structure of earth as imaged by seismic waves
DESCRIPTION
Structure of Earth as imaged by seismic waves. crust. upper mantle. transition zone. lower mantle. D”, core-mantle boundary layer. core-mantle boundary. outer core. inner core. 2000. 4000 km. 6000. 8000. 10,000. 12,000. radius of earth = 6371 km. - PowerPoint PPT PresentationTRANSCRIPT
upper mantle
outer core
inner core
D”, core-mantle boundary layer
2000 4000 km 6000 8000 10,000 12,000
lower mantle
core-mantle boundary
transition zone
crust
Structure of Earth as imaged by seismic waves
radius of earth = 6371 km
Seismic waves involve stress, strain, and density
Two important types of stresses and strains:
Pressure, P and volume change per unit volume, V/V
Shear stress and shear strain
For linear elasticity, Hooke’s law applies:
stress = elastic_constant x strain
For elastic waves, two elastic constants are key:
And density of the material,
= mass/volume
Two types of elastic waves
Compressional or P waves involve volume change and shear
Shear or S waves involve only shear
P wave particle motions
S wave particle motions
Click on these links to see particle motions:
Elastic wave velocities determined by material properties
43
p
s
V
V
P wave velocity
S wave velocity
epicenter
expanding wavefront at some instant of time after earthquake occurrence
ray perpendicular to wavefront
seismograph station
Earth surface
Earthcenter
epicenter
ray
seismograph station
= epicentral distance in degrees
Earth surface
Earthcenter
tt() = total travel time along ray from earthquake to station
Globally recorded earthquakes during the past 40 years
earthquake depth 0-33 km 33-70 70-300 300-700
Partial map of modern global seismograph network
2,538,185 travel time observations from International Seismological Centre (ISC), for earthquakes with depths between “0” and 60 km.
These are the commonly reported phases as reported to the ISC from seismograph stations from around the world; see phase types on next page
distance, degrees
tim
e, m
inu
tes
These lines represent plus or minus one minute errors in reading arrival times
P
diffracted P
PKIKP
PKP
PcP
PP
PPP
ScS
SKS
S
PcS
SS
SSS
PS
PPSPKPPKP
PKKP
PKS
SKKS
PPP
surf
ace w
aves
wat
er w
aves
click on link to P and S phases in the earth
Nomenclature for seismic body phases
c = reflection at core mantle boundary
K
P or S
I or Ji = reflection at inner core-outer core boundary
P wave segments in blueS wave segments in red
inner core
outer core
mantle
PS
Mantle
Inner core
Outer core
Single path refracted through mantle
seismic wave source
2,538,185 travel time observations from International Seismological Centre (ISC), for earthquakes with depths between “0” and 60 km.
These are the commonly reported phases as reported to the ISC from seismograph stations from around the world; see phase types on next page
distance, degrees
tim
e, m
inu
tes
P
S
P diffracted around core
PP
SS
Mantle
Outer core
Single reflection at surface
Inner core
2,538,185 travel time observations from International Seismological Centre (ISC), for earthquakes with depths between “0” and 60 km.
These are the commonly reported phases as reported to the ISC from seismograph stations from around the world; see phase types on next page
distance, degrees
tim
e, m
inu
tes
PP
SS
PcP
Single reflection at core-mantle boundary
reflection
ScS
Single reflection at core-mantle boundary
PcS
Single reflection with conversion of P to S
2,538,185 travel time observations from International Seismological Centre (ISC), for earthquakes with depths between “0” and 60 km.
These are the commonly reported phases as reported to the ISC from seismograph stations from around the world; see phase types on next page
distance, degrees
tim
e, m
inu
tes
PcP
ScS
PcS
PKP
P in mantle, refracting to P in the outer core (K) and out through the mantle as P
PK
P
PKIKP
P segments in mantle, P segments in outer core (K), and P segment in inner core (I)
P
KPK
I
2,538,185 travel time observations from International Seismological Centre (ISC), for earthquakes with depths between “0” and 60 km.
These are the commonly reported phases as reported to the ISC from seismograph stations from around the world; see phase types on next page
distance, degrees
tim
e, m
inu
tes
PKIKP
PKP
SKSS in mantle, refracting and converting to P in outer core,
then refracting back out and
converting back to S in the
mantle
S
KS
SKKS
S in mantle, refracting and converting to P in outer core, P reflects once at
inner side of core-mantle
boundary, then refracting back out back with
conversion to S in the mantle
S
KS
Kreflection
2,538,185 travel time observations from International Seismological Centre (ISC), for earthquakes with depths between “0” and 60 km.
These are the commonly reported phases as reported to the ISC from seismograph stations from around the world; see phase types on next page
distance, degrees
tim
e, m
inu
tes
SKS
S
SKKS
outer core
inner core
lower mantle
upper mantlekm/sec
km
transition zone
D’’ layer
0
1000
2000
3000
4000
5000
6000
7000
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14
dep
th
seismic wave velocity
Compressional (P) and Shear (S) wave velocities, Vp and Vs
43
p
s
V
V
modulus of incompressibility/
shear stressmodulus of rigidity
shear straindensity
(R) = "seismic parameter" derived from Vp(R) and Vs(R)
P
V V
2 24( )
3p sV V R
From Vp and Vs to seismic parameter
For self compression of homogeneous material
(R) = /
= - dP/(dV/V) = dP/(d/)
dP = - g dR
where R = radius to a point in the earth, andg = gravitational acceleration at that radiusg = GMR/R2
where MR = mass within sphere of radius R
d/dR = -/g
For self compression of homogeneous material
d/dR = -/g
This is the gradient in density determined by the seismic wave velocities. To obtain density, one must integrate by fixing the density, , and gravity, g, at the top of the layer and calculating both and g as one proceeds downwards.
The calculation assumes
a simple compression of material that does not change chemistry or phase.
the compression as one goes deeper produces an adiabatic temperature increase.
For self compression of homogeneous material
d/dR = -/g
The method is applied to the following layers:upper mantlelower mantleouter coreinner core
To determine the jumps in density between these layers, the following constraints are used:
Mass of earthMoment of Inertia of EarthPeriods of free oscillations of Earth
Density,
0
1000
2000
3000
4000
5000
6000
0 2000 4000 6000 8000 10000 12000 14000
core-mantle boundary
kg/m3
km
dep
th, k
m
Gravitational acceleration, g
0
1000
2000
3000
4000
5000
6000
0 2 4 6 8 10 12
core-mantle boundary
km
m/s2
dep
th, k
m
Pressure, P
core-mantle boundary
GPa.
km
0
1000
2000
3000
4000
5000
6000
0 50 100 150 200 250 300 350 400
dep
th, k
m
Density vrs pressure
0
2000
4000
6000
8000
10000
12000
14000
0 50 100 150 200 250 300 350 400GPa.
kg/m3
Density vrs pressure
0
2000
4000
6000
8000
10000
12000
14000
0 50 100 150 200 250 300 350 400GPa.
kg/m3
compression
compression
com
pos
itio
n
chan
ge
ph
ase
chan
ges liq
uid
to
solid
mantle density
crustal density
core-mantle boundary
Inner core/outer core boundary
fluid, 90% iron
solidified iron
2000 4000 km 6000 8000 10,000 12,000
Mg(Fe) silicates
phase changes
basaltic-granitic crust
chemical stratification and differentiation
cool, strong lithospheric boundary layer
slowly convecting mantle:plate tectonic engine
rapidly convectingouter core:
geomagnetic dynamo
solid inner core
subd
uctio
n
seafloor spreading
core-mantle thermo-chemical boundary layer
2000 4000 km 6000 8000 10,000 12,000
crust
Earth’s convective systems
upper mantle
lower mantle
transition zone
outer core
CMBD”
0 2000 4000 500030001000Temperature, degrees C
iron melting
Adiabatic gradient
near surface thermal boundary layer = lithosphere
D” = Lower mantle thermo-chemical boundary layer
mantle convectionadvective heat flow
conductive heat flow
conductive heat flow
Adiabatic gradient
Temperature in mantle
?
mantle m
elting
Temperature profile through
entire earth
Lowrie, Fundamentals of Geophysics, Fig. 6.26
Mantle convection, hot spots and plumes
Average P-wave velocity perturbation in the lowermost 1000 km of the mantle
abstract
inner core
mantle
The geomagnetic dynamo:• turbulent fluid convection• electrically conducting fluid• fluid flow-electromagnetic interactions• effects of rotation of earth
Generation of Earth’s magnetic field in the outer core
outer core
Geomagnetic field