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