Treatise on Geochemistry это первая в своем роде работа, с глобальным интегрированным подходом к современному состоянию Геохимии. В Трактате освещены все основные разделы геохимии, от химии солнечной системы, до экологической геохимии. В Treatise on Geochemistry использованы труды выдающихся мировых ученых и включены ссылки на самые современные публикации. Труд включает в себя 9 томов, каждый из которых состоит из 15-25 разделов, составленных ведущими авторитетами в данной области знаний. Treatise on Geochemistry в электронном формате включает в себя систему разветвленных ссылок, облегчающих навигацию по документу.

In general usage, the term ‘earthquake’ describes a sudden shaking of the ground. Earth scientists, however, typically use the word ‘earthquake’ somewhat differently – to describe the ‘source’ of seismic waves, which is nearly always sudden shear slip on a fault within the Earth (see Figure 1). In this article, we follow the scientific usage of the term, and focus our review on how earthquakes are studied using the motion of the ground remote from the earthquake source itself, that is, by interpreting the same shaking that most people consider to be ‘the earthquake’. The field defined by the use of seismic waves to understand earthquakes is known as earthquake seismology. The nature of the earthquakes makes them intrinsically difficult to study. Different aspects of the earthquake process span a tremendous range in length scales – all the way from the size of individual mineral grains to the size of the largest plates. They span a tremendous range in timescales as well. The smallest micro-earthquakes rupture faults for only a small fractionof a second andthe durationof even the verylargest earthquakes can be measured in hundreds of seconds. Compare this with the length of strain accumulation in the earthquake cycle, which can be measured in decades, centuries, and even millenniums in regions of slow strain rate. The evolution of fault systems spans longer times still, since that can require the action of thousands of earthquakes. At different physical dimensions or temporal scales, different physical mechanisms may become important, or perhaps negligible. Earthquakes occur in geologically, and hence physically, complicated environments. The behavior of earthquakes has been held up as a type example of a complex natural system. The sudden transformation of faults from being locked, or perhaps slipping quasistatically, to slipping unstably at large slip speeds, as is nearly universally observed for earthquakes, also makes them a challenging physical system to understand. Despite these challenges, seismologists have made tremendous progress in understanding many aspects of earthquakes – elucidating their mechanisms based on the radiated seismic wavefield, determining where they occur and the deep structure of faults with great precision, documenting the frequency and the regularity (or irregularity) with which they occur (and recur) over the long-term, gaining insight into the ways in which they interact with one another, and so on. Yet, the obvious goal of short-term prediction of earthquakes, that is specifying the time, location, and size of future significant earthquakes on a timescale shorter than decades, remains elusive. Earthquakes are different in this sense from nearly all other deadly natural hazards such as hurricanes, floods, and tornadoes, and even volcanic eruptions, which to varying degrees are predictable over a timescale of hours to days. The worst earthquakes rank at the very top of known disasters. The deadliest known earthquake killed over half a million people in a matter of minutes.

It has been known since the pioneering work of Joseph Barrell during the early part of the last century that the outermost layers of the Earth comprise a strong upper layer, the lithosphere, which overlies a weak lower layer, the asthenosphere. Barrell (1914a) argued that because river deltas such as the Niger and Nile lack a flanking topographic depression, they must be supported by the strength of the lithosphere. He used (Barrell, 1914b) Pratt isostatic gravity anomalies over North America as a proxy for the magnitude of the stress differences that could be supported by the lithosphere and showed, using the equations of Darwin (1882), that stresses increase and then decrease with depth, passing by transition into the weak underlying asthenosphere. Today, we distinguish the lithosphere from the asthenosphere not only on the basis of its strength, but its physical properties such as temperature, density, and seismic velocity structure. The lithosphere, for example, is generally associated with cooler temperatures, higher average densities, and higher average S-wave velocities than the asthenosphere. Plate tectonics is based on the assumption that the lithosphere is rigid on long timescales and is moving across the surface of the Earth with the plates. The positive density contrast between the lithosphere and the asthenosphere suggests, however, that the rigid layer may be gravitationally unstable. Indeed, oceanic lithosphere – after it is created at a mid-oceanic ridge – cools, subsides, and sinks into the underlying asthenosphere, for example, at a deep-sea trench–outer-rise system.Continental lithosphere may also be unstable. In rifts (e.g., East Africa) the lithosphere is regionally heated, thinned, and uplifted and only subsides locally below sea level. In collisional systems (e.g., Himalaya, Betics), however, continental lithosphere is thickened (Molnar et al., 1998) or is infiltrated by fluids released during metamorphic reactions (Le Pichon et al., 1997). Both processes may cause dense rocks of the lower crust to enter the eclogite stability field. As a result, the lower crust becomes denser than the underlying mantle, detaches, and, as at trenches, may sink into the underlying asthenosphere. Isostatic considerations, however, suggest that the crust – which comprises the uppermost part of the lithosphere – is buoyant and is in a state of flotation on the underlying mantle. Furthermore, flexure studies suggest that when it is subject to long-term geological loads such as volcanoes and sediment, the lithosphere, rather than behaving as a number of independent floating blocks, as local models of isostasy such as Airy and Pratt predict, responds by bending – in a similar manner as would an elastic plate that overlies an inviscid fluid substrate.

Modern geodesy as discussed in this volume started with the development of distance measurement using propagating electromagnetic signals and the launch of Earth-orbiting satellites. With these developments, space-based geodesy allowed global measurements of positions, changes in the rotation of the Earth, and the Earth’s gravity field. These three areas (positioning, Earth rotation, and gravity field) are considered the three pillars of geodesy. The accuracy of current measurement systems allows time variations to be observed in all three areas. Also the complexity of problems is such that each of the pillars interacts with each other and also with many other branches of Earth Science. This interaction is most apparent in the role that water plays in modern geodetic measurements. Every chapter in this volume mentions the role of water. It is critical because it can move rapidly and over large distances; it can exist in all three phases, gas, fluid, and solid; and modern geodetic methods are accurate enough that their measurements are sensitive to its effects. In its vapor form, its refractive properties delay microwave signals propagating through the Earth’s atmosphere. For geodetic positioning, this is a noise source but it is a signal for metrological applications. In the liquid form, it forms oceans that affect both the tidal signal and the rotation of the Earth. Also in liquid form, its mass changes the gravity field as it is moved through the hydrologic cycle. In solid form, it has a gravitational and deformation signal that changes if melting of the ice unloads the surface of the Earth. The interactions between the pillars include the elastic loading effects of changing mass loads that can be seen in the gravity field and in the positions of ground stations. The movement of water to and from the oceans can be seen with altimeter satellites whose orbital information is derived from measurements from ground stations whose positions are affected by the changing mass load. In modern, time-dependent geodetic data analysis these interactions need to be accounted for. The common interface between the geodetic methods is the coordinate systems and reference frames used to analyze data. Coordinate systems and the associated reference frames form a core theme of the chapters in this volume. Two other unifying themes are the measurement systems of geodesy that are used again throughout the volume, and interplay of errors in measurements, signals, and noise. In this chapter weexamine these three themes.