Seismic Source Descriptions of Underground Explosions and a Depth Discriminate

Synthetic seismograms of both body waves and Rayleigh waves are used to determine the radiation field of a few large contained underground explosions. A number of possible source descriptions are investigated. A reduced displacement potential of the form, ø(t) = ø_0t^ξ exp(-ηt), fits the long- and short-period data. The source parameters appropriate for the Boxcar event are ξ = 0·5 and η = 0·15. Synthetic PL and Rayleigh waves are compared with observations from a number of different size events to determine the dependence of η on yield. The amplitude of the long period synthetic body wave responses at ranges greater than about 12° increases rapidly as the source depth is increased. Thus the difference in spectral properties of explosions and earthquakes can be largely explained by the depth effect. The theoretical ratio SP/LP, that is the short period divided by the long-period amplitude, is computed from 12 to 25° for the Johnson upper mantle model and the Boxcar source. A study of an earthquake which cannot be distinguished from an explosion using the m_b vs. M_s criterion is investigated by the SP/LP discriminate

Evidence for a chemical-thermal structure at base of mantle from sharp lateral P-wave variations beneath Central America

Compressional waves that sample the lowermost mantle west of Central America show a rapid change in travel times of up to 4 s over a sampling distance of 300 km and a change in waveforms. The differential travel times of the PKP waves (which traverse Earth's core) correlate remarkably well with predictions for S-wave tomography. Our modeling suggests a sharp transition in the lowermost mantle from a broad slow region to a broad fast region with a narrow zone of slowest anomaly next to the boundary beneath the Cocos Plate and the Caribbean Plate. The structure may be the result of ponding of ancient subducted Farallon slabs situated near the edge of a thermal and chemical upwelling

Fine structure of an oceanic crustal section near the East Pacific Rise

In this study we model synthetically a complete seismic profile of roughly 20-m.y.-old crust located to the west of the East Pacific Rise, 3.37S, 114.13W. The results indicate a rather strong velocity gradient below the sediments with little evidence of layering in the upper crust and a slightly dipping oceanic layer. The crust-to-mantle transition zone appears sharp providing a relatively good wave guide for multiple Moho reflections which are modeled synthetically to further test the usefulness of a layered earth model in explaining entire seismograms. The mantle head waves decay abruptly near 50 km which can be explained by the onset of a low-velocity zone in the upper mantle at about a depth of 12 km below the ocean surface

Approximation of Rupture Directivity in Regional Phases Using Upgoing and Downgoing Wave Fields

Recent broadband modeling of regional events suggests that vertical directivity is particularly important at high frequency. Conventionally, such directivity is obtained by summing a grid of point sources. This relatively time-consuming procedure can be greatly reduced by introducing directivity time histories appropriate for the various crustal phases in terms of upgoing and downgoing paths that are calculated at only one depth. To achieve this, we formulated frequency-wavenumber solutions for a simultaneous computation of surface displacement for three wave fields, upgoing, downgoing, and the total from a seismic source buried in a layered medium (Appendix A). The concept of upgoing and downgoing wave field is introduced in the source layer matrix explicitly before allowing the source coefficients to interact with the propagation of the stress-displacement vector. Using this new algorithm, we generated a set of upgoing and downgoing wave fields at a fixed depth for different crustal models. We also simulated the effects of rupture propagation using distributed point-source summations and predicted the same effect by summing the upgoing and downgoing wave fields calculated at a single depth, each convolved with a separate analytical boxcar function representing the far-field rupture. A library of these new Green's functions should prove much more effective in modeling recorded motions than using point-source Green's functions alone

Time functions appropriate for nuclear explosions

The source-time function of megaton class nuclear explosions has been determined by modeling teleseismic short- and long-period body waves with synthetic seismograms. A simple analytic expression for the time function was used to closely match observations from both Novaya Zemlya and the U.S. test site at Amchitka. It was found that the time functions of all the events have a substantial overshoot. It was also found that, although the durations of the time functions did appear to depend on yield, the effect was very difficult to observe even in short-period records. All synthetics were computed by assuming a simple point source in a layered elastic half-space. It was not necessary to appeal to any nonlinear processes in the source region to explain the observations. Numerical calculations are presented to show that tectonic release triggered by earthquakes does not have a substantial effect on the P waves unless the long-period level of the tectonic event is as large or larger than the long-period level of the explosion. The pS wave, on the other hand, is shown to be very sensitive to even a moderate amount of tectonic release

Time functions appropriate for deep earthquakes

The seismic signatures of isolated body phases from many deep-focus earthquakes were analyzed in the time domain. Most shocks were found to be multiple events when examined in detail. The time history derived from P waves for single events predict synthetic S-wave shapes that match the observations, indicating compatibility with shear dislocation theory. Several other features of source functions in the time domain have been brought to light

Modeling the long-period body waves from shallow earthquakes at regional ranges

A procedure for modeling P and PL waves recorded on long-period WWSSN instruments at ranges 1° to 12° is presented. Following the experience gained by modeling explosions (Helmberger, 1972), we demonstrate that these long-period phases are adequately treated by a single crustal layer for most of Western United States. After generating the Green's functions at the various ranges for the three fundamental dislocation types, we need only construct linear combinations of these vectors to represent any arbitrary oriented earthquake. The waveform patterns produced from the various fault types are quite diagnostic with the dip-slip orientations showing a strong ringing nature which is caused by the vertical SV lobes. To test the usefulness of this technique, we construct synthetics for some well-studied west coast earthquakes where the orientation, time history, and moment have been determined independently. Comparing the predicted seismograms with observations, we find good agreement in waveshapes and amplitudes

Modeling two-dimensional structure at the core-mantle boundary

Recent studies of SKS waveform modeling emphasize the strong variation of seismic properties at the core-mantle boundary (CMB) and the need for two-dimensional and three-dimensional waveform modeling capabilities. In particular, the bifurcation of SKS into SP _dKS and SKP _dS near 110° shows strong regional variations. The first of these phases has a P wave diffraction along the bottom of the mantle near the source, while the latter phase occurs at the receiver end. Generalized ray theory proves effective in generating theoretical seismograms in this type of problem because each of these diffractions is associated with a particular transmission coefficient: T_(sp) which transmits shear waves into primary waves when crossing the CMB and T_(sp) which transmits the primary waves back into shear waves at the receiver end. Each region can then be isolated and have its separate fine structure, sharp or gradational. Two classes of boundaries are explored: the CMB as a simple, sharp interface and the CMB with a very low velocity transition layer (10% slower than reference models). The two diffractions produced by these structures have diagnostic arrival times and wave shapes and when combined with the geometric SKS produce distinct waveform characteristics not easily generated by other means. Since the ray paths associated with these three phases are virtually identical in the mantle and only differ along a short sample of CMB and in the one-dimensional fluid core, we can isolate the small localized CMB region sampled. Thus the waveform character of the extended SKS in the range of 105° to 120° becomes an excellent CMB probe which we demonstrate on a small sample of observations from the Fiji-Tonga region as recorded in North America

Crustal complexity from regional waveform tomography: Aftershocks of the 1992 Landers earthquake, California

We construct a two-dimensional velocity section sampling the Mojave crustal block in southern California by modeling shear wave (SH) seismograms. Our approach uses individual generalized rays computed from a layered model. The model is divided into blocks with variable velocity perturbations such that ray responses are allowed to shift relative to each other to maximize synthetic waveform fits to data. An efficient simulated annealing algorithm is employed in this search. The technique is applied to a collection of 25 aftershocks (Landers earthquake) as recorded at two stations, GSC and PFO, separated by ∼200 km, which bracket the event population along the Landers fault system. The events are assumed to have known mechanisms and epicenters, but both their depths and origin times are allowed to vary. The results indicate considerable variation, especially in the top layer (up to ±13%), which mirrors surface geology. Best fitting models contain a low-velocity zone in the lower crust if we constrain the crustal thickness (29 km) from receiver function analysis. Reduced lower crustal velocities imply crustal weakening, which appears compatible with the shallow seismogenic zone found in the northern end of this section. There is also evidence for a lateral jump in velocity of several percent across the San Andreas with the faster velocities on the west