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IBM Workshop in Honolulu. Seismological studies on mantle upwelling in NE Japan: Implications for the genesis of arc magmas. Junichi Nakajima & Akira Hasegawa Research Center for Prediction of Earthquakes & Volcanic Eruptions Graduate School of Science, Tohoku University, JAPAN. - PowerPoint PPT Presentation
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Seismological studies on mantle upwelling in NE Japan:
Implications for the genesis of arc magmas
Junichi Nakajima & Akira Hasegawa
Research Center for Prediction of Earthquakes & Volcanic Eruptions
Graduate School of Science, Tohoku University, JAPAN
November 8, 2007
IBM Workshop in Honolulu
Outline1. Review previous
results on mantle-wedge structure in 2000’s and their interpretations.
2. Introduce recent (preliminary) results of velocity structure in Tohoku and Hokkaido
Miller et al. (2006)
Study Study areaarea
Seismological observations in 1990’s
SeismologyHasegawa et al. [1991]Zhao et al., [1992-1994]
Hasegawa et al. [1991]
Zhao et al. [1992]
Travel-time tomography in NE Japan
Seismic tomography study - Eqs. and stations(Nakajima et al., 2001, JGR)
•169,712 P-wave arrivals & 103,993 S-wave arrivals
•Method : Zhao et al. [1992, JGR]
•Grid separation: 15-25 km in both horizontal and vertical directions
Neqs. = 4338
Inclined low-Vs zones in mantle wedge
Low-F eventVolcano
Pacific plate
Nakajima et al. [2001]
Inclined low-V zone ~50 km above the slab
Velocity reductions of 4-6 % in Vp and 6-10 % in Vs
Velocity structure at a 40 km depth (below the Moho)
Nakajima et al. JGR, [2001]
dVp Vp/VsdVs
(Eberle et al., PEPI, 2002)
Flow pattern
Numerical simulation
Inclined low-V zone
= upwelling flow induced by slab subduction
Predicted low-V zone is consistent with the observation.
Flow pattern (wedge)
Upward flow (high-T) is generated in the mantle wedge.
Karato [1993, GRL]
Question
What causes an inclined low-velocity zone ?
- thermal heterogeneity?- melts?- chemical heterogeneity?
Qp structure in NE Japan
(Tsumura et al., 2000)
Conversion from Qp to Temperature[Nakajima and Hasegawa, GRL, 2003]
]/)(exp[),,( 00*000
10 RTPHfPTfQ
]/)(exp[),,( *1 RTPHfPTfQ
( f: frequency [Hz] , P : pressure [GPa], T: temperature[K], H*: activation enthalpy [kJ/mol] )
1
10
1
*0
*0
*
ln)(
1
)(
)(
Q
Q
PH
R
TPH
PHT
Simple relationship between Q, temperature, pressure and frequency [e.g., Karato, 2004]
Given T0, P0 and Q0 as reference values….
References
T0 : 1025℃ ( 40 km depth ) [Kushiro, 1987]
Q0-1 = 0.0035 [Tsumura et al., 2000]
a=0.20, H*(P)=500 + 16×P kJ/mol, H0*=500 kJ/mol [Karato,
2004]
Thermal structure [Nakajima and Hasegawa, GRL, 2003]
Wet solidus of peridotite
Correction of thermal effect
Observed low-velocity anomalies-> 4-6 % in Vp and 6-10 % in V
sExpected velocity reductions from therm
al anomalies-> 1-2 % in Vp and 2-3 % in Vs
Residuals of velocity anomalies-> -dlnVp=0.03-0.04
-dlnVs=0.04-0.07 dlnVp/dlnVs = 1~2
2
*1,
,,
),()(
lnln
RT
HPTQ
F
T
V
T
V
sp
anh
spsp
Karato (1993)
Takei’s model ( Takei, JGR, 2002 )
dlnVs/dlnVp( Velocity reduction rate )
Aspect ratio ( α )
Volume fraction from dlnVs ( φ )
Takei (2002)
physical properties of fluids
Melt distribution in low-V zoneNakajima, Takei and Hasegawa (2005, EPSL)
Partial melting with fractions of 0.3-5 vol% in low-V zone.
Depth (km) Aspect ratio Melt fraction (%)
40 0.01-0.1 ~1
65 0.001-0.05 0.05-1
90 0.1-0.2 3-5
Interpretation of depth variation in pore shapes
dike/crack
dike/crack
GeneratioGeneration?n?
MigrationMigration??
Accumulation below Accumulation below Moho?Moho?
Depth variation in aspect ratio of melt-filled pores
Which direction does mantle upwelling flow?
Seismic velocity/attenuation structures are the present-day snap shot and do not provide the direction of mantle flow.
Shear-wave splitting could provide an important and independent information on mantle dynamics.
Results of shear-wave splitting(Nakajima and Hasegawa, EPSL, 2004)
Assuming A-type olivine in back arc, flow direction is inferred to be EW.
A model of return flow in NE Japan
Hasegawa & Nakajima (2004)
Summary
1. An inclined-low-velocity zone in the mantle wedge sub-parallel to the slab
2. Temperatures in the mantle wedge of 1000-1300 C
3. Depth variation in aspect ratio of melt-filled pores and melt fractions of 0.05-5 vol% in the low-velocity zone
4. Flow direction parallel to the slab dip
Recent tomographic results in NE Japan
@ Update previous results by Nakajima et al. (2001)
@ Obtain clearer images of inclined low-velocity zone
@ Understand whole fluid circulation
Kawakatsu & Watada (2007)
Data set
Zhao Nakajima This study
Eqs. 450 4400 8700
Stations ~50 150 >300
Grid int. 20-30km 15-25 km 10-20 km
P arrivals 16,000 160,000 600,000
S arrivals 5,000 100,000 350,000
Comparison with Nakajima et al. (2001)
This studyThis study Nakajima et al. (2001)Nakajima et al. (2001)
Central part of Tohoku
dVp
dVs
Results
Sheet-like low-velocity zoneSheet-like low-velocity zone
Larger velocity reductions in S wave than P waveLarger velocity reductions in S wave than P wave
( -dlnVp=3-6%, -dlnVs=5-10%)
Thickness of low-velocity zone of 10-30 km with an along-arc variation (seThickness of low-velocity zone of 10-30 km with an along-arc variation (seems to be thinner in C and D)ems to be thinner in C and D)
dd VVpp
dd VVss
Low-velocity zone beneath back-arc volcanoes
Diapirs from the upwelling?
Path of fluids from slab to mantle
Low-V zone at a dept of 150 km -> Supply of fluids from slab to mantle?
Summary of recent results
1: Low-velocity zone corresponding to oceanic crust down to a depth of 100 km (Tsuji et al., unpublished).
2~3: Low-velocity zone at a depth of ~150 km. Supply of fluids to mantle there?3~4: Sheet-like low-velocity zone
-dlnVs > -dlnVp . Thickness of 10-30 km with along-arc variation5: Segregated diapirs from upwelling?. Source of magmas of back-arc volcanoes?
11
2233
4455
Low-velocity zone in mantle wedge
Hasegawa and Nakajima (2004), AGU Geophys. Monog.
NE Japan : Zhao et al. (1992), Nakajima et al. (2001)
Alaska & Aleutian : Abers (1994), Zhao et al. (1995)
Kamchatka : Gorbatov et al. (1999)
Tonga : Zhao et al. (1997)
Inclined low-V zone (from back-arc to the VF)
Hokkaido : Wang and Zhao (2006)
Kyushu : Wang and Zhao (2006)
New Zealand : Reyners et al. (2006)
Alaska : Eberhart-Phillips et al. (2006)
Tonga : Conder and Wiens (2006)
After 2004After 2004
Is inclined low-V zone a common feature in subduction zones?
S-wave velocity structure -HOKKAIDO