Analysis Of Seismograms Research Articles

The propagation of short-period seismic surface waves across oceanic areas Part II—Analysis of seismograms

Abstract The propagation of short-period oceanic surface waves with predominant periods between 5 and 20 seconds was studied for a large number of paths in the Atlantic, Indian and Pacific oceans. In the ocean basins these waves are controlled largely by the sedimentary layer. The amplitudes of the short-period waves are greatly dependent upon the nature of the propagation path and on the properties of the source. Surface waves with periods between 5 and 10 seconds are often observed for paths that cross the continental margins of the Pacific, but are recorded only rarely for similar transmission paths in the Atlantic. Probably this effect is due to the differences in the structural configuration of the margins, particularly to such differences in the sedimentary strata. Seismic refraction and reflection measurements indicate that most of the oceanic paths for which the 5 to 10 second waves are observed are characterized by sedimentary thicknesses that do not average more than a few tenths of a kilometer. The predominant periods of the short-period wave train are increased to as much as 12 to 20 seconds for paths that traverse the thick sediments of the Argentine Basin. This increase in period, which was predicted theoretically for regions of relatively thick sediments, indicates that the low-rigidity sediments play a prominent role in determining the character of the short-period wave train. The increase in the predominant periods of the waves associated with the first Love and first shear mode also accounts for the absence of the 5 to 10 second waves in areas of thick sediments such as the continental margins of the Atlantic. Dispersion data for the Rayleigh, first Love and first shear modes were used in conjunction with reflection and refraction results to estimate the average shear velocity in the sediments of the Argentine Basin. The average shear velocity in the upper 0.5 km of sediments in about 0.2 to 0.4 km/sec, and the velocity in the kilometer of sediments below this is about 0.5 to 0.7 km/sec. Sedimentary shear velocities of a few tenths of a kilometer per second were also obtained for paths along which the average sedimentary thicknesses are a few tenths of a kilometer. For group velocities between 4.2 and 3.4 km/sec the particle motion in the first shear mode is retrograde; ratios of horizontal-to-vertical motion as large as 3.5 are observed at island and coastal stations. Seismic waves with periods of 10 to 30 seconds and with group velocities of 4.0 to 4.4 km/sec are sometimes recorded on the vertical components of long-period seismographs. These waves may be explained as higher modes of the Rayleigh type. Oscillatory waves with periods of 5 to 10 seconds are observed to follow the P wave in many oceanic areas. These arrivals are attributed to a type of leaking mode that results from the multiple reflection of SV waves in a low-rigidity layer.

Microearthquakes near Socorro, New Mexico

Since June 1960, several hundred natural microearthquakes having S-P intervals of less than 2.3 seconds have been recorded by high-magnification seismographs located in the mountains 3 miles west of Socorro, New Mexico. Two seismographs have been used in this study, a single-channel seismograph (magnification 9 million at 20 cps) recording 24 hours a day at 3.5 mm/sec and a three-channel seismograph (magnification variable from 0.08 to 10 million at 20 cps) recording a few hours each day at either 15, 30, or 60 mm/sec. Data from continuous recording indicate: (1) Rate of microearthquake activity is significantly greater in the fall months; (2) more shocks occur in groups than were predicted for a random and mutually exclusive distribution of events. Analysis of high-resolution seismograms (three-component, tripartite, and others) indicates: (1) Epicenters for most shocks are within a 36-km2 area southwest of Socorro; (2) depths of focus range from 2.7 to 6.3 km; (3) energy release for the weakest shocks recorded is of the order of 107 ergs; (4) number of microearthquakes and energy release are related by log10 N = C2 - 0.496 log10 E.

Long-period surface waves from the Chilean Earthquake of May 22, 1960, recorded on linear strain seismographs

Phase and group velocities of mantle Love and Rayleigh waves obtained from strain seismograph records of the Chilean earthquake are presented. The velocities of mantle Rayleigh waves of period from 300 to 550 seconds agree with those predicted from periods of free spheroidal oscillation of the earth and do not show a flattening of the group velocity curve for periods greater than 380 seconds. Group velocities for mantle Rayleigh waves reach a maximum of 7.8 km/sec at a period of about 1000 sec. Study of initial phases of Rayleigh waves indicates a difference of phase of π between the azimuth to Isabella and the azimuths to Ñaña and Ogdensburg. Determinations of phase and group velocities of Love waves have been extended to periods of 700 seconds. The phase velocity data of Satô [1958] has been corrected for the polar phase shift. The correct curve has been identified from the numerous possible curves which result from a 2π ambiguity in the phase correlation made by Satô. Values of phase velocities are presented for periods in the range of 60 to 700 seconds. The group and phase velocities of both Love waves and Rayleigh waves agree well with those predicted for the Gutenberg-Bullen A model of the earth. It is verified that analysis of seismograms in terms of progressive wave trains is equivalent to analysis in terms of standing waves. In the presence of absorption, as for the earth, the analysis in terms of progressive wave trains has many advantages. Material supplementary to this article has been deposited with the ADI Auxiliary Publications Project, Photoduplication Service, Library of Congress, Washington 25, D.C. A copy may be secured by citing the document number 6816 and remitting $1.75 for 35-mm microfilm. Advance payment is required. Make check or money order payable to: Chief, Photoduplieation Service, Library of Congress.

Free oscillations of the Earth observed on strain and pendulum seismographs

Spectral analyses of seismograms of the great Chilean earthquake of May 22, 1960, from a newly installed strain seismograph at Ogdensburg, New Jersey, and from pendulum seismographs at Palisades, New York, have revealed spectral peaks corresponding to fundamental spheroidal modes 2 to 34, fundamental torsional modes 2 to 9, and the first overtone of the second spheroidal mode. Other peaks, some of which may be overtones, occur in the spectra but are not yet identified. Amplitudes of some observed spectral peaks vary radically between two time intervals on the same record. The peaks do not decrease in amplitude according to any simple law, and, rarely, an increase in amplitude with time is observed, indicating the apparent acquisition of energy in the mode between the time intervals. The periods of the graver modes of oscillation, both spheroidal and torsional, are in agreement with the theoretical values of Alterman, Jarosch, and Pekeris. The periods of the fundamental spheroidal oscillations between 250 and 500 seconds have been determined very accurately. These periods show excellent agreement with theoretical values calculated by Bolt and Dorman for a mantle with velocities according to the Gutenberg model and densities according to the Bullen model A. Also, phase velocities obtained from these periods are in agreement with Rayleigh wave phase velocities observed directly from the same earthquake by Brune, Nafe, and Alsop. Good agreement is also observed between torsional periods and theoretical values of Sato, Landisman, and Ewing, based on velocities of Jeffreys and densities of Bullen model A.

Progress Report, Seismological Laboratory of the California Institute of Technology, 1957

On July 1, 1957 Beno Gutenberg retired as Director of the Seismological Laboratory and was succeeded by Frank Press. Beno Gutenberg remains a member of the staff. On February 5, 1958, Harry 0. Wood, who founded the Seismological Laboratory, passed away after a long illness.The long‐standing condition of overcrowding it the Seismological Laboratory was corrected by the acquisition of an adjacent property. The new quarters, known as the Donnelley Laboratory, house offices for staff, visitors, and students as well as a library, map room, seismogram analysis room, and ultrasonic seismic model laboratory. An important feature of the Donnelley Laboratory is a tunnel 150 ft long, located in granite at a depth of 60 ft beneath the building. A linear strain seismograph and ultra long‐period pendulums will be installed in the tunnel. The old quarters are being extensively remodeled and will be known as the Kresge Laboratory, housing the main seismograph vaults and recording rooms, experimental piers, electronics laboratory, machine shop, storage and stock rooms, photographic dark room, and offices for technicians.

ON THE RELIABILITY OF HARMONIC ANALYSIS OF SEISMOGRAMS

ABSTRACTSince harmonic analysis of seismograms seems to become more familiar in the interpretation of seismic data, the author presents the results of a thorough check analysis. Following a fixed scheme, one seismogram is analysed by means of a mechanical analyser (system Mader‐Ott). Standard deviation of Fourier coefficients is examined, and scattering in phase values is discussed. At the best, in the range of maximum amplitudes of the spectrum, the method yields 6% in phase scattering. At the worst, on either side of the spectrum, 13 % are obtained. Generally 8 % have to be taken in account. (Percentages in periods of partial‐waves of the analysis). The results are in accordance with former routine analysis (Korschunow, 1955).

Correlogram Analyses of Seismograms by Means of a Simple Automatic Computer.

Correlogram Analyses of Seismograms by Means of a Simple Automatic Computer.

ON SURFACE‐WAVES IN LOOSE MATERIALS OF THE SOIL*

ABSTRACTFor the first time a method of observing the effects of hammering and of small blasts has been applied in a series of experiments on various soils, using a three component set of small mechanical leaf spring seismographs. The seismograms have been subjected to a thorough harmonic analysis. By means of these experiments, the theoretical concept of Rayleigh waves generated in a layer overlying a semi infinite medium has been confirmed. The dispersion curves resulting from different experiments correspond well with theoretical dispersion curves.It may be. concluded that every seismic pulse generates a specific Rayleigh wave mechanism in loose layers. The depth of penetration of the Rayleigh wave mechanism and its frequency spectrum depend upon the delivery of energy and upon the consistence of the particular soil.Most of the energy delivered is transformed into surface‐waves, and it is obvious that the direct reading of pulses in seismograms effects mainly group velocities of the Rayleigh wave mechanism. Thus, all endeavours to procure information about near surface layers of loose soils by refraction seismic work, as the present investigations show, meet with far more difficulties than has been assumed in the past.With regard to the determination of the critical frequencies of the generated surface waves, the method of harmonic analysis of seismograms is superior to the usual dynamic method of soil mechanics, since rotating weights do not give the characteristic phase velocities and transversal wave velocities necessary for judging the consistency of loose soils.Finally, a method of determining the absorption of surface waves is discussed.

Review of Seismology

THE National Research Council of the National Academy of Sciences at Washington is issuing a series of bulletins on the physics of the earth, to give scientific workers who are not specialists in the subjects treated an idea of the position and problems of various branches of geophysics. Among the bulletins in this series which have already been issued are those on volcanology, the figure of the earth, meteorology, the age of the earth and oceanography. Recently, Bulletin No. 90 on seismology has appeared. It has been prepared by a Committee of which Prof. J. B. Macelwane is chairman. Within 219 pages it includes twenty chapters, and gives a very valuable and interesting general view of the subject. Chapters by the chairman include-the definition and classification of earthquakes, tectonic earthquakes, plutonic earthquakes, rock fall earthquakes, body waves, reflection and refraction of seismic waves, surface waves and paths and velocities of seismic waves within the earth; H. O. Woods contributes articles on volcanic earthquakes, field investigation and surface geology in relation to the ‘apparent’ intensity; articles by H. F. Reid deal with magnetic effects, earthquake mechanics and with the focus. The principle of the seismograph is described by J. A. Anderson, and P. Byerly contributes five articles on analysis of seismograms in earthquakes, records at intermediate and great distances, time distance curves, reduction of trace amplitude, and seismic geography. The Bulletin has numerous bibliographies and is priced at 2 dollar.