ÖNCEL AKADEMİ: SOLID EARTH GEOPHYSICS
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Seismic Waves and Earth’s Interior
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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
- 5. Composition of the Crust See for more detail at pp. 513, Fowler-2005
- 6. How can we infer structure at depth?
- 20. 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
- 22. Notation 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.
- 23. Class Exercise Write up phases of from 1 to 6?
Editor's Notes
- Pages 326-331
- The continental crust, despite its complexity and variation, has a fairly standart “average” composition (see Table 10.1, pp.514). This composition is more slica-rich than that of oceanic basalts. In general, the composition of the continental upper crust is similar to that of granodiorite, and the lower crust is probably granulite. pp.252- Richter-1959 The term crust or continental crust has been used with various meanings. In this book, “crust” means only that part of the earth which is above the Mohorivicic discontinuity,
- Oceanic Crust: The pressure at an arbitrary depth in mantle is the same beneath continent and oceans. This means that, at arbitrary depth in the mantle, a column of continental crust and underlying mantle and a column of oceanic crust and its underlying mantle have the same mass. This fact enables us to make a simple estimate of the thickness of the oceanic crust (see Section 5 for the method). If we assume Airy type compensation, densities of those things: Water: 1.03x 10E3 kgm-3 Crust: 2.9x10E3 kgm-3 Mantle: 3.3 E3 kgm-3 An overage ocean depth of 7 km, then a typical 35 km thick continental crust would be isostatic equilibrium with an oceanic crust 6.6 km in thickness. This rough calculation tells us the important fact that oceanic crust is approximately one-fith the thickness of the continental crust.
- The most directly way to determine the composition of the oceanic crust is to collect rock samples from each of the oceanic plates. Dredging samples from seabed is not particularly difficult or expensive but it is often frustrating: trying to man oeuvre a large bucket, hanging on 5 km of wire, over a scarp slope, which you can see only the ship’s echo sounder, and then attempting to collect rocks from the debris at the scarp base. Drilling into oceanic crust is an expensive and difficult operation compared with drilling ship and the top of the drill hole. Not only is the rock hard and frequently fractured, but also there are many kilometers of sea water between the drilling ship and the top of the drill hole. Drilling , which started in 1968 as the Deep Sea Drilling Project (DSDP), in 1985 entered a new phase as the Ocean Drilling Program (ODP) and then in 2003 became the Integrated Ocean Drilling Program (IODP), has tremendously advanced our knowledge of the geological and geophysical structure of the uppermost crust.
- 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 . The seismic-velocity structure of the crust is determined from long seismic-refraction lines. The advent of deep reflection lines has delineated the fine structure of the crust very well, but such data usually can not yield accurate velocity estimates (see 4.5.3). Teleseismic earthquake recordings can be used to confirm gross crustal and upper mantle interfaces through the use of P to S mode conversions. The technique is referred t as the “ receiver function ” method since interfaces are identified for each “receiver” or seismograph location. The direct wave which travels in the crystalline, continental basement, beneath surface soli and sedimentary cover, termed Pg, normally travels with a velocity of about 5.9-6.2 kms-1. The velocity of upper 10 km of crust is usually in the range 6.0-6.3 kms-1; beneath that, in the middle crust, the velocity generally exceeds 6.5 km s-1.
- The upper most mantle is very heterogeneous, its structure being dependent upon plate process and history. There does not seem to be a universal discontinuity at 220 km , but the region above 220 km is sometimes referred to as lid . Standard velocity models vary in representation of upper mantle depending upon the data used and assumptions made. A low velocity zone for S-waves down to about 220 km is well established by the surface-wave-dispersion data . Beneath LVZ, P- and S-wave velocities increase markedly until about 400 km depth. At depths of 400 and 670 km, there are sharp changes in velocity; both P- and S-wave velocities increase by 5%-7%. Earthquake activity in subduction zones ceases at about 670 km depth , and this depth is also commonly taken as the boundary between upper and lower mantle. The entire region between 400 and 670 km depth is often mantle-transition zone. The lower mantle at depths down to 2700 km is referred to as the D’ shell. The lower most 150-200 km of the mantle (~2700-2900 km) is referred to as the D” shell. This could be due to chemical heterogeneity and interaction between the core and mantle and/or to a thermal boundary layer that would conduct, not convect, heat (refer to Sections 8.2 and 8.3).
- Sci-Tech Encyclopedia: Olivine It is generally accepted that the Earth's upper mantle consists mainly of olivine, an orthorhombic silicate with the composition (Mg 1.8 ,Fe 0.2 )SiO 4 , together with some pyroxene and garnet . The natural occurrence of two high-pressure forms (polymorphs) of olivine—orthorhombic wadsleyite, and cubic ringwoodite (with a spinel structure)—was predicted from high-pressure experiments and was later confirmed by meteorite investigations. The names olivine, wadsleyite, and ringwoodite refer only to naturally occurring compositions [(Mg,Fe) 2 SiO 4 ]. Sci-Tech Encyclopedia: Spinel Any of a family of important AB 2 O 4 oxide minerals, where A and B represent cations. Spinel minerals are widely distributed in the earth, in meteorites , and in rocks from the Moon. While the ideal spinel formula is MgAl 2 O 4 , some 30 elements, with valences from 1 to 6, are known to substitute in the A or B cation sites, resulting in well over 150 synthetic compounds having the spinel crystal structure. The term spinel is derived from spina (Latin, thorn) in reference to its pointed octahedral, crystal habit, and also to its dendritic snowflake form in rapidly chilled high-temperature slags and lavas.
- See pp. 329, Fowler-2005
- Lehman 5144 Fe solid against FeO, FeS fluid (inner/outer core boundary) Gutenberg: 2885, it was discovered by Oldham but corrected depth for this discontinuity was carried out by Gutenberg. D'' 2870 thin, mixing of mantle and core material? (D”=D double-prime) 670 km 670 worldwide, no earthquakes deeper, debates over whether a composition, phase, or viscosity change 400 km 400 worldwide LVZ 50-200 regionally variable depth Moho 4-55 (abbreviation of Mo-ho-RHO-vi-chik) - sharp compositional change Conrad ? 5-30 mafic to felsic crust, often absent
- Richard Dixon Oldham Richard Dixon Oldham ( July 31 , 1858 – July 15 , 1936 ) was a British geologist who, in 1906 , argued that the Earth must have a molten interior as S waves were not able to travel through liquids nor through the Earth's interior. Inge Lehmann Inge Lehmann ( May 13 , 1888 – February 21 , 1993 ), Fellow of the Royal Society ( London ) 1969 , was a Danish seismologist who, in 1936 , argued that the Earth must not only have a molten
- Ray paths for PKIKP, the direct P-wave passing through the mantle, outer core and inner core (1959). pp.254-Richter-1959 The surface of the core, when finally found by Gutenberg, proved to be halfway down to the center. Nevertheless, the core is large; it is larger than the planet Mars, and its radius is a little greater than diameter of the moon.
- P waves are refracted at the Mantle-Outer Core interface S waves are stopped at this interface Both create shadow zones and S wave attenuation implies a fluid outer core
- P waves are refracted at the Mantle-Outer Core interface S waves are stopped at this interface
- 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. 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)).
- Modified after Gutenberg and Richter, 1939