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_4.01 Comprehensive Overview_, G. C. Beroza, Stanford University, Stanford, CA, USA; H. Kanamori, California Institute of Technology, Pasadena, CA, USA © 2007 Elsevier B.V. All rights reserved. ### 4.01.1 Introduction In general usage, the term 'earthquake' describes a sudden shaking of the ground. Earth scientists, however, typically use the word 'earthquake' 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 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 earthquakes makes them intrinsically difficult to study. Different aspects of the earthquake process span a tremendous range in length scales — from the size of individual mineral grains to the size of the largest plates. They also span a tremendous range in timescales, with micro-earthquakes rupturing faults for only a small fraction of a second and even the very largest earthquakes lasting hundreds of seconds. Compare this with the length of strain accumulation in the earthquake cycle, which can be measured in decades, centuries, and even millennia 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 regularity (or irregularity) with which they occur 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. A level of sudden destruction that no other catastrophe in recorded history — either natural or human made — has attained. Our inability to predict earthquakes is one reason they cause such apprehension. This lack of a precursory warning is compounded by the fact that they strike so abruptly. No one can see an earthquake coming and it is only a matter of seconds from the initial perception of the first arriving waves of a large earthquake before dangerous strong ground motion begins. Moreover, large, damaging earthquakes occur infrequently (fortunately) at any given point on the Earth relative to a human lifespan. This means that most people who experience a major earthquake are doing so for the first time. Finally, there is something fundamentally unsettling about the movement of the 'solid' earth. The unpredictability, sudden onset, and their unfamiliarity make earthquakes a uniquely terrifying phenomenon. As testament to this, other extreme and catastrophic events in the affairs of humankind — if they are devastating enough — are described with the simile, 'like an earthquake'. The point of origin of an extreme event of any kind is often described as 'the epicenter', a term borrowed from seismology. The unpredictability of earthquakes also renders them difficult to study. Since we do not know where and particularly when large earthquakes will strike, collecting data on earthquakes has to be approached passively. Seismologists deploy instruments to measure seismic waves where they expect earthquakes to occur and then wait for nature to carry out the experiment. The wait can last decades or more for a large earthquake. Inevitably, with finite budgets and finite patience, this leads to seismic monitoring instruments being widely and hence too thinly dispersed in an attempt to gather data from at least some earthquakes, wherever and whenever they might occur. Finally, the combination of unpredictability, sudden onset, long intervals between events, and unfamiliarity means that the risk created by earthquake hazards is extremely difficult for both policymakers and the general public to contend with. Because earthquakes are not predicted and occur infrequently relative to other hazards, it is understandably tempting for governments and individuals to focus on the many immediate and predictable problems that impact society more frequently. The unpredictability and sudden onset of earthquakes, however, mean that once an earthquake begins, it is too late to do much more than duck and cover. ### 4.01.2 Seismicity #### 4.01.2.1 Earthquake Size Perhaps the best known earthquake source parameter is magnitude. Earthquake magnitude, and the characteristics of earthquake behavior that it is used to define, such as Gutenberg–Richter statistics (discussed further in Section 4.01.2.5), are purely empirical observations in the sense that they rely only on measurements of seismic waves as recorded on seismographs and do not require much in the way of assumptions about earthquake source physics. It is impossible to completely represent a complex physical process like an earthquake with a single number that measures its size. Nevertheless, several definitions of earthquake size have proven extremely useful for reaching a better understanding of earthquakes. Earthquake size is traditionally measured by one of various magnitude scales. ML, the local magnitude scale, was devised by Richter in the early 1930s (Richter, 1935). He was cataloging data from the southern California seismic network, and although locations, depths, and origin times for many earthquakes were available, there was no measurement of earthquake size. So, he invented one using the relation, \[ ML \approx \log_{10} A - \log_{10} A_0 \] where \(A\) is the measured amplitude of the seismic trace on a standardized Wood–Anderson seismograph (Figure 2) at a distance of 100 km, and \(A_0\) is the amplitude of a reference earthquake with ML = 0. The particular definition of the zero level is that an earthquake of magnitude 0 has an amplitude of 0.000 001 m at a distance of 100 km. Thus, an earthquake of magnitude 3.0 has an amplitude of 1 mm at the same distance and a magnitude 7.0 earthquake would have an amplitude of 10 m. Wood–Anderson instruments record on photographic paper and saturate at an amplitude of about 20 cm. Until recently, quoted magnitudes for local earthquakes were... Ключевые слова: e, r, o