized in many studies for oceanic upper mantle, suggesting cool oceanic lithosphere andhotter asthenosphere, respectively. The thickness of the lithospheric resistive mantle is about
150 km and the conductivity value of the asthenospheric mantle is 0.03-0.1 S/m. The resistive
layer is much thinner than the previous studies in the off Bonin Trench area (>200km) and in
the petit-spot area (~200 km), although the seafloor age of the areas are very old (130~155
Ma) and not different very much. We expected that the conductivity structures beneath thearea A and off Bonin Trench are comparable and that beneath the petit-spot is abnormallyhigh because the petit-spot area is the field of volcanic activity. However, the actual models
are thinner for the resistive layer and more conductive in the underlying layer in order of area
A, the petit-spot area, and off Bonin Trench. The cause of this feature has not been clear yet.
We will revise the 1-D model beneath the area A and also estimate a model for area B, bycollecting further data in on-going and future observations. Interpretation of resulting models
using laboratory experiments and seismic results will help to understand the status of thenormal oceanic upper mantle.

18

Session 3: Mantle imaging

Uppermost mantle structure beneath oceanic regions from broadband
array analysis of surface waves recorded by ocean bottom seismometers
Akiko Takeo1 and the NOMan Project team1

Earthquake Research Institute, University of Tokyo, Japan,

Seismic structure and seismic anisotropy in the uppermost mantle beneath oceanic
regions are essential for understanding the thermal state and deformation related to plate
tectonics. The structure including low velocity zone (LVZ) and high velocity lid (LID) isespecially needed, as they seem to correspond to the oceanic asthenosphere and lithosphere,
respectively. From 1980s, shear-wave velocity structure in the upper mantle has been
estimated by global surface-wave tomography studies. As tomography studies usually
analyze surface waves at periods longer than 40 s, however, the depth resolutions of theirmodels are limited especially in the uppermost mantle. As a result, it has been difficult to
discuss the detailed structure related to LID and LVZ such as the velocity gradient in the LID
and the depth of transition from LID to LVZ.
The array records of ocean bottom seismometers (OBSs) enables us the broadband
surface-waves analysis. At periods of about 30-100 s, phase velocities beneath the stationarray can be measured by the array analysis of teleseismic surface waves. At periods ofabout 2-30 s, phase velocities between a pair of stations can be measured by cross-correlating
continuous records obtained at the stations. By combining these two methods, we can obtainphase velocities of surface waves (Rayleigh and Love waves) in a broadband period range of
2-100 s, and can estimate shear-wave velocity structure from the crust to the uppermost
mantle beneath station array including radial anisotropy (azimuthally averaged anisotropy)
and azimuthal anisotropy.
We obtained preliminary results by applying above array-analysis methods torecords of OBSs deployed in the northwest of Shatsky Rise by the pilot observation of
NOMan Project. By using similar methods, Takeo et al. (submitted) obtained results for the
Shikoku Basin region, whereas Takeo et al. (2012) obtained results for the French Polynesia
region and the southwest of Shatsky Rise region. We can therefore compare results fromfour regions (Table). The one-dimensional structures beneath each region indicate thetransition from LID to LVZ at depths of about 50-70 km, and the increasing shear-wave
velocity within the LVZ with seafloor age. The intensity of radial (VSH>VSV) and azimuthal
anisotropy is about 3-4%. The azimuth of maximum shear-wave velocity at depths of 20-50
km seem to reflect the ancient plate motion in the French Polynesia region, the seafloor
spreading in the southwest of Shatsky Rise, and the ancient seafloor spreading or the current
plate motion in the northwest of Shatsky Rise, indicating various types of deformations
affects the seismic anisotropy in the uppermost mantle.

19

Session 3: Mantle imaging

Table.

Summary of four datasets
Project
Regionpart of SSP (’05-’08)
Shikoku Basin
TIARES (’09-’10)
French Polynesia
PLATE (’09-’10)
SW of Shatsky Rise
pilot of NOMan (’10-’11) NW of Shatsky Rise

Instruments
7 OBSs9 OBSs9 OBSs and 9 DPGs
4 OBSsSeafloor age
15-30 Ma
50-70 Ma
145-155 Ma
125-135 Ma

References[1] Takeo, A., D. W. Forsyth, K. Nishida, T. Isse, H. Kawakatsu, H. Shiobara, D. Suetsugu, H.
Sugioka, A. Ito, D. S. Weeraratne, T. Kanazawa, Uppermost mantle anisotropy beneath south
and northwest Pacific by ambient noise interferometry analysis of OBS records, DI21B-2355,
AGU Fall Meeting, 2012.
[2] Takeo, A., K. Nishida, T. Isse, H. Kawakatsu, H. Shiobara, H. Sugioka, T. Kanazawa,
Radially anisotropic structure beneath the Shikoku Basin from broadband surface wave
analysis of ocean bottom seismometer records, J. Geophys. Res., submitted.

20

Session 4: Seismic anisotropy

Oceanic Mantle Structure from Global Tomography
Göran Ekström1
1Lamont-Doherty Earth Observatory, Columbia University, New York, USA

Seismic tomography would be a significantly different endeavor if we could assert
with some certitude (and truthfulness) that the Earth were elastically isotropic. New data andmethods could confidently be used and focused on improving the 3-D resolution of just twoelastic-wave velocities, Vp and Vs, and interpretations of heterogeneity could proceed withthe expectation that temperature and composition were the main underlying geophysicalparameters that explain the seismological observations. The Earth is not, of course, elasticallyisotropic, except as an approximation. In some areas this approximation may be appropriate;in others it is a convenient simplification that is likely to be inadequate. On the one hand, theseismological expressions of anisotropy offer opportunities to uncover new information on
petrological fabric and strain in the Earth. Some observations, such as the geometry andmagnitude of splitting of various S phases, are sought and valued for their direct sensitivity toanisotropy. On the other hand, anisotropy contributes an additional signal, often weakly
resolved, to many seismological observations, confounding a straightforward interpretation of
the data but demanding that the effects of anisotropy be accounted for in some manner.
Possible approaches range from modeling the anisotropic structure as part of the inverse
problem, to relying on geometrical averaging of wave paths to eliminate the anisotropic
effects, to excluding data types that have large sensitivity to anisotropy, to ignoring thecomplexity. Each one of these approaches can be appropriate for a particular imaging problem,
but the adequacy of the result clearly depends on the choice and its validity. These imaging
challenges remain significant in global and regional tomography of the oceanic mantle. In thecontext of mapping the lithosphere and asthenosphere in the Pacific Basin using intermediateperiod surface waves, coverage is not problem, and several hundred thousand measurementsof phase speeds provide an excellent path distribution. Theoretically, isotropic velocity
variations over length scales of a few hundred kilometers are resolvable without damping or
regularization. However, the effects of azimuthal anisotropy are large and, depending on the
inversion approach, the mapping of the signal may distort both the isotropic and anisotropic
models retrieved. Experiments show that relying on azimuthal averaging provided by manycriss-crossing paths does not lead to a good isotropic result. Anisotropy must be included torealize the improved resolution of the isotropic structure that is expected from the greater pathcoverage offered by the growing number of permanent and temporary seismic stations.
Decades ago, a similar inference was made regarding the effects of radial anisotropy on
retrieved profiles of shear-velocity structure in the Earth. This concern remains directly
relevant today for the mapping of the oceanic asthenosphere, where strong and laterally
varying radial anisotropy is evident. The necessity of addressing these tradeoffs throughmodeling of the complexity grows when more and better observations are brought to bear on
the tomographic problem, and additional structural detail is sought.

21

Session 4: Seismic anisotropy

Seismic anisotropy below oceanic plates and large-scale mantle convection
Thorsten W Becker
University of Southern California, Los Angeles

I review progress in our quantitative understanding of uppermost mantle, seismic anisotropy acrossdifferent spatio-temporal scales of mantle convection, and what this says about the forces thatgovern plate motions and plate boundary evolution. Mineral physics tools constrained by laboratory
experiments allow to model the formation of lattice preferred orientation (LPO) textures of olivinethat is deformed over millions of years within global circulation models. This approach yields
synthetic fabrics whose heterogeneity matches that seen in xenolith databases, and provides
petrologically-motivated rules for new kinds of seismological inversions. When compared withsurface wave models for anisotropy, LPO from mantle flow models which include the effects ofthermal anomalies (inferred from isotropic seismic tomography) predict patterns of azimuthal
anisotropy significantly better than plate-scale shear, or absolute plate motion (APM) models. The
global average of radial anisotropy, as well as patterns of vSH vs. vSV, are matched well by the
global flow models overall, implying that regular, “A-type” LPOs and active asthenospheric
convection, or another effect that mimics those patterns closely, are the causes of most upper mantle
anisotropy underneath oceanic plates. I also discuss how high resolution global models can be used
to explore regional tectonic questions, such as in the northern South America margin, selfconsistently using a detailed analysis of shear wave splitting measurements and an inverse approachto geodynamic modeling. Deviations between model synthetics and observations may be used toinfer the distribution of frozen-in, shallow, lithospheric anisotropy, the degree of net rotations of thelithosphere with respect to the lower mantle, the extent of partial melt, and the degree of lateralviscosity and volatile variations in asthenosphere.

22

Session 4: Seismic anisotropy

Possible lateral variation of seismic anisotropies in the oceanic lithosphere due toan active mantle flow in the mid-ocean ridge: some insights from the Oman
Ophiolite
Katsuyoshi MICHIBAYASHI
Department of Geosciences, Faculty of Science, Shizuoka University, Shizuoka 422-8529 JAPANsekmich@ipc.shizuoka.ac.jp

Crystal-preferred orientation (CPO) is a common feature of peridotites and is developed during
intense homogeneous plastic deformation of peridotitic minerals with a dominant slip system.
Whereas an olivine CPO classification (A, B, C, D and E types) has been proposed by Karato andco-workers to illustrate the roles of stress and water content as controlling factors of olivine slip
systems (e.g., Karato et al., 2008 Annu. Rev. Earth Planet. Sci.), an additional CPO type (AG) has
also been proposed in recognition of its common occurrence in nature (Mainprice, 2007 Treatise on
Geophysics). AG-type has been experimentally formed in sheared partially-molten samples, in whicha-axes of olivine grains are aligned predominantly normal to the shear direction, rather than parallel
to it (Kohlstedt & Holtzman, 2008 Annu. Rev. Earth Planet. Sci.). Thus, we can expect thedevelopment of AG-type olivine fabrics to be related to the occurrence of melt during deformation,
most likely in the vicinity of mid-ocean ridges, where strong upflow is related to active mantle ascent(Nicolas et al., 2000 Marine Geophysical Researches; Michibayashi et al., 2000 MGR). Results from
our analysis of peridotites from the Hilti mantle section of the Oman ophiolite show that olivine inthat section is dominated more commonly by AG-Type than A-type CPO. This section preserves
subhorizontal uppermost mantle lithosphere (Michibayashi & Mainprice, 2004 Jour. Petrology;
Onoue & Michibayashi, 2013 JpGU abstract). Since olivine contains intrinsic elastic anisotropies,
the development of CPO within peridotite during plastic deformation at mid-ocean ridges gives riseto seismic anisotropy in the upper mantle. Seismic properties of AG-type olivine fabrics reveal that
whereas Vp velocity is maximum parallel to the flow direction (X) and minimum normal to the flow
plane (Z), the intermediate direction (Y) has relatively higher Vp velocity than the median velocity.
This feature of AG-type fabric is different from that of A-type, which occurs commonly undermelt-free conditions, resulting in the different degrees of seismic anisotropies between AG-type andA-type. Thus, we propose, based on our results for the Oman ophiolite, that the intensity distributionof seismic anisotropy in the uppermost mantle could vary laterally depending on various strength ofmantle ascent along a given segment of mid-ocean ridges in conjunction with various degree of melt
impregnation.

23

Session 4: Seismic anisotropy

Subduction of oceanic asthenosphere: Evidence from sub-slab seismic
anisotropy
Teh-Ru Alex Song1 and Hitoshi Kawakatsu2
1

2

IFREE, JAMSTEC, Japan,
Earthquake Research Institute, University of Tokyo, Japan,

The oceanic asthenosphere is characterized as a low viscosity channel down to200-300 km depth separating the cold lithosphere from above, and it is intimately linked to alayer of low seismic velocity and prominent seismic anisotropy observed globally beneathocean basins. While subduction of tectonic plates in convergent margins is well recognized,
the fate of oceanic asthenosphere remains enigmatic. We demonstrate that subduction of theoceanic asthenosphere characterized by weak azimuthal anisotropy and strong radialanisotropy explains the essence of sub-slab shear-wave splitting patterns, where the fast
splitting direction changes from predominantly trench-parallel (or sub-parallel) underrelatively steep subduction zones to frequently trench-normal under shallow subduction zones.
To explain the observed splitting time, the thickness of the subducted asthenosphere isestimated to be 100±50 km. The general validity of such a scenario is crucial to fundamental
understandings of the development of mantle anisotropy in sub-lithosphere or/and sub-slab
conditions, the nature and formation of oceanic asthenosphere as well as the flow pattern andmass transport near subduction zones.
To validate this scenario, we examine SKS splitting patterns observed across thefore-arc in 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 anddown-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 thedominant source of seismic anisotropy in central Alaska and potentially in many subduction
zones. Furthermore, this result also provides alternative views to models of seismic anisotropy
in the mantle wedge and on the length scale in which the 3D mantle flow may be important.

24

Session 4: Seismic anisotropy

A database of crystal preferred orientation of olivine in upper mantle rocks– the direct mantle sample
David Mainprice
Géosciences Montpellier, CNRS-UMR 5243, Université Montpellier 2, 34095 Montpellier, France,

Olivine is the most volumetrically abundant mineral in the Earth’s upper mantle, assuch it dominates the mechanical and physical properties and has a controlling influence ofthe geodynamics of plate tectonics. Since the pioneering work of Hess and others we knowthat seismic anisotropy of the shallow mantle is related to olivine and it’s crystal preferredorientation (CPO). With advent of plate tectonics the understanding of the key role ofperidotite rocks became a major scientific objective and the measurement CPO of olivine inupper mantle samples became an important tool for studying the kinematics of these rocks.
Our group originally lead by Adolphe Nicolas introduced the systematic use of CPO
measured by U-stage for field studies all over the world for over 30 years, this tradition was
extended in last 15 years by the use of electron back-scattered diffraction (EBSD) to study of
CPO and the associated digital microstructure. It is an appropriate time to analysis thissignificant database of olivine CPO of over 700 samples and 15 million measurements, which
represents the work of our group, both present and former members (e.g. Prof. K.
Michibayashi, Shizuoka), as well as collaborating colleagues. It is also interesting to compare
the natural record as illustrated by our database in the light of recent experimental results
stimulated by the extended ranges in temperature, pressure and finite strain, as well as
intrinsic olivine variables such as hydrogen content.
To analysis the database, which is heterogeneous because it is constructed from the individual
work of many people over a 45 year period containing U-stage data and EBSD measurements(manual indexing point per grain, automatic indexing one point per grain, automatic indexing
gridded mapping data) of various formats, we need a flexible software tool that can handle
large volumes of data in consistent way. We have used the state-of-art open source MTEX
toolbox for quantitative texture analysis. MTEX is a scriptable MATLAB toolbox, whichallows all aspects of quantitative texture analysis using the modern methods and anisotropic
physical property calculations (e.g. seismic anisotropy).

25

Session 5: Physical properties

Volatile-rich partial melt at the base of the oceanic lithosphere and in
the asthenosphere
Marc Hirschmann
Dept. of Earth Sciences University of Minnesota, Minneapolis, MN 55455 USA
mmh@umn.edu

Mounting observational evidence for small amounts of partial melt beneath theoceanic lithosphere has arisen at the same time that experimental studies havedemonstrated strong stabilization of volatile-rich partial melts of peridotite at
appropriate temperatures and pressures. However, this fortunate coincidence does not
fully resolve the question of melt in the sub-lithospheric mantle, and significant
challenges remain. Chief among these are the relative roles of H2O and CO2 instabilizing melt, plausible volumes of melt derived from melting of volatile-poor
mantle, and the dynamic stability of finite melt accumulations.
Several recent studies have argued that small amounts of H2O present in the upper
mantle are sufficient to incite partial melting in the asthenosphere (Mierdel et al.
2007; Green et al. 2010; Other Ref?). However, experiments on the H2O storage
capacity of mantle minerals (Ardia et al. 2012; Tenner et al. 2012a), together withmineral/melt partitioning of OH determinations (Aubaud et al. 2004; 2008; Tenner et
al. 2009) and quantification of the influence of H2O on the extent of melting of
peridotite (Tenner et al. 2012b) suggest that hydrous melting is unlikely along thegeotherm beneath mature oceanic lithosphere unless bulk mantle H2O contents areelevated compared to those typically judged from MORB compositions.
In contrast, experimental evidence clearly demonstrates that the solidus of carbonatedperidotite exceeds any reasonable oceanic geotherms. Previously, the data suggestdthat small-degree partial melts would be carbonatitic in the coldest parts of theasthenosphere, but new experiments at low pressure (Dasgupta et al. 2013) suggest
instead that they are more likely to be CO2-rich silicate melts similar to CO2-enriched
nephelinites. Melt fractions, however, remain very small, far less than 1% forreasonable bulk C concentrations. Whether these melt fractions are dynamicallystable and can account for observed geophysical anomalies remains an open question.

26

Session 5: Physical properties

Experimental study on anelasticty of polycrystalline material
for seismological application.
Yasuko Takei1, Fumiya Karasawa1, Ayako Suzuki1
1

Earthquake Research Institute, University of Tokyo, Japan,

Due to the recent progress in seismology, we can obtain highly-resolved seismic
velocity structures in the upper mantle. In order to interpret the velocity structures in termsof temperature heterogeneity, chemical heterogeneity, fluid/melt distribution, etc, it isimportant to assess the quantitative effects of temperature, chemical composition, andfluid/melt on Vp and Vs. Although these effects at the ultrasonic frequencies (anharmonic
effect, poroelastic effect) have been measured and assessed quantitatively, these effects at theseismic frequencies are subject to large uncertainty due to the uncertainty in rock anelasticity,which additionally causes modulus relaxation at lower frequencies (anelastic effect). Rockanelasticity has been poorly understood yet, due to the difficulty of high temperature and highpressure experiments performed on the rock samples. We, therefore, developed a newexperimental method by using organic polycrystalline material as a rock analogue. Previous

studies have shown that anelasticity of polycrystalline materials follows the similarity rule inwhich frequency normalized to the Maxwell frequency, f/fM, can be used as a master variable(McCarthy et al, 2012). The Q−1 spectra obtained from olivine aggregates (e.g., Jackson et al,2002) also collapsed onto the same master curve, when plotted as a function of f/fM. The
general applicability of this Maxwell frequency scaling shows that anelastic relaxation in
those experiments is caused by diffusionally accommodated grain boundary sliding (GBS)(Gribb & Cooper, 1998; McCarthy et al, 2012). However, normalized frequency of the existing
experimental data is usually considerably lower than the seismic frequencies normalized to
the Maxwell frequency of the upper mantle (f/fM=106-1010). Therefore, in order to clarify themechanism and scaling law applicable to the seismic waves, we have to measure anelasticity
at higher normalized frequencies.
By improving the experimental apparatus, we measured anelasticity at higher
frequencies and lower temperatures than before. We prepared one high-purity borneol sample
and one impure borneol sample containing 0.83 wt% diphenylamine, so that effect of impurity
on anelasticity can be investigated. Data could be obtained up to f/fM = 107 (more than 102
higher than the previous study), which is within the normalized seismic frequencies.
Attenuation spectra at higher normalized frequencies did not collapse onto a single mastercurve, but were considerably scattered. This result demonstrates that the Maxwell frequency
scaling can not fully capture the effect of temperature and impurity at higher normalized
frequencies. It is revealed that Q−1 spectra of the high purity borneol sample decrease
monotonically with increasing frequencies, but on the Q−1 spectrum of the impure borneol
sample, a high and broad attenuation peak is superimposed. To clarify the underlying
mechanism of this attenuation peak and to clarify the scaling law applicable to the seismic
waves, further experimental studies are needed.

27

Session 5: Physical properties

Estimating electrical conductivity of melt from observation
Hisashi Utada1 and Kiyoshi Baba1
1

Earthquake Research Institute, University of Tokyo, Japan,

The presence of upper mantle high conductivity layer has been recognized and called as the‘electrical asthenosphere’ since the very beginning of seafloor magnetotelluric study. Effectof water (proton) is considered as a promising cause of the high conductivity, and has been
adopted to interpret obtained conductivity profiles (Baba et al., 2010).
If CO2 content in the upper mantle exceeds 5 ppm wt., conductivity profile may beinterpreted by partial melting model because of the extremely high conductivity of carbonatite
melt (Gaillard et al., 2009). Under such a situation, electrical conductivity profile will
constrain the product of melt conductivity and volume fraction. If melt volume fraction isconstrained by independent information (by model or seismology, for example), we canestimate the conductivity of melt (carbonatite and/or silicate).
Here we try to interpret three recent results (1-Dimensional conductivity profiles )of ocean bottom magnetotelluric study made by our group. Two profiles are obtained by the
Stagnant Slab Project (SSP) in the Philippine Sea and the Western Pacific Ocean (Baba et al.,2010), which are called SSP-PS and SSP-WP, respectively. Another is obtained in the
Northwestern Pacific Ocean by the NOMan project. As for distribution of melt fraction, we
refer to the calculation by Hirschmann (2009), assuming mean seafloor age for SSP-PS to be
30 ma and for SSP-WP and NOMan to be 100 ma. Although actual seafloor age of the latteris much older, we use this value assuming the age dependence is weak after 100 ma. Melt
conductivity profiles were estimated from three conductivity profiles by using
Hashin-Strickman upper bound model.
We found the results of SSP-PS and NOMan are compatible with the present
framework. Relatively low conductivity below the LAB close to that of wet silicate melt wasestimated for the SSP-PS result. From the NOMan profile higher conductivity between thosefor silicate and carbonatite melts was indicated. However, if we assume large melt fraction at
the shallowest part of the asthenosphere just below the LAB for the NOMan profile assuggested by Kawakatsu et al. (2010) at WP2, resulting conductivity value becomes lower
and closer to that of silicate melt. If acceptable model of partially molten asthenosphere is
obtained in this way, the volatile (carbon) content in the upper mantle will be estimated fromresulting melt conductivity values with a systematic laboratory measurement of melt
conductivity with accurate control of volatile contents. On the other hand, the profile from
SSP-WP cannot consistently be explained by the present approach. Difference between
SSP-WP and NOMan results is so large that it should be interpreted by other effects.
The solution (cause of asthenosphere) may not be unique, but stronger constraints
will be obtained by seeking for consistency among multi-parameters from seismology, EM,and mineral physics.

28

Session 6: Future

Imaging	  of	
  Lithosphere	
  Asthenosphere	  Boundary	
  Across	
  Atlantic	
  Ocean:	  	
  A	
  Dream	  	
  Satish	  Singh	
  Institut	  de	
  Physique	
  du	
  Globe	
  de	
  Paris,	  France	
  (singh@ipgp.fr)	
  	
  The base of the lithosphere, Lithosphere Asthenosphere Boundary (LAB), is the lower
boundary of the plate. Although the lithosphere is a basic building block of the plate tectonics
theory, its nature, its thickness, its boundary with the asthenosphere (LAB) is still matter ofheated debates. For example, some studies suggest that the LAB could be a sharp boundary,but the others propose a thick transition. Some people argue that the thickness of thelithosphere increases with the age of the lithosphere and others argue that it lies at a constantdepth at 70 km. In order to address these fundamental questions we propose to image the
LAB and internal structure of the lithosphere in a systematic manner across the Atlantic
Ocean for a lithosphere of 0-100 Ma age using a combination of different geophysicalmethods. Along with using seismological and magnetotelluric methods, we propose to use atechnology newly developed for the oil and gas exploration that is capable of providing aseismic reflection image down to 120 km depth with a few hundred metres resolution. Subject
to funding, we propose first to acquire a short seismic reflection profile (800 km) across MidAtlantic Ridge covering 0-15 Ma of the lithosphere and then crossing three fracture zones of
28 Ma, 40 Ma and 12 Ma age contrasts, covering a lithosphere of 0-75 Ma. In the second
stage, we propose to acquire coincident seismic reflection, refraction, broadband OBS and
MT data along a 2200 km long profile covering the oceanic lithosphere of 0-100 Ma of age.
These new seismic data should also provide images of melt lenses in the mantle beneath the
spreading centre axis, if present, or frozen melt lenses in old lithosphere which will help us tobuild a new model of melt generation and migration from the mantle. We should also be able
to image deep penetrating faults that might have been generated due to the cooling of thelithosphere as it moved away from the spreading centre, allowing the development of a newmodel of hydration of the oceanic lithosphere.
In this talk, I will discuss the design of the survey. I will show some results that have alreadybeen obtained using deep seismic profiling of the Indian ocean lithosphere where we haveimaged the 2012 Mw=8.6 earthquake rupture zone down to 50 km depth. We have also
imaged a band of reflectivity at 10-15 km below the oceanic Moho, which could be due toeither a phase change from plagioclase to spinel or the presence of gabbroic melt lenses, or a
combination of the above two. These images were possible because I was able to persuade
seismic industry to provide their best technology, very long streamers (12-15 km long) and
low frequency energy source capable of penetrating deep.
	
  
	
  
	
  

29

Poster

Flow and thermal structure around the junction of Japan-Kurile arcs
Manabu Morishige1,21Institute for Research on Earth Evolution, Japan Agency for Marine-Earth Science and Technology,
Japan,
2
Department of Earth and Environmental Sciences, University of Michigan, USA,The junction of the Japan-Kurile arc is the best studied junction in the world and we
can see several characteristic features in this region. These include trench-normal fastpolarization direction of S-wave splitting in the mantle wedge and along-arc variation of theangle of subduction. In this presentation, I show the dynamic effects of the junction using a
numerical model of mantle convection with a realistic curved shape of the trench. I obtain 3D
flow in the mantle wedge which is consistent with the observation of seismic anisotropy. The
fast polarization direction of S-wave anisotropy beneath the slab is nearly parallel to that ofplate motion at least in the shallow part. I also find that the subduction angle varies
along-strike, which agrees with the observations. This variation can be explained by a torquebalance acting on subducting slabs in the case of oblique subduction.
The results obtained using numerical model with higher resolution show that thetemperature of slab surface also changes along the strike of the trench, although themagnitude is not so large (~70K). This might affect intermediate seismicity in this region.

30

Poster

Importance of hot anomaly adjacent to the subducting slab
S. Honda1, A. Ismail-Zadeh2, M. Morishige,3, I. Tsepelev,4
1

2

ERI, Univ. Tokyo
IAG, Karlsruhe Univ., IPGP, IEPTMG, Russ. Acad. Sci.
1
JAMSTEC
2
IMM, Russ. Acad. Sci.

The origin and past evolution of seismically detected hot material in the sub-slab
mantle under the subducting Pacific plate are studied. Forward modeling studies show that its
origin is likely to be originated from the hot material next to the cold sinking material
typically observed in the internally heated convection. Backward modeling of thermal
structure under Japan and its surrounding show the common source of present hot materials inthe sub-slab mantle and the mantle wedge. The results also suggest the leakage of hot material
from the sub-slab to the back-arc regions which is also observed in the forward modelingstudies. Thus, this thermal anomaly plays potentially important role in controlling the
dynamics of subduction zone and back-arc mantle.
.

31

PosterThree-dimensional simulation of the EM fields induced by 2011 Tohoku
tsunamiLuolei Zhang1, Hisashi Utada1, Hisayoshi Shimizu1, Kiyoshi Baba1, Takuto Maeda1
1

Earthquake Research Institute, University of Tokyo, JAPANThe motion of seawater induces electromotive force of significant intensity(Sanford, 1971) due to Faraday’s law, and resulting electromagnetic (EM) field can berecorded by instruments installed on land or at ocean bottom (Tyler, 2005; Toh et al. 2011).
However, few studies were successfully simulating Tsunami induced EM fields by an exact
and accurate application of Maxwell equations that is essential for a quantitative interpretation
to get geophysical information from observations of tsunami-related EM signals.
There are a number of observations of such EM fields that were caused by thedevastating Tohoku tsunami of 2011 not only on land observatories but also at some seafloorsites (e.g. Utada et al., 2011; Ichihara et al., 2012). Here we present a 3-D modeling scheme tosimulate these observed fields. We apply a 3-D EM induction code in Cartesian coordinate
system with the heterogeneous source term, which is based on the modified iterative
dissipative method (MIDM) (Zhang et al. 2012). The source current distribution is predictedby the flow data calculated by a tsunami simulation (Maeda and Furumura, 2011) based on
the linear long-wave theory. The flow data provided in the time domain are transformed to thefrequency domain, and the induced current distribution was calculated by applying Faraday’slaw. The frequency domain solutions of Maxwell’s equations with heterogeneous distributionof the source current in the sea are then transformed back to the time domain to compare withobservations. Here we show examples of comparison for one land observatory in Tohoku
district, one island observatory in the Izu-Bonin arc, and one seafloor station installed by the
NOMan Project.
The water motion generates source current in the sea both of poloidal (without
vertical currents) and toroidal modes (with vertical currents). Lateral heterogeneities would
convert the toroidal to poloidal mode but the amplitude of converted mode is generally muchsmaller than that of original poloidal mode. Therefore the source of the toroidal mode can beignored in case of modeling observations on land. Our numerical simulation shows that thetoroidal mode can be effective for seafloor observations, but only when the seafloor(sedimentary layer) is very conducting. In such a case, the effect of toroidal mode would bias
the estimation of the wave propagation direction (the wave number vector) by seafloor
electromagnetic data. We also examined how the tsunami-induced EM field observations
constrain the conductivity of the shallower part of the seabed, which is difficult to be resolved
by using ordinary seafloor magnetotelluric signals. We also defined the motional impedance
just as the case of ordinary magnetotellurics, and showed that both model and observed
impedances indicate the phase velocity of the long wave as predicted by a simple theory.

31