Long-period Surface Waves Research Articles

New constraints on the rupture process of the 1999 August 17 Izmit earthquake deduced from estimates of stress glut rate moments

SUMMARY This paper illustrates the use of integral estimates given by the stress glut rate moments of total degree 2 for constraining the rupture scenario of a large earthquake in the particular case of the 1999 Izmit mainshock. We determine the integral estimates of the geometry, source duration and rupture propagation given by the stress glut rate moments of total degree 2 by inverting long-period surface wave (LPSW) amplitude spectra. Kinematic and static models of the Izmit earthquake published in the literature are quite different from one another. In order to extract the characteristic features of this event, we calculate the same integral estimates directly from those models and compare them with those deduced from our inversion. While the equivalent rupture zone and the eastward directivity are consistent among all models, the LPSW solution displays a strong unilateral character of the rupture associated with a short rupture duration that is not compatible with the solutions deduced from the published models. With the aim of understand this discrepancy, we use simple equivalent kinematic models to reproduce the integral estimates of the considered rupture processes (including ours) by adjusting a few free parameters controlling the western and eastern parts of the rupture. We show that the joint analysis of the LPSW solution and source tomographies allows us to elucidate the scattering of source processes published for this earthquake and to discriminate between the models. Our results strongly suggest that (1) there was significant moment released on the eastern segment of the activated fault system during the Izmit earthquake; (2) the apparent rupture velocity decreases on this segment.

Response of a 14-Story Anchorage, Alaska, Building in 2002 to Two Close Earthquakes and Two Distant Denali Fault Earthquakes

The recorded responses of an Anchorage, Alaska, building during four significant earthquakes that occurred in 2002 are studied. Two earthquakes, including the 3 November 2002 M7.9 Denali fault earthquake, with epicenters approximately 275 km from the building, generated long trains of long-period (>1 s) surface waves. The other two smaller earthquakes occurred at subcrustal depths practically beneath Anchorage and produced higher frequency motions. These two pairs of earthquakes have different impacts on the response of the building. Higher modes are more pronounced in the building response during the smaller nearby events. The building responses indicate that the close-coupling of translational and torsional modes causes a significant beating effect. It is also possible that there is some resonance occurring due to the site frequency being close to the structural frequency. Identification of dynamic characteristics and behavior of buildings can provide important lessons for future earthquake-resistant designs and retrofit of existing buildings.

Contribution of Dense Array Analysis to the Identification and Quantification of Basin-Edge-Induced Waves, Part I: Methodology

Recent earthquakes have shown that edge-generated surface waves can significantly contribute to increased damages. Most observations of edge-generated surface waves are concerning long-period surface waves propagating in large-size valleys. Since travel times of such waves between valley edges can reach several tens of seconds, they are quite easy to isolate. In small-size structures, reverberating wave trains are mixed and very dense array analysis is required for the identification of basin-induced surface wave trains. The city of Grenoble (French Alps) is located in a small-size deep alluvial valley and faces important site effects (Lebrun et al. , 2001). In order to identify and quantify multidimensional site effects in this basin, a very dense array of 29 three-component seismometers over a 1-km aperture was installed within the city. The wave-field complexity as well as the in situ noise characteristics (colored/correlated noise and low signal-to-noise ratio) led us to develop a procedure based on time-frequency coherence of signal waveforms and the multiple signal classification (MUSIC) (Schmidt, 1981) algorithm to identify the main energetic contributions crossing the array. Next, the nature and energy of waves were estimated using some properties of the analytical three-component covariance matrix. Careful methodological investigations were performed in order to better understand and quantify the effects of site constraints on the estimation of wave parameters with the MUSIC technique. Simulations outline the ability of array antennas first to handle difficult scenarios involving multiple, nonstationnary, and correlated propagating phases and second to estimate the polarization and energy of waves. The velocity estimation is shown to be much more unstable than backazimuth estimation, and a low signal-to-noise ratio introduces some variation in estimates. Finally, considering the very large number of identified waves, a statistical view of final estimates is suggested for improving the reliability of analysis. In an accompanying article (Cornou et al. , 2003), we use this method to investigate the entire wave field of seismic events recorded by the array in order to isolate basin-induced waves. Manuscript received 21 June 2002.

Centroid–moment tensor solutions for October–December, 1999

Centroid–moment tensor solutions are presented for 258 earthquakes that occurred during the fourth quarter of 1999. The solutions are obtained using corrections for aspherical earth structure represented by a whole mantle shear velocity model SH8/U4L8 of Dziewonski and Woodward [Dziewonski, A.M., Woodward, R.L., 1992. Acoustic imaging at the planetary scale, in: Emert, H., Harjes, H.-P. (Eds.), Acoustical Imaging, Plenum, 19, 785–797]. A model of anelastic attenuation of Durek and Ekström [Durek, J.J., Ekström, G., 1996. A radial model of anelasticity consistent with long-period surface wave attenuation, Bull. Seismol. Soc. Am., 86, 144–158] is used to predict the decay of the waveforms.

Centroid-moment tensor solutions for April–June, 1999

Centroid-moment tensor solutions are presented for 218 earthquakes that occurred during the second quarter of 1999. The solutions are obtained using corrections for aspherical earth structure represented by a whole mantle shear velocity model SH8/U4L8 of [Dziewonski, A.M., Woodward, R.L., 1992. Acoustic imaging at the planetary scale. In: Emert, H., Harjes, H.-P. (Eds.), Acoustical Imaging, Plenum, 19, 785–797]. A model of anelastic attenuation of [Durek, J.J., Ekström, G., 1996. A radial model of anelasticity consistent with long-period surface wave attenuation, Bull. Seismol. Soc. Am., 86, 144–158.] is used to predict the decay of the waveforms.

Centroid-moment tensor solutions for July–September 1999

Centroid-moment tensor (CMT) solutions are presented for 231 earthquakes that occurred during the third quarter of 1999. The solutions are obtained using corrections for aspherical earth structure represented by a whole mantle shear velocity model SH8/U4L8 of Dziewonski and Woodward (Dziewonski, A.M., Woodward, R.L., 1992. Acoustic imaging at the planetary scale, In: Emert, H., Harjes, H.-P. (Eds.), Acoustical Imaging, Plenum, New York, Vol. 19, pp. 785–797.). A model of anelastic attenuation of Durek and Ekström (Durek, J.J., Ekström, G., 1996. A radial model of anelasticity consistent with long-period surface wave attenuation. Bull. Seismol. Soc. Am. 86 144–158.) is used to predict the decay of the waveforms.

Seismic waveform modeling and surface wave tomography in a three-dimensional Earth: asymptotic and non-asymptotic approaches

We investigate the impact of the theoretical limitations brought by asymptotic methods on upper-mantle tomographic models deduced from long-period surface wave data (period >80 s), by performing a synthetic test using a non-asymptotic formalism. This methodology incorporates the effects of back and multiple forward scattering on the wave field by summing normal modes computed to third order of perturbations directly in the 3D Earth, and models the sensitivity to scatterers away from the great-circle path. We first compare the methods we used for the forward problem, both theoretically and numerically. Then we present results from the computation of 7849 synthetic Love waveforms in an upper mantle model consisting of two heterogeneities with power up to spherical harmonic degree 12. The waveforms are subsequently inverted using a 0th order asymptotic formalism (equivalent to a path-average approximation in the surface waves domain). We show that the main structures are retrieved, but that the theoretical noise on the output model is of the same order as the noise due to the path-coverage and a priori constraints.

Resonant oscillations between the solid earth and the atmosphere

Continuously excited free oscillations of the whole Earth have been found in the ground noise in a frequency band of long-period surface waves. Here we report evidence of an annual variation of this phenomenon, indicating that the excitation source is not within the solid Earth. There is evidence that these seismic free oscillations resonate with acoustic free oscillations of the atmosphere. The observed amplitudes suggest that the excitation source is at or just above Earth's surface.

Strong Motion from Surface Waves in Deep Sedimentary Basins

It is widely recognized that long-period surface waves generated by conversion of body waves at the boundaries of deep sedimentary basins make an important contribution to strong ground motion. The factors controlling the amplitude of such motion, however, are not widely understood. A study of pseudovelocity response spectra of strong-motion records from the Los Angeles Basin shows that late-arriving surface waves with group velocities of about 1 km/sec dominate the ground motion for periods of 3 sec and longer. The rate of amplitude decay for these waves is less than for the body waves and depends significantly on period, with smaller decay for longer periods. The amplitude can be modeled by the equation \batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \[\mathrm{log}{ }y=f(\mathbf{M},R\_{\mathrm{E}})+c+bR\_{\mathrm{B}}\] \end{document} where y is the pseudovelocity response, f ( M , R E) is an attenuation relation based on a general strong-motion data set, M is moment magnitude, R E is the distance from the source to the edge of the basin, R B is the distance from the edge of the basin to the recording site, and b and c are parameters fit to the data. The equation gives values larger by as much as a factor of 3 than given by the attenuation relationships based on general strong-motion data sets for the same source-site distance. It is clear that surface waves need to be taken into account in the design of long-period structures in deep sedimentary basins. The ground-motion levels specified by the earthquake provisions of current building codes, in California at least, accommodate the long-period ground motions from basin-edge-generated surface waves for periods of 5 sec and less and earthquakes with moment magnitudes of 7.5 or less located more than 20 km outside the basin. There may be problems at longer periods and for earthquakes located closer to the basin edge. The results of this study suggest that anelastic attenuation may need to be included in attempts to model long-period motion in deep sedimentary basins. To obtain better data on surface waves in the future, operators of strong-motion networks should take special care for the faithful recording of the long-period components of ground motion. It will also be necessary to insure that at least some selected recorders, once triggered, continue to operate for a time sufficient for the surface waves to traverse the basin. With velocities of about 1 km/sec, that time will be as long as 100 sec for a basin the size of the Los Angeles Basin.

Centroid-moment tensor solutions for January–March 1999

Centroid-moment tensor solutions are presented for 240 earthquakes that occurred during the first quarter of 1999. The solutions are obtained using corrections for aspherical earth structure represented by a whole mantle shear velocity model SH8/U4L8 of Dziewonski and Woodward [Dziewonski, A.M., Woodhouse, J.H., 1983. An experiment in the systematic study of global seismicity: centroid-moment tensor solutions for 201 moderate and large earthquakes of 1981. J. Geophys. Res. 88, 3247–3271]. A model of anelastic attenuation of Durek and Ekström [Durek, J.J., Ekström, G., 1996. A radial model of anelasticity consistent with long-period surface wave attenuation. Bull. Seism. Soc. Am. 86, 144–158.] is used to predict the decay of the waveforms.

Non‐double‐couple moment tensor of the March 25, 1998, Antarctic Earthquake: Composite rupture of strike‐slip and normal faults

The 1998 Antarctic intraplate earthquake (Mw 8.1) has a significant non‐double‐couple component in the moment tensor solution as determined from long‐period surface waves. In the teleseismic P and SH wave‐forms, there are two distinctive wave packets that can be explained by multiple strike‐slip subevents. We propose a composite rupture model comprising two different episodes: two predominant clusters of en echelon strike‐slip segments (∼16 × 1020 Nm) and normal faulting with a long duration of approximately 100 s (∼4 × 1020 Nm). This model agrees with observations of both teleseismic body waves and long‐period surface waves, including the large non‐double‐couple component. The normal faulting with as long a duration as the main strike‐slip rupture might be secondarily induced by the en echelon segments of strike‐slip faulting.

Centroid-moment tensor solutions for October–December, 1998

Centroid-moment tensor solutions are presented for 242 earthquakes that occurred during the fourth quarter of 1998. The solutions are obtained using corrections for aspherical earth structure represented by a whole mantle shear velocity model SH8/U4L8 of Dziewonski and Woodward [Dziewonski, A.M., Woodward, R.L., 1992. Acoustic imaging at the planetary scale. In: Emert, H., Harjes, H.-P. (Eds.), Acoustical Imaging, Vol. 19. Plenum, pp. 785–797]. A model of anelastic attenuation by Durek and Ekström [Durek, J.J., Ekström, G., 1996. A radial model of anelasticity consistent with long-period surface wave attenuation. Bull. Seismol. Soc. Am. 86, 144–158] is used to predict the decay of the waveforms.

Centroid-moment tensor solutions for July–September, 1998

Centroid-moment tensor solutions are presented for 214 earthquakes that occurred during the third quarter of 1998. The solutions are obtained using corrections for aspherical earth structure represented by a whole mantle shear velocity model SH8/U4L8 of Dziewonski and Woodward [Dziewonski, A.M., Woodward, R.L., 1992. Acoustic imaging at the planetary scale. In: Emert, H., Harjes, H.-P. (Eds.), Acoustical Imaging. Plenum, New York, 19, 785–797]. A new model of anelastic attenuation of Durek and Ekström [Durek, J.J., Ekström, G., 1996. A radial model of anelasticity consistent with long-period surface wave attenuation. Bull. Seism. Soc. Am. 86, 144–158] is used to predict the decay of the waveforms.

Evolution of oceanic upper mantle structure

Love and Rayleigh wave phase velocities increase systematically with increasing age of oceanic lithosphere up to 150 Ma. The rates of increase differ between oceans and vary for different age intervals. Modeling of lithospheric age–phase velocity relations indicates that the high velocity seismic lid thickens and the velocities in the sub-lithospheric low velocity zone (LVZ) increase with age at different rates between oceans. The initial thickness of the lithosphere near mid-ocean ridges, as averaged by long-period surface waves, varies between 10 and 45 km, and 100 Ma lithosphere has thickness from 83 to 110 km, for models with isotropic shear velocities. Hotspots in oceanic areas appear to modify oceanic lithosphere evolution, producing deviations from average patterns with increasing age.

Centroid-moment tensor solutions for April–June, 1998

Centroid-moment tensor solutions are presented for 219 earthquakes that occurred during the second quarter of 1998. The solutions are obtained using corrections for aspherical earth structure represented by a whole mantle shear velocity model SH8/U4L8 of Dziewonski and Woodward [Dziewonski, A.M., Woodward, R.L., 1992. Acoustic imaging at the planetary scale. In: Emert, H., Harjes, H.-P. (Eds.), Acoustical Imaging. Plenum, Vol. 19, pp. 785–797.]. A new model of anelastic attenuation of Durek and Ekström [Durek, J.J., Ekström, G., 1996. A radial model of anelasticity consistent with long-period surface wave attenuation. Bull. Seismol. Soc. Am. 86, 144–158.] is used to predict the decay of the waveforms.

Centroid-moment tensor solutions for April–June, 1997

Centroid-moment tensor solutions are presented for 220 earthquakes that occurred during the second quarter of 1997. The solutions are obtained using corrections for aspherical earth structure represented by a whole mantle shear velocity model SH8/U4L8 of Dziewonski and Woodward [Dziewonski, A.M., Woodward, R.L., 1992. Acoustic imaging at the planetary scale. In: Emert, H., Harjes, H.-P. (Eds.), Acoustical Imaging. Plenum Press, Vol. 19, pp. 785–797]. A new model of anelastic attenuation of Durek and Ekström [Durek, J.J., Ekström, G., 1996. A radial model of anelasticity consistent with long-period surface wave attenuation, Bull. Seism. Soc. Am., 86, 144–158] is used to predict the decay of the waveforms.

Crust and upper mantle heterogeneities in the southwest Pacific from surface wave phase velocity analysis

Direct earthquake-to-station Rayleigh and Love wave data observed on high gain broadband records are analyzed in order to improve the lateral resolution of the uppermost mantle in the southwest Pacific region. We used data of nine permanent Geoscope and Iris stations located in the southern hemisphere and nine other stations as part of two temporary networks, the first one installed in New Caledonia and Vanuatu (hereafter named Cavascope network) by ORSTOM and the EOST from Louis Pasteur University in Strasbourg (France) and the second one installed in the Fiji, Tonga and Niue islands (hereafter named Spase network) by Washington University in St. Louis (USA). In order to collect more significant details on the surficial structures, we included the analysis of short period waves down to 8 s. A multiple frequency filtering technique has been used to recover phase velocities of Rayleigh and Love waves for selected earthquakes with magnitude greater than 5.5 and with known centroid moment tensor (CMT). About 1100 well-distributed seismograms have been processed in the period range 8–100 s and corrections for topography and water depth have been applied to the observed phase velocities. The geographical distribution of phase velocity anomalies have then been computed using the tomographic method developed by Montagner [Montagner, J.P., 1986a. Regional three-dimensional structures using long-period surface waves. Ann. Geophys. 4 (B3), 283–294]. Due to a poor knowledge of dense, well-distributed, crustal thickness values and corresponding velocity models, we did not perform or speculate on the construction of an S-wave 3D velocity model; therefore, we limited this study to the interpretation of the phase velocity distribution. The location of phase velocity anomalies are well determined and the deviations are discussed within the framework of the geological context and compared with other tomographic models. At long periods, from 40 s to 100 s, our results agree well with most of previous studies: the tomographic imaging shows a large contrast between low and high phase velocities along the Solomon, New Hebrides and Fiji–Tonga trenches. The lowest phase velocity anomalies are distributed beneath northern and southern Fiji basins and the Lau basin (corresponding to the volume situated just above the dipping slabs), whereas the highest values are displayed beneath the Pacific plate and the eastern part of Indian plate downgoing under the North Fiji basin. At shorter periods, our results show that the phase velocity distributions are well correlated with the large structural crustal domains. The use of local temporary broadband stations in the central part of the studied area gives us the opportunity to observe surface waves showing well-dispersed trains, allowing extended velocity measurements down to 8 s although aliasing due to multipaths become important. The continental regions (Eastern Australia, New Guinea, Fiji islands and New Zealand) show low velocities which are likely due to thick continental crust, whereas the Tasmanian, D'Entrecasteaux, and the Northern and Southern Fiji basins are characterized by higher velocities suggesting thinner oceanic crust. Additional analysis including the anisotropic case and S-wave velocity inversion with depth is in progress.

Centroid-moment tensor solutions for October–December, 1997

Centroid-moment tensor solutions are presented for 238 earthquakes that occurred during the fourth quarter of 1997. The solutions are obtained using corrections for aspherical earth structure represented by a whole mantle shear velocity model SH8/U4L8 of Dziewonski and Woodward [Dziewonski, A.M., Woodward, R.L., 1992. Acoustic imaging at the planetary scale. In: Emert H., Harjes, H.-P. (Eds.), Acoustical Imaging, Plenum, 19, 785–797]. A new model of anelastic attenuation of Durek and Ekström [Durek, J.J., Ekström, G., 1996. A radial model of anelasticity consistent with long-period surface wave attenuation, Bull. Seism. Soc. Am., 86, 144–158] is used to predict the decay of the waveforms.

Centroid-moment tensor solutions for January–March 1997

Centroid-moment tensor solutions are presented for 162 earthquakes that occurred during the first quarter of 1997. The solutions are obtained using corrections for aspherical earth structure represented by a whole mantle shear velocity model SH8/U4L8 of Dziewonski and Woodward [Dziewonski, A.M., Woodward, J.H., 1992. Acoustic imaging at the planetary scale. In: H. Emert, H.-P. Harjes (Eds.), Acoustical Imaging. Plenum, New York, 19, 785–797]. A new model of anelastic attenuation of Durek and Ekström [Durek, J.J., Ekström, G., 1996. A radial model of anelasticity consistent with long-period surface wave attenuation, Bull Seismol. Soc. Am., 86, 144–158] is used to predict the decay of the waveforms.

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.