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Conclusions… Challenge: 1)Seismic waves are affected by variations in temperature, pressure, composition, mineralogy, structure (layering, scales and distribution.

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Presentation on theme: "Conclusions… Challenge: 1)Seismic waves are affected by variations in temperature, pressure, composition, mineralogy, structure (layering, scales and distribution."— Presentation transcript:

1 Conclusions… Challenge: 1)Seismic waves are affected by variations in temperature, pressure, composition, mineralogy, structure (layering, scales and distribution of multiphase materials, texture, fabric, grain size, etc.) and water content.

2 Conclusions… Challenge: 1)Seismic waves are affected by variations in temperature, pressure, composition, mineralogy, structure (layering, scales and distribution of multiphase materials, texture, fabric, grain size, etc.) and water content. 2) There are differences depending upon whether the water is in the form of hydrous melts, hydrous phases, or incorporated into the crystal structure of nominally anhydrous minerals.

3 Solution: There are ways to distinguish between them Ex/ Increase in Water vs. Temperature S VelocityDECREASEDECREASE P VelocitydecreaseDECREASE AttenuationINCREASEincrease

4 Solution: There are ways to distinguish between them Ex/ Increase in Water vs. Temperature S VelocityDECREASEDECREASE P VelocitydecreaseDECREASE AttenuationINCREASEincrease (Amounts vary GREATLY depending upon compositions and values of temperature and pressure)

5 Solution: There are ways to distinguish between them Ex/ Increase in Water vs. Temperature S VelocityDECREASEDECREASE P VelocitydecreaseDECREASE AttenuationINCREASEincrease (Water increase in ringwoodite of 1% would lower S velocities by about 5.4% and P velocities by about 1.5% [Jacobsen et al., 2004; Jacobsen and Smyth, 2006; Karato, 2006]; ----> Water increases P-to-S velocity ratio)

6 Solution: There are ways to distinguish between them Ex/ Increase in Water vs. Temperature S VelocityDECREASEDECREASE P VelocitydecreaseDECREASE AttenuationINCREASEincrease (Temperature affects on attenuation are greater at higher temperatures and lower pressures)

7 Solution: There are ways to distinguish between them Ex/ Increase in Water vs. Temperature S VelocityDECREASEDECREASE P VelocitydecreaseDECREASE AttenuationINCREASEincrease 410 Heightelevatedepress 660 Heightdepresselevate TZ Widthincreasedecrease

8 Solution: There are ways to distinguish between them Ex/ Increase in Water vs. Temperature S VelocityDECREASEDECREASE P VelocitydecreaseDECREASE AttenuationINCREASEincrease 410 Heightelevatedepress 660 Heightdepresselevate TZ Widthincreasedecrease (Water increase could elevate 410 by km, depress the 660 by up to 4 km, and so increase TZ Width [Smyth and Frost, 2002; Hirschmann et al., 2005; Higo et al., 2001])

9 Solution: There are ways to distinguish between them Ex/ Increase in Water vs. Temperature S VelocityDECREASEDECREASE P VelocitydecreaseDECREASE AttenuationINCREASEincrease 410 Heightelevatedepress 660 Heightdepresselevate TZ Widthincreasedecrease (Temperature increase of 400ºC could depress 410 by km, elevate the 660 by 7-40 km, so greatly reduce TZ Width [Litasov et al., 2006])

10 Solution: There are ways to distinguish between them Ex/ Increase in Water vs. Temperature S VelocityDECREASEDECREASE P VelocitydecreaseDECREASE AttenuationINCREASEincrease 410 Heightelevatedepress 660 Heightdepresselevate TZ Widthincreasedecrease 410 Thicknessbroadensharpen 660 Thicknessbroaden sharpen

11 Solution: There are ways to distinguish between them Ex/ Increase in Water vs. Temperature S VelocityDECREASEDECREASE P VelocitydecreaseDECREASE AttenuationINCREASEincrease 410 Heightelevatedepress 660 Heightdepresselevate TZ Widthincreasedecrease 410 Thicknessbroadensharpen 660 Thicknessbroaden sharpen (Wet mantle --> broaden 410 by up to 40 km, broaden 660 by up to 8 km [Smyth and Frost, 2002; Hirschmann et al., 2005; Higo et al., 2001])

12 Solution: There are ways to distinguish between them Ex/ Increase in Water vs. Temperature S VelocityDECREASEDECREASE P VelocitydecreaseDECREASE AttenuationINCREASEincrease 410 Heightelevatedepress 660 Heightdepresselevate TZ Widthincreasedecrease 410 Thicknessbroadensharpen 660 Thicknessbroaden sharpen (Hot mantle --> sharpen 410 and 660 by by around 5 km (Helffrich and Bina, 1994)

13 Most Seismically- Observed Mantle Water is in Subduction Zones (not surprisingly!)

14 Gerya et al. [2006] Petro-thermo-mechanical models can now predict what kinds of features we would expect to see seismically

15 Gerya et al. [2006] Flow and Temperature…

16 Gerya et al. [2006] P Velocity…

17 Gerya et al. [2006] S Velocity… Presence of cold, wet plumes predict >20% Poisson ratio variations, as opposed to ~2% variations due to only temperatures

18 Wiens et al. [2008] Inability of thermal models to explain seismic parameters is seen in subduction zone tomography observations:

19 Wiens et al. [2008] Thermal Model Predictions Tonga Observations

20 Wiens et al. [2008] Thermal Model Predictions Tonga Observations

21 van der Lee et al. [2008] Velocity Tomography away from subduction zones also show features that may be associated with water.

22 van der Lee et al. [2008]

23

24 Tibi et al. [2008] “Reciever Functions” of P- to-S converted phases find LZVs in forearc mantles interpreted as serpentinization from slab-expelled water

25 Tibi et al. [2008] LVZ S-velocities as low as 3.6 km/s suggest serpentinization of 30-50%, corresponding to chemically bound water contents of 4-6 wt%

26 Kawakatsu and Watada [2008]

27

28 Percentages show S-velocity reductions relative to slab velocities; parentheses show suggested water content in wt%)

29 The presence of water can determine the magnitude and orientation of seismic anisotropy in olivine

30 Seismic shear- wave splitting results from Nakajima and Hasegawa [2004]

31 Leahy and Bercovici [2007] Geodynamic models suggest the possibility of water just above the 410 discontinuity….

32 Leahy and Bercovici [2007]

33 Courier and Revenaugh [2007] Mantle reverberations show discontinuity depth and impedance

34 Courier and Revenaugh [2007] Discontinuity depths…..

35 Courier and Revenaugh [2007] Reflection Coefficients… Results show a LVZ above the 410 with reduced 410 impedance attributed to partial melt from volatile-induced melting.

36 Gao et al. [2007] Seismic Arrays (in this case RISTRA) can identify layer velocities from traveltime moveouts.

37 Gao et al. [2007] Triplication patterns reveal vertical velocity structures

38 Gao et al. [2007] Water on Top of Transition Zone? LVZ above 410 is interpreted as partial melting due to water released from the Transition Zone

39 Water causes increased Seismic Attenuation Stein and Wysession [2003]

40 Large High- Attenuation region km deep (Q < 100 !!!) with only slightly negative velocities Lawrence and Wysession [2006]

41 A Vertical Cross-Section through Earth’s mantle at 1000 km depth shows the high-attenuation region above circum-Pacific subducted lithosphere Depth = 1000 km Lawrence and Wysession [2006]

42 Shieh et al. [1998]

43 Lawrence and Wysession [2006]


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