Fundamental Mode Surface Waves Research Articles

A study of the Barisakho, Georgia, earthquake of 1992 October 23 from broad-band surface and body waves

SUMMARY We study the 1992 October Georgian earthquake located in the centra) part of the Great Caucasus. This is the last strong (Ms = 6.5) shallow earthquake of a Series of three that struck this region in the two-year period of 1991-1992. In order to determine the source parameters we analysed surface and body waves from the IRIS and GEOSCOPE very broad-band digital networks. The fundamental mode of surface waves was retrieved using frequency-time analysis (FTAN), a technique that makes inversion more stable and enables the measurement of the spectral features from the signal in the presence of stationary noise. We used long-period surface waves (60-150 s) to obtain the seismic moment (Mo =4.8~10'~ N m) and the focal mechanism (strike 8=300, dip 6 = 15 and rake A= 135). The space-time distribution of the source moment density tensor was determined from intermediate-period (35-60 s) surface waves. We found the following results: duration = 6 s, rupture velocity = 3.2 km s-I, major and minor axes of the fault surface I,,, = 32 km and Zmin = 20 km, respectively. For the inversion of body waves we performed a shape waveform modelling using the gradient method of NabElek. We considered the source as a point source of 6 s duration. The results of body-wave inversion (Mo = 3.0~ 10 N m, 8 = 300°, 6 = 17 and 1 = 137) are in good agreement with estimates from surface waves. One of the nodal planes of the fault-plane solution corresponds to a thrust fault which agrees with the general seismotectonic features of the region of the Kartli and Kokheti ridges in the central part of the Great Caucasus. The southern slope of the Great Caucasus strikes WNW-ESE and the underlying crystalline basement plunges towards the NNE under the main axial zone. The source plane coincides with the location of the contact between the crystalline basement and the upper-block overthrust.

Examination of slow and late moment release of the 1988 Spitak and 1991 Racha, Caucasus, earthquakes

SUMMARY Two large shallow earthquakes occurred in the Caucasus, part of the collisional belt caused by the northward movement of the Arabian plate with respect to the Eurasian plate: the 1988 December 7 Spitak (Armenian) (M,6.7) and 1991 April 29 Racha (M,6.9) earthquakes. In the present study, slow and late moment releases of the earthquakes, which were previously proposed based on the body-wave analyses, are examined using long-period surface waves. Results from spectral inversions using fundamental-mode surface waves show that the source durations of both earthquakes are about 20 s. Although additional investigations utilizing the long-period surface waves, which are optimized to detect slow and late moment release, were employed, no evidence of slow and late moment release was found for either earthquake. For the Racha earthquake, the obtained source duration is consistent with the result from body-wave analyses. Within the resolution of the body-wave data, the simple P and SH waveforms are well modelled by a source time history model with a duration of 20 s. However, for the Spitak earthquake, the source duration obtained from the surface waves is about 30 s shorter than the source model suggested in the body-wave analysis. This discrepancy is attributed to a slow and late thrust-fault slip in the body-wave source model, which was proposed in order to interpret the late arrivals of P waves as the result of source complexity. Numerical experiments predict that Rayleigh-wave radiation is sensitive to the radiation from the proposed slow and late slip. A comparison between the observed and predicted Rayleigh-wave radiation patterns, however, provides no indication of the surface-wave radiation from the slow and late slip. It is unlikely that the discrepancy in the source model is only due to errors in modelling the propagation of long-period surface waves, because for the Racha earthquake the source models obtained from body and surface waves are consistent. The present results thus indicate that the late arrivals of P waves are probably caused by something other than source complexity, for example complex local structure near the earthquake source in the continental collisional belt. For the Spitak earthquake, a large non-double-couple component is observed in the moment tensor solution obtained at long periods. If the non-double-couple component is due to source complexity, it should be produced within the 20 s rupture process.

Dispersion of laser generated surface waves in an epoxy-bonded layered medium

In this paper, the dispersion of laser generated surface wave in an epoxy bonded copper-aluminum layered specimen is studied. A laser ultrasonic experiment based on the point-source/point-receiver (PS/PR) technique was conducted to measure the surface wave signals in the layered specimen. The received wave signals were then processed in the frequency domain to obtain the dispersion relation of the fundamental surface wave mode. Theoretical calculations of the dispersion relations of the fundamental surface wave modes in two-layered and three-layered specimens were conducted to explore the influence of the bonding layer thickness on the dispersion relation. The experimental dispersion relation for the epoxy bonded copper-aluminum layered specimen is in good agreement with the calculated dispersion relation. The influence of the bonding layer thickness on the dispersion relation is studied and the potential application of the present results to the NDE of bonded layered media based on laser ultrasonics is also addressed.

Using differential waveform data to retrieve local S velocity structure or path‐averaged S velocity gradients

An algorithm is presented which uses the surface waveforms originating from one event recorded in two stations which are located close to each other. The waveforms are inverted for local laterally homogeneous S velocity structure or for path‐averaged horizontal S velocity gradients. In the first case the stations are situated on the same great circle to the source. In the second case the two stations lie at approximately the same epicentral distance, but with a slightly different azimuth. In both cases, the frequency‐dependent phase differences between the two recordings are used as data for an inversion for S velocity values. No a priori knowledge about the source parameters and the medium between the source and the nearest receiver is needed. These factors are estimated in the first step of the inversion (linear). In the second step of the inversion (nonlinear) the S velocity information is obtained. Synthetic tests show that the algorithm is a powerful tool to image structure with only two fundamental mode surface wave recordings as data. In addition, they show that for the case where a horizontal gradient is determined, higher modes can be included in the analysis to display gradients at larger depths, without the need to identify the individual modes separately. The power of the method is illustrated with some real data examples that are all related to a pronounced tectonic boundary in western Eurasia: the Tornquist‐Teisseyre zone. The results of the experiments with real data coincide well with previously obtained models for this area.

Shear wave velocity structure over the eastern Indian subcontinent

Fundamental-mode surface wave generated by twelve earthquakes, which occurred in northeastern India and nearby regions and were recorded at the Shillong (SHL) and Chiengmai (CHG) seismic stations are used to determine the shear wave velocity structure beneath these regions. The Frequency Time Analysis method is used to determine the group velocities for periods ranging from 4 to 50 s for fundamental-mode Rayleigh and Love waves. A high shear wave velocity (4.2–4.7 km s −1) is estimated in the lower 30–34 km of the crust. Crustal thickness is found to vary from 36 to 56 km with an overlying 2–4 km sedimentary thickness of shear wave velocity of 2.55–2.75 km s −1. The shear wave velocity in the upper 10 km of crust is 3.5–3.7 km s −1 below the sedimentary layer. The inferred high shear velocity for the lower crust beneath these regions suggests an oceanic affinity and they are different from the Indian shield structure. The lower crust must have an oceanic origin as derived from the reminiscent of a certain oceanic lithosphere. The available data do not permit us to estimate the upper mantle structure precisely; however, it shows a higher upper mantle shear velocity of 5.1–5.3 km s −1. This may be indicative of active tectonism beneath these regions in the uppermost mantle. The Moho boundary is not sharply defined here. The increase in crustal thickness may be due to the collision of the Indian and Eurasian plates in the north and subduction of the Indian plate beneath the Burmese Arc in the east.

Improved resolution of earthquake source parameters from long-period surface wave inversions

Source centroid location optimization moment tensor inversions of long-period fundamental mode surface wave spectra. Joint inversions of Rayleigh and Love wave spectra are used to find source moment tensors for the 18 October 1989 Loma Prieta, California, earthquake ( M w = 6.9) and the 22 April 1991 Valle de la Estrella, Costa Rica, earthquake ( M w = 7.6). For the Loma Prieta event, the focal mechanism at the optimal centroid location is consistent with that at the epicenter, and both agree with independent information. For the Costa Rica event, the solution at the optimal centroid location is consistent with geodetic and body wave observations, whereas the solution found for the epicenter is not. We conclude that optimizing for centroid location in spectral inversions is possible and improves the determination of long-period surface wave source parameters.

Love-wave propagation in a three-dimensional sedimentary basin

Abstract A simplified three-dimensional finite-element method has been developed for simulation of Love-wave propagation in three-dimensional sedimentary basins. The eigenfunctions for the fundamental-mode surface waves are employed as interpolation functions in the finite-element scheme. By reducing the number of degrees of freedom, the method enables us to analyze wave propagation in an area of 2000 km2 as large as the southern part of the Kanto plain, Japan. Time histories of the near Izu-Ohshima earthquake of 1990 are calculated and compared with observation. Calculated displacement snapshots show the effect of three-dimensional topography on direction of Love-wave propagation. The three-dimensional simulation is also compared with a two-dimensional one, demonstrating amplitude increase and extended duration. Time histories and their spectra from the three-dimensional model show better agreement with the observations than those from the two-dimensional model.

Anisotropy beneath Hawaii from surface wave particle motion observations

Observed polarization ellipses for fundamental-mode surface waves observed at a digital station in Hawaii deviate from those expected for isotropic models of crust and mantle structure for that region. The anomalous motion occurs as rotations of the ellipse about all three axes in a cartesian corrdinate system. The largest and most consistent deviations occur as anomalous slopes of the ellipse about the horizontal axis transverse to the direction of propagation. The observed orientations and magnitudes of these angles can be explained by models of the upper mantle which contain olivine for which thea-axis dips significantly from the horizontal and which includes a sufficiently thick sedimentary layer (1 km) and a thicker than normal oceanic crust (15 km). The ellipses are also generally inclined from great circle paths about the vertical axis and are tilted about the axis aligned with the propagation direction. Both angles are small and difficult to measure, but the inclination angles are consistent with a model of the upper mantle in which thea-axis of olivine is preferentially oriented in an east-west direction.

Crust and upper mantle velocity structure beneath north and central India from the phase and group velocity of Rayleigh and Love waves

Crust and upper mantle velocity structure is determined using the fundamental mode Rayleigh and Love waves for 23 different paths across north and central India. The earthquakes occurring in Kashmir, Nepal Himalaya, Burma and nearby regions and recorded at Hyderabad, New Delhi and Kodaikanal seismic stations are used for dispersion studies. Frequency Time Analysis (FTAN) using the multiple filter technique is done for the fundamental mode surface waves to estimate group velocities from the time periods of 13–71 s. The single station technique is used to estimate Rayleigh and Love wave phase velocities at time periods of 15–75 s. Velocity structure down to a depth of 120 km is determined using both Rayleigh and Love wave phase and group velocities. The crustal thickness is found to be 50 km, with 4 km sedimentary thickness, for the northern India across Burma to New Delhi path, and 44 km for the central India across Kashmir, Nepal Himalaya and northeast India to Hyderabad and Kodaikanal path. A low-velocity zone ( V p = 8.4 km/s, V s = 4.15 km/s ) is found from 100 km below the surface. Velocity structure shows that the shield structure of the peninsula extends up to northernmost India below sub-Himalayan regions with an overlying sedimentary thickness of 4 km. The lower value of observed group velocity and higher phase velocity across northern and central India may be caused by the presence of varying sedimentary thickness in the northernmost part of India (the Indo-Gangetic Plain) below the sub-Himalayan region. The observed lower value of Rayleigh wave group velocities below 40 s time periods can be explained by assuming a thin layer of a high contrast material with a P-wave velocity of 4.6 km/s, an S-wave velocity of 2.5 km/s, a density of 3.24 g/cm 3 and a thickness of 3 km, centred above the Moho.

Three dimensional reconstruction of Pangea

The three dimensional seismic velocity structure from the surface of the Earth to a depth of about 900 km was determined from fundamental and higher mode surface waves. Velocity anomalies near the surface correlate well with the age of the surface rocks. Reconstruction of the ancient continent Pangea connects all fast velocity anomalies near the center of Pangea. Fast velocity anomalies extend to 200‐400 km. Considering the resolving power of long period surface waves, this indicates that Pangea was a continent with a lithospheric thickness of about 200 (±100) km or larger, which broke up right in the middle (Central Atlantic). Examinations of seismic velocity sections with respect to depth show different depth penetration of fast velocity anomalies under various continents such as North America, South America, Eurasia and Africa. We speculate that continents possessed different thicknesses before Pangea was formed and had moved over the surface of the Earth independently.

Higher‐mode surface waves and structure of the Great Basin of Nevada and western Utah

Observed seismograms and dispersion data for crust and mantle higher mode surface waves in the Great Basin are compared with theoretical seismograms and dispersion curves computed for the Great Basin model of Priestley and Brune [1978]. This structure was originally derived from fundamental mode surface wave and refraction data. Phases identified as Sa [T∼13 s] are observed to have a phase velocity of 4.50±0.03 km s−1. Crustal second Rayleigh mode (first shear mode) waves have predominant periods varying from about 5 s on some paths to about 8 s on others. The observed excitation and phase velocity of the Sa phase are in agreement with theoretical seismograms and computed phase velocities for a modified Great Basin model. The agreement provides added support, in an unexpected way, for the existence of a mantle lid of velocity about 4.5 km s−1 in the Great Basin. The crustal higher mode group velocity observations, i.e., the predominant periods of the second Rayleigh mode along various paths, reflect variations in crustal thickness within the Great Basin. Crustal thicknesses of approximately 25 km are indicated for some paths in northwestern Nevada and southeastern Oregon, whereas crustal thicknesses of greater than 35 km are indicated for east central Nevada. The crustal thickness of 35 km in the Great Basin model is probably most appropriate for the central part of the Great Basin.

Fault parameters of the St. Elias, Alaska, earthquake of February 28, 1979

abstractFault parameters of the Ms 7.1 St. Elias, Alaska, earthquake of February 28, 1979, are determined from an analysis of P-wave first motions, fundamental-mode surface waves, and aftershock data. The preferred P-wave nodal plane has a shallow (12°) angle of dip and indicates underthrusting in a northerly (N13°W) direction, which is also close to the azimuth (N8°W) of the deviatoric compression (P) vector. Aftershock activity during the 24-hr interval immediately following the main shock extends over an area of 3200 km2, which is taken to represent the fault area of the main shock. Because aftershock activity outlines a fault area with nonrectangular geometry, fault length (50 to 80 km) and width (50 to 65 km) are not well defined. Estimates of focal depth from aftershock activity fall in the range 10 to 20 km, which surface-wave analysis is unable to restrict further. For the main shock, seismic moment, Mo, is 2.5 × 1027 dyne-cm (0.25 × 1021 N.m) and average dislocation, 2 meters. Average rupture velocity falls in the range 2.5 to 3 km/sec. Apparent stress, ησ, is 5 bars (0.5 MPa) and stress drop, Δσ, 35 bars (3.5 MPa). These values are considered typical of large-magnitude interplate earthquakes.

Thrust faulting and crust--upper mantle structure in East Australia

Summary. The mid-crustal earthquake of 1973 March 9 (mb= 5.5, h≤ 20 km) located 60 km south-west of Sydney, Australia, provides unambiguous evidence of contemporary thrust faulting in South-eastern Australia — a region of high heat flow and Cenozoic basaltic volcanism. Aftershock locations suggest a steeply dipping fault in the depth range from 8 to 24 km with a lateral extent of about 8 km. The mechanism solution is consistent with a tectonic stress field that is dominated by east—west horizontal compression. A seismic moment of 5.7 ± 1023± 20 per cent dyne-cm was computed from surface-wave amplitudes. Minimum values of slip and stress drop, 2 cm and 1 bar respectively, were estimated from the moment and a fault size taken’ from aftershock locations. Refinement modelling by a controlled Monte Carlo technique was used to provide unbiased models directly from multimode group velocities. The dispersion of fundamental and higher mode surface waves recorded at the digital high-gain station at Charters Towers, Queensland, and the WWSSN station at Adelaide, South Australia, is satisfied by crust- and upper-mantle models which have neither pronounced S-wave low-velocity zones nor thick high-velocity lids within 140 km of the Earth's surface. These models have subcrustal shear velocities of 4.20–4.32 km/s which are 0.4–0.5 km/s slower than Canadian shield shear velocities (CANSD).

Reflection, refraction and mode conversion of long-period surface waves and the measurement of Q−1 for free oscillations

abstract Filtered time series of Rayleigh waves from 4 events recorded on the quartz accelerometer at the IGPP, Camp Elliott Station, were analyzed. Attenuation (Q−1) was computed for spheroidal fundamental modes (oS19 to oS24) from several sequences of time-lapsed records for each event. A five-fold variation in measured Q−1 (and some variations in peak frequency) was assumed to be the result of lateral inhomogeneities in earth structure. Utilizing the duality between Rayleigh waves and fundamental-mode spheroidal oscillations, model power spectra were computed by summing the simulated Fourier transforms of dispersed wave trains. The effect of lateral variations in earth structure resulting in reflection, refraction and mode conversion of fundamental-mode surface waves was simulated by changes in amplitude, phase angle, and group and component travel times. Assuming an anelastic 10,000/Q of 33.3 (Q = 300), the observed range of measured Q−1 (and peak frequency) variations was duplicated by models with up to 5 per cent of the fundamental-mode Rayleigh-wave energy being “scattered”, i.e., reflected, refracted or converted to higher modes. In the real Earth, this would call for lateral variations in velocity structure well below the upper few hundred kilometers of the mantle. Recent seismological investigations have suggested lateral variations at these depths.

Radiation and attenuation of rayleigh waves from the Southeastern Missouri Earthquake of October 21, 1965

The southeastern Missouri earthquake of October 21, 1965, generated fundamental mode and higher-mode surface waves that were widely recorded throughout North America. Amplitude radiation patterns for the fundamental and first higher Rayleigh modes were determined and compared with patterns computed for various fault plane solutions. The favored solution is that of a fault 4 km deep, oriented N70°E, dipping 50°S, and having a slip vector oriented 85° downward from the horizontal on the fault face. A least squares fitting process was devised to determine the source spectrum and the values for the Rayleigh wave attenuation coefficient at each period. The fundamental mode attenuation coefficient is slightly greater than 0.001 km−1 at a period of 4 sec, decreases rapidly to 0.0002 km−1 or less at periods between 17 and 25 sec, and increases slowly to about 0.00017 km−1 at 50 sec. The first higher mode attenuation coefficients parallel and are slightly lower than those of the fundamental mode at periods between 4 and 10 sec. The source spectrum peaks between 5 and 9 sec and appears to attain a lower dc level at periods of greater than 20 sec. The similarity in the shapes and the orientations of the short-period radiation patterns and the area of perceptibility for this earthquake suggests that the size of the ‘felt area’ of an earthquake is related to the nature of the radiation and the attenuation of short-period Rayleigh waves. This observation and the lower short-period Rayleigh wave attenuation coefficient values observed in the eastern United States in comparison with those observed in the western United States indicate that the larger areas of perceptibility of eastern United States earthquakes occur because short-period Rayleigh waves are transmitted more efficiently in that region.