Treatise on geophisics. Earthquake seismology. Volume 4/ Трактат о геофизике. Сейсмология. Том 4

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.