seismic waves and earth’s interior

Solid Earth Geophysics-Geop503

Ali Onceloncel@kfupm.edu.sa

Department of Earth Sciences, KFUPM

Seismic Waves and Earth’s Interior

Reading: Fowler Chapter 8- Section 8.1

Summary: Lecture 4

Central Core: P-wave Shadow Zone

P-wave slower in liquid and downwardly refracted. Thus, the shadow zone is an evident that the Earth has a central Core (5100 km), which deflects the P-waves.

P-waves travel much more slowly through the core, so they are refracted inwards. Thus, more detailed observations show that some weak p-waves are actually observed within the shadow zone at about 120°from the epicenter.

What is some of possible inferences for P-wave shadow zone (103-143°) for?

What is the speed of P-wave through the core?

S-wave Shadow Zone

What is some of possible inferences for S-wave shadow zone (>103, no direct S waves) for?

Core must be liquid which produces a

large shadow zone covering nearly half

the earthThis cannot be

explained by seismic refraction - the core

simply does not allow s-waves to pass

Liquid Core: S-wave Shadow Zone

P-wave shadow zone S-wave shadow zone

Overview for Shadow Zones

Core by P-wave Liquid Core by S-wave

Overview: Body Wave

Phases

P and S: Direct ray paths p and s: Upgoing paths SP: S wave through the mantle reflected at the surface as P. c: Reflection at the core boundary PcP: P wave reflected at the core.

PcS: P wave reflected at the core as S. i: Reflection at he inner core’s boundary.

Stein (2003)

Class Exercise

Write up phases of from 1 to 6?

Low velocity zone

Distance ( )

V1 > V2

What is the relation between the shadow zone and low velocity zone?

or example, v(r) behaviour is no longer simple.

See Figure 8.2, Fowler-2005

103 143

Core PhasesCore reflections are of great interest because the core-mantle boundary (CMB) is a solid-liquid boundary, and thus a strong reflector for shear-waves (pp.166, Stein-2003).

Top Left: Paths for direct waves (i.e., excluding reflections and diffractions). Right: Ray paths for others phases:

PKP: passes outer core PKIKP: passes through the outer core. PKiKKP: reflects from the boundary between outer cores and inner cores. Pd: diffracts the along the core-mantle boundary.

Stein (2003)

http://wwwrses.anu.edu.au/seismology/SHon2002/

Guttenberg (1959) inferred its existence from changes in the amplitude of arrivals, at distances of around 15o, which he attributed to the defocusing effect of a low-velocity region.

There are two possible scenarios that produce hidden layers: Low velocity layers and thin layers underlain by a large velocity contrast.

Layers that can not be distinguished from first arrival time information are known as hidden layers.

Low-Velocity Layer