Seismological studies on mantle upwelling in NE Japan: Implications for the genesis of arc magmas

1. 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 November 8, 2007

2. Outline Miller et al. (2006) • Review previous results on mantle-wedge structure in 2000’s and their interpretations. • Introduce recent (preliminary) results of velocity structure in Tohoku and Hokkaido Study area

3. Seismological observations in 1990’s Seismology Hasegawa et al. [1991] Zhao et al., [1992-1994] Hasegawa et al. [1991] Zhao et al. [1992]

4. Travel-time tomography in NE Japan

5. 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

6. Inclined low-Vs zones in mantle wedge Inclined low-V zone ~50 km above the slab Velocity reductions of 4-6 % in Vp and 6-10 % in Vs Pacific plate Low-F event Volcano Nakajima et al. [2001]

7. Velocity structure at a 40 km depth (below the Moho) dVp dVs Vp/Vs Nakajima et al. JGR, [2001]

8. Flow pattern (wedge) (Eberle et al., PEPI, 2002) Numerical simulation Flow pattern Upward flow (high-T) is generated in the mantle wedge. Karato [1993, GRL] Predicted low-V zone is consistent with the observation. Inclined low-V zone = upwelling flow induced by slab subduction

9. Question What causes an inclined low-velocity zone ? - thermal heterogeneity? - melts? - chemical heterogeneity?

10. Qp structure in NE Japan (Tsumura et al., 2000)

11. Conversion from Qp to Temperature[Nakajima and Hasegawa, GRL, 2003] Simple relationship between Q, temperature, pressure and frequency [e.g., Karato, 2004] （ f: frequency [Hz]，P： pressure [GPa], T: temperature[K], H*: activation enthalpy [kJ/mol]） 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]

12. Thermal structure[Nakajima and Hasegawa, GRL, 2003] Wet solidus of peridotite

13. 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=0.03-0.04 -dlnVs=0.04-0.07 dlnVp/dlnVs = 1~2 Karato (1993)

14. Takei’s model （Takei, JGR, 2002） dlnVs/dlnVp （Velocity reduction rate） physical properties of fluids Aspect ratio（α） Volume fraction from dlnVs（φ） Takei (2002)

15. Melt distribution in low-V zone Nakajima, Takei and Hasegawa (2005, EPSL) Partial melting with fractions of 0.3-5 vol% in low-V zone.

16. Interpretation of depth variation in pore shapes Accumulation below Moho? dike/crack Migration? dike/crack Generation? Depth variation in aspect ratio of melt-filled pores

17. 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.

18. (Nakajima and Hasegawa, EPSL, 2004) Results of shear-wave splitting Assuming A-type olivine in back arc, flow direction is inferred to be EW.

19. A model of return flow in NE Japan Hasegawa & Nakajima (2004)

20. Summary • An inclined-low-velocity zone in the mantle wedge sub-parallel to the slab • Temperatures in the mantle wedge of 1000-1300 C • Depth variation in aspect ratio of melt-filled pores and melt fractions of 0.05-5 vol% in the low-velocity zone • Flow direction parallel to the slab dip

21. Kawakatsu & Watada (2007) 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

22. Data set

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

24. Results ｄVs ｄVp 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 10-30 km with an along-arc variation (seems to be thinner in C and D)

25. Low-velocity zone beneath back-arc volcanoes Diapirs from the upwelling?

26. Path of fluids from slab to mantle Low-V zone at a dept of 150 km -> Supply of fluids from slab to mantle?

27. Summary of recent results 4 5 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 variation 5: Segregated diapirs from upwelling?. Source of magmas of back-arc volcanoes? 1 3 2

28. Low-velocity zone in mantle wedge Inclined low-V zone (from back-arc to the VF) 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) After 2004 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) Is inclined low-V zone a common feature in subduction zones?

29. S-wave velocity structure -HOKKAIDO