Econstudentlog

Geophysics (I)

“Geophysics is a field of earth sciences that uses the methods of physics to investigate the physical properties of the Earth and the processes that have determined and continue to govern its evolution. Geophysical investigations cover a wide range of research fields, extending from surface changes that can be observed from Earth-orbiting satellites to unseen behaviour in the Earth’s deep interior. […] This book presents a general overview of the principal methods of geophysics that have contributed to our understanding of Planet Earth and how it works.”

I gave this book five stars on goodreads, where I deemed it: ‘An excellent introduction to the topic, with high-level yet satisfactorily detailed coverage of many areas of interest.’ It doesn’t cover these topics in the amount of detail they’re covered in books like Press & Siever (…a book which I incidentally covered, though not in much detail, here and here), but it’s a very decent introductory book on these topics. I have added some observations and links related to the first half of the book’s coverage below.

“The gravitational attractions of the other planets — especially Jupiter, whose mass is 2.5 times the combined mass of all the other planets — influence the Earth’s long-term orbital rotations in a complex fashion. The planets move with different periods around their differently shaped and sized orbits. Their gravitational attractions impose fluctuations on the Earth’s orbit at many frequencies, a few of which are more significant than the rest. One important effect is on the obliquity: the amplitude of the axial tilt is forced to change rhythmically between a maximum of 24.5 degrees and a minimum of 22.1 degrees with a period of 41,000 yr. Another gravitational interaction with the other planets causes the orientation of the elliptical orbit to change with respect to the stars […]. The line of apsides — the major axis of the ellipse — precesses around the pole to the ecliptic in a prograde sense (i.e. in the same sense as the Earth’s rotation) with a period of 100,000 yr. This is known as planetary precession. Additionally, the shape of the orbit changes with time […], so that the eccentricity varies cyclically between 0.005 (almost circular) and a maximum of 0.058; currently it is 0.0167 […]. The dominant period of the eccentricity fluctuation is 405,000 yr, on which a further fluctuation of around 100,000 yr is superposed, which is close to the period of the planetary precession.”

“The amount of solar energy received by a unit area of the Earth’s surface is called the insolation. […] The long-term fluctuations in the Earth’s rotation and orbital parameters influence the insolation […] and this causes changes in climate. When the obliquity is smallest, the axis is more upright with respect to the ecliptic than at present. The seasonal differences are then smaller and vary less between polar and equatorial regions. Conversely, a large axial tilt causes an extreme difference between summer and winter at all latitudes. The insolation at any point on the Earth thus changes with the obliquity cycle. Precession of the axis also changes the insolation. At present the north pole points away from the Sun at perihelion; one half of a precessional cycle later it will point away from the Sun at aphelion. This results in a change of insolation and an effect on climate with a period equal to that of the precession. The orbital eccentricity cycle changes the Earth–Sun distances at perihelion and aphelion, with corresponding changes in insolation. When the orbit is closest to being circular, the perihelion–aphelion difference in insolation is smallest, but when the orbit is more elongate this difference increases. In this way the changes in eccentricity cause long-term variations in climate. The periodic climatic changes due to orbital variations are called Milankovitch cycles, after the Serbian astronomer Milutin Milankovitch, who studied them systematically in the 1920s and 1930s. […] The evidence for cyclical climatic variations is found in geological sedimentary records and in long cores drilled into the ice on glaciers and in polar regions. […] Sedimentation takes place slowly over thousands of years, during which the Milankovitch cycles are recorded in the physical and chemical properties of the sediments. Analyses of marine sedimentary sequences deposited in the deep oceans over millions of years have revealed cyclical variations in a number of physical properties. Examples are bedding thickness, sediment colour, isotopic ratios, and magnetic susceptibility. […] The records of oxygen isotope ratios in long ice cores display Milankovitch cycles and are important evidence for the climatic changes, generally referred to as orbital forcing, which are brought about by the long-term variations in the Earth’s orbit and axial tilt.”

Stress is defined as the force acting on a unit area. The fractional deformation it causes is called strain. The stress–strain relationship describes the mechanical behaviour of a material. When subjected to a low stress, materials deform in an elastic manner so that stress and strain are proportional to each other and the material returns to its original unstrained condition when the stress is removed. Seismic waves usually propagate under conditions of low stress. If the stress is increased progressively, a material eventually reaches its elastic limit, beyond which it cannot return to its unstrained state. Further stress causes disproportionately large strain and permanent deformation. Eventually the stress causes the material to reach its breaking point, at which it ruptures. The relationship between stress and strain is an important aspect of seismology. Two types of elastic deformation—compressional and shear—are important in determining how seismic waves propagate in the Earth. Imagine a small block that is subject to a deforming stress perpendicular to one face of the block; this is called a normal stress. The block shortens in the direction it is squeezed, but it expands slightly in the perpendicular direction; when stretched, the opposite changes of shape occur. These reversible elastic changes depend on how the material responds to compression or tension. This property is described by a physical parameter called the bulk modulus. In a shear deformation, the stress acts parallel to the surface of the block, so that one edge moves parallel to the opposite edge, changing the shape but not the volume of the block. This elastic property is described by a parameter called the shear modulus. An earthquake causes normal and shear strains that result in four types of seismic wave. Each type of wave is described by two quantities: its wavelength and frequency. The wavelength is the distance between successive peaks of a vibration, and the frequency is the number of vibrations per second. Their product is the speed of the wave.”

“A seismic P-wave (also called a primary, compressional, or longitudinal wave) consists of a series of compressions and expansions caused by particles in the ground moving back and forward parallel to the direction in which the wave travels […] It is the fastest seismic wave and can pass through fluids, although with reduced speed. When it reaches the Earth’s surface, a P-wave usually causes nearly vertical motion, which is recorded by instruments and may be felt by people but usually does not result in severe damage. […] A seismic S-wave (i.e. secondary or shear wave) arises from shear deformation […] It travels by means of particle vibrations perpendicular to the direction of travel; for that reason it is also known as a transverse wave. The shear wave vibrations are further divided into components in the horizontal and vertical planes, labelled the SH- and SV-waves, respectively. […] an S-wave is slower than a P-wave, propagating about 58 per cent as fast […] Moreover, shear waves can only travel in a material that supports shear strain. This is the case for a solid object, in which the molecules have regular locations and intermolecular forces hold the object together. By contrast, a liquid (or gas) is made up of independent molecules that are not bonded to each other, and thus a fluid has no shear strength. For this reason S-waves cannot travel through a fluid. […] S-waves have components in both the horizontal and vertical planes, so when they reach the Earth’s surface they shake structures from side to side as well as up and down. They can have larger amplitudes than P-waves. Buildings are better able to resist up-and-down motion than side-to-side shaking, and as a result SH-waves can cause serious damage to structures. […] Surface waves spread out along the Earth’s surface around a point – called the epicentre – located vertically above the earthquake’s source […] Very deep earthquakes usually do not produce surface waves, but the surface waves caused by shallow earthquakes are very destructive. In contrast to seismic body waves, which can spread out in three dimensions through the Earth’s interior, the energy in a seismic surface wave is guided by the free surface. It is only able to spread out in two dimensions and is more concentrated. Consequently, surface waves have the largest amplitudes on the seismogram of a shallow earthquake […] and are responsible for the strongest ground motions and greatest damage. There are two types of surface wave. [Rayleigh waves & Love waves, US].”

“The number of earthquakes that occur globally each year falls off with increasing magnitude. Approximately 1.4 million earthquakes annually have magnitude 2 or larger; of these about 1,500 have magnitude 5 or larger. The number of very damaging earthquakes with magnitude above 7 varies from year to year but has averaged about 15-20 annually since 1900. On average, one earthquake per year has magnitude 8 or greater, although such large events occur at irregular intervals. A magnitude 9 earthquake may release more energy than the cumulative energy of all other earthquakes in the same year. […] Large earthquakes may be preceded by foreshocks, which are lesser events that occur shortly before and in the same region as the main shock. They indicate the build-up of stress that leads to the main rupture. Large earthquakes are also followed by smaller aftershocks on the same fault or near to it; their frequency decreases as time passes, following the main shock. Aftershocks may individually be large enough to have serious consequences for a damaged region, because they can cause already weakened structures to collapse. […] About 90 per cent of the world’s earthquakes and 75 per cent of its volcanoes occur in the circum-Pacific belt known as the ‘Ring of Fire‘. […] The relative motions of the tectonic plates at their margins, together with changes in the state of stress within the plates, are responsible for most of the world’s seismicity. Earthquakes occur much more rarely in the geographic interiors of the plates. However, large intraplate earthquakes do occur […] In 2001 an intraplate earthquake with magnitude 7.7 occurred on a previously unknown fault under Gujarat, India […], killing 20,000 people and destroying 400,000 homes. […] Earthquakes are a serious hazard for populations, their property, and the natural environment. Great effort has been invested in the effort to predict their occurrence, but as yet without general success. […] Scientists have made more progress in assessing the possible location of an earthquake than in predicting the time of its occurrence. Although a damaging event can occur whenever local stress in the crust exceeds the breaking point of underlying rocks, the active seismic belts where this is most likely to happen are narrow and well defined […]. Unfortunately many densely populated regions and great cities are located in some of the seismically most active regions.[…] it is not yet possible to forecast reliably where or when an earthquake will occur, or how large it is likely to be.”

Links:

Plate tectonics.
Geodesy.
Seismology. Seismometer.
Law of conservation of energy. Second law of thermodynamics (This book incidentally covers these topics in much more detail, and does it quite well – US).
Angular momentum.
Big Bang model. Formation and evolution of the Solar System (…I should probably mention here that I do believe Wikipedia covers these sorts of topics quite well).
Invariable plane. Ecliptic.
Newton’s law of universal gravitation.
Kepler’s laws of planetary motion.
Potential energy. Kinetic energy. Orbital eccentricity. Line of apsides. Axial tilt. Figure of the Earth. Nutation. Chandler wobble.
Torque. Precession.
Very-long-baseline interferometry.
Reflection seismology.
Geophone.
Seismic shadow zone. Ray tracing (physics).
Structure of the Earth. Core–mantle boundary. D” region. Mohorovičić discontinuity. Lithosphere. Asthenosphere. Mantle transition zone.
Peridotite. Olivine. Perovskite.
Seismic tomography.
Lithoprobe project.
Orogenic belt.
European Geotraverse ProjectEuropean Geotraverse Project.
Microseism. Seismic noise.
Elastic-rebound theory. Fault (geology).
Richter magnitude scale (…of note: “the Richter scale underestimates the size of very large earthquakes with magnitudes greater than about 8.5”). Seismic moment. Moment magnitude scale. Modified Mercalli intensity scale. European macroseismic scale.
Focal mechanism.
Transform fault. Euler pole. Triple junction.
Megathrust earthquake.
Alpine fault. East African Rift.

November 1, 2018 - Posted by | Astronomy, Books, Geology, Physics

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