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Solid Earth Geophysics-Geop503
Ali [email protected].
saDepartment of Earth Sciences, KFUPM
Seismic Waves and Earth’s Interior
Reading: Fowler Chapter 8- Section 8.1
Summary: Lectures 1-2 Course OutlineCourse Outline
Term-Project OutlineTerm-Project Outline
Earth FormationEarth Formation
The past/present knowledge on Earth’s Interior
Examples from the paper published in 1924, 31, 40, 42
Increased temperature/pressure versus depth
The Three Major Chemical Radial Divisions
How do we know the internal structure?How do we know the internal structure?
Continental Scientific Drilling Program
Seismology based on seismic wave propagation
Internal structure of the Earth
Seismic velocities for the whole Earth
Reading: Pages 326-331, Chapter 8
use reflections to find interfacesuse refraction (travel time curves), surface waves, and normal modes for inversion
Bulk composition
Composition of the Crust
See for more detail at pp. 513, Fowler-2005
How can we infer structure at depth?
Drilling into Crust
History
DSDP-1968
OCDP-1985
IODP-2003Advanced knowledge of the geological and geophysical structure of the uppermost crust.
See more detail on pp. 397-398
Seismology to the rescue
ReflectionRefractiontomography
Crustal Structure
The thickest crust is found beneath the Tibetan plateau, the Andes and Finland. The global average thickness of continental crust is 38 km, but the thickness typically ranges between 30 and 45 km.
Al-Damegh, K., et al., 2005.
3D Crustal Structure
Southern California
Kohler., et al., 2003. , BSSA., Vol. 93, No. 2, pp. 757–774,
Determined upper mantle seismic velocity heterogeneities below Southern California from the inversion of teleseismic travel-time residuals.
1D Earth Models
220 km- Discontinuity
Low velocity zone for S-waves above 220 km is well detected by Surface wave dispersion data
PREM was determined by a joint inversion of the free oscillation periods of the Earth, its mass and moment of inertia as well as the travel-time-distance data.
See Figure 8.1 and pp. 328-329, Fowler-2005
The upper mantle
olivine – spinel transition at ~410 km
spinel – perovskite transition at ~660 km
phase transition determined by Clapeyron slope dT/dp
Strength of heterogeneity
amplitudes are larger in boundary layers
spectrum changes, too: long wavelengths at
top (ocean/continents) and bottom (slabs?)
short wavelengths in mid mantle
Bullen's (1940) layers
A: crust, 33 km thick B: Upper mantle, 33 – 413 km depthC: Upper mantle transition zone: 413 – 984 kmD': Lower mantle: 984 – 2400 kmD'': Lower mantle/core transition: 2400 – 2900
kmE: outer core (CMB discovered by Gutenberg)F: inner core (OC/IC boundary discovered by
Lehmann, 1936, by PKIKP; SKJKP and pPKJKP by Deuss et al., 2000)
See pp. 329, Fowler-2005
Seismic Seismic DiscontinuitiesDiscontinuities Note that velocities
increase gradually within layers (phase
changes) but jump at discontinuities, which are thought mostly to
be compositional changes.
LVZ (“low velocity zone”) is uppermost
part of asthenosphere, with low Vp due to less viscous nature
(partial melts). Plates ride on the LVZ
YOU DON’T HAVE TO REMEMBER THESE!
Ray Paths for PKIKPRay Paths for PKIKP MovieMovie
Nature of the Core
The direct P-wave passing through the mantle, outer core and inner core (1959).
Seismic shadow zones
P-wave shadow zoneP-wave shadow zone S-wave shadow zoneS-wave shadow zone
Shadow Zones
Low velocity zone
Distance ( )
V1 > V2
What is the relation between the shadow zone and low velocity
zone?
For example, v(r) behaviour is no longer simple.
See Figure 8.2, Fowler-2005
Seismic waves (wave fronts shown by dotted lines) and ray paths through the Earth’s interior that indicate interior structure (crust, mantle, outer core, inner core, etc.)
Travel Time (minutes)
Distance (degrees)
Source
Mantle
Inner Core
Outer core
Wavefronts
Raypaths PPdiffracted
PKPPKIKP
PKiKP
PKPP
Seismic phases Seismic phases in the Earthin the Earth
PKP – The direct P-wave passing through the mantle and outer core.
C – Mantle P-wave which has grazing incidence on the core has an epicentral distance of 103.
C’ – The PKP ray with the shallowest angle of incidence on the outer core is refracted and finally at an epicentral distance of 188.
A
C
E
F
D
C’
B
Figure modified after Gutenberg and Richter, 1939
Notation for seismic phasesNotation for seismic phases
Thus PKIKP is a P wave which has traveled through the mantle and both the inner and outer cores, whilst PKiKP is reflected back from the surface of the inner core. Similarly an S wave reflected at the core-mantle boundary is indicated by ScS, and if conversion occurs in reflection we have ScP.
ContinentalContinental
OceanicOceanic
Class Exercise
Write up phases of from 1 to 6?
