ABSTRACT
Average crustal models for the northeastern United States are computed on the basis of the travel times of P and S waves from regional earthquakes. The Precambrian Grenville Province in New York State has a relatively homogeneous crust. The Paleozoic New England Appalachians have a well-defined, two-layer crust that is slightly thicker and shows a high-velocity lower layer relative to the Grenville. A time-term analysis based on P(n) data (waves refracted from the Moho) shows that a relatively thick or low-velocity crust parallels northeast-trending geologic structures in central New England. The observed differences between the two orogenic belts may reflect contrasts in their tectonic evolution.
ABSTRACT
Observed features of moonquakes are combined with theoretical calculations of the tidal stresses to interpret the moonquake mechanisms. Tidal stresses, together with a postulated ambient tectonic stress, are sufficient to explain the depth, periodicity, and polarity reversal of moonquakes. Both of these stresses are small (on the order of 1 bar) and consistent with the small magnitudes of moonquakes.
ABSTRACT
The earthquake return periods (the mean interoccurrence time) and the probability of earthquake occurrences for southern New England are calculated from the available seismic data for the period 1725 through 1974. For this region the occurrence of larger earthquakes varied with time and the seismic activity was higher in the period 1725 through 1824 than in the next 100 years (1825 through 1924). This variation introduces large uncertainties into calculations of the earthquake hazard. The estimated return period, based on data covering the time period 1725 through 1974, for earthquakes in the southern New England area of intensity VI or greater is 25 years and for intensity VIII or greater is 130 years.
ABSTRACT
Lunar seismic data from artificial impacts recorded at three Apollo seismometers are interpreted to determine the structure of the moon's interior to a depth of about 100 kilomneters. In the Fra Mauro region of Oceanus Procellarum, the moon has a layered crust 65 kilometers thick. The seismic velocities in the upper 25 kilometers are consistent with those in lunar basalts. Between 25 and 65 kilometers, the nearly constant velocity (6.8 kilometers per second) corresponds to velocities in gabbroic and anorthositic rocks. The apparent velocity is high (about 9 kilometers per second) in the lunar mantle immediately below the crust.
ABSTRACT
The tectonic strain energy released by several underground nuclear explosions has been calculated through an analysis of seismic surface waves. The proportionally great amount of energy released in certain events suggests the possible uses for, as well as the hazards of, underground testing.
ABSTRACT
Frequency-wave number spectra of microseisms were obtained by use of a set of short-period and long-period seismometers at LASA (Large Aperture Seismic Array, Montana). At times of relatively high microseismic activity short-period (shorter than 5 seconds) microseisms consist of both body waves and higher-mode surface waves. From the phase velocity and direction of body waves, source areas were determined, coinciding with low-pressure regions on the weather map. At longer periods, microseisms consist of fundamental- mode Rayleigh and Love waves, the former being dominant. Most microseismic energy arrives at LASA from the northeast and the west.
ABSTRACT
Travel-time residuals of seismic P-waves were expanded in spherical harmonics to determine their global variations. The expansion cofficients were correlated with similar coefficients of geopotential, variations in heat flow, and the surface topography of Earth. Although none of the correlation coefficients is very high, best values are between the seismic delay times and variations in gravitational potential and between heat flow and surface topography.
