Frequency‐dependent regional wave attenuation is estimated for continental paths to the NORESS array in Norway. Regional Lg and Pn spectra from 186 events at ranges between 200 and 1400 km and local magnitudes between 1.1 and 4.8 are inverted for both seismic moment and apparent attenuation. The Lg spectra were inverted between 1 and 7 Hz, and the Pn spectra were inverted between 1 and 15 Hz. The method uses both the spectral and spatial decay of observed signal amplitudes to separate source and path contributions. The assumptions include the geometric spreading rate and the source spectrum to be uniquely defined by its long‐period level. Most events considered have local magnitudes less than 3.0, so the source corner frequencies are near or beyond the upper limit of the inverted bandwidth. The Q results, particularly for Lg, are therefore not very sensitive to the details of our source parameterization. The inversion parameters are source moment (for each event), a constant relating corner frequency and moment for the entire data set, and two parameters describing a power law frequency dependence of Q in the region. For fixed source and spreading assumptions the inversion defines clear trade‐offs among model parameters. These trade‐offs are resolved by adding the constraint that the separately derived source parameters for Lg and Pn are consistent. The “preferred” estimates for the apparent attenuation are QLg(f) = 560f0.26 and QPn(f) = 325f0.48. These Q values correspond to assumed geometric spreading rates of r−0.5 for Lg and r−1.3 for Pn. For fixed Lg spreading, the Pn spreading rate is constrained by requiring that the relative Lg amplitude for earthquakes and explosions of the same moment be consistent with well‐supported results from previous empirical studies. The relationship between the inverted seismic moment values and local magnitude is generally consistent with values from near‐field studies. Since magnitude does not enter the inversion, this result lends considerable support to the derived Q models. Whatever the physical interpretation of the results, they certainly provide an accurate parameterization of observed amplitude spectra in this region. This is valuable for representing wave propagation in the region, and it provides important data for assessing the event monitoring capabilities of small regional networks.

We present models for the structure of the crust and upper mantle beneath lunar impact basins from an inversion of gravity and topographic data from the nearside of the moon. All basin models display a thinner crust and an elevated Moho beneath the central basin region compared to surrounding areas, a signature of the processes of basin excavation and mantle uplift during collapse of the transient cavity.There is a general decrease in the magnitude of apparent uplift of mantle material with increasing basin age; we attribute this relation primarily to enhanced rates of ductile flow of crustal material early in lunar history when crustal temperatures were relatively high and the effective elastic lithosphere was thin.

We use over 500 seismograms collected by five ocean bottom seismometers to examine the structure and variability of 0.5-m.y.-old crust in the Rivera Ocean Seismic Experiment (ROSE) area of the East Pacific Rise between 11 ø and 13øN. There are no significant differences among the first arrival travel times for 10 independent refraction line segments along the isochron. Amplitude patterns recorded on the individual lines, however, suggest that structural boundaries within the upper crust vary by hundreds of meters in depth over lateral distances of tens of kilometers within this 200-km segment of lithosphere. On the basis of the modeling of amplitude patterns at ranges of 5-10 km and of comparisons with the upper crust sampled at Deep Sea Drilling Project hole 504B and in ophiolite complexes, layer 2 can be divided into three units: 2A corresponds to extrusive volcanics with a steep velocity gradient, 2B to extrusives with a near-zero velocity gradient, and 2C to a complex region of transition from extrusives to sheeted dikes. The total thickness of layer 2 varies by about 0.7 km over the length of the line. Another amplitude peak which varies in range between 14 and 21 km may be related to the downward transition from sheeted dikes to isotropic gabbros within layer 3. The variability of these two amplitude patterns provides direct evidence that the along-strike structure of oceanic crust in the ROSE area is definably heterogeneous and that the process of crustal accretion is likewise variable on the scale resolved by amplitude and travel time information.

We present a series of exploratory models which test the hypothesis that thermoelastic stress is a significant contributor to the state of stress in young oceanic lithosphere (less than 35 m.y. in age). In support of this hypothesis is the concentration of seismicity in lithosphere younger than 15 m.y. where cooling rates are relatively high. Most near‐ridge earthquakes have focal depths between the Moho and the depth of the 800°C isotherm. Thrust and strike‐slip events dominate the shallowest seismicity, while the focal mechanisms of the deeper events are generally characterized by normal faulting. To assess the importance of thermal stress in the generation of near‐ridge earthquakes, we model the lithosphere as an elastic half‐space in which the response to cooling is computed using the method of thermoelastic displacement potentials. A key assumption in the models is that stress is relieved on time scales generally short compared with the age of the lithosphere. A further assumption in some models is that material will not contribute thermal stress until it has cooled below an elastic blocking temperature. Normal faulting is predicted throughout the lithosphere in thermal stress models based on simple half‐space cooling. Models in which a cooled surface layer is incorporated to simulate the effect of hydrothermal circulation on shallow thermal structure predict stresses that can match the locations of both thrust‐ and normal‐faulting earthquakes near mid‐ocean ridges. The addition of a “ridge push” stress to the computed thermal stress enhances the likelihood of thrust faulting in the uppermost lithosphere at ages greater than about 15 m.y. but probably contributes little to the stress field at younger ages. Though these simple models are limited by several simplifying assumptions, particularly in the immediate vicinity of the ridge axis, they support the hypothesis that thermoelastic stress can play a major role in the tectonics of young oceanic lithosphere.

Potentially important contributors to the topography and tectonics of multi-ring impact basins are the thermal contraction and thermal stress that accompany the loss of heat emplaced during basin formation. Heat converted from impact kinetic energy and contributed from the uplift of isotherms during cavity collapse are important components in the energy budget of a newly-formed basin. That the subsequent cooling may have been an important factor in the tectonic evolution of the Orientale basin is suggested by the deep central depression and by a surrounding region of extensive fissuring. To test these concepts, we develop models for the anomalous temperature distribution immediately following basin formation, and we calculate the resulting elastic displacement and stress fields that then would accompany cooling of the basin region. All models predict subsidence of the basin floor and a near-surface stress field consistent with fissuring. In addition, the rates of cooling and of accumulation of thermal stress are in agreement with the inferred timing of fissure formation in Orientale. The sensitivity of the predicted displacements and stresses to the initial temperature field allows us to place bounds on the quantity and distribution of impact heat emplaced during basin formation. In order to be consistent with the observed topography and the distribution of fissures in the Orientale basin, the buried heat deposited during the basin-forming event was between 1032 and 1033 erg. It is likely that most of this heat was concentrated within a distance of 100-200 km from the point of impact.