mechanics have led to constitutive relations and dynamical models that have found applications in materials engineering. The power-law scaling relations between earthquake frequency and size, combined with the recognition that nearly all of the Earth’s crust may be close to its critical state of failure, have stimulated theories of nonlinear system dynamics (14).
The fundamental interactions that govern active faulting are distributed over an enormous range of spatial and temporal scales—sequences of great earthquakes on thousand-kilometer faults over hundreds of years are coupled dynamically to deformation processes operating in milliseconds over millimeters. These processes, difficult to study directly in the field, can be replicated in the laboratory only at the lower end of this range. Sampling and in situ observation by trenching, tunneling, and drilling are confined to the outer few kilometers. Understanding the physical processes at the much greater depths where earthquakes typically nucleate depends on the ability to construct models that combine surface observations with seismic imaging and other remote-sensing data.
Earthquakes in nearly all tectonic environments share similar scaling laws (e.g., frequency-magnitude statistics, aftershock decay rates, stress drops), which suggests that some of the most basic aspects of earthquake behavior are universal and not sensitive to the details of deformation processes. However, differing theories have sparked considerable controversy about how small-scale processes such as rock damage and fluid flow are involved in active faulting. A comprehensive theory must integrate earthquake behavior across all dimensions of the problem. This challenge is a primary motivation for the National Science Foundation’s ambitious EarthScope initiative, which will employ four new technologies for observing active deformation in the United States over a wide range of scales (15).
Educating people about earthquakes can effectively reduce human and economic losses during seismic disasters. The pedagogical responsibilities of earthquake science are expanding as concerns about the vulnerability of the built environment increase. The issues encompass the delivery of earthquake information to the public; earthquake-oriented curricula in schools at all levels; the career focus of young researchers, and the transfer of knowledge to engineers, emergency managers, and government officials.
The Internet has greatly enhanced the capability for delivering a wide variety of earthquake information. Many public and private organizations maintain web sites that host a variety of earthquake information services. These sites display up-to-the-minute maps of earthquake epi-