SCG60-01

Room:201A

Time:May 25 11:00-11:15

Thermal structure and melt fraction distribution of mantle from a 3-D electrical conductivity structure beneath Kyushu
HATA, Maki1∗ ; UYESHIMA, Makoto1
1

Earthquake Research Institute, The University of Tokyo

The Kyushu Island in the Southwest Japan Arc has many Quaternary active volcanoes, which exist along the volcanic frontof N30o E-S30o W, in relation to the subduction of the Philippine Sea Plate (PSP). The volcanoes are located in northern andsouthern regions of the island, and no volcano is located in the central region between the two volcanic regions of the island.
We have performed three-dimensional (3-D) inversion analyses to obtain a lithospheric-scale electrical conductivity structure(model) beneath the entire Kyushu Island using the Network-Magnetotelluric (MT) data [Hata et al. , 2015]. One of two major
findings from a distribution of conductive anomalies in the model is that the volcanoes in the northern and southern volcanic
regions have two different origins bordering the non-volcanic region at deep depths. Secondly, the degrees of magmatism and therelative contributions of slab-derived fluids to the magmatism vary spatially in the one non-volcanic and two volcanic regions.
Then, in this study, we try to verify whether the respective conductivity anomalies impart a different effect on temperature andmelt fraction.
We use laboratory work results to determine thermal structure and melt fraction distributions derived from the electrical conductivity structure beneath the Kyushu Island. The laboratory work results are relation between electrical conductivity andtemperature for four nominally anhydrous minerals (Olivine, Orthopyroxene, Clinopyroxene, and Garnet) in solid phase, relation between electrical conductivity and temperature for hydrous basaltic melt in liquid phase, and a parameterization result ofisobaric hydrous mantle melting. In this presentation, we will show our approach to determine temperature and melt fraction
as a function of the water contents among the four mantle minerals and the basaltic melt, which integrate laboratory-derived
conductivity and field-derived conductivity. We will also show thermal structure profiles and melt fraction distribution profilesof the mantle wedge beneath the one non-volcanic region and the two volcanic regions of the Kyushu Island.

1/1

SCG60-02

Room:201A

Time:May 25 11:15-11:30

Subduction Initiation and spatial and temporal variation of magma generating conditionin the Miocene SW JapanMORISATO, Fumitoshi1∗ ; OZAWA, Kazuhito1
1Department of Earth and Planetary Science, Graduate School of Science, University of Tokyo

Introduction
The relationships between arc magmatism and thermal and chemical structures of subduction zones have been investigated in
two dimensions (e.g., Spiegelman & McKenzie, 1987; Tatsumi, 1989). However the necessity of a three-dimensional point ofview is indicated in Northeast Japan (Tamura et al., 2002; Ueki & Iwamori, 2007) and 3-D numerical simulations (Honda, 2011).
Processes involved in the initiation of 3-D structures in actual subduction have not been well studied because of the fragmental
nature of geological records during subduction initiation (Umino & Ishizuka, 2010 etc.).
In the Miocene Southwest Japan, temporarily (12˜18Ma) and spatially (˜900km) wide spread magmatism occurred during theinitiation of subduction (Sumii, 2000; Tatsumi et al., 2001, etc.). The migration of magmatism has been discussed with 2-D
model (Kimura et al., 2005). Clustering of volcanic rocks spacing of ˜50km and elongating crossing the arc (west Shikoku,
east Shikoku, and Kii Peninsula) and corresponding crustal structures strongly suggests 3-D mantle dynamics must be invoked.
We tried to decipher 3-D heat and material transportation in the mantle by estimating spatial and temporal change of magma
generating conditions of the Setouchi Volcanic Belt.
In the Setouchi Volcanic Belt, magma generating condition has been estimated from multiple-saturation experiments on andesitic lavas (Tatsumi & Ishizaka, 1982, etc.), geochemical characteristics (Shimoda et al., 1998, etc.), and modeling thermal
structure (Furukawa & Tatsumi, 1999). The primitive andesitic magmas with more than 4 wt% H 2O was inferred to have been
generated through reaction between wedge mantle and slab-derived melt (Tatsumi et al., 2006). We try to put proper constraints
on melting conditions of mantle with least assumptions by exploiting useful information from volcanic rocks.
We focus on the eastern Shikoku cluster. We analyzed whole-rock compositions of andesites by using XRF and LA-ICP-MS
in the central area (Kanayama, Kiyama, and Goshikidai area) and western margin (Shichiho-san area). We also used literature
data from eastern margin (Shodo-shima area: Tatsumi et al., 2006).
Result and Discussion
In the central area, the variation of major element composition can be explained by fractional crystallization of a parental
magma containing up to 2wt% H2 O (alphaMELTS) and magma mixing between the daughter magmas. Extensive degassing
is not required if the water content (0.48˜1.7 wt%:Henmi et al., 1976) and a small amount of vesicles in the lavas are taken
into consideration. We constrain a range of primary melt compositions from the most magnesian zone in oscillatory zoningof orthopyroxenes. The primitive magma was estimated to be SiO2 =56.8˜57.1 wt%, MgO=13.2˜11.7 wt%, and H2 O=0.9˜1.5
wt%. Such melt can be in equilibrium with harzburgitic mantle at ˜30km in depth (alphaMELS) and 1240˜1200 degrees Celsius(Sugawara, 2000; Medard & Grove, 2008).
The variations of HFSEs and HREEs against SiO2 as a differentiation proxy, indicate later andesites in the central area were
derived from more fertile material, although common sub-arc mantle was melted in three areas in the initial stage. The abundanceof LILEs indicates the source materials of the central area and the eastern margin were significantly affected by slab-derived fluidin the later stage. The potential temperature and the initial depth of melting are estimated to be 1300˜1250 degrees Celsius and
˜75km respectively by assuming melting of a fertile lherzolite with 25% melting degree.
We proposed a model that the clustering of magmatism formed by upwelling of forced convection induced by subduction
initiation. The temporal change of the source material is attributed to upwelling of deeper part of the mantle and involvement ofslab components by deeper penetration of the slab. 3-D thermal structures in the steady-state subduction may be inherited fromto dynamics of subduction initiation.
Keywords: Subduction initiation, Primary magma, Southwest Japan, Setouchi Volcanic Belt, Mantle dynamics

1/1

SCG60-03

Room:201A

Time:May 25 11:30-11:45

Development of a 3D thermal structure in the subduction zone due to a low viscosity layer
at the plate interface
MORISHIGE, Manabu1∗
1

IGS, Kyoto University

It is essential to understand the detailed 3D thermal structure in the subduction zone to better constrain the transport of fluid
and melt there. In this presentation, I will propose a simple and new mechanism to produce the along-arc variation in the thermal
structure. In the northeast Japan, the earthquakes at the plate interface may occur down to 50 km depth. On the other hand, theslab and mantle may need to decouple down to 80 km depth in this region to explain the observed low surface heatflow in theforearc. These observations may show that the slab and mantle are decoupled by non-brittle deformation from 50 down to 80 km
depth. Based on this idea, I set a thin low viscosity layer (LVL) just above the subducting slab within the depth range.
3D finite element models are used to investigate the effects of LVL on the thermal structure. The model domain is divided
into four parts: the crust, a small portion of the mantle wedge tip which is rigid, the viscous mantle wedge, and the subducting
slab. The model is exactly the same in the along-arc direction. The flow is computed only in the viscous mantle wedge, whereas
temperature is computed for the whole model domain. When the viscosity in LVL is relatively high, the slab and mantle are
effectively decoupled but there is no along-arc variation in the flow and thermal structure. I find, however, that when the viscosity
in LVL is sufficiently low the corner flow starts to show 3D features and it leads to the along-arc temperature variation. It is
well known that the distribution of Quaternary volcanoes in the northeast Japan forms clusters whose characteristic wavelength
is around 80 km. The model proposed here successfully explains the observed wavelength based on an assumption which issimpler and better constrained by observations compared to previous models.
A previous study has proposed that slab and mantle are decoupled down to a common depth (70-80 km) for most subduction
zones. It means that the LVL considered in this study could exist for other subduction zones as well. Therefore, the new model
proposed here can be applied to a wide range of regions.

Keywords: subduction zone, plate interface, low viscosity layer, distribution of volcanoes, slab-mantle coupling

1/1

SCG60-04

Room:201A

Time:May 25 11:45-12:00

S-wave ray path analysis constrains the distribution and dynamics of the hydrated mantle
wedge
NAGAYA, Takayoshi1∗ ; WALLIS, Simon1 ; WALKER, Andrew3 ; WOOKEY, James2 ; KENDALL, Michael2
1

Department of Earth and Planetary Sciences, Nagoya University, 2 School of Earth Sciences, University of Bristol, 3 School ofEarth 9 and Environment, University of Leeds
Subduction fluids including H2 O are a key component of understanding the dynamics in convergent plate margins and recyclingof materials from the Earth’s surface into the deep interior. H2 O-rich fluids in the wedge mantle are dominantly derived fromantigorite, dragged down by plate motion. Antigorite is generally thought to be distributed in the relatively warmer subduction
zones where the dehydration of the subducting slab is predicted to occur. Seismic observations are commonly used to identify
the location and proportion of antigorite-rich domains (low seismic velocities Vp = ~ 6.5 ̶ 6.7 km/s, Vs = ~ 3.4 ̶ 3.7 km/s
and high Vp/Vs = ~ 1.8 ̶ 1.9 for single antigorite crystals).
However, antigorite has a very strong seismic anisotropy (∆ Vp ≤ 46 % and ∆ Vs ≤ 66 %), meaning that seismic velocities
and the Vp/Vs ratio can show very large variations (Vp = 5.6 ̶ 8.9 km/s, Vs = 2.5 ̶ 5.1 km/s and Vp/Vs = 1.2 ̶ 3.4) dependingon the propagation path of the seismic wave. This means the analyses based on the observation of average seismic velocities
cannot necessarily distinguish antigorite nearing rocks from dry olivine-rich mantle. The distribution and proportion of antigorite
and hence water has thus far been impossible to determine.
In this study we calculated the shear wave splitting caused within the wedge mantle using the geometry of seismic ray paths
observed in Ryukyu arc where trench-parallel S-wave anisotropy with large delay times (~ 1 s) has been observed. To match theshear wave splitting observations of both ray that arrived from forearc side and backarc side to the seismic stations, we find thealignment of antigorite must change from parallel to the subducting slab in the deepest part of the wedge to vertically aligned atintermediate depths in the wedge mantle and the proportion of these antigorite must be more than 65 %.
This type of analysis that takes into account ray paths geometry and the seismic anisotropy of deformed antigorite-bearing
mantle can constrain the distribution, amount and orientation of antigorite.
The change in orientation of antigorite in Ryukyu arc suggests the presence of convective flow in the hydrated forearc mantle
associated with a bulk long-term viscosity of less than 1019 Pa s.
The analysis of the shear wave splitting observations in other subduction zones shows that large delay times with trenchparallel fast Vs within the wedge mantle, similar to the Ryukyu example, have been observed from even cold subduction zones(e.g Izu-Bonin and Tonga-Kermadec subduction zones) regardless of the age of the slab. This results implies the large-scaleserpentinization and hydrous mantle flow associated with the dehydration of the slab are likely to be more widespread than
generally recognized and the widespread view that the forearc mantle of cold subduction zones is dry, needs to be reassessed.
However, subduction zones with small delay times in the wedge mantle are also reported irrespective of the thermal structurein the wedge. The lack of a strong shear wave splitting in some subduction zones and the thermal structure in the subduction
zones may be related to the tectonic erosion of hydrous material from the base of the mantle wedge. An additional factor may be
subduction angle. Subduction zones with large splitting times show relatively steep slab dips (~ 40 ̶ 60 o ) (Ryukyu, Aleutians,
Izu-Bonin and Tonga-Kermadec subduction zones) and no subduction zones with shallow slab dips are associated with large
shear wave splitting. Steep subduction zone may be an important geometry requirement for convection cells to develop in thewedge mantle with both slab-parallel and sub-vertical foliation domains.
Keywords: Shear waves splitting, Antigorite, Hydrated mantle wedge, Seismic anisotropy, Seismic ray path, Mantle convection

1/1

SCG60-05

Room:201A

Time:May 25 12:00-12:15

The mantle anisotropy obtained from shear-wave splitting in the region of 1891 Nobi
earthquake
IIDAKA, Takashi1∗ ; HIRAMATSU, Yoshihiro2 ;THE RESEARCH GROUP FOR THE JOINT SEISMIC, Observations at the nobi area1
1

Earthquake. Res. Inst., University of Tokyo, 2 Kanazawa University

The Nobi earthquake which occurred in 1891 was one of the largest inland earthquakes in Japan. The magnitude of the 1891
Nobi earthquake was 8. The magnitude is much larger than those of other inland earthquakes in Japan. To know the cause of
Nobi earthquake and crustal structure of the area, a temporary seismic observation was conducted. The seismic structure of thecrust and uppermost mantle around the 1891 Nobi earthquake area is very important. We researched shear-wave splitting analysis
to obtain seismic image around the 1891 Nobi earthquake area including uppermost mantle structure. The earthquakes which
occurred from 2009 to 2014 are used. All of the earthquakes used here are deep earthquakes with depths are deeper than 200 km.
The seismic stations of Hi-net and temporary seismic network were used for the shear-wave splitting analysis.
Shear-wave splitting is usually expressed by two parameters, the fast polarization azimuth φ (in degrees), and the time-lag τ(in seconds), which is the delay time between the fast and slow components of a shear wave. We use the following techniques
to determine the shear-wave splitting. We pick S waves from individual seismograms. We calculated the cross-correlation of thetwo horizontal seismogram components over a grid -90o to +90o for φ and a window of 4 sec for τ with increments of 1o and0.01 sec., respectively. The time-lag τ (in seconds) and the fast polarization azimuth φ (in degree) are defined to be the values
that yielded the maximum correlation.
The results suggested very interesting and remarkable pattern of the polarization directions. The northeastern part of theresearch area showed polarization direction of NE-SW. The eastern part of the research area suggested that the polarization
directions are almost E-W. However, the polarization direction at the 1891 Nobi earthquake area was NW-SE. Most of theaveraged time lag values are larger than 0.5 sec. The value is much larger than that of the maximum time-lag value cause by
crustal anisotropy in this area (Hiramatsu et al. submitted to EPS). The maximum crustal anisotropy was considered to be lessthan 0.1 sec. We consider that the observed shear-wave splitting was caused by the mantle anisotropy. The shear-wave splittingof the mantle anisotropy was studied by previous studies. The polarization direction with E-W at the eastern part of the researchand the polarization direction with NE-SW are consistent with the results of previous studies (e.g., Ando et al. 1983, Iidaka
et al., 2009). The polarization direction can be explained by the preferred orientation cause by the mantle flow relating to thesubducting Pacific and Philippine Sea plates. However, the polarization direction with NW-SE is not consistent with both of the
subduction directions of oceanic plates. The cause of anisotropy is considered based on the two models. One is the model thatthe shear-wave splitting is caused by the heterogeneous structure which is related to the fluid or magma in the mantle wedge.
The other model is that the shear-wave splitting is cause by some irregular mantle flow. The cause of the polarization direction
will be discussed based on other observations.

Keywords: mantle, Splitting, Nobi earthquake

1/1

SCG60-06

Room:201A

Time:May 25 12:15-12:30

Reconsideration of serpentinite in the shallow wedge mantle -Importance of bruciteKAWAHARA, Hirokazu1∗ ; ENDO, Shunsuke2 ; WALLIS, Simon1 ; NAGAYA, Takayoshi1 ; MORI, Hiroshi2 ;YAMAMOTO, Koshi1 ; ASAHARA, Yoshihiro1
1

Earth & Planetary Sciences, Nagoya University, 2 National Institution of Advanced Industrial Science and Technology (AIST)The flux of H2 O rich fluid is an important aspect of geological processes operating in the shallow wedge mantle. In the duniteH2 O system, two of the most important hydration-dehydration reactions are as follows:
antigorite (Atg) = olivine (Ol) + orthopyroxene (Opx) + H2 O (1)Atg + brucite (Brc) = Ol + H2 O (2)Natural examples of the breakdown dehydration of Atg (reaction 1) have been closely studied (e.g. Padron-Navarta et al.,2011). However, there are relatively few well-documented studies describing the dehydration associated with reaction between
Atg and Brc (reaction 2). Formation and breakdown of Brc (reaction 2) is potentially important when considering the H2 O fluxin the shallow wedge mantle. It is also important to note that the product of Ol reacting with an SiO2 -bearing H2 O fluid is Atg
and Brc is not formed. This can be expressed by the following reaction:
Ol + SiO2 aq + H2 O = Atg (3)This means that presence of Brc has the potential to be used as an indicator of the former action of an SiO2 -rich fluid.
Brc is much weaker than Ol or Atg and its presence in sufficient quantities may have an important influence on the physicalproperties of the wedge mantle, for example development of shear zones. It is important therefore to obtain more information
about the abundance and microstructure of Brc in serpentinite when considering a wide range of physical processes occurring inthe shallow wedge mantle. However, Brc is not generally considered an important component of the shallow wedge mantle inprevious studies.
Numerous ultramafic bodies that originated in the wedge mantle are distributed throughout the high-pressure/temperature
subduction-type Sanbagawa metamorphic belt (Aoya et al., 2013). Here, we focus on the Shiraga (SG) body, which is a kilometerscale ultramafic body located in the low-grade metamorphic region (corresponding to depths of about 30km). The SG body isa metaserpentinite, which originated as dunite. The body underwent metamorphism after serpentinization and metamorphic Ol(m-Ol) formed by reaction 2 is widely present (Kunugiza, 1980). This study shows that Brc was once widely distributed in
the SG body. The major exception is the eastern margin of the body where a 100m thick serpentinite zone containing only
Atg is found. Microstructural observations and compositional mapping show that Brc can be divided into two distinct types.
Brc Ⅰ existed before peak metamorphism (within the Ol + Atg or Ol + Brc stability field), and Brc Ⅱ formed during theretrograde metamorphism. Brc Ⅰ occurs as inclusion in m-Ol and veins composed of coarse grains. Brc Ⅰ is separated fromAtg by m-Ol. Brc Ⅱ occurs as veins and is in direct contact with Atg. Brc Ⅰ locally shows a clear zonal structure with abrown core containing significant amounts of magnetite (Mgt) exsolution lamellae surrounded by a colorless rim. Estimationsof the initial composition of Brc before exsolution yield Mg# (= Mg / (Mg + Fe)) of Brc Ⅰ that increase from core to rim. In
contrast, Mg# of Brc Ⅱ decreases from core to rim. m-Ol also shows a zonal structure with increasing Mg# from the core to
the rim. In an Fe-bearing system, reaction 2 is a continuous reaction, which shows that the observed compositional changes ofBrc Ⅰ and m-Ol are formed during the rising temperature period and those of Brc Ⅱ are formed in descending temperature
period. Representative pseudosections of metaserpentinite in an Fe-bearing system were constructed to estimate the amount of
Brc before peak metamorphic condition. The results suggest that the original rock contained about 20 vol% assuming weak Si
metasomatism. If such large amounts of Brc are present in the shallow wedge mantle, they would significantly influence thephysical property of the wedge mantle.
References: Aoya, M. et al. (2013) Geology, 41, 451-454; Kunugiza, K. (1980) J. Jpn. Assoc. Min. Petrol. Econ. Geol, 75,
14-20; Padron-Navarta, J. A. et al. (2011) J. Petrol, 52, 2047-2078.
Keywords: brucite, antigorite, wedge mantle, serpentinite, subduction zone

1/1

SCG60-07

Room:201A

Time:May 25 12:30-12:45

Brittle-ductile transition of serpentinites in subduction zones: Roles of pore fluid pressure
SHIMIZU, Ichiko1∗
1

Faculty of Earth and Planetary Science, University of Tokyo

Geophysical observations suggest that the mantle wedge and subducting slabs and partly serpentinized. Low-velocity andhigh-Vp/Vs anomaly zones were identified along the subducting plate boundary in SW Japan, where slow earthquakes anddeep tremors occur, whereas no such velocity anomaly zones have been observed in the source region of the 2011 Tohokuoki earthquake (M9) in the NE Japan subduction zone (Shimizu, 2014). These facts indicate that seismogenic processes on
the plate interfaces are greatly influenced by the presence or absence of serpentinites and overpressurized fluids. Relationships
between intermediate-depth earthquakes and dehydration of serpentinites have been discussed based on high-PT deformation
experiments and the studies of thermal structures in subduction zones. Several different mechanisms have been proposed formechanical instability in double seismic zones in subducting slabs triggered by dehydration reaction. In this talk, I briefly review
the rheological and frictional properties of serpentinites at high-PT conditions and show some experimental results of ourselves,
and then discuss the influence of pore fluid pressure on brittle-ductile transition of serpentinites and seismogenic processes insubduction zones.
Reference
Shimizu, I., 2014, Rheological profile across the NE Japan interplate megathrust in the source region of the 2011 Mw9.0
Tohoku-oki earthquake, Earth, Planets and Space, 66:73, doi:10.1186/1880-5981-66-73(http://www.earth-planets-space.com/content/66/1/73)

Acknowledgement: I would like to thank K. Michibayashi, Y. Watanabe, M. Takahashi, S. Uehara, M. Nakanani, N. Katsuta,
and T. Murakami for their collaboration in experimental works of antigorite-serpentinites.
Keywords: serpentine, pore pressure, rheology, brittle-ductile transition, seismogenic zone, high-PT deformation experiment

1/1

SCG60-08

Room:201A

Time:May 25 14:15-14:30

Streak process due to irregular slab topography and the classification of subduction zones
EGUCHI, Takao1∗
1

Dept. Earth and Ocean Sci., N.D.A.

We demonstrate some physical features of streak (or abrasion) process associated with the surface convex topography ofdowngoing oceanic slab at subduction zones.
Then, we reconsider more realistic classification model of subduction zones, based on not only the streak process due to theirregular slab topography and/or with the degree of accreting process at the leading edge of the overriding lithosphere, but also
the other tectonics parameters such as slab age etc.
Keywords: streak, subduction zone, oceanic slab, convex topography

1/1

SCG60-09

Room:201A

Time:May 25 14:30-14:45

Split of the Philippine Sea plate and non-volcanic seismic swarm in Wakayama district,
SW Japan
KATO, Aitaro1∗ ; IWASAKI, Takaya2
1

Graduate School of Environmental Studies, Nagoya University, 2 Eathquake Research Institute, the University of Tokyo

To understand the driving force of an intensive non-volcanic seismic swarm in Wakayama district of the Kii Peninsula, Japan,
we used a series of dense seismic linear arrays to measure fine-scale variations of seismic structures beneath the seismic swarm
area. Kato et al. (2014, EPS) revealed that a low-velocity anomaly confined to just beneath the seismic swarm area is clearly
imaged, which correlates spatially with an uplifted surface area and a highly conductive and strong attenuative body, implying
the presence of fluids therein. In addition, they suggested that dehydration conversion from oceanic basalt to eclogite within the
subducting Philippine Sea Plate takes place at depths greater than 50 km. Fluids released from the subducting oceanic crust could
cause serpentinization of the mantle wedge.
In 2013, we conducted a new seismic experiment (deployment of 40 portable seismic stations) to investigate the structure
between Wakayama district and Awaji Island at the western extension of the seismic swarm area, where the Philippine Sea
Plate has been proposed to split (Ide et al., 2010). From a new receiver function image with high spatial resolution, we foundthat the dehydrated oceanic crust (high-velocity without intra-slab seismicity) steeply gets to be shallow toward offshore from
Wakayama district and close to the bottom of the overlying crust beneath Awaji Island. Interestingly, we found out a split or
gap of the oceanic crust beneath the center of Awaji Island. However, the gap width appears to be significantly smaller than one
proposed by Ide et al. (2010). Due to this split of the oceanic crust, hot mantle is easily leaking into the mantle wedge beneath
Wakayama district, resulting to warm thermal condition. These anomalous structures of the oceanic crust and mantle wedge
may locally promotes dehydration reactions of the subducting oceanic crust, leading to local increase in fluid flux to the shallow
seismic swarm area.

1/1

SCG60-10

Room:201A

Time:May 25 14:45-15:00

Insight into earthquake generation from evolution of pore fluid pressures in a stimulated
geothermal reservoir
TERAKAWA, Toshiko1∗ ; DEICHMANN, Nicholas2
1

Graduate School of Environmental Studies, Nagoya University, 2 ETH Zurich

We developed an inversion method to estimate the evolution of pore fluid pressure fields from earthquake focal mechanism
solutions. The basic assumption in this approach is that seismic slip occurs in the direction of the resolved shear traction acting
on pre-existing faults, controlled by the Coulomb failure criterion with a constant friction coefficient. Application of the method
to induced seismicity in the Basel enhanced geothermal system (EGS) in Switzerland shows the evolution of pore fluid pressure
in response to fluid injection experiments. For a few days following the initiation of the fluid injection, overpressurized fluids
were concentrated around the injection well and then anisotropically propagated within the reservoir until the well was shut inand bled off. At four representative locations the pore fluid pressure increased together with the wellhead pressure for the first
3?5 days, and reached a ceiling by the time of shutting in. The peak pressure in the reservoir was less than the minimum principal
stress at each depth, indicating that hydraulic fracturing did not occur during the stimulation. This suggests that seismic events
may play an important role in promoting the development of permeable channels, particularly southeast of the borehole where
the largest seismic event (Mw 2.95) occurred. The induced events were primarily controlled by a decrease in fault strength due
to an increase in pore fluid pressures. However, the largest event (the mainshock) was not directly related to a drastic decrease infault strength at the hypocenter. The precise relative location of the hypocenters indicated that substantial stress loading by thepreshocks on the same fault plane promoted the dynamic rupture of the mainshock.
Keywords: pore fluid pressure, stress, earthquake, focal mechanism, inversion theory, fluid injection

1/1

SCG60-11

Room:201A

Time:May 25 15:00-15:15

Characteristics of slab-derived fluids beneath Kii Peninsula inferred from seismic traveltime tomography
SHIBUTANI, Takuo1∗ ; HIRAHARA, Kazuro2
1

DPRI, Kyoto Univ, 2 Science, Kyoto Univ

1. Introduction
Deep low frequency events (DLFEs) are distributed at the depths of 30 - 40 km near the subducting Philippine Sea plate widelyfrom western Shikoku to central Tokai (Obara, 2002). Hot springs with high 3 He/4 He ratios are found in an area between central
Kinki and Kii Peninsula despite in the forearc region (Sano and Wakita, 1985). Arima-type deep thermal water with CO2 andhigh salt contents is found at hot springs in the area. These phenomena suggest the process that H2 O subducting with the oceanic
crust dehydrates at the depths of 30 - 40 km, causes the DLFEs, and uprises to shallower depths.
2. Receiver function analyses
We carried out seismic observations in Kii Peninsula since 2004 in order to estimate the structure of the Philippine Sea plate
and the surrounding area. We deployed seismometers along profile lines with an average spacing of ˜5 km. We applied receiver
function analyses and obtained images of S wave velocity discontinuities. We estimated 3D configurations of the continental
Moho, the slab top and the oceanic Moho from receiver function images for four profile lines in the NNW-SSE direction whichis the dip direction of the Philippine Sea plate and for two profile lines in the NNW-SSE direction that is almost perpendicular tothe dip direction.
3. Seismic travel time tomography
We carried out the tomography with FMTOMO (Rawlinson et al., 2006) in which a robust wavefront tracking (de Kool et al.,2006) is implemented for the theoretical travel time calculation and the ray tracing. We used a velocity model with 3D geometriesof the three discontinuities derived from the receiver function analysis. We also used observed travel times at temporary stationsin the dense linear arrays in addition to permanent stations. A dense distribution of the temporary stations contributed to higher
resolutions of tomographic images.
A result of the seismic tomography is shown in Figure 1. The generating area of the DLFEs (red circles) shows low velocity
anomaly of ˜5 %. As mentioned above, H2 O is discharged from hydrous minerals in the oceanic crust at the depths of 30 - 40
km. This can cause the low velocity anomaly.
Another strong low velocity anomaly more than 10 % is widely distributed in the lower crust beneath northern Wakayama
Prefecture (N34.0 - 34.5o ). It is known that seismic activity is very high in the upper crust above this low velocity anomaly.
This can be explained by a mechanism that fluids upwelling from the low velocity anomaly in the lower crust increase the porepressure in existing cracks in the brittle upper crust.
The Vp/Vs ratio of the low velocity anomaly in the lower crust beneath northern Wakayama Prefecture has small values near
1.6. Contrastingly the Vp/Vs ratio of the low velocity anomaly in and around DLFEs shows larger values 1.75 - 1.8. This difference in the Vp/Vs ratios of the two low velocity anomalies can be explained by the difference in the aspect ratios of the poresfilled with the fluids. And/or the small Vp/Vs ratio in the lower crust beneath northern Wakayama Prefecture might be due tosilica-saturated fluids (Manning, 1996).
We used waveform data from permanent stations of NIED; JMA; ERI, Univ. of Tokyo; Nagoya Univ. and DPRI, Kyoto Univ.

Keywords: tomography, receiver function, Philippine Sea slab, Kii Peninsula, slab-derived fluids, Nankai Trough megaquake

1/2

SCG60-11

Room:201A

Time:May 25 15:00-15:15

2/2

SCG60-12

Room:201A

Time:May 25 15:15-15:30

Coseismic fluid-rock interactions in subduction-zone faults: Constraints from geochemical analyses of fault rocks
ISHIKAWA, Tsuyoshi1∗
1

JAMSTEC Kochi

In this paper, we report recent progress on geochemical method for evaluating coseismic fluid-rock interactions in subductionzone fault zones on the basis of trace element and isotope analyses. Compositions of slip-zone rocks from the Taiwan Chelungpu
fault and the Boso accretionary complex showed distinct decreases of Li, Rb and Cs and an increase of Sr, indicating coseismic fluid-rock interaction at high temperatures of >350 deg. C (Ishikawa et al., 2008; Hamada et al., 2011). In the Shimanto
accretionary complex at the Kure area, the slip-zone rocks exhibited trace element characteristics consistent with coseismic
high-temperature fluid-rock interactions followed by frictional melting to form pseudotachylite (Honda et al., 2011). Such geochemical anomaly induced by coseismic hydrothermal processes can be reproduced by high-velocity friction experiments at wet
condition (Tanikawa et al., submitted).
Geochemical evaluation of coseismic fluid-rock interactions in fault zones is useful means not only to understand the slip
mechanism (e.g. thermal pressurization) but also to constrain compositions of the fluids in the fault zones. Trace element andisotope characteristics of slip-zone rocks from the Midian Tectonic Line in Japan (Ishikawa et al., 2014) and from the Kodiak
accretionary complex (Yamaguchi et al., 2014) both requires interactions with high-Li fluids, which implies that coseismic hydrothermal processes took place in the presence of fluids that had migrated from the depth.
Keywords: fluid-rock interactions, fault rocks, earthquake, geochemistry, subduction zones

1/1

SCG60-13

Room:201A

Time:May 25 15:30-15:45

Effect of swelling cray minerals on permeability and occurrence condition of abnormal
pore pressure
KAKU, Nahomi1∗ ; KATAYAMA, Ikuo1
1

Department of Earth and Planetary Systems Science, Hiroshima University

The March 11, 2011, devastating Tohoku-Pacific Ocean Earthquake had its epicenter at the Sanriku coast as the Hokuriku
plate tip had slipped significantly. However, it raises question on the traditional theory that plate tip never slips on a large scale.
Geological survey in this area brings out the fact that, smectite, one of the swelling clay minerals, is present in 78% of the fault
samples. Hence, smectite seems to be controlling the generation of the earthquake. In this study, it is hypothesized that swollen
clay minerals reduced the permeability and caused the abnormal pore pressure. By measuring permeability of clay minerals, thishypothesis is validated and discussed about the earthquakes that occur in the plate boundary.
Permeability is measured using water and nitorogen gas as pore fluid by the constant differential pressure flow method. The
method helps to determine the permeability based on the flow rate through the downstream in a state of constant pore pressure
difference applied across the sample. The vessel deformation permeability testing machine is used in case of gas, whereas thesyringe pump is used in case of water as pore fluid. During the experiment the confining pressure is controlled up to 10MPa,
while pore pressure up to 5MPa. Two types of clay minerals i.e. illite, and montmorillonite are used as sample. At the beginning
gas is used as the pore fluid, and then measured using water and compared for results.
Measurement results indicate that permeability of water is lower than the value measured using gas as pore fluid. Moreover,the decrease is 1.5 orders of magnitude in illite, while about 4.4 orders of magnitude in montmorillonite.
The reduction in permeability from gas to the water is due to water absorption and swelling of clay minerals. It decreases
the gap between the clay particles reducing the flow paths. Montmorillonite has large decrease in permeability, leading to larger
control of water effect. It is considered that a local presence of smectite will reduce the permeability in the underground. In
addition, this result is applied to the abnormal pore pressure producing conditions. In case of illite, abnormal pore pressure does
not occur to meet the condition. On the other hand, in case of montmorillonite the results satisfied the required condition, andthere is a high possibility that abnormal pore pressure occurs when present locally in the subduction zone.

1/1

SCG60-14

Room:201A

Time:May 25 15:45-16:00

Fracture sealing and permeability change induced by silica dissolution and precipitation
OKAMOTO, Atsushi1∗ ; YAMADA, Ryo1 ; TANAKA, Hiroto1 ; SAISHU, Hanae2 ; TSUCHIYA, Noriyoshi1
1

Tohoku University, 2 National Institute of Advanced Industrial Science and Technology

Solubility of silica significantly varies with depth within the crust; therefore, dissolution and precipitation of silica minerals
control the hydrological properties of the crust (Saishu et al., 2014), and may affect earthquake cycles (Audet and Burgmann,2014). However, permeability is usually measured as a static property of a rock, and it is unclear how porosity structure of afracture and permeability changes during dissolution or precipitation of minerals (i.e., silica); especially the effects of saturation index and effective confining pressure. In this study we conducted two-types of the hydrothermal experiments; (1) silica
precipitation in porous media and (2) dissolution of granite with a fracture. For both experiments, we used novel flow-throughreactors with tube-in-tube vessel, which make possible, after the experiments, to analyze porosity structure by micro X-ray CT(revolution is 10 micron/boxel).
The silica precipitation experiments were conducted under the supercritical (420 degreeC, 30 MPa) and vapor conditions (380degreeC, 20 MPa). The inner tube of the precipitation vessel (4 mm diameter, ˜200 mm long) was filled with alumina balls (1mm diameter). The input solution was made by dissolution of granite + quartz sands under liquid conditions, and thus, highsupersaturated solutions were brought into the precipitation vessel. In both conditions, nucleation of silica minerals occurred but
showed the contrasting porosity patterns. In the supercritical condition, amorphous silica was surrounded the surfaces of alumina
balls and walls, and discrete quartz grains and cristobalite formed within the amorphous silica. In contrast, in the vapor condition,fine-grained quartz crystals were nucleated, and settled on the bottom. As the results, the developed porosity in the supercriticalfluids was more tortuous that in the vapor. At the end of the experiments under the supercritical condition, the oscillation of
upstream fluid pressure (and thus permeability) was observed; such an oscillation was probably caused by repeated sealing andbreak of the bottleneck of the pore network.
In the granite dissolution experiments, the granite core (Aji granite, 8 mm diameter, 45 mm long) with a tensile fracture was
input into the inner SUS jacket with 0.1 mm thick. The P-T condition for the dissolution was 350 degreeC and 25 MPa, underthe effective confining pressure of 0 ? 15 MPa. The X-ray CT images revealed that (1) quartz was preferentially dissolved toform the convex surfaces, whereas that (2) mean fracture aperture decreased with time. The decrease in the mean aperture wasconsistent with the decrease in permeability from 10−13 to 10-15 m2 during experiments. According to the solution chemistries,
feldspars (Pl + Kfs) dissolution also occurred, which volume was about one-third of that of quartz. The dissolution of feldspars
at the contact regions (pressure solution) is critical for the permeability decrease in the granite fracture under confining pressure.
References
Saishu, H., Okamoto, A., Tsuchiya, N., 2014. Terra Nova, 26, 253-259.
Audet, P., Burgmann, R., 2014. Nature, 510, 389.

Keywords: silica, granite, dissolution/precipitation, porosity structure, X-ray CT, permeability

1/1

SCG60-15

Room:201A

Time:May 25 16:15-16:30

Experimental study on the hydration rate of peridotites at forearc mantle wedge conditions
NAKATANI, Takayuki1∗ ; NAKAMURA, Michihiko1
1Graduate School of Science, TOHOKU Univ.

Recent ground water studies in southwestern Japan suggest that slab-derived fluid upwells through the forearc mantle wedge
without significant reaction with the country rocks (e.g., Kazahaya et al. 2014; Kusuda et al., 2014), which may provide a clue tounderstanding the hydrological budget in forearc regions. The rate of serpentinization is one of the primary parameter for constraining the flow regime of slab-derived fluid in the stagnant forearc mantle wedge. Hydration experiments for kinetic studies
have been vigorously conducted previously at relatively low P-T conditions (up to ca. 350 ℃ and 0.3 GPa), in which olivine
reacts with water to form the low T serpentine variety lizardite (or chrysotile) and brucite. However, antigorite is expected to be
the dominant serpentine variety under the higher P-T condition corresponding to the forearc mantle wedge (350 to 650 ℃ and 1.0
to 2.0 GPa). Moreover, serpentine formation needs a silica source in addition to olivine (e.g., orthopyroxne) at the temperature
above 450 to 500 ℃ due to the instability of phase assemblage serpentine + brucite.
In order to constrain the serpentinization rates of peridotite under the mantle wedge conditions, we conducted piston-cylinder
experiments at temperature of 400 ℃ (brucite-present condition) and 500 to 580 ℃ (brucite-absent condition), and pressure of
1.3 and 1.8 GPa. Three types of starting materials were prepared from the crushed powder of a San Carlos lherzolite xenolith:1) olivine (Ol), 2) orthopyroxene (Opx) + clinopyroxene (Cpx) and 3) Ol + Opx. Hereafter these systems are abbreviated as
OL, OPX+CPX and OL+OPX respectively. The starting materials were reacted with 15 wt% distilled water for 4 to 19 days.
The hydration reaction proceeded in all the experiments, except for the OL system under the brucite-absent conditions. Basedon Raman spectroscopy results and crystal shapes, the synthesized serpentine minerals were identified as lizardite in most of theruns except for antigorite in the OL+OPX system at 1.8 GPa. The Al2 O3 in the system possibly stabilized the aluminous lizardite(Caruso and Chernosky, 1979). In the OL+OPX system, the reaction progress followed a diffusion-controlled rate law in the
brucite-present condition and an interface-controlled rate law in the brucite-absent conditions. The rate constants were estimatedto be 1.5 × 10−16 m2 /s and 8.7 × 10−12 to 1.5 × 10−11 m/s in the brucite-present and the brucite-absent condition, respectively.
We applied the experimentally-obtained hydration rates of peridotites to a reactive-transport model for the stagnant mantle
wedge hydration. In the case of grain-scale pervasive flow, slab-derived fluid is completely fixed in the mantle wedge peridotite.
Otherwise, aqueous fluid possibly penetrate all the way through the mantle wedge via crack-like pathways (we assumed thevertical distance of 10 km) with a spacing >0.025 to 0.80 m in the brucite-present conditions and >2.6 to 4600 m in the bruciteabsent condition. This indicates that slab-derived fluid may upwell easily through a cold forearc mantle wedge like in Western
Shikoku rather than a warm forearc mantle wedge like in Cascadia.

Keywords: forearc, mantle, fluid, serpentinite, antigorite, kinetics

1/1

SCG60-16

Room:201A

Time:May 25 16:30-16:45

Hydrothermal experiments on the metasomatic reactions at crust-mantle boundary.
OYANAGI, Ryosuke1∗ ; OKAMOTO, Atsushi1 ; TSUCHIYA, Noriyoshi1
1Graduate School Of Environmental Studies, Tohuku University

Hydration and dehydration reactions play crucial roles on global circulation of H2O in the earth’s interior and serpentinization(hydration of ultramafic rocks) is representative hydration process at slow-spreading ridge, bending faults and subduction zone.
It is known that silica activity has a large impact on controlling the reaction path during serpentinization [e.g., 1 and 2]. At
the crust?mantle boundary, a steeper silica activity gradient is expected, and a large mass transfer including silica would cause
metasomatic zoning. However, detailed mechanism to produce metasomatic reactions and its relation to the mass transfer are
still poorly understood.
In present study, hydrothermal experiments (250C, Psat) were carried out in two systems: the olivine (Ol)?quartz (Qtz)?H2O
and Ol?plagioclase (Pl)?H2O system, as analogues of crust-mantle boundary. Especially, by using unique tube-in-tube typehydrothermal experiments vessel, spatio-temporal evolution for metasomatic reactions as a function of distance from Ol?Qtz or
Ol?Pl boudaries were evaluated.
In the Ol?Qtz?H2O experiments, the mineralogy of the reaction products in the Ol-hosted region changed with increasing
distance from the Ol?Qtz boundary, from smectite + serpentine (Smc zone) to serpentine + brucite + magnetite (Brc zone). Massbalance calculations revealed that olivine hydration occurred without any supply of silica in the Brc zone. In contrast, the Smczone was formed by silica metasomatism via competitive hydration and dehydration reactions. In the Smc zone, smectite formed
via the simultaneous progress of olivine hydration and serpentine dehydration, and around the boundary of the Smc and Brc
zones, serpentine formation occurred by olivine hydration and brucite dehydration.
In the Ol?Pl?H2O experiments, the mineralogy of the reaction products in the Ol-hosted region changed with increasing
distance from the Ol?Pl boundary, from chlorite + serpentine (Chl zone) to serpentine + brucite + magnetite (Brc zone). Olivine
hydration proceeded in both zones, but the total H2O content in the products was greatest at the boundary than other part of theinner tube in the Ol?Pl?H2O experiments.
Our result indicates that in Ol?Qtz?H2O experiments, the competitive progress of serpentinization and silica metasomatic
reactions would cause fluctuations in pore fluid pressure. However, in Ol?Pl?H2O experiments, fluid pressure was not raised.
This metasomatic reaction possibly affects the mechanical strengths of the crust?mantle boundary.

References[1] Frost, B. R., & Beard, J. S. (2007). Journal of Petrology, 48(7), 1351?1368.
[2] Ogasawara, Y., Okamoto, A., Hirano, N., & Tsuchiya, N. (2013). Geochimica et Cosmochimica Acta, 119, 212?230.

Keywords: Ultramafic rocks, Metasomatism, Serpentinization, Pore fluid pressure

1/1

SCG60-17

Room:201A

Time:May 25 16:45-17:00

Hydrogen and oxygen isotope of glaucophane in lawsonite eclogites preserving pillow
structure
MASUYAMA, Yuya1 ; HAMABATA, Daichi1 ; TOMIYASU, Fumiya2 ; UENO, Yuichiro2 ; OKAMOTO, Kazuaki1∗
1

Saitama University, 2 Tokyo Institute of Technology

In order to understand evolution of life, change of seawater chemistry from Hadean, Archean to present is significant. Pillow
structure is well-preserved in the Archean greenstone belt (e.g. Komiya et al., 1999). Oxygen and hydrogen isotope of rims
in the pillow is useful conventional tool to decipher chemistry of Paleao-seawater from Archean to Present. However, Archean
greenstone belt suffered regional metamorphism from greenschist to Amphibolite facies condition. Therefore, it is necessary totestify the validity of pillow chemistry from recent (Phanerozoic) metamorphosed greenstone. We have systematically collectedpillowed greenstone from blueschist and eclogites. Two eclogite exhibiting pillow structures were chosen for oxygen and hydrogen isotope analysis. One is from Corsica (lawsonite eclogite collected with Dr. Alberto Vidale Barbarone) and another isfrom Cazadero, Franciscan belt (collected by Dr. Tatsuki Tsujimori). The both are ascribed as MORB from major and trace
bulk chemistry and Ca is rich in the core and Na is poor in the rims. The former exhibits garnet, omphacite, lawsonite, andglaucophane. Phengite is in core of the pillow and chlorite is in the rims. In the latter, besides garnet, omphacite, epdiote andglaucophane, chlorite is recognized with phengite in the core. Glaucophane is richer in the rims from the both samples, therefore
istope analysis of glaucophane was done. Mineral separation was carefully done using micro-mill, heavy liquid and isodynamic
separator. 20 mg specimens were used for oxygen isotope analysis and 2mg were for hydrogen analysis. δ 18 O of the all analysis(7.7 to 8.3) is within the range of unaltered igneous oceanic crust and high temperature hydrothermal alteration although rims
(8.3 for Franciscan and 8.0 for Corsica) are higher than cores (7.7 for Franciscan and Corsica). δD data is also consistent withhydrothermal alteration. It is relative higher in core from the Corsica and Franciscan (-45 and -56) than of the rims (-49 and -57,
respectively), suggesting dehydration in deep subduction zone.
Keywords: lawsonite eclogite, pillow structure, glaucophane, oxygen isotope, hydrogen isotope, seawater chemistry

1/1

SCG60-18

Room:201A

Time:May 25 17:00-17:15

Structure and physico-chemical properties of interfacial water on a quartz revealed bymolecular dynamics simulation
ISHIKAWA, Satoru1∗ ; SAKUMA, Hiroshi2 ; TSUCHIYA, Noriyoshi1
1Graduate School of Environmental Studies, Tohoku, 2 National Institute for Materials Science

Structure of water in thin film may have different characteristics compared with that of bulk water. Thin film water can bepresent at mineral grain boundaries or fractures, and its structure might be influenced by mineral surface. In this study, classicalmolecular dynamics (MD) simulations were performed to investigate the structure and physico-chemical characteristics of thinfilm water on a quartz surface.
The model of the quartz surface was characterized by the termination of silanol (Si-OH) group. Water molecules were confined
between these surfaces, and we simulated several thicknesses of water in the slab geometry.
Density of water oscillates to be about 1 nm from the quartz surface at 298 K. This structure became less clear with heating,and almost disappeared above 473 K. The self-diffusion coefficients of the confined water were calculated from mean squaredisplacements. These values were lower than that of bulk water indicating that the mobility of confined water is low between
quartz surfaces.
These changes in the physical properties of interfacial water on quartz might affect such as slips in the fracture surface of the
Earth’s crust.
Keywords: interfacial water, quartz, molecular dynamics, self-diffusion coefficient

1/1