Mantle Rayleigh waves from the Kamchatka earthquake of November 4, 1952

Mantle Rayleigh waves from the Kamchatka earthquake of November 4, 1952, are analyzed. The new Palisades long-period vertical seismograph recorded orders R_6–R_(15), the corresponding paths involving up to seven complete passages around the earth. The dispersion data for periods below 400 sec. are in excellent agreement with earlier results and can be explained in terms of the known increase of shear velocity with depth in the mantle. Data for periods 400-480 sec. indicate a tendency for the group velocity curve to level off, suggesting that these long waves are influenced by a low or vanishing shear velocity in the core. Deduction of internal friction in the mantle from wave absorption gives a value 1/Q = 370 × 10^(−5) for periods 250-350 sec. This is a little over half the value reported earlier for periods 140-215 sec

Crustal structure and surface-wave dispersion. Part II. Solomon Islands earthquake of July 29, 1950

Rayleigh waves from the Solomon Islands earthquake of July 29, 1950, recorded at Honolulu, Berkeley, Tucson, and Palisades are analyzed. Both the direct waves and those propagated through the Antipodes were observed for all stations except Honolulu. Application of a correction for land travel results in a dispersion curve for the oceanic portion of the path. It is found that the observed dispersion could be accounted for by propagation through a layer of water 5.57 km. thick overlying simatic rocks having shear velocity 4.56 km/sec. and density 3.0 gm/cc. Basement structure in the Pacific, Indian, South Atlantic, and North Atlantic oceans is identical within the limits of accuracy of the method. The sinusoidal nature and duration of the coda is explained by the effect of the oceans on the propagation of Rayleigh waves. The results are compatible with seismic refraction measurements in the Atlantic and Pacific oceans

Crustal structure and surface-wave dispersion

The observed dispersion of Rayleigh waves across the Atlantic and Pacific oceans can be accounted for by considering the propagation of such waves through a system consisting of water and unconsolidated sediments overlying a thick layer of ultrabasic rock. This contrasts with all former treatments, which have considered the effect of the water layer to be negligible. The depth of the water-sediment layer and the speed of shear waves in the underlying ultrabasic layer are obtained for several paths across the Atlantic and Pacific oceans. The results for the Atlantic are in good agreement with the data obtained in a recent seismic refraction measurement made 120 miles northwest of Bermuda, and offer strong evidence that the result of this single refraction measurement will be found to be typical of the entire ocean. No significant difference in the nature of the suboceanic basement of the Atlantic and Pacific has been found, since the velocity of shear waves in the uppermost 50 to 100 km. was calculated to be 4.45 km/sec. for both oceans. Previously reported differences in Atlantic and Pacific velocities for Rayleigh waves of some selected period are now believed to be due primarily to differences in the depth of water plus sediment in the two oceans

Two slow surface waves across North America

Surface shear waves (Lg) with initial period about 1/2 to 6 sec. with sharp commencements and amplitudes larger than any conventional phase have been recorded for continental paths at distances up to 6,000 km. These waves have a group velocity of 3.51±.07 km/sec. and for distances greater than 20° they have reverse dispersion. For distances less than about 10° the periods shorten and Lg merges into the recognized near-earthquake phase Sg. An additional large amplitude phase in which the orbital motion of the particle is retrograde elliptical and the velocity is 3.05±.07 km/sec. has also been observed for continental paths. It is believed that these phases are propagated through a wave guide formed by a superficial sialic layer. The problem of explaining the propagation of these surface waves is that of finding a crustal structure which is consistent with the other data of geology and geophysics and which will provide a suitable wave guide for the new phases. A possible nature of the wave guide is described

An investigation of mantle Rayleigh waves

Dispersion of Rayleigh waves for a new range of periods ranging from 1 to 7 minutes is described. The group velocity curve shows a long-period and a short-period branch merging at a minimum value of 3.54 km/sec. with a corresponding period of about 225 sec. It is suggested that the known variation of velocity with depth in the mantle can account for the observed dispersion. The small scatter in the velocities and the absorption of these waves suggests that, unlike shorter-period surface waves, refraction and attenuation effects are negligible at the continental margins. From the absorption of mantle Rayleigh waves the internal friction in the upper mantle for periods of 140 and 215 sec. is found to be given by 1/Q = 670 × 10^(−5). This is of the same order as that reported from vibration measurements at audio frequencies on laboratory samples of crystalline rocks at normal pressure and temperature

Ground roll coupling to atmospheric compressional waves

A theoretical treatment of ground roll originating from air shots and hole shots is given. It is shown that coupling of ground roll to compressional waves in the atmosphere exists for both air shots and hole shots. Experimental data obtained in the field are in excellent agreement with the theoretical results; namely, that the effective coupling exists for surface waves whose phase velocity is equal to the speed of sound in air. In regions where Rayleigh wave velocities vary with period due to layering in such a way that they are less than the speed of sound in air for short periods and exceed this value for longer periods, this coupling gives rise to a unique surface wave pattern on seismic records. It is shown that body wave and surface wave character is almost independent of charge elevation in the range from 0 (on the ground) to 30 feet. In a reciprocal manner ground roll from hole shots was recorded with air microphones as predicted by the theory

Seismic measurements in ocean basins

This paper will include a discussion of seismic effects and measurements in ocean basins as determined from natural earthquakes and from explosions. Earthquake seismology has been related to problems of the oceans since its beginning. In fact, establishment of the international network of seismograph stations, suggested by John Milne in 1897, was due in large part to the correlation of submarine cable breaks with oceanic earthquakes. Much of the great destruction of life and property caused by earthquakes at the borders of oceans results from the great sea waves or tsunamis occasionally generated by them. Recently it has become possible to draw quantitative information about the structure of the sea floor and its underlying rock layers from study of the dispersion of earthquake surface waves along oceanic paths

Crustal structure and surface-wave dispersion in Africa

In 1953, through cooperation of the Bernard Price Geophysical Institute of the University of the Witwatersrand and the University of Natal, a Columbia University-type long-period seismometer (To = 15 sec., Tg = 75 sec.) was installed in the seismological observatory of the University of Natal at Pietermaritzburg, Union of South Africa. This instrument was well situated for receiving surface waves from the shock in northern Algeria of September 9, 1954, and the aftershock next day, which was of such intensity that its seismogram supplemented that of the original shock for the larger phases. The dispersion of the Rayleigh waves from these seismograms can be measured with greater precision than has been practicable heretofore for continents, because the path is longer (7,890 km.) than any which has been available for a long-period vertical instrument, and is remarkably free from obvious anomalies such as major mountain ranges. The scarcity of suitable seismograms for the study of Raleigh-wave dispersion along continental paths is due to the fact that suitably placed long-period vertical seismographs have not been available until recently

Performance of resonant seismometers

A group of resonant vertical seismometers, each tuned to cover a part of the spectrum of microseism frequencies, has been operated for about one year. These instruments (a) clearly distinguish between simultaneous microseisms from two separate sources; (b) show an improved signal-to-noise ratio for microseisms from a single storm, permitting earlier detection of storm onsets; (c) show clearly the increase in period of frontal microseisms as cold fronts move seaward from the east coast of North America; (d) record only the envelope of the oscillations, which greatly facilitates measurement of intensity as a function of time; and (e) appear to be very useful tools in continued attempts at hurricane location by means of microseism amplitude studies. The performance of the instruments is demonstrated by seven case histories in which microseismic readings of seismometers tuned to different frequencies are related to the meteorological conditions which are apparently responsible for the microseismic activity