Download Wait! How do we know this??

Simplest assumption: suppose the
Earth!s interior was uniform...
Wait! How do we know this??
We measure the travel
times of seismic waves
from earthquakes, and
compare them with
what we would expect
from different layered
models until we get a
match
•! P wave, S wave, and
surface waves would
arrive at all stations
•! we would compute
their arrival times at
different seismometers
assuming a gradual
increase in velocity due
to pressure
“inversion” of the
seismic travel time
data for velocity
structure
•! this would work
perfectly
Here is what we
actually see!
we could make a plot of P, S, and surface
wave arrival time with distance “ ∆ “
surface
∆=
50°
time
S
time (minutes)
If the Earth!s interior were uniform...
P
∆
∆
“Seismic Phases”
•
•
•
•
•
•
P,S : P,S waves in the mantle, e.g. PPP, SS
K : P-wave in the outer core, e.g. PKP, SKP
I,J : P,S waves in the inner core
c : reflection from CMB, e.g. PcP, ScS
i : reflection from IOB, e.g. PKiKS
SUMMARY: many phases are created by
Earth’s stratification (reflections, and
sometimes conversion from one wave type
to another)
P wave velocities drop suddenly at 2900 km depth,
and S waves cannot pass through this layer
Fig. 4.7
S wave shadow zone
P wave shadow zone
Fig. 4.8
Fig. 4.9
Why are there seismic velocity jumps inside
the mantle?
We get this picture by calculating arrival times for all seismic
phases in a stratified model of the Earth, and making sure they
match the observed arrival times at all points on the Earth
• mantle has fairly
uniform composition
• same chemical elements
arrange into different
minerals at different depths
• minerals that are stable
at great depth are the densest
• seismic wave speeds
change as minerals change
• Low Velocity Zone:
close to melting temperature
Meteorites: analogues to
composition of the Earth!s interior?
Engdahl and Kennett 1991
dots (phase
travel times)
match curves
(modelpredicted
travel times)
very well.
We can get seismic wavespeeds, but does this
tell us the chemical composition of the layers?
METEORITES
Abundances of elements in the solar system are
estimated from meteorites, solar corona, etc.
Crust and mantle: too little iron (and nickel
and lead etc.) relative to oxygen, silicon, etc.
Missing: we need lots of
iron and nickel inside the Earth
+
~
=
a small amount of
+
“Primordial” (never
melted or re-processed)
4.5 BY old meteorite
Likely close to average
composition of the
Earth
Core - mantle boundary
What is the Earth!s Core
made of?
• liquid outer core: must be a liquid at the P,T
conditions deep in the Earth
• magnetic field generation: it must be a metal
• densities: the core must be dense
– Earth’s average: 5.5 g/cm^3
– crust, mantle: 2.7-3.3 g/cm^3 (85% by volume)
– therefore, core: 10+ g/cm^3
• meteorites suggest Fe core with trace O, Si, Ni, S
CORE-MANTLE BOUNDARY
• dramatic density and seismic velocity
change
• slab graveyard
• mantle plume birthplace
• site of “anti-crust’’ (ULVZ) and “antilithosphere’’ (D’’ layer)
• is the core reacting with the mantle? or is it
melting the mantle?
D’’ layer and ultra low velocity zone (ULVZ): what’s going on?
Rising plumes, sinking slabs... The Earth
is not exactly radially symmetric
Seismic tomography: let!s find
Texas
seismographs
earthquakes
suppose that
the Earth is
flat and that
seismic
waves travel
unusually
slowly
through
Texas...
Global Seismic Tomography
Finding Texas.
on-time
late
late
late
on-time
seismic velocity
anomalies usually
differ by less than 2%
from surroundings
Global Seismic Tomography
upper
mantle
• similar to CAT scan in medical imaging
• compares real travel times with travel times
predicted by the radially symmetric Earth
model
• small differences in travel times are
translated to seismic velocity variations
• Blue (cold) is fast & Red (hot) is slow
• snapshots of mantle convection: hot
material rises and cold material sinks
core - mantle
boundary
Van der Hilst et al., 1998
Subducting slabs: stronger and colder
than their surroundings
Giant superplume rising from the CMB
under the Pacific Ocean
Do they all sink to the
core-mantle boundary
or not?
There!s another one
of these under Africa too.
Van der Hilst et al., 1998
Seismic Tomography of the Mantle
a “snapshot” of current mantle velocity
• primary influences:
• composition
• temperature
Tomography at a finer scale - P wave
velocity anomalies at 100 km depth
Subducting
slab
3-D image:
• orange-red: hot
(upward convection)
Tomography at a finer scale - the
Yellowstone hot spot
they* could not
resolve the
bottom of this
low-velocity
feature
small plumes
associated with
hot spots are too
small to show up
on the global
tomographic
images
*Derek Schutt, Eugene Humphreys, Rebecca Salzer (P and S wave studies)
Strong mantle
under the Sierra
Nevada batholith
Yellowstone
hot spot
Maximum
perturbation
is 2%