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

Mysteries of the Inner Earth

http://ourworld.compuserve.com/homepages/dp5/inner1.htm

Solid Earth Processes may contribute to the weakness of the asthenosphere. They include some phase changes which can be listed as:

dehydration or partial melting change in composition of mineralogy increase in stress or dislocation density very rapid in increase of the temperature gradient.

Upper MantleReference: Anderson, D. L., 1995. Lithosphere, asthenosphere and perisphere, Reviews of Geophysics, 33, 125-149.

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).

Probing the Earth’s

Interior with

seismic waves

Constant Velocity Variable VelocityConstant Velocity Variable Velocity

Seismic waves - uniform density

In a homogeneous planet with uniform

density seismic waves are neither refracted or

reflected Seismic ‘rays’ drawn

perpendicular to the wave fronts follow

straight lines But early seismic

observations showed the Earth is not like this

Seismic waves - increasing density Because, In a planet

where density increases gradually with depth, seismic waves are refracted, i.e. they are bent

Seismic velocities would increase steadily with depth and rays would follow a curved path

This is similar to the paths of seismic waves in the Earth - up to about 11,000 km from the epicenter

Why do seismic waves generally curve upward in Earth’s mantle?

Ray Theory 1

1

v

isin =

2

'1

v

isin

Ray Wavefront

Ray Paths and Travel Times

As the angle of incidence at the hypocenter decreases, the rays descend more steeply and to greater depth and emerge at the surface at increasing distance. When the distance corresponds to a central angle of about 103 between hypocenter and station (for ordinary shallow earthquakes) the P ray grazes the core. Stein, 2003

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.

ContinentalContinentalOceanicOceanic

Body wave Phases

After Bolt (1982)

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