PL Waves and Crustal Structure in Canada

1972 ◽  
Vol 9 (8) ◽  
pp. 1014-1029 ◽  
Author(s):  
G. Poupinet
Keyword(s):  
Crustal Structure ◽  
Rayleigh Waves ◽  
Degrees Of Freedom ◽  
Wave Train ◽  
Reliable Estimate ◽  
Period Range ◽  
Simple Method ◽  
Gravity Measurements ◽  
In The Beginning ◽  
Lateral Variations

A study of the group velocity of PL for about fifty paths in Canada has been made. It is difficult to measure the dispersion of PL for long periods because two Airy phases arrive in the beginning of the wave train. It is also concluded that like Rayleigh waves PL waves cannot really give more than an S-velocity distribution because the partial derivatives in SV are too large compared to those in P for the period range where a reliable estimate of the dispersion can be obtained. The different dispersion curves are interpreted by looking for lateral variations of PL dispersion. As these curves have only one or two degrees of freedom, we label a curve with an index of dispersion. As in Santo's studies, this index is attributed to each region crossed by fitting the propagation times for a given period. Diagrams are then used giving the variation of the index with the average S velocity and the depth of the Moho. The structures found by this rather simple method are well correlated with tectonic regions and gravity measurements.

scholarly journals Velocities of mantle Love and Rayleigh waves over multiple paths

1963 ◽  
Vol 53 (4) ◽  
pp. 741-764 ◽  
Author(s):  
M. Nafi Toksöz ◽  
Ari Ben-Menahem
Keyword(s):  
Rayleigh Waves ◽  
Great Circle ◽  
Love Waves ◽  
Period Range ◽  
Phase Velocities ◽  
Multiple Paths ◽  
Lateral Variations ◽  
Phase Spectra ◽  
Group Velocities ◽  
Good Agreement

Abstract Phase velocities of Love waves from five major earthquakes are measured over six great circle paths in the period range of 50 to 400 seconds. For two of the great circle paths the phase velocities of Rayleigh waves are also obtained. The digitized seismograph traces are Fourier analyzed, and the phase spectra are used in determining the phase velocities. Where the great circle paths are close, the phase velocities over these paths are found to be in very good agreement with each other indicating that the measured velocities are accurate and reliable. Phase velocities of Love waves over paths that lie far from each other are different, and this difference is consistent and much greater than the experimental error. From this it is concluded that there are lateral variations in the structure of the earth's mantle. One interpretation of this variation is that the mantle under the continents is different from that under the oceans, since the path with the highest phase velocities is almost completely oceanic. This interpretation, however, is not unique and variations under the oceans and continents are also possible. Group velocities are computed from the phase velocities and are also directly measured from the seismograms. The group-velocity curve of Love waves has a plateau between periods of 100 and 300 seconds with a shallow minimum at about 290 seconds. The sources of error in both Fourier analysis and direct time domain methods of phase velocity measurement are discussed.

Observations of phase velocity for Rayleigh waves in the period range 100 to 400 seconds

1960 ◽  
Vol 50 (3) ◽  
pp. 427-439
Author(s):  
John E. Nafe ◽  
James N. Brune
Keyword(s):  
Phase Velocity ◽  
Rayleigh Waves ◽  
Wave Train ◽  
Velocity Versus ◽  
Published Data ◽  
Single Pair ◽  
Final Phase ◽  
Period Range ◽  
Better Than ◽  
Alaska Earthquake

ABSTRACT Phase velocity as a function of period has been determined for Rayleigh waves in the period range 100 to 400 seconds. The results were derived from a study of seismograms from the southeastern Alaska earthquake of July 10, 1958, and from published data on the Assam earthquake of August 15, 1950. The method depends on measurement of the travel time of wave crests along an arc of known length, with proper correction for change of period with distance. For observations of a single Rayleigh wave train at a single pair of observing stations, crest identification is uncertain, and so too is the resulting curve of phase velocity versus period. A set of phase velocity curves may be computed, each one corresponding to a different choice of crest identification. Only one of these is consistent with the data from several earthquakes and several pairs of observing stations. In the present work, high precision in phase velocity measurement is achieved by using the observations of the Rayleigh waves R3 and R5 at Pasadena of the Assam earthquake. Data from the southeastern Alaska earthquake are used to resolve the ambiguity resulting from uncertainty in crest identification. The final phase velocity curve is estimated to be accurate to better than one per cent in the range of periods 100 to 400 seconds.

Crustal structure of the Tibetan Plateau: A surface-wave study by a moving window analysis

1977 ◽  
Vol 67 (3) ◽  
pp. 735-750
Author(s):  
Kin-Yip Chun ◽  
Toshikatsu Yoshii
Keyword(s):  
Tibetan Plateau ◽  
Crustal Structure ◽  
The Tibetan Plateau ◽  
Moving Window ◽  
Low Velocity ◽  
Period Range ◽  
Window Analysis ◽  
Dispersion Data ◽  
Moving Window Analysis ◽  
Group Velocities

abstract Group velocities of fundamental-mode Rayleigh and Love waves are analyzed to construct a crustal structure of the Tibetan Plateau. A moving window analysis is employed to compute group velocities in a wide period range of 7 to 100 sec for 17 individual paths. The crustal models derived from these dispersion data indicate that under the Tibetan Plateau the total crustal thickness is about 70 km and that the crustal velocities are generally low. The low velocities are most probably caused by high temperatures. A low-velocity zone located at an intermediate depth within the crust appears to be strongly demanded by the observed dispersion data. The main features of the proposed crustal structure will place stringent constraints on future tectonic models of the Tibetan Plateau which is generally regarded as a region of active deformation due to the continent-continent collision between India and Asia.

An algorithm for automated tsunami warning in French Polynesia based on mantle magnitudes

1989 ◽  
Vol 79 (4) ◽  
pp. 1177-1193
Author(s):  
Jacques Talandier ◽  
Emile A. Okal
Keyword(s):  
Rayleigh Wave ◽  
Seismic Moment ◽  
Rayleigh Waves ◽  
French Polynesia ◽  
Tsunami Warning ◽  
Period Range ◽  
Wave Duration ◽  
T Wave ◽  
The Moment ◽  
Historical References

Abstract We have developed a new magnitude scale, Mm, based on the measurement of mantle Rayleigh-wave energy in the 50 to 300 sec period range, and directly related to the seismic moment through Mm = log10M0 − 20. Measurements are taken on the first passage of Rayleigh waves, recorded on-scale on broadband instruments with adequate dynamical range. This allows estimation of the moment of an event within minutes of the arrival of the Rayleigh wave, and with a standard deviation of ±0.2 magnitude units. In turn, the knowledge of the seismic moment allows computation of an estimate of the high-seas amplitude of a range of expectable tsunami heights. The latter, combined with complementary data from T-wave duration and historical references, have been integrated into an automated procedure of tsunami warning by the Centre Polynésien de Prévention des Tsunamis (CPPT), in Papeete, Tahiti.

Seismic phases and scaling associated with small high-explosive surface shots

1981 ◽  
Vol 71 (6) ◽  
pp. 1731-1741
Author(s):  
I. N. Gupta ◽  
R. A. Hartenberger
Keyword(s):  
Blast Wave ◽  
Rayleigh Waves ◽  
Wave Train ◽  
Simple Wave ◽  
P Wave ◽  
Peak Ground Velocity ◽  
Air Blast ◽  
Geological Conditions ◽  
Wide Range ◽  
First Arrival

Abstract An analysis of seismic field data from surface shots in two radically different geologic environments shows significantly different seismic phases at the two sites. At the first site, which has a layered sedimentary section, five distinct phases are observed: the P-wave first arrival; a complex wave train consisting of higher mode Rayleigh waves; a precursor to air-blast wave; the air blast wave; and the air-coupled Rayleigh waves. Records from the second site, overlying an unlayered mass of igneous rocks, show only three distinct seismic phases: the P-wave first arrival; a simple wave train of fundamental-mode Rayleigh and Love waves; and an air blast wave. Peak ground velocity, based on the average of the three largest amplitudes in the surface waves preceding the air blast wave, scales well with yield for both sites. Measurements of peak ground velocity may be used to estimate yields of explosive charges at either site within a factor of about 2 if the source distance is known. The scaling relationship appears to be valid over a wide range of yields and site geological conditions.

Short-period seismic noise

1967 ◽  
Vol 57 (1) ◽  
pp. 55-81
Author(s):  
E. J. Douze
Keyword(s):  
Surface Waves ◽  
Rayleigh Waves ◽  
Seismic Noise ◽  
Body Waves ◽  
Deep Hole ◽  
Period Range ◽  
Short Period ◽  
The Subject ◽  
Hole Arrays

abstract This report consists of a summary of the studies conducted on the subject of short-period (6.0-0.3 sec period) noise over a period of approximately three years. Information from deep-hole and surface arrays was used in an attempt to determine the types of waves of which the noise is composed. The theoretical behavior of higher-mode Rayleigh waves and of body waves as measured by surface and deep-hole arrays is described. Both surface and body waves are shown to exist in the noise. Surface waves generally predominate at the longer periods (of the period range discussed) while body waves appear at the shorter periods at quiet sites. Not all the data could be interpreted to define the wave types present.

Wave Buoy Performance in Short and Long Waves, Evaluated Using Tests on a Hexapod

2018 ◽  
Author(s):  
Sanne van Essen ◽  
Kevin Ewans ◽  
Jason McConochie
Keyword(s):  
Degrees Of Freedom ◽  
Wind Waves ◽  
Long Waves ◽  
Normal Operation ◽  
Period Range ◽  
Multiple Degrees Of Freedom ◽  
Hydrodynamic Response ◽  
Vertical Displacements ◽  
Internal Integration ◽  
Offshore Wave

Offshore wave conditions can be measured using wave buoys, which are generally designed for wind waves. Longer waves (swell or bound second-order waves) are very relevant for certain maritime structures. The accuracy of the instrumentation in a typical wave buoy in long and short waves was therefore studied, and it was investigated if the buoy can be applied in longer waves. A Waverider buoy was placed on a hexapod, which applied regular and irregular prescribed motions in multiple degrees of freedom. The hydrodynamic response of the buoy in waves and the effect of its mooring system were not evaluated; the buoy was assumed to follow the orbital motions of a wave and to rotate with its slope. The tests showed that the buoy sensors measure accelerations and rotations with periods between 1.5 and 35 s very well. Vertical displacements derived from the accelerations by the buoy are accurate for the period range of 2 to 20 s. In longer waves, the motions are significantly underestimated, even though the accelerations are accurately measured. This will not lead to large errors in normal operation, as the energy of such long waves is generally low. This explains why the buoy also performs well when it is subjected to irregular motions (less than 2% error in the significant wave height of a half-hour measurement in realistic irregular sea states with peak periods between 5 and 20 s can be expected). It can be concluded that the buoy accurately measures accelerations. The accuracy of the derived displacements decreases when very long swell wave energy (> 20 s) is present. Review of the internal integration procedure may be considered when there is a specific interest in measuring longer waves.

Ellipticity of Rayleigh waves recorded in the Midwest

1977 ◽  
Vol 67 (2) ◽  
pp. 369-382
Author(s):  
John L. Sexton ◽  
A. J. Rudman ◽  
Judson Mead
Keyword(s):  
Local Structure ◽  
Rayleigh Waves ◽  
Signal To Noise Ratio ◽  
Interference Effects ◽  
Signal To Noise ◽  
Period Range ◽  
Model Studies ◽  
Single Station ◽  
Special Value ◽  
The Time Domain

Abstract Measurements of ellipticity of Rayleigh waves recorded in the U.S. Midwest have been examined for azimuth dependence, effects of interference, and repeatability, as well as the hypothesis that a single station may be used to determine local structure. Time- and frequency-domain analyses were performed for each event, with more consistent results from the time-domain method. Results indicate that for the period range of 10 to 50 sec, ellipticity depends primarily upon local structure and does not exhibit significant azimuthal dependence. Most ellipticity values for a given period are repeatable within 5 per cent of other measured values from all source regions, with the greatest deviation being about 10 per cent. The cause of the deviations is attributed to interfering waves and/or poor signal-to-noise ratios. Interference effects result in scatter in ellipticity values. An ellipticity peak in the period range of 18 to 22 sec has variable magnitude for different events, depending upon the amount of interference present and the signal-to-noise ratio. Interference effects also manifest themselves as sharp decreases in group-velocity observations even after filtering. Model studies show that ellipticity peaks can exist, which are due to the layered structure and not necessarily to interference effects. Ellipticity measurements (10- to 50-sec-period range) from a single station are useful for determination of a crustal model for the vicinity of the recording station, but should be used in conjunction with other available geophysical and geological data. Ellipticity measurements are shown to be of special value for model determination in areas with sedimentary layering, a result in agreement with the Boore-Toksöz 1969) study.

Sign in / Sign up

Export Citation Format

Share Document