Seismic Waves and Earth’s Interior

Solid Earth Geophysics-Geop503

Ali Onceloncel@kfupm.edu.

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?