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

Outline 1.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 area

Seismological observations in 1990’s Seismology Hasegawa et al. [1991] Zhao et al., [ ] 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: km in both horizontal and vertical directions Neqs. = 4338

Inclined low-Vs zones in mantle wedge Low-F event Volcano 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] dVpVp/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] （ f: frequency [Hz] ， P ： pressure [GPa], T: temperature[K], H*: activation enthalpy [kJ/mol] ） Simple relationship between Q, temperature, pressure and frequency [e.g., Karato, 2004] Given T0, P0 and Q0 as reference values…. References T 0 ： 1025 ℃ （ 40 km depth ） [Kushiro, 1987] Q 0 -1 ＝ [Tsumura et al., 2000] a=0.20, H * (P)= ×P kJ/mol, H 0 * =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 Vs Expected velocity reductions from thermal anomalies -> 1-2 % in Vp and 2-3 % in Vs Residuals of velocity anomalies -> -dlnVp= dlnVs= dlnVp/dlnVs = 1~2 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 zone Nakajima, Takei and Hasegawa (2005, EPSL) Partial melting with fractions of vol% in low-V zone. Depth (km)Aspect ratioMelt fraction (%) ~

Interpretation of depth variation in pore shapes dike/crack Generation? Migration? Accumulation below 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 C 3.Depth variation in aspect ratio of melt- filled pores and melt fractions of vol% in the low-velocity zone 4.Flow direction parallel to the slab dip

Recent tomographic results in NE Update previous results by Nakajima et al. Obtain clearer images of inclined low-velocity Understand whole fluid circulation Kawakatsu & Watada (2007)

Data set ZhaoNakajimaThis study Eqs Stations~50150>300 Grid int.20-30km15-25 km10-20 km P arrivals16,000160,000600,000 S arrivals5,000100,000350,000

Comparison with Nakajima et al. (2001) This study Nakajima et al. (2001) Central part of Tohoku dVp dVs

Results Sheet-like low-velocity zone Larger velocity reductions in S wave than P wave （ -dlnVp=3-6%, -dlnVs=5-10%) Thickness of low-velocity zone of km with an along-arc variation (seems to be thinner in C and D) ｄ Vp ｄ Vs

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 km with along-arc variation 5: Segregated diapirs from upwelling?. Source of magmas of back-arc volcanoes?

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 2004 Is inclined low-V zone a common feature in subduction zones?

S-wave velocity structure -HOKKAIDO