Radiated energies from shallow earthquakes with magnitudes ≥5.8 that occurred between 1986 and 1991 are used to examine global patterns of energy release and apparent stress. In contrast to traditional methods which have relied upon empirical formulas, these energies are computed through direct spectral analysis of broadband seismic waveforms. Velocity‐squared spectra of body waves are integrated after they have been corrected for effects arising from depth phases, frequency‐dependent attenuation, and focal mechanism. The least squares regression fit of energy Es to surface wave magnitude Ms for a global set of 397 earthquakes yields log Es = 4.4 + 1.5Ms, which implies that the Gutenberg‐Richter relationship overestimates the energies of earthquakes. The least squares fit between Es and seismic moment M0 is given by the relationship Es = 1.6 × 10−5 M0, which yields 0.47 MPa as the average global value of apparent stress. However, the regression lines of both Es–Ms and Es–M0 yield poor empirical predictors for the actual energy radiated by any given earthquake; the scatter of data is more than an order of magnitude about each of the regression lines. On the other hand, global variations between Es and Mo, while large, are not random. When subsets of Es–M0 are plotted as a function of seismic region and faulting type, the scatter in data is substantially reduced. The Es–M0 fits for many seismic regions and tectonic environments are very distinctive, and a characteristic apparent stress τc can be derived. The lowest apparent stresses (<1.5 MPa) are associated with thrust earthquakes at subduction zones. The highest apparent stresses (>3.0 MPa) are associated with strike‐slip earthquakes that occur at oceanic ridge‐ridge transforms and in intraplate environments seaward of island arcs. Intermediate values of apparent stress (1.5 < τa < 3.0 MPa) are associated with strike‐slip earthquakes at incipient or transitional plate boundaries. In general, the dominant mode of failure for a tectonic environment is associated with the faulting type that has the lowest apparent stress. An energy magnitude ME can complement moment magnitude Mw in describing the size of an earthquake. ME, being derived from velocity power spectra, is a measure of seismic potential for damage. Mw, being derived from the low‐frequency asymptote of displacement spectra, is more physically related to the final static displacement of an earthquake. When earthquake size is ranked by moment, a list of the largest events is dominated by earthquakes with thrust mechanisms. When earthquake size is ranked by energy, the list of the largest events is dominated by strike‐slip earthquakes.

We consider how variations in fault frictional properties affect the phenomenology of earthquake faulting. In particular, we propose that lateral variations in fault friction produce the marked heterogeneity of slip observed in large earthquakes. We model these variations using a rate‐ and state‐dependent friction law, where we differentiate velocity‐weakening behavior into two fields: the strong seismic field is very velocity weakening and the weak seismic field is slightly velocity weakening. Similarly, we differentiate velocity‐strengthening behavior into two fields: the compliant field is slightly velocity strengthening and the viscous field is very velocity strengthening. The strong seismic field comprises the seismic slip concentrations, or asperities. The two “intermediate” fields, weak seismic and compliant, have frictional velocity dependences that are close to velocity neutral: these fields modulate both the tectonic loading and the dynamic rupture process. During the interseismic period, the weak seismic and compliant regions slip aseismically, while the strong seismic regions remain locked, evolving into stress concentrations that fail only in main shocks. The weak seismic areas exhibit most of the interseismic activity and aftershocks but can also creep seismically. This “mixed” frictional behavior can be obtained from a sufficiently heterogeneous distribution of the critical slip distance. The model also provides a mechanism for rupture arrest: dynamic rupture fronts decelerate as they penetrate into unloaded complaint or weak seismic areas, producing broad areas of accelerated afterslip. Aftershocks occur on both the weak seismic and compliant areas around a fault, but most of the stress is diffused through aseismic slip. Rapid afterslip on these peripheral areas can also produce aftershocks within the main shock rupture area by reloading weak fault areas that slipped in the main shock and then healed. We test this frictional model by comparing the seismicity and the coseismic slip for the 1966 Parkfield, 1979 Coyote Lake, and 1984 Morgan Hill earthquakes. The interevent seismicity and aftershocks appear to occur on fault areas outside the regions of significant slip: these regions are interpreted as either weak seismic or compliant, depending on whether or not they manifest interevent seismicity.

The energy flux contained in the P‐wave groups (P + pP + sP) or the S‐wave groups (S + pS + sS) radiated by a shallow earthquake is modeled assuming that the energy flux in the direct and depth phases adds incoherently. By defining generalized radiation patterns which incorporate this neutral interference, the wave groups are analyzed as though they were comprised of a single phase. Measurements of the energy flux in the wave groups are corrected in the frequency domain for both the body‐wave attenuation and the frequency band of the recording. The corrected measurements are then used to estimate the seismic energy radiated by the earthquake. This analysis is applied to digital recordings of the teleseismic wave groups radiated by the May 2, 1983, Coalinga, California, earthquake and the October 28, 1983, Borah Peak, Idaho, earthquake. For the Coalinga earthquake, an estimate of Es = 1.6 ± 0.4×1021 dyn cm was determined from six P‐wave groups, while the SH wave group recorded at station COL returned an estimate of Es = 1.2×1021 dyn cm. For the Borah Peak earthquake, an estimate ofEs = 3.2 ± 0.5×1021 dyn cm was determined from seven P‐wave groups. The distribution of the isoseismals for the Borah Peak earthquake indicate that the energy radiated by this event was focussed to the northwest, in the direction of rupture propagation. Correcting the teleseismic estimate for this focussing gives Es = 3.6 ± 0.5×1021 dyn cm. Combining these estimates of the radiated energy with broad‐band estimates of the seismic moment yields estimates of the apparent stress of 17 and 7 bars for the Coalinga and the Borah Peak earthquakes, respectively.

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