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.

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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A frequency-dependent full wave theory is successfully employed to synthesize long-period seismograms of the core phases SmKS (m = 1,2, . . .) in the distance range 100' --125". Body-wave displacements are calculated by numerically integrating in the complex ray parameter plane. Langer's method is employed to obtain a uniformly asymptotic approximation to the vertical wave functions. Plane-wave reflection and transmission coefficients are adequately corrected for the effect of the curvature at the core -mantle discontinuity by the use of generalized cosines. Results are presented in the time domain, after a numerical Fourier (inverse) transform.The computed seismograms exhibit many non-ray effects that the SmKS incur upon interacting with the core--mantle boundary. For SKS, the amplitude, group delay and phase delay are very strong functions of frequency at less than 0.5 Hz, both because of the frequency dependence of the reflection/ transmission coefficients at the core--mantle boundary, and because of the presence of diffracted energy, called SP(diff)KS, perturbing the waveform. The diffracted energy of the type that perturbs SKS may also interact with shear waves to give rise to a precursor to the body-wave ScS, called SP(diff)S. The major complication in synthesizing the portion of the seismogram containing SmKS for m > 2 is that the arrival time of each successively higher order reflection is within the waveform of previous lower order reflections. It is found that a summation of body-wave displacements from S2KS to SISKS gives an adequate seismogram in the distance range 100" -125". Each individual reflection has an amplitude spectrum, group delay and phase delay which are strongly frequency-dependent at less than 0.2 Hz. It is shown that arrival times for SmKS, m 2 2 , cannot be picked accurately by conventional methods. Furthermore, neglecting the frequency-dependence of reflection/ transmission coefficients can significantly distort the interpretation of amplitude and phase data.The seismograms generated by this method agree so remarkably well with observed records that the synthetic waveforms provide a powerful test of the validity of particular earth models. In particular, we find that the waveforms

S U M M A R YThe behavior of apparent stress for normal-fault earthquakes at subduction zones is derived by examining the apparent stress (τ a = µE S /M 0 , where E S is radiated energy and M 0 is seismic moment) of all globally distributed shallow (depth, h < 70 km) earthquakes with normal-fault mechanisms that occurred in or near subduction zones between 1987 and 2001 for which E S and M 0 are available. Accurately determined hypocentres from the Engdahl-HilstBuland (EHB) catalogue establish the fine detail of the Wadati-Benioff zone. In many cases, we can relate trends in apparent stress to specific features within the subduction zone and compare these with trends for interplate-thrust earthquakes in the same subduction zones. There are two depth ranges over which τ a for normal-fault earthquakes shows maxima. The highest and most anomalous values of τ a are found in the deeper depth range from 35 to 70 km. The high-τ a events (up to 5 MPa) are characteristically intraslab and in proximity to zones of intense deformation such as a sharp slab bend or where opposing slabs collide. High-τ a events in the region of the shallower maximum (hypocentres between 10-35 km and τ a > 1 MPa) are also generally intraslab, but occur where the lithosphere has just begun subduction beneath the overriding plate. They usually occur in cold slabs near trenches where the direction of plate motion across the trench is oblique to the trench axis, or where there are local contortions or geometrical complexities of the plate boundary. Lower τ a (<1 MPa) is associated with events occurring at the outer rise (OR) complex (between the OR and the trench axis), as well as with intracrustal events occurring just landward of the trench. The average apparent stress of intraslab-normal-fault earthquakes is considerably higher than the average apparent stress of interplate-thrust-fault earthquakes. In turn, the average τ a of strike-slip earthquakes in intraoceanic environments is considerably higher than that of intraslab-normal-fault earthquakes. The variation of average τ a with focal mechanism and tectonic regime suggests that the level of τ a is related to fault maturity. Lower stress drops are needed to rupture mature faults such as those found at plate interfaces that have been smoothed by large cumulative displacements (from hundreds to thousands of kilometres). In contrast, immature faults, such as those on which intraslab-normal-fault earthquakes generally occur, are found in cold and intact lithosphere in which total fault displacement has been much less (from hundreds of metres to a few kilometres). Also, faults on which high τ a oceanic strike-slip earthquakes occur are predominantly intraplate or at evolving ends of transforms. At subduction zones, earthquakes occurring on immature faults are likely to be more hazardous as they tend to generate higher amounts of radiated energy per unit of moment than earthquakes occurring on mature faults. We have identified earthquake pairs in which an interplate-thrust and an intraslab-normal e...

The cataloging of earthquakes of m b (USGS) 5.1 and larger is essentially complete for the time period except for the first half-day following the 26 December mainshock, a period of about two hours following the Nias earthquake of 28 March 2005, and occasionally during the Andaman Sea swarm of 26-30 January 2005. Moderate and larger (m b Ն5.5) aftershocks are absent from most of the deep interplate thrust faults of the segments of the Sumatra-Andaman Islands subduction zone on which the 26 December mainshock occurred, which probably reflects nearly complete release of elastic strain on the seismogenic interplate-thrust during the mainshock. An exceptional thrust-fault source offshore of Banda Aceh may represent a segment of the interplate thrust that was bypassed during the mainshock. The 26 December mainshock triggered a high level of aftershock activity near the axis of the Sunda trench and the leading edge of the overthrust Burma plate. Much near-trench activity is intraplate activity within the subducting plate, but some shallow-focus, near-trench, reverse-fault earthquakes may represent an unusual seismogenic release of interplate compressional stress near the tip of the overriding plate. The interplate-thrust Nias earthquake of 28 March 2005, in contrast to the 26 December aftershock sequence, was followed by many interplate-thrust aftershocks along the length of its inferred rupture zone.

On January 22, 1988, three large intraplate earthquakes (with MS 6.3, 6.4, and 6.7) occurred within a 12‐hour period near Tennant Creek, Australia. These earthquakes, which occurred over a small interval of time and within a small volume of space, present a unique opportunity to study the rupture process of the class of intraplate earthquakes that occur as multiple main shocks. Broadband displacement and velocity records of body waves from teleseismically recorded data are analyzed to determine source mechanisms, depths, and complexity of rupture of each of the three main shocks. Hypocenters of an additional 150 foreshocks and aftershocks constrained by local arrival time data and field observations of surface rupture are used to complement the source characteristics of the main shocks in order to derive as complete a description of the rupture process as possible. The interpretation of the combined data sets suggests that the overall rupture process involved unusually complicated stress release. As locations of the main shock hypocenters progressively moved from west to east, we infer that the first and third main shocks, denoted as MS1 and MS3, produced the southeast‐northwest trending scarps observed at the western end (the Kunayungku fault) and at the eastern end (the east end of the Lake Surprise fault), respectively, of the rupture zone. The epicenter of the only immediate foreshock was located in the gap between these two fault scarps. MS1 nucleated near this epicenter and ruptured upward and to the northwest from a depth of 6.5 km. MS3 ruptured predominantly to the SE at a depth of 4.5 km. The second main shock, MS2, is inferred to have produced the deformation of the southwest trending central scarp segment (the western end of the Lake Surprise fault). From the sense of thrusting seen at the surface and from the distribution of aftershock hypocenters, the south dipping nodal planes derived from waveform modeling are identified as the fault planes for earthquakes MS1 and MS3. In contrast, the dip of the central fault scarp is reversed relative to the dips of the western and eastern fault scarps. The rupture process Of MS2 turns out to be commensurately complex and sufficiently explains the geological complexity. MS2 consisted of three subevents. The southeast dipping nodal plane of the first two subevents is coplanar with a southeast dipping plane implied by locations of aftershocks which did not break the surface. Choice of the north dipping plane as the rupture plane of the third subevent, consistent with the surface deformation and coplanar with a second plane delineated by aftershocks, would imply conjugate faulting. The majority of the aftershocks are concentrated near the edges of the fault planes, and there is an absence of activity in the center of the planes. The areas of absent activity may represent the failed asperities of the main shocks in which substantial stress relief occurred. The rupture process of each main shock is characterized by the rapid release of energy followed by a much slower...