Two-station Method Research Articles

The Structure of the Upper Mantle under the United States from the Dispersion of Rayleigh Waves

Summary Seismograms from vertical-component, long-period instruments of the World-Wide Standard Seismographic Network (WWSSN) within the United States have been analysed to determine the dispersion of Rayleigh waves by the two-station method. Phase velocities from fundamental mode Rayleigh waves over the period range 20–250s have been obtained to study the structure of the upper mantle of the continent. The phase velocities show systematic variations in specific sub-regions of the United States. The phase velocities observed along five representative paths have been inverted into sub-surface structural cross-sections by the Hedgehog method. The results of the inversion indicate that there are significant regional variations in S-wave velocities, particularly from the Moho down to about 400km. We make no statement about lateral variations in structure at greater depths. The aseismic south-central part of the continent and the tectonically active western Cordillera are characterized by a well-developed low-velocity channel for S waves in the upper mantle. The north-central United States has dispersion data which are consistent with structures which either have a low velocity channel of marginal properties or no low velocity channel at all; if a channel is present for this region, the velocity contrast with the layers of the mantle above and below it is very small.

Simultaneous determination of the seismic moment and attenuation of seismic surface waves

Abstract A comprehensive spectral analysis of Love and Rayleigh waves was made on the Parkfield earthquake of June 28, 1966. Using tentative Q values obtained by the usual two-station method for continental and oceanic paths, the observed spectra were corrected for dissipation, and used for obtaining a tentative source model. The resultant model, having the fault length 37 km, the strike direction N43°W and the rupture velocity 2.2 km/sec, is consistent with various near field measurements. Based on this tentative source model, a least-squares method is applied to separate source, dissipation and interference effects on the spectra. The logarithm of the ratio of the observed to the theoretical spectra was plotted against the epicentral distance for various frequencies. The slope and zero-crossing value of the fitted straight line were used for revising the attenuation coefficient and the seismic moment respectively. The variability of individual station spectra was attributed to the interference effect. The revised value of the seismic moment of the Parkfield earthquake is e (0.33±0.40) ×10 25 dyne-cm. The revised attenuation coefficients indicate that Love waves have consistently higher dissipation than Rayleigh waves at periods 15 to 40 seconds. Both show marked minimum of dissipation (Q over 800 for Love and over 1000 for Rayleigh waves) at periods between 20 and 25 seconds. We found that our Q data are consistent with a model which has a high-Q zone in the lower crust and uppermost mantle, overlying a low-Q zone which correlates well with the Gutenberg low-velocity zone. There is an indication that the intrinsic Q is dependent on period at depths shallower than 90 kilometers in such a way that it increases with period up to 20 seconds. For periods beyond 20 seconds, the intrinsic Q may be constant for all periods and all depths above 90 kilometers.

Air pressure dependence of cosmic ray intensity: Methods and results of a statistical analysis on neutron monitor data

A general regression model is derived which describes the dependence of the cosmic-ray neutronic component on the air pressure. As the residuals are correlated the ordinary least-squares method is not the best one to estimate the pressure effect. Alternative methods are discussed. Five methods are used to estimate the pressure coefficient at Upsala during the years 1957–1964. It turns out that a modified two-station method gives the best result and also that there exists a solar cycle dependence of the pressure coefficient. DOI: 10.1111/j.2153-3490.1968.tb00361.x