Rayleigh Wave Signals Research Articles

Teleseismic inversion for rupture process of the 27 February 2010 Chile (Mw 8.8) earthquake

The 27 February 2010 Chile (Mw 8.8) earthquake is the fifth largest earthquake to strike during the age of seismological instrumentation. The faulting geometry, slip distribution, seismic moment, and moment‐rate function are estimated from broadband teleseismic P, SH, and Rayleigh wave signals. We explore some of the trade‐offs in the rupture‐process estimation due to model parameterizations, limited teleseismic sampling of seismic phase velocities, and uncertainty in fault geometry. The average slip over the ∼81,500 km2 rupture area is about 5 m, with slip concentrations down‐dip, up‐dip and southwest, and up‐dip and north of the hypocenter. Relatively little slip occurred up‐dip/offshore of the hypocenter. The average rupture velocity is ∼2.0–2.5 km/s.

Broadband ambient noise surface wave tomography across the United States

This study presents surface wave dispersion maps across the contiguous United States determined using seismic ambient noise. Two years of ambient noise data are used from March 2003 through February 2005 observed at 203 broadband seismic stations in the US, southern Canada, and northern Mexico. Cross‐correlations are computed between all station‐pairs to produce empirical Green functions. At most azimuths across the US, coherent Rayleigh wave signals exist in the empirical Green functions implying that ambient noise in the frequency band of this study (5–100 s period) is sufficiently isotropically distributed in azimuth to yield largely unbiased dispersion measurements. Rayleigh and Love wave group and phase velocity curves are measured together with associated uncertainties determined from the temporal variability of the measurements. A sufficient number of measurements (>2000) is obtained between 8 and 25 s period for Love waves and 8 and 70 s period for Rayleigh waves to produce tomographic dispersion maps. Both phase and group velocity maps are presented in these period bands. Resolution is estimated to be better than 100 km across much of the US from 8–40 s period for Rayleigh waves and 8–20 s period for Love waves, which is unprecedented in a study at this spatial scale. At longer and shorter periods, resolution degrades as the number of coherent signals diminishes. The dispersion maps agree well with each other and with known geological and tectonic features and, in addition, provide new information about structures in the crust and uppermost mantle beneath much of the US.

Non-linear enhancement of laser generated ultrasonic Rayleigh waves by cracks

Laser generated ultrasound has been widely used for detecting cracks, surface and sub-surface defects in many different materials. It provides a non-contact wideband excitation source, which can be focused into different geometries. Previous workers have reported enhancement of the laser generated Rayleigh wave, when a crack is illuminated by pulsed laser beam irradiation. We demonstrate that the enhancement observed is due to a combination of source truncation, the free boundary condition at the edge of the crack and interference effects. Generating a Rayleigh wave over a crack can lead to enhancement of the amplitude of the Rayleigh wave signal, a shift in the dominant frequency of the wideband Rayleigh wave and strong enhancement of the high frequency components of the Rayleigh wave.

Surface wave tomography of the western United States from ambient seismic noise: Rayleigh and Love wave phase velocity maps

SUMMARY We present the results of Rayleigh wave and Love wave phase velocity tomography in the western United States using ambient seismic noise observed at over 250 broad-band stations from the EarthScope/USArray Transportable Array and regional networks. All available threecomponent time-series for the 12-month span between 2005 November 1 and 2006 October 31 have been cross-correlated to yield estimated empirical Rayleigh and Love wave Green’s functions. The Love wave signals were observed with higher average signal-to-noise ratio (SNR) than Rayleigh wave signals and hence cannot be fully explained by the scattering of Rayleigh waves. Phase velocity dispersion curves for both Rayleigh and Love waves between 5 and 40 speriod were measured for each interstation path by applying frequency‐time analysis. The average uncertainty and systematic bias of the measurements are estimated using a method based on analysing thousands of nearly linearly aligned station-triplets. We find that empirical Green’s functions can be estimated accurately from the negative time derivative of the symmetric component ambient noise cross-correlation without explicit knowledge of the source distribution. The average traveltime uncertainty is less than 1 s at periods shorter than 24 s. We present Rayleigh and Love wave phase speed maps at periods of 8, 12, 16,and 20 s. The maps show clear correlations with major geological structures and qualitative agreement with previous results based on Rayleigh wave group speeds.

Enhancement of the Rayleigh wave signal at surface defects

We present details of a non-contacting technique that uses ultrasound to detect reliably surface defects with depths that are significantly smaller than the average ultrasonic wavelength to many times this depth. Two electro-magnetic acoustic transducers (EMATs) are used to generate and detect a broadband Rayleigh wave signal with a central frequency close to 200 kHz and extending to approximately 400 kHz. These transducers are moved with a fixed separation between them along a sample towards a defect. Enhancement of the ultrasonic signal is observed when the detector is very close to the defect, with the enhanced signal about 1.6 times the signal away from the defect. A similar enhancement is observed when using an EMAT for generation and a laser interferometer to measure the absolute surface displacement of the sample. We attribute the enhancement of the signal to straightforward interference between the ultrasonic signal that passes directly from the generating coil to the detector and that reflected from the defect. The enhancement can be used to pinpoint accurately a surface or subsurface defect, with the wide-band nature of the signal allowing us to gauge the depth of a wide range of surface-breaking defects.

Quantitative imaging of Rayleigh wave velocity with a scanning acoustic microscope.

An acoustic microscope operating with impulse excitation has been used to perform measurements of the Rayleigh wave velocity by measuring the time difference between the direct reflected signal and the Rayleigh wave signal. The accuracy and precision of the methodology have been examined by performing measurements at a single location on an elastically isotropic sample of E6 glass. The accuracy of the Rayleigh wave velocity measurement has been determined to be better than 0.5%. The measured Rayleigh wave velocity of (3035+/-5) m/s differs by 0.3% from measurements reported in the literature for a similar sample, using two different techniques. The methodology has been extended to acquire the Rayleigh wave velocity while raster scanning the sample to develop a quantitative velocity image. The background noise in the Rayleigh wave velocity image has been investigated by mapping the velocity on elastically isotropic E6 glass. Possible reasons for background noise in the images is discussed. The methodology has been extended to acquire quantitative Rayleigh wave velocity images on Ti-6Al-4V. The contrast in the images is attributed to the variation of the Rayleigh wave velocity in individual grains or regions. Applicability of the technique to investigate crystallographic texture in materials is discussed.

Eurasian fundamental mode surface wave phase velocities and their relationship with tectonic structures

We automatically analyzed 32,000 fundamental mode Love and Rayleigh wave signals with earthquake‐station paths traversing Eurasia and Indonesia and obtained robust average phase velocity measurements between 20 s and 170 s periods along 4389 Love and 4020 Rayleigh paths. These were inverted to give phase velocity maps at 14 fixed periods. Resolution tests suggest that features with diameter >750 km and >500 km are resolved over most of Eurasia and central/SE Asia respectively. Low‐period Love waves image areas with thick sedimentary cover as low‐velocity zones, and almost all periods image mountainous regions since these have thick crust and hence low average lithospheric shear velocity. At long periods, both Love and Rayleigh waves define high phase velocity zones across shield and cratonic areas reflecting their deep lithospheric roots. We observe significant along‐strike heterogeneity in the Zagros fold belt and Tien Shan‐Altai system. Taking sections across Eurasian phase velocity space allows us to make approximate interpretations in terms of shear velocity structure directly. For example, the Red River and East Vietnam Boundary faults are traced on their eastern side by low velocities which extend at depth into Indonesia. We relate this to mantle upwelling associated with early Eocene rotation of Indochina and reversal of the sense of shear across the Red River fault post‐20 Ma. We observe dipping subduction of the Mediterranean beneath the Aegean, of the Philippine Sea beneath Indonesia, and of the Indian shield beneath Tibet. We also image a fossil subducted plate beneath NE Borneo which we associate with subduction of the proto‐South China Sea between 50 Ma and 15 Ma.

HIGH-FREQUENCY ULTRASOUND DETECTION OF SMALL DEFECTS IN Si3N4 CERAMIC

Surface cracks and artificial volume defects in silicon nitride ceramic valves were studied using high-frequency ultrasound in the 10–80MHz range. The volume defects were pores of l00μm diameter. A focusing transducer with a focal length of 25 mm in water was operated in pulse-echo technique. Longitudinal, and mode converted transverse and Rayleigh waves were excited in the ceramic by variation of the angle of incidence of the sound beam in water. The echo amplitude of individual defects was studied as a function of the angle of incidence and defect depth. The amplitude of the backscattered longitudinal wave from the defect was always higher than that of the backscattered transverse wave. Leaky Rayleigh wave signals from surface cracks were obtained over a range of angles of incidence much beyond the calculated angle for a plane wave.

Observation and analysis of acoustic signals following the second or third front surface reflections in an acoustic microscope

In the normal mode of operation of an acoustic microscope, the properties of the sample are inferred from the time delay of the leaky Rayleigh wave signal with respect to the first specular reflection from the sample surface. In this paper, observations of similar leaky Rayleigh wave signals following the second and third front surface signals from the sample are reported. Included is a ray analysis of their origin, which identifies the second front surface as involving a Rayleigh wave in the lens and the third as involving a Rayleigh wave in the sample. It is suggested that the latter could be of significant practical interest since, for a given lens, leaky Rayleigh waves can be excited on samples with lower velocities than those following the first specular reflection.

Split spectrum processing of backscattered Rayleigh wave signals to improve detectability of fatigue microcracks: 51613 Resch, M.T.; Karpur, P. Cyclic Deformation, Fracture and Nondestructive Evaluation of Advanced Materials. Edited by M.R. Mitchell and O. Buck. pp. 323–333. ASTM STP 1157 (1992)

Split spectrum processing of backscattered Rayleigh wave signals to improve detectability of fatigue microcracks: 51613 Resch, M.T.; Karpur, P. Cyclic Deformation, Fracture and Nondestructive Evaluation of Advanced Materials. Edited by M.R. Mitchell and O. Buck. pp. 323–333. ASTM STP 1157 (1992)

Propagation of Rayleigh and Scholte waves along edge of quarter-space

A surprisingly large leaky Rayleigh wave component has been observed propagating along the edge of a laboratory nonsaline-ice quarter-space in water with both source and receiver positioned along the edge of one of the two surfaces of the quarter-space. Studies on different solids (aluminum, Plexiglas, limestone) indicate that all surface and interface edge waves travel slower than their corresponding waves on a solid half-space. The measured ratio of the edge Rayleigh wave velocity to the half-space Rayleigh wave velocity is 0.9617 for aluminum 6061, 0.9665 for Plexiglas, and 0.9954 for limestone. Ultrasonic experimental results are presented on edge Rayleigh waves, and on edge leaky Rayleigh waves and Scholte waves for the liquid/solid case. The edge wave has no geometrical spreading and dominates the received signal when the receiver is moved slightly away from the water loaded edge of the quarter space. The Rayleigh waves along the horizontal and vertical faces at the edge are out of phase influencing the radiation of the leaky Rayleigh wave for the water-loaded case. Near the edge it is observed that a decrease in the Scholte wave signal is accompanied by an increase in the leaky Rayleigh wave signal. The leaky Rayleigh wave velocity for the water/ice is slightly smaller than the shear wave velocity at the onset of the signal. [Work supported by DREP and ONR.]

Review of seismic source models for underground nuclear explosions

abstractA number of seismic source models for underground nuclear explosions have been developed over the past 2 decades. These models include the spherically symmetric compressional source model, the wave conversion source model, the tectonic strain release source model, the spall slapdown source model, and the near-regional source model. These model are reviewed in this study and are shown to be inconsistent with various geophysical data associated with underground nuclear explosions. In particular, the Rayleigh and Love wave signals generated by underground nuclear explosions have not been explained satisfactorily by any of these source models. To explain the observed explosion data, it may be necessary to model the explosion seismic source as a sequence of mechanisms producing seismic signals. These mechanisms all act within the first few seconds following the explosion detonation. One of the most important of these mechanisms is probably explosion-induced thrust faulting.

The Focal Mechanisms of a Small Intra Caribbean Plate Earthquake as Determined From Rayleigh Waves

The focal mechanism and depth of a small yet tectonically important, intra-Caribbean plate earthquake have been determined using observed Rayleigh wave signals. This earthquake occurred on August 14, 1972 and is believed to be the first known hypocenter in this area of the Caribbean Sea; about 330 kilometers from any previous event. Thus, the source mechanism of this earthquake should yield important information about the stresses in the interior of the Caribbean plate. Only about 5 reliable P-wave first motions were recorded at WWSSN stations, due to the low magnitude (M b = 4.7) of this earthquake. Therefore, the focal mechanism of this event can not be determined from the body wave data alone. Nonetheless, coherent surface wave trains were recorded as far away as 7,000 kilometers. The deduced mechanism is obtained from a comparison of the observed Rayleigh wave phase from this earthquake with that of a master event (September 13, 1971: M b = 5.7) located along the northern boundary of the Caribbean plate. The mechanism which yields the best fit to both the surface wave data and the body wave data is of the strike–slip type, with the compression axis trending approximately NW–SE.

Observation of Rayleigh wave dispersion at very long periods

The LaCoste-Romberg earth tide gravimeter with a Weber-Larson sensor plus a band-pass filter for rejecting the large semidiurnal and diurnal tidal signals is an excellent ultralong-period seismometer. As an accelerometer its passband extends from 140 to 7200 s at the 3-dB points; as a displacement transducer the peak of the instrumental response is about 100 s. The seismograms display Rayleigh wave signals at periods rarely seen visibly on World-Wide Standard Seismographic Network long-period seismograms. Phase velocities from fundamental mode Rayleigh waves at periods in excess of 400 s have been computed for several moderate-sized earthquakes for globe-circling paths; these phase velocities for the traveling wave spectrum of Rayleigh waves are in good agreement with values obtained from studies of the free oscillations of the earth.