Horizontal Particle Motion Research Articles

Effects of Multiple Modes on Rayleigh Wave Dispersion Characteristics

The effects of multiple modes on Rayleigh wave dispersion are discussed to reduce the ambiguity of uniqueness of shear wave velocity (Vs) profiles estimated by the surface wave method. Based on a review of previous studies, dispersion curves of multiple‐mode Rayleigh waves induced by harmonic vertical point loading are derived for both vertical and horizontal particle motions. Also presented is the variation with frequency of the amplitude ratio between horizontal and vertical particle motions. Numerical studies indicate that a stiff soil layer overlying a softer soil layer induces a higher mode or multiple modes, leading to an inversely dispersive characteristic. Consideration of the effects of higher modes is strongly recommended in the inverse process when the observed data show an inversely dispersive trend. The ambiguity of uniqueness of the inverted soil profiles may be reduced by using either the dispersion data of horizontal motion or the amplitude ratio of particle motions in addition to the disp...

Coherence measurements of deep-ocean seismoacoustic noise and signals

In April 1992, an array of six ocean bottom seismometer/hydrophone instruments was deployed on the seafloor in 3800-m-deep water. The instrument site was approximately 300 km west of San Diego, California and was within a few miles of the location of a smaller aperture array used in an experiment conducted in 1987. Initial calculations of the spatial coherence between vertical component seismometer recordings of ambient noise show remarkable similarities to data collected in the earlier experiment. The ratio of vertical to horizontal particle motion at the seafloor during ambient noise recording periods also is consistent with the earlier observations. The combination of coherence structure and particle motion ratios derived from the earlier experiment is used to infer that the ambient noise field at the site is dominated by seismic interface waves. With the new data set, this hypothesis is tested by examining: (1) the behavior of spatial coherence over a larger aperture (∼400 m) array, (2) the spatial coherence of the near-bottom pressure field and horizontal particle motion field, (3) the ratios of particle motion to near-bottom pressure, and (4) the particle motion ratios and spatial coherence of signals from near sea-surface and on-bottom explosive shots. [Work funded by ONR and ONT.]

Estimating the Hypocenter and Mechanism of the August 15, 1991 Centre Hall, Pennsylvania Earthquake Using Single-Station Data

Abstract On August 15, 1991 a small (mbLg = 3.0) earthquake occurred near the town of Centre Hall, Pennsylvania. Based on early reports of felt effects and earthquake-generated sounds, the epicenter was placed somewhere ENE of State College, Pennsylvania. Three-component short-period digital data from the DWWSSN station SCP were analyzed to determine the hypocenter. Often, for small earthquakes in regions without dense seismic networks, information about an event must be obtained from single-station data. In this case, since no shallow velocity model exists for the area, simple ideas of wave propagation are invoked to estimate the distance and back-azimuth to the event. The horizontal P-wave particle motion constrained the back-azimuth, after calibrating the horizontal components by measuring the back-azimuth of quarry blast P-waves of known location. Distance determination was hampered by lack of a detailed upper crustal velocity model. Using iterative forward waveform modeling, a velocity model was generated that fit the observed S-minus-P and Rg-minus-P times and Rg-wave dispersion, and which was consistent with known upper crustal velocities in the area. A source depth of less than 1 km was inferred from the Rg-to-S ratio, the depth phase sP, and reports of earthquake-generated sounds. Estimates of the focal mechanism were obtained by a grid search procedure using Green’s functions computed with wavenumber integration for shear dislocation sources. Theoretical and observed amplitudes of sP, direct SH and SV (taken as ratios to the direct P), along with P polarity were compared for all possible combinations of strike, dip, and rake. Though fault plane orientation is poorly constrained, E-W to WNW-ESE P-axis orientation is a robust result of the search. Normal-faulting mechanisms are inconsistent with the data. However, the theoretical SV-to-P ratio is up to a factor of two larger than the observed ratio. This is probably related to an inadequate structure model and waveform sensitivity to source depth. Mechanism P-axis trends are consistent with other regional stress field indicators in the area.

Seismic noise in northcentral North Carolina

Ambient seismic noise in the frequency range 0.5 to 7 Hz is investigated for a portion of northcentral North Carolina. Three-component, digital data were collected as part of the state's bid for the Superconducting Super Collider. Stations were established along the proposed collider ring position and were intentionally located near sources of transient vibration. The absence of transient vibrations over all or a significant portion of the recording interval at six stations allowed seismic noise to be investigated. Noise spectra display similarities in shape; all spectra decay with approximately constant slope (roughly f −3) below 1 Hz and all flatten in the frequency range 2 to 3 Hz. Vertical-component spectra decay with roughly constant slope ( f −2.1) for f > 4 Hz. At most stations, spectral levels for horizontal components are greater than vertical-component levels for f > 4 Hz. Vertical-component spectra are similar in shape and magnitude to vertical spectra computed for RSTN station RSCP. The most likely source for the ambient seismic noise is higher-mode surface waves produced by cultural activity. Ambient noise cannot be attributed to system noise or to the direct effects of traffic. Higher-mode surface waves are suggested by low coherence values computed for orthogonal components and extremely complex particle motion diagrams. Large horizontal particle motions and high spectral levels for horizontal components suggest that a significant portion of the noise, particularly for f > 4 Hz, is being produced by Love waves. Rayleigh waves contribute to the noise in the band 2 to 3 Hz.

Borehole wave particle motion in anisotropic formations

Recent observations of particle motion in vertical seismic profile surveys suggest the existence of borehole waves for which the formation horizontal particle motion is polarized in a specific azimuthal direction, possibly the direction of a local principal axis of stress. From an analysis of the static displacement of a pressurized vertical cylindrical borehole in a transversely isotropic formation with a horizontal axis of symmetry, we conclude that horizontal particle motion should not be azimuthally polarized for “tube waves” (lowest mode symmetric borehole wave) even in an ideal azimuthally anisotropic medium. Azimuthally polarized particle motion does exist for the “bending mode” of the borehole (lowest antisymmetric mode) even in isotropic formations; the polarization direction being determined by the source direction. Finite element calculations for a vertical borehole in a transversely isotropic formation with a horizontal axis of symmetry show that two bending modes with different velocities exist. The horizontal particle motion of one is polarized along the symmetry axis and the other is polarized orthogonal to that axis. At low frequencies the velocity of each approaches the velocity of a vertically propagating formation body shear wave with the same polarization direction. The recent experimental observations include one borehole arrival with a propagation velocity and a ratio of horizontal to vertical formation particle motion consistent with the hypothesis that it is a bending mode in an azimuthally anisotropic medium. The polarization direction and excitation of such a mode do not seem unreasonable. Other polarized arrivals cannot be easily explained.

Lateral variation of upper mantle structure beneath New Caledonia determined from P-wave receiver function: evidence for a fossil subduction zone

SUMMARY Short-period teleseismic P-waves recorded at Noumea, New Caledonia, from intermediate and deep focus earthquakes in SE Asia and the SW Pacific are used to investigate crustal and upper mantle structure of the Norfolk ridge, which is currently described as a zone of convergence in its caledonian part. Radial components recorded from various source backazimuths show a succession of large P-to-S converted waves whose timing and polarity suggest the presence of a north-dipping low-velocity zone at approximately 60km depth below the station. The inferred strong lateral variations of structure in the upper mantle are also supported by observations of complex horizontal particle motions and large tangential components which exhibit clear P-to-S phases. The high velocity contrast observed across these dipping interfaces is interpreted as a structural remnant of the New Caledonia basin oceanic crust which would have been underthrust beneath the New Caledonia crust. Using geometrical ray theory in dipping structure, a model valid for the Noumea station has been derived. The crust is apparently quite homogeneous. It is 26 ±1 km thick beneath the station and seems to thicken eastward as deduced from the orientation of the Moho. The low average crustal velocities suggest a continental nature for New Caledonia. Toward the thrust zone, the Moho is a thick transition zone whereas it can be modelled by a sharp boundary elsewhere. The inferred structure can be interpreted in terms of either a dead subduction zone or an underthrusting zone. According to the history of compressive deformations observed in land, the underthrusting would likely have been syntectonic of the New Caledonia late Eocene obduction.

Determination of in situ stress orientation from borehole guided waves

We have analyzed the horizontal particle motion of borehole guided waves from three‐component vertical seismic profile data recorded from three experiments. The polarization directions of the guided waves are consistent with the direction of horizontal principal stress determined independently. We suggest that stress‐induced velocity anisotropy in the rock adjacent to the borehole appears to cause antisymmetric flexural guided modes to be polarized in the direction of the maximum horizontal principal stress. Thus analysis of the horizontal particle motion of borehole guided waves appears to represent a new method for determining the orientation of horizontal principal stresses.

Hot-film anemometry measurement of hydroacoustic particle motion

Hydroacoustic particle motion may be detected by hot-film anemometry. Free convection causes a vertical flow that acts as a bias for vertical particle motion, resulting in a linear dependence of output voltage on the displacement. For horizontal particle motion, the output frequency is twice that of the acoustic field, and the output depends quadratically on the motion. An imposed bias flow increases the sensitivity and provides independence of the direction of gravity. The measurements are performed for water and ethylene glycol with sensors of diameters 25, 50, and 150 μm and temperature differences between sensor and medium of 15°, 30°, and 45 °C. The frequency range for horizontal motion is 20–160 Hz and for vertical motion is 20–500 Hz. The results are presented in terms of dependence of the ac Nusselt number as a function of relative displacement, Fourier number, and Rayleigh or Grashof number. At constant intensity, the output is inversely proportional to frequency to the 3/2 power for the horizontal case, and inversely proportional to the frequency for the vertical case.

Analysis of the Phase Difference Between Particle Motion Components of Sound By Teleosts

ABSTRACT The hypothesis that fish may remove 180° ambiguities concerning the sound source direction by a timing analysis between particle motion components is addressed. Analysis of the phase difference ψ (w, v) between the horizontal (v) and the vertical (w) components of the particle velocity of sound signals by teleost fish is investigated by cardiac conditioning. Standing wave sound stimuli (90 Hz) are used with ψ (w, v) = + or – 90° and equal amplitudes of v and w. The water particles move along circular trajectories for such stimuli: however, the direction of revolution is reversed in the two stimuli. These stimuli were discriminated by one whiting, one catfish and several cod, indicating that these teleosts are able to discriminate signals on a pure timing cue. If the w/v ratio is lowered, the cod can discriminate the resulting elliptical motions on the direction of revolution down to a w/v ratio of –12 dB. Moreover, down to the same w/v ratio the cod discriminates an elliptical particle motion signal from a pure translatory particle motion signal. At this level, the threshold signal-to-noise ratio for the w component is exceeded by 9 dB. Apparently phase analysis can be limited by ‘cross talk’ between horizontal and vertical particle motion detectors. The results are discussed with respect to models of directional hearing by fish.

Evidence for the subducting lithosphere under southern Vancouver Island and western Oregon from teleseismic P wave conversions

Long‐period teleseismic P waves recorded at VIC (Victoria, British Columbia) and COR (Corvallis, Oregon) show anomalously large Ps conversions and later arriving P‐to‐S reverberations not observed from typical continental crustal sections or from previously proposed structures for these stations determined from refraction surveys. The timing and large amplitude of the Ps phase, relative to direct P, suggests a high velocity‐contrast interface at 45 to 50 km depth under VIC and COR forming the base of a distinct low velocity zone. This interface is proposed to be the oceanic Moho which is being subducted under North America. Off azimuth Ps recorded at COR is consistent with a 20° eastward dip for the interface. Horizontal particle motion at both sites show evidence for lateral heterogeneity in local crustal structure. The distinct low velocity zone and its negative velocity gradient with depth has important consequences for refraction interpretation in the region since the usual assumption of increasing velocity with depth is violated. Crustal thicknesses derived from such misinterpretation may be overestimated. In principle, this type of structure suggests a solution for the Vancouver Island crustal thickness problem in which the observed positive Bouguer gravity anomaly is inconsistent with the 50 km thick crustal thickness derived from previous refraction work.

Triple-Doppler Observations of a Convective Storm

Abstract The advantages of three or more radars is illustrated in terms of extended areal coverage, error variance reduction and mitigation of poor sampling effects of divergence fields near the ground. Errors in the horizontal and vertical particle motion fields are illustrated for a three-, four- and five-radar network, and the importance of experimental design is illustrated. A variationally formulated multiple-Doppler radar analysis, which includes data from a surface network, and an illustrative analysis of storm data are presented.

Effect of an Ice Layer on Arctic SOFAR Propagation

The effect of an ice layer on Arctic SOFAR propagation is investigated from the normal-mode solution of the sound field in multilayered media. For an ice layer 3 m thick, typical of the central Arctic Ocean, the effect on pressure amplitudes at depth and dispersion is small, but even this thin ice sheet causes a large change in particle motions near the surface. Waves are elliptically polarized in the ice in the plane of propagation. Particle motion is retrograde elliptical at the surface and prograde elliptical at the bottom of the ice. This orbital motion is similar to that of flexural waves in the ice. Flexural waves generated by large-scale ice movements at the boundaries of floes are the principal background noise in the ice. It is significant that a node of horizontal particle motion often does not occur at the same depth in the ice for both hydroacoustic waves and flexural waves. This effect may be useful to improve the signal-to-noise ratio when horizontal component geophones are used as listening devices.

Waves, weather, and ocean bottom microseisms

The latest model of the Lamont Geological Observatory Ocean Bottom Geophysical Station (OBS III), located approximately 200 km WNW of San Francisco at a depth of 3.9 km, has been in operation since May 1966. In addition to long- and short-period seismic data, this station provides data on water current direction and speed, water temperature, long- and short-period pressure variations, and horizontal and vertical accelerations at tidal periods. Twenty-three microseismic storms have been detected on the seismic and pressure sensors during the first eight months of operation (May-December 1966). The characteristics of two of these storms have been studied in detail, and the results have been compared with data from the Berkeley seismographs. Both peaks in microseismic activity appear to be related to specific weather systems, one local and one distant. Correlation between local water waves and microseismic amplitudes is observed in one case but not in the other. Particle motion amplitudes on the ocean bottom were 3 to 5 times larger than those at a near-land station (Berkeley) for the vertical component and 7 to 10 times larger for the horizontal component. Predominant periods (6 to 8 sec) were the same at both sites. Ratios of horizontal to vertical particle motion and pressure to vertical motion were measured for microseisms and compared with theoretical values for Rayleigh waves. The combination of these two ratios proves to be a very sensitive method of determining sediment properties. Coherence between pressure and vertical motion is consistently high. It is concluded that (1) the observed microseisms propagate primarily as Rayleigh waves of the fundamental mode, and (2) the thickness of the sediment layer is 0.65 km. The predominant direction of microseismic propagation during both storms was approximately perpendicular to the coastline. Study of the average phase relationship of pressure, vertical particle motion, and horizontal particle motion shows that the direction of propagation is from sea to land if fundamental mode Rayleigh waves are assumed. The microseismic energy flux was approximately 4 times larger in the oceanic structure than in the continental structure near Berkeley for both storms.