The energy balance of the elastodynamic field generated by a spherical cavity in a homogeneous, isotropic, and unbounded medium is studied. The near-field waves are found to play an essential role in the energy balance, transforming static energy into wave energy and vice versa, thus displaying a surprising ability to transmit wave energy in the direction opposite to wave propagation. The occurrence of such a negative wave energy clarifies the energy balance of sources, in which the waves are associated with energy dissipa- tion rather than radiation. 241 241 242 242 243 243 244 244 245 245 245 246 246 246 This Colloquium was inspired by a study of the energy balance in earthquakes. Earthquakes arise to relieve stat- ic strains accumulated in the Earth's crust over a long time, radiating their energy suddenly in the form of se- ismic waves. An earthquake is initiated by a rupture pro- cess on a fault, starting with a small area but rapidly ex- panding to the entire fault. As a result of rupture, one side begins to move along the other, creating a displace- ment discontinuity. This shear displacement is generally assumed, but is also often observed directly (a typical value of the displacement for a large earthquake is 5 m (Turcotte and Schubert, 1982)). The evolution of the dis- placement discontinuity generates a seismic wave, whose wave1ength is smaller than, or at most on the same order as, the fault's length. No larger wavelengths are radiated because a rupture front, controlling seismic wave genera- tion, propagates at a rate comparable with the velocity of seismic waves (a few kilometers per second). The static strain energy released in an earthquake is not concentrated along a fault but throughout the stressed medium surrounding it. Since the density of the strain energy decreases rapidly with the distance from the fault, most of the strain energy lies in a rather small area with a length scale no larger than 2— 3 lengths of the fault. A part of the released strain energy is expended in irreversible deformations during rupturing and in friction between the fault's sides (Freund, 1990; Kostrov and Das, 1989), the other part being radiated away into a dis- tant medium. Both parts of the energy are transported by seismic waves: the first, from a surrounding medium to the fault by short-range waves; the second, from the surrounding medium into a distant medium by long- range waves. The short-range w'aves are called the near- field waves, whose amphtude decreases with the square of 1 lr or faster; the long-range waves are called the far-field waves, whose amplitude decreases as 1 jr (Aki and Richards, 1980).