Download The velocity structure of the Earth Nomenclature

Global seismology:
The velocity structure of the Earth
Reading:
Fowler p126-130, 326-331, 336-348
EPS 122: Lecture 13 – The velocity structure of the Earth
Seismic phases
Nomenclature
EPS 122: Lecture 13 – The velocity structure of the Earth
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Waves through the Earth
P-waves
S-waves
EPS 122: Lecture 13 – The velocity structure of the Earth
Seismic phases
Arrival times
EPS 122: Lecture 13 – The velocity structure of the Earth
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Seismic phases
Short period arrival times
EPS 122: Lecture 13 – The velocity structure of the Earth
Seismic phases
All arrival times
Love and
Rayleigh
S
P
EPS 122: Lecture 13 – The velocity structure of the Earth
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1D velocity model for the Earth
Using the arrival times of all these phases at stations around the
globe we can calculate a 1D average velocity model for the Earth
Uppermost mantle low-velocity zone
Transition zone: 410-660 km
Earthquakes stop at ~660km
Lower mantle
VS
VP
Core-mantle boundary
Outer core: must be fluid as VS = 0
Also required for Earth’s magnetic field
Inner core solid
EPS 122: Lecture 13 – The velocity structure of the Earth
Velocity and mineralogy
EPS 122: Lecture 13 – The velocity structure of the Earth
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3D velocity variations?
EPS 122: Lecture 13 – The velocity structure of the Earth
Mantle convection
Features we expect will cause 3D velocity anomalies:
• Hot, upwelling molten rock at mid-ocean ridges
• Old, cold subducting lithosphere
• Hot, upwelling mantle plumes
EPS 122: Lecture 13 – The velocity structure of the Earth
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Imaging 3D velocity variations
Tomography
Use the variations in traveltimes of seismic
arrivals to determine regions of high and
low velocity
Similar methods
used in CAT scan
EPS 122: Lecture 13 – The velocity structure of the Earth
Borehole tomography
Tomography result:
velocity variations
between the holes
Raypaths between holes
Low velocities
due to a cavity
EPS 122: Lecture 13 – The velocity structure of the Earth
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Whole Earth tomography
Use variations in the
arrival times of seismic
energy
If ray has passed through
a high-velocity region then
phase arrived early
If low velocity region
phase arrived late
Use this information to
reconstruct an image of
the mantle
EPS 122: Lecture 13 – The velocity structure of the Earth
Subduction
Western
pacific
EPS 122: Lecture 13 – The velocity structure of the Earth
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Subduction
Western pacific
Kuril slab
Japan trench
Izu slab
EPS 122: Lecture 13 – The velocity structure of the Earth
Subduction
Farallon slab
• Originates from a time
when there was
subduction all along the
western US.
• We find evidence of this
slab extending all the
way to the core-mantle
boundary
EPS 122: Lecture 13 – The velocity structure of the Earth
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Mid-ocean ridges
EPS 122: Lecture 13 – The velocity structure of the Earth
Mid-ocean ridges
Mid-Atlantic Ridge
and Iceland
EPS 122: Lecture 13 – The velocity structure of the Earth
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Hotspots and
mantle plumes
Strings of volcanic ocean
islands recording the motion
of the plate
Hawaiian chain
The formation process
EPS 122: Lecture 13 – The velocity structure of the Earth
Iceland
The plume beneath the
Iceland hotspot
EPS 122: Lecture 13 – The velocity structure of the Earth
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Iceland
EPS 122: Lecture 13 – The velocity structure of the Earth
Azores
Canary
Cape Verde
Montelli et al
EPS 122: Lecture 13 – The velocity structure of the Earth
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Hawaii plume?
Montelli et al
EPS 122: Lecture 13 – The velocity structure of the Earth
African cratons
The difference between
oceanic and continental
crust
EPS 122: Lecture 13 – The velocity structure of the Earth
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Velocity and mineralogy
EPS 122: Lecture 13 – The velocity structure of the Earth
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