ÖNCEL AKADEMİ: SOLID EARTH GEOPHYSICS
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This document discusses seismic waves and their interactions with Earth's interior structures. It describes how observations of P-wave and S-wave shadow zones provide evidence for Earth's liquid outer core. The P-wave shadow zone between 103-143 degrees indicates the core deflects P-waves, traveling slower within the liquid core. The full S-wave shadow zone beyond 103 degrees suggests the core does not allow S-waves, indicating it is liquid. Various seismic phases are also described, such as PcP, PKP, that involve reflections or refractions at the core-mantle boundary.
![Solid Earth Geophysics-Geop503 Ali Oncel [email_address] Department of Earth Sciences, KFUPM Seismic Waves and Earth’s Interior Reading: Fowler Chapter 8- Section 8.1](https://image.slidesharecdn.com/lecture05-091219225616-phpapp02/85/ncel-akademi-solid-earth-geophysics-1-320.jpg)
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ÖNCEL AKADEMİ: SOLID EARTH GEOPHYSICS
- 1. Solid Earth Geophysics-Geop503 Ali Oncel [email_address] Department of Earth Sciences, KFUPM Seismic Waves and Earth’s Interior Reading: Fowler Chapter 8- Section 8.1
- 7. Class Exercise Write up phases of from 1 to 6?
- 8. Low velocity zone Distance ( ) V 1 > V 2 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 103 143
- 11. Low-Velocity Layer 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.
Notes de l'éditeur
- http://physics.fortlewis.edu/Astronomy/astronomy%20today/CHAISSON/AT307/HTML/AT30703.HTM
- Existence of Core
- S-waves within Core (see. Pp.168, Stein): Some core phases begin as S waves. Although no S waves propagate in the liquid outer core, phases like SKS travel through mantle as an S wave and through the core as a P-wave. SKKS is a similar to SKS, but also involves an underside reflection at the CMB. Because the P velocity of the uppermost core (about 8.1 km/s) is not much larger than the S velocity of lower mantle (about 7.2 km/s), SKS and SKKS waves do not change direction significantly as they cross the CMB. Thus SKS, SKKS, SKKKS, etc. are the only waves that bottom near the top of the core and are used to constrain the outer core’s velocity structure.
- The shadow zone resulting from a low velocity zone. As an example, consider a two layered sphere for which the seismic velocity increases gradually with depth each layer. The seismic velocity immediately above the discontinuity in the upper layer is V1 and that immediately below the discontinuity is V2. 103 The ray paths for the case V2>V1 (the velocity increases at the discontinuity) are shown in a. If V2 < V1 (the velocity decreases at the discontinuity, resulting in a low velocity zone at the top of the second layer, then the ray refracted into the inner layer bends towards the normal (Snell’s law), yielding the ray paths shown in ( c ). The travel time curves for ( a ) and ( c ) are shown in ( b ) and ( d ), respectively. When V2>V1, arrivals are recorded at all distances, but when V2<V1, there is a distance interval over the shadow zone. The angular extent of the shadow zone (b to B) are dependent on the depth and extent of low-velocity zone and on the reduction of velocity in the velocity zone (After Gutenberg (1959)).
- Figure 3.5-7 of Stein-2003, pp. 168: It shows that the travel time curve for the core phases is complicated because it combines the effects of a geometric shadow zone, which gives two PKP branches, a triplication-like feature containing the PKIKP and PKiKP branches, and a diffraction branch. In reality, even these models are simplifications of a more complex reality.