Structural Setting and Seismicity in the Vicinity of the Great Sumatra - Andaman Islands Earthquake

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Structural Setting and Seismicity in the Vicinity
of the Great Sumatra - Andaman Islands Earthquake
M.H. Ritzwoller, N.M. Shapiro, E.R. Engdahl, A.L.
Levshin University of Colorado at Boulder J.
Park Yale University

• Upper mantle structure revealed by a
• global model derived from surface waves.
• Include relocations of regional eventshistoric,
  modern, and recent.

3. Conclude with speculative connections
between observed structures with regional
tectonics and the rupture of the great earthquake.
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Outline

• Brief summary of the relocation and
• model construction procedures.
• II. Thermal structure of the incoming plate.
• Characteristics of the subducting plate
• (e.g., existence?, dip).
• Thermal state of the back-arc beneath the
• Andaman Sea.

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Relocation Procedure

• EHB procedure 2523 events in the
• study region from 1906 - Feb 2005
• met the EHB criteria.
• Depth phases reviewed
• 1486 well-constrained by
• depth phases (/- 10 km),
• remainder were set at optimal depths
• relative to well-constrained events.
• Additional events analyzed near Java
• Trench.

E.R. Engdahl, Van der Hilst, R.D., and Buland,
R.P., 1998, Global teleseismic earthquake
relocation with improved travel times and
procedures for depth determination Bulletin of
the Seismological Society of America, v. 88, p.
3295-3314.
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Relocation Procedure
A
A
B
B
Blue triangles -- trench locations. Red triangles
-- volcanoes. Circles White - historical
event Red Star Great Green --
modern event Earthquake rupture
Red -- relocated event initiation
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Seismic Model Construction

• Global surface wave tomography
• (20 x 20 grid)
• broadband (15-150 s) dispersion measurements,
• group phase speeds (Harvard, Utrecht Univs.)
• construct dispersion maps using diffraction
• tomography,
• invert for a 3D Vs crust upper mantle model,
• oceanic areas additional inversion with a
  thermal
• parameterization physical constraints.

2. Refine grid regionally to 10 x 10.
50 km
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Seismic Model Construction

• Global surface wave tomography
• (20 x 20 grid)
• broadband (15-150 s) dispersion measurements,
• group phase speeds (Harvard, Utrecht Univs.)
• construct dispersion maps using diffraction
• tomography,
• invert for a 3D Vs crust upper mantle model,
• oceanic areas additional inversion with a
  thermal
• parameterization physical constraints.

2. Refine grid regionally to 10 x 10.
50 km
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Path Coverage
Seismic Model Construction
global model (20 x 20 grid)
Path Density 40 sec Rayleigh group
50 km
rays per 20 x 20 cell
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Vs 50 km
Seismic Model Construction
global model (20 x 20 grid)
regional model (10 x 10 grid)
Wharton Fossil Ridge
50 km
50 km
Vs Anomaly ()
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Vs 100 km
Seismic Model Construction
global model (20 x 20 grid)
regional model (10 x 10 grid)
100km
100km
Vs Anomaly ()
10
Vs 150 km
Seismic Model Construction
global model (20 x 20 grid)
regional model (10 x 10 grid)
150km
150km
Vs Anomaly ()
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Outline

• Brief summary of the relocation and
• model construction procedures.
• II. Thermal structure of the incoming plate.
• Characteristics of the subducting plate
• (e.g., existence?, dip).
• Thermal state of the back-arc beneath the
• Andaman Sea.

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Physically Constrained Thermal Parameterization
lithosphere
asthenosphere
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Lithospheric Age
Apparent Thermal Age
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Outline

• Brief summary of the relocation and
• model construction procedures.
• II. Thermal structure of the incoming plate.
• Characteristics of the subducting plate
• (e.g., existence?, dip).
• Thermal state of the back-arc beneath the
• Andaman Sea.

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500
300
450
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Comparison with Western Aleutian Subduction Zone
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Outline

• Brief summary of the relocation and
• model construction procedures.
• II. Thermal structure of the incoming plate.
• Characteristics of the subducting plate
• (e.g., existence?, dip).
• Thermal state of the back-arc beneath the
• Andaman Sea.

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Conclusions

• The incoming plate at N. Sumatra (near the
  initiation of rupture of the Great earthquake) is
  warmer, presumably more buoyant, and dips more
  shallowly upon subduction than elsewhere along
  the Sunda Trench. (Affect coupling strength?)
• A subducting slab is observed everywhere along
  the trench (off-board the Andaman Sea along the
  Sunda and Java Trenches) even given oblique
  incidence. There is no slab portal, as there is
  with oblique incidence in the W. Aleutians.
  (Convergence comes from the direction of the
  back-arc at the Burma Plate.)
• The upper mantle wedge beneath the Andaman Sea is
  very warm to a depth of 50 km or so, due to
  active divergence along the Andaman Spreading
  Center. (Affect late stages of rupture
  propagation of the Great earthquake?)

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Putting it All Together into A Sensitivity Kernel
R
Full kernel
First Fresnel zone approximation
S
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Forward Problem Spatially extended sensitivity
kernels model diffraction and wave-front
healing.
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Dataset

• More than 200,000 individual paths across the
  globe.
• Emphasis on short periods to improve resolution
  of the crust from the mantle.
• Use of regional data (e.g., PASSCAL) improves
  resolution in some areas.

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Dispersion maps result of a linearized inversi
on differ with period and with wave type of
measurement Rayleigh vs Love and phase vs group
speed azimuthal anisotropy estimated at the
same time.
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I. Inversion Method

• Data Rayleigh and Love wave group speeds. (20 -
  150 sec). Phase speeds (40 -150 s) from Harvard
  Utrecht Universities.
• Surface wave tomography creation of maps of
  surface wave speeds -- diffraction effects.
• Monte-Carlo inversion
• Seismic parameterization
• Temperature parameterization based on a thermal
  model

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Surface Wave Inversion Without Physical
Constraints
Two Stage Inversion Process

• 1. Linearized Inversion for the Dispersion Maps
• Measurements of dispersion are inverted for maps
  of local wave speed at different periods and wave
  types.

• 2. Conventional Monte-Carlo Inversion for a 3-D
  Vs Model
• The dispersion maps are inverted on a global
  grid to estimate the 3-D distribution of shear
  wave speed in the earths crust and uppermost
  mantle.

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Seismic Inversion Dispersion maps
100 s Rayleigh wave group velocity
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Seismic Inversion Local dispersion curves
All dispersion maps Rayleigh and Love wave group
and phase velocities at all periods
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Inversion of dispersion curves
All dispersion maps Rayleigh and Love wave group
and phase velocities at all periods
Monte-Carlo sampling of model space to find an
ensemble of acceptable models
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C. Details of the inversion seismic
parameterization

1. Ad-hoc combination of layers and B-splines
2. Seismic model is slightly over-parameterized
3. Non-physical vertical oscillations

Physically motivated parameterization is required
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D. Conversion between seismic velocity and
temperature
Computed with the method of Goes et al. (2000)
using laboratory-measured thermo-elastic
properties of the principal mantle minerals and a
model of mantle composition.
non-linear relation