1. Large-Scale Seismological Imaging of the Mariana Subduction Zone
Douglas Wiens, James Conder, Sara Pozgay, Mitchell Barklage, Moira Pyle, Rigobert TIbi
Dept. of Earth and Planetary Sciences, Washington University, St. Louis, MO
Hajime Shiobara
Earthquake Research Institute, University of Tokyo, Tokyo, JAPAN
Hiroko Sugioka
IFREE, JAMSTEC, Yokosuka, JAPAN
Eruption of Anatahan Volcano,
Northern Mariana Islands,
June 10, 2003

2. Outline Passive Ocean Bottom Seismograph Deployments in the
Mariana Arc
Seismic Anisotropy - constraints on mantle flow
Interpreting seismic velocity and attenuation
Seismic velocity and attenuation tomography results
Implications of results for
- forearc
- arc
--- backarc spreading center

3. Mariana Passive Ocean Bottom Seismograph Deployments
ERI deployment
9 OBSs
Joint US-Japan
deployment
- 50 US OBS (LDEO)
- 8 Japanese OBS (ERI)
- 20 broadband land stations
- deployed from R/V Kaiyo
- recovered with R/V Wecoma
- problems with 35 LDEO OBSs

4. Results from initial 2001-2002 9 OBS deployment: Intermediate Depth Double Seismic Zone
Shiobara et al., in prep.

5. P and S velocity tomography from initial 9 OBS deployment
Shiobara et al., in prep.

6. Mantle flow, seismic anisotropy, and lattice preferred orientation
Shear velocity of olivine
Relationship of anisotropy and strain
Mainprice & Silver [1993]
Data from Kumazawa & Anderson [1969]

7. Mariana Arc Shear Wave Splitting
Rose Diagrams - plotted at station
for sources in upper 250 km
Spatial Averaging - for paths in the
upper 250 km
Pozgay et al. [2007]

8. Anisotropy and Structure from Rayleigh Waves
Moira Pyle, see poster
Rayleigh waves show low velocities in the arc and backarc spreading center
Average fast direction is EW
We suggest that portions of the forearc and backarc with EW fast directions dominate

9. Interpretation of anisotropy in arcs… along-strike flow or effect of water?
Modeling (Kneller et al, 2005)
3 types of fabric (Jung and Karato, 2001)
Modeling shows that conditions favorable to type-b
fabric (low T, high stress) occur in the forearc
Yet observations from several arcs show that
along-strike fast directions extend well into the backarc
This suggests that along-strike fast directions result
from mantle flow

10. How do material properties affect mantle seismic observables?
Temperature effect on seismic velocity and attenuation -- no melt
P and S velocities are controlled by anharmonic temperature derivatives at
temperatures below about 900 C relatively linear
VP/T ~ 0.6 m/s/K ( 0.8 % per 100C); VS/T ~ m/s/K (1 % per 100C)
Above 900 C attenuation increases rapidly and the velocity derivatives are non-linear
Both attenuation and velocity are also a function of frequency, grain size, and depth (Jackson et al., 2002; Faul and Jackson, 2005)
Shear Velocity
Velocity derivative

11. Melt and Water Geometry
The effect of fliuds on seismic velocity is a function of the fluid geometry
There is still a controversy about melt geometry and how it varies with percent melt
Melt geometry is also related to porosity and permeability and how fast melt escapes
q  60
q < 60
Node q q
Tubule
Wark et al., 2003

12. The seismic effect of partial melt as a function of inclusion aspect ratio
S velocity derivative wrt melt fraction
Fractional change in Vs relative to
fractional change in Vp
 is ratio of solid bulk modulus
to liquid bulk modulus
After Takei [2002]

13. Shear Velocity Reduction and Attenuation for Olivine containing Melt
Modulus Reduction and Attenuation Mechanism
Melt and seismic attenuation
Line thickness gives melt content;
line color gives grain size
For a given grainsize, 1% melt gives nearly an order
of magnitude increase at 1 Hz
Seismic velocity reduction occurs through
both “melt squirt” and grain boundary sliding
Faul et al., 2004

14. Effect of dissolved water?
Karato, 2003
Experiments: Aizawa et al [submitted] shows effects but no quantitative relationship
Karato [2003] extrapolates from the rheological effect
810 ppm H/Si = .005 wt % water - normal MORB
Mariana backarc to 0.25 wt % H2O in the mantle source [Kelley et al., 2006]
At 100 km depth water < 0.01 to 0.02 wt % [Hirschmann, 2006]
Lowers Qs from 80 to 60; 2% decrease in seismic velocity

15. Velocity Tomography P velocity S velocity
Depth
(km)
Barklage et al, in prep.
Preliminary tomography results using 3-D double difference tomography
Show low velocity region beneath the arc at depths of km
Separate low velocity region beneath backarc spreading center
Backarc poorly resolved due to high attenuation, OBS failures
Low velocities in shallow slab and outer forearc
Improvements will add teleseismic arrivals; incorporate crustal thickness variations

16. P and S wave attenuation tomography
2-D attenuation tomography along the line of OBSs
Frequency Hz; Assumes  = 0.27
~ 2300 P wave t* measurements, ~ 440 S wave; P resolution better than S
Highest attenuation in sheet-like anomaly beneath the spreading center
High attenuation beneath arc, in shallow slab and outer forearc
Pozgay et al, in prep, see poster

17. P Attenuation Comparison

18. P Attenuation Comparison

19. P velocity and attenuation - forearc
100
200
Most forearcs show high mantle seismic velocity and low attenuation - “cold nose”
Mariana forearc shows regions of low velocity and high attenuation
trenchward of the arc
outer forearc and shallow slab
Low seismic velocity may indicate serpentinization
Effect of serpentinization on attenuation is unknown; high attenuation may result from fluids

20. Receiver function evidence for widespread serpentinization in the forearc
Tibi et al. submitted; see poster
Widespread low velocity body ~ km depth above the slab in the forearc

21. P velocity and attenuation tomography - arc and backarc
Slow velocity high attenuation beneath the arc at km depth
Sheet-like high attenuation anomaly beneath spreading center
75 km wide, extends to 100 km depth
Arc and spreading center anomalies separated at shallow depths in both images

22. P Attenuation Modeling (assume all anomalies due to temperature)
Thermal
Model
(J. Conder)
Modeled
Attenuation
(using Faul &
Jackson 2005)
P Attenuation
(Pozgay in prep)
Assume thermal model and use Q-temperature relationships to predict attenuation
Observed attenuation is similar except in high attenuation regions (note scale)
Spreading center and arc have much higher attenuation than model
We interpret as effect of in-situ melt; cannot quantitatively estimate % melt
Narrow back-arc anomaly may suggest dynamic upwelling

23. Mantle temperature variations between active backarcs
Seismic Structure
Major Element systematics
Large solid symbols are basin averages
(with error bars)
Data from Kelley et al [2005], dry (< 0.65 wt
% in melt) samples only
Na2O and FeO corrected to equilibrium with
Fo90 olivine
Significant variations between active backarcs – Lau slowest, and Mariana fastest
Variations largely confined to the “melt producing” region - 40 to 100 km depth
Wiens, Kelley, Plank, EPSL, 2006

24. Attenuation structure comparison - Mariana vs Lau (Tonga) backarc spreading centers
Pozgay et al, in prep.
Roth et al, 1999;
Reprocessed by J. Conder
Mariana image uses  = 0 for direct comparison with Tonga results
Tonga image has lower spatial resolution
Tonga shows much higher attenuation, greater depth extent
Consistent with higher temperatures and greater melt productivity
Much broader anomaly - passive vs active upwelling at ridge?

25. Conclusions
Shear wave splitting shows along-strike fast directions in the mantle wedge beneath the arc and extending to the backarc; Interpreted as along-strike mantle flow in the low-viscosity part of the wedge
Low velocity and high attenuation regions exist in the forearc, perhaps due to serpentinization and fluids
Low velocity and high attenuation extends from km depth beneath the arc, likely defining the melt production region
Arc and backarc spreading center anomalies are separated at depths < 80 km
A 75 km wide sheet-like high attenuation anomaly extends to 100 km depth beneath the backarc spreading center
We interpret this as evidence of focused dynamic upwelling
The seismic data provide evidence of in-situ melt in the upper mantle but cannot yet estimate melt %