The effect of crustal structure on the regional phases P g and L g at the Nevada test site

S U M M A R Y Broad-band data recorded by the Lawrence Livermore Laboratory Network are used to investigate whether or not Lg Q and Lg coda Q have similar values in the Basin and Range province of North America. In order to retrieve spectral estimates of the broad-band Lg and Lg coda signals, a hybrid method is used which retains high fidelity at lower frequencies while strongly smoothing the spectral estimates and minimizing the number of spectral points at high frequencies. A stochastic approach is used to model the Lg signal and ambient noise, and to analyse results from two-station determinations of Lg Q in the Basin and Range. The effects of ambient noise, frequency-dependent station site response and focusing/defocusing on Lg Q inversion are analysed. It is shown that after averaging spectral ratios determined from repeated observations over four two-station paths, a modest site effect will not cause significant errors in the resulting Lg Q relation. Using this inversion scheme and spectral ratio estimates averaged over those paths at frequencies between 0.2 and 2.5 Hz, we obtain the relation Q L g ( f ) = (267 f 56)f0.37*0.06 for the Basin and Range province. Further constraints on Lg Q were obtained from Lg coda, which, due to the natural averaging effect of scattering, is insensitive to effects of focusing/defocusing. To investigate the validity of the single isotropic scattering model for Lg coda in the Basin and Range province, we calculated ischrons for scatterers corresponding to later Lg coda signals, as required by the single scattering model. It is found that when significant portions of these isochrons are oceanic, Lg coda amplitude decay becomes irregular and abnormally slow, suggesting a strong scattering/reflection effect caused by the rapidly changing crustal structure along the coast. Applying the single isotropic scattering model to Lg coda records with isochrons mostly confined to the continental crust results in Q , ( f ) = (275 f 26) f 0.36*0.03 , which is nearly identical to our QLg relation. This indicates that even in a complicated geological province, a joint interpretation of the dynamic properties of Lg and Lg coda can be successful and fruitful. Inconsistent Lg Q and coda Q relations found in earlier studies in the Basin and Range province may be attributed to different settings of parameter trade-offs, different seismic phases, or different dynamic properties being used.

Section: Theoretical Modellingmentioning

confidence: 99%

“…Several studies have been conducted on the site response of seismic stations to the Lg phase, assuming the absence of both focusing and defocusing effects and lateral variations of Lg Q(Baker et al 1981;Campillo et al 1985;Chun et al 1987).…”

mentioning

confidence: 98%

Attenuation of multiphase surface waves in the Basin and Range province, part I:LgandLgcoda

Xie¹,

Mitchell²

1990

“…mbLg measurements are also used to derive y, the coefficient of anelastic attenuation, which is important in many types of wave propagation and attenuation studies and can be employed to assess the possible destructiveness of earthquakes. It is important to understand if reflections, refractions, or diffractions from changes in crustal thickness, generally ignored in studies measuring mbLg or y will produce significant effects not accounted for in the interpretations given (Nuttli 1973(Nuttli , 1978(Nuttli , 1981Street 1976Street , 1984Street, Herrmann & Nuttli 1975;Street & Turcotte 1977;Jones, Long & McKee 1977;Bollinger 1979;Barker, Der & Mrazek 1981;Nicolas et al, 1982;Dwyer, Herrmann & Nuttli 1983;Chung & Bernreuter 1981;Singh & Herrmann 1983;Campillo, Bouchon & Massinon 1984;Herrmann & Kijko 1983).…”

Section: Introductionmentioning

confidence: 99%

Numerical modelling of SHL_gwaves in and near continental margins

Regan

Harkrider

1989

The effect of transition regions between continental and oceanic structures on the propagation of L, waves from continental sources is examined. In particular, the attenuation due to variations in layer thickness in such transition regions is calculated and explained for a suite of simple models. The measured attenuation, due tq the geometry of the transition regions between the oceanic and continental structures within a partially oceanic path with source and receiver in a continental structure, is at most a factor of four for frequencies from 0.01 to 1 Hz. This is inadequate to explain the observed extinction of L, along such paths. This extinction has previously been attributed to the effects of the transition region geometry. The method used to calculate the results presented in this study is developed and its validity and accuracy are demonstrated. Propagator matrix seismograms are coupled into a Finite Element calculation to produce hybrid teleseismic SH mode sum seismograms. These hybrid synthetics can be determined for paths including any regional transition zone or other heterogeneity that exists as part of a longer, mostly plane-layered, path. Numerical results presented for a suite of transition models show distinct trends in each of the regions through which the wavefield passes. The wavefield passes through a continent-ocean transition region, then a region of oceanic structure, and finally through an ocean-continent transition region. When an L, wavefront passes through a continent-ocean transition, the amplitude and coda duration of the L, wave at the surface both increase. At the same time, much of the modal L, energy previously trapped in the continental crust is able to escape from the lower crust into the subcrustal layers as body waves. The magnitude of both these effects increases as the length of the transition region increases. When the wavefront passes through the region of oceanic structure further energy escapes from the crustal layer, and produces a decrease in L, amplitude at the surface. The rate of amplitude decrease is maximum near the transition region and decreases with distance from it. When the wavefield passes through the oceancontinent transition region a rapid decrease in the L, amplitude at the surface of the crust results. The energy previously trapped in the oceanic crustal layer spreads throughout the thickening crustal layer. Some of the body wave phases produced when the wavefield passes through the continent-ocean transition region are incident on the continental crust in the ocean-continent transition region. These waves are predominantly transmitted back into the crust. The other body wave phases reach depths below the depth of the base of the continental crust before reaching the ocean-continent transition and, thus, escape from the system. Key w o r k synthetic seismograms, L, waves, continental margins, finite element, attenuation half-space. The wavefields transmitted through the transition region models are calculated to model L,. The modal INTRODUCTION This pap...

“…It was not the intention to m6der any data; it was rather to understand the influence of non-uniform structures on the wavefield. It may be worth pointing out that Barker et al (1981) were also successful, to some degree, in simulating the amplification and envelope characteristics of L, waves recorded at Yucca Flat using a similar 2-D calculation. The presence of embedded scatterers in a medium also helps the excitation of L, waves (Aki & Chouet 1975).…”

Section: Discussion a N D Conclusionmentioning

confidence: 94%

“…Previous studies by Kawase & Aki (1989), Barker, Der & Mrazek (1981 and others show that interaction of incident SH, SV, P, L, or long-peiod Rayleigh waves with 2-D structure can significantly lengthen the duration of seismic wavefield. In a recent study, Saikia et af.…”

Section: Lg Waves As Affected By Irregular Receiver Crust and Embeddementioning

confidence: 98%

Modified frequency-wavenumber algorithm for regional seismograms using Filon's quadrature: modelling ofL_gwaves in eastern North America

Saikia

1994

255

170

S U M M A R YThe main focus of this study is to develop an efficient frequency-wavenumber code to synthesize high-frequency (say, 10 Hz) regional seismograms (up to a distance of more than 1000 km) in a medium consisting of a large number of crustal layers, and investigate effects of regional waveguides on L, waves. In this newly developed code, the frequency-wavenumber response is evaluated by using the Filon's quadrature at high frequency and the polynomial approximation of Bessei's functions (implementing trapezoidal integration) at low frequency. This has reduced the computation time by several folds compared to the time needed for the trapezoidal integration. Using this algorithm, both broad-band seismograms at Harvard and short-period ECTN (Eastern Canadian Telemetered Network) seismograms from 1988 November 25 Saguenay earthquake were successfully modelled. A moment magnitude of 5.9 was used. Initially an average waveguide was calibrated to successfully excite the regional P,,, and S,,, waves at HRV, its near-surface structure constrained by matching the recorded frequency content. The crust-mantle transition zone was constrained by modelling the strength of the broad-band S,,, waves, consisting of S,,, sSmS,,, SmSSmS, and sSmSSmS phases, relative to the initial P,,, waves. The model was modified to consist of thin layers with an alternating highand low-velocity distribution to match the high-frequency ECTN Lg waves observed from 300 to 600 km away. In addition, the effect of a near-surface irregular receiver structure on the duration of L, waves was also investigated by convolving its response with the regional L, seismograms. Initial study suggests this method as viable for modelling the entire L, waves including the coda. Numerical studies suggest that the shapes and peak amplitudes on L, seismograms depend more strongly on the source depth, anelasticity and crustal waveguide than do the P,, seismograms.