Attenuation Of Lg Waves Research Articles

Numerical modelling of Lg wave propagation across the North Sea Central Graben

SUMMARY In an effort to study the effect of strong crustal lateral variations on the propagation characteristics of Lg waves, we perform a numerical modelling of these waves at 1 Hz in a graben structure. We use a coupled mode method and a simplified 2-D model of the North Sea Central Graben. We show that at 1 Hz the energy reflected by the structure is negligible, and that on the average only 20 per cent of the Lg incoming energy is converted to waves propagating into the mantle. The energy remaining in the Lg wavetrain is subject to a strong redistribution with depth, which has an effect at the surface of the model which is very dependent on source type and depth, multiplying the surface amplitude by a factor of 0.6 for shallow sources and 0.9-1.4 for mid-crustal sources, on the average. The phase of the wavetrain is shown to be stable enough with period to avoid a destructive interference of the signal. Crossing the Graben perpendicularly or at an angle leads to equivalent results. Lg wave propagation appears to be surprisingly robust in crustal structures with strong lateral variations. The large-scale geometric features of the Graben prove insufficient to explain the strong attenuation of the Lg waves which has been observed in the North Sea area.

A novel technique for measuringLgattenuation—results from Eastern Canada between 1 to 10 hz

AbstractSpectral amplitudes of regionally recorded Lg waves are studied in detail between 0.6 and 10 Hz, using vertical-component, velocity seismograms of the Eastern Canada Telemetered Network stations and a supplementary Seismic Research Observatory-type station at Glen Almond (GAC), Québec. We find that the site responses vary among these stations by more than a factor of 3 within the frequency range of interest. Furthermore, they are found to be strongly frequency dependent. Consequently, it is essential that they be taken into consideration in studies of Lg wave attenuation and Lg source spectra of regionally recorded seismic events.We present a new method of measuring interstation surface wave attenuation, which is closely related to the conventional two-station method. While retaining all the desirable features of the conventional two-station method, the new technique, which we will call the “reversed two-station method,” allows simple, direct (one-parameter) determination of the Lg wave attenuation from sparse spectral data in a manner unaffected by station site effects and associated instrument error.The reversed two-station method is successfully tested over weakly attenuating, short (53 to 210 km) interstation paths in eastern Canada, a difficult experimental condition by normal standards. The Lg attenuation coefficient (0.6 to 10 Hz) in eastern Canada is found to be frequency dependent and of the form γ(f) = 0.0008 f0.81 km−1. At higher frequencies, the Lg attenuation appears to be essentially frequency independent. This latter finding is preliminarily interpreted as evidence that regionally recorded Lg waves in the Canadian Shield are, as in the case of Lg waves propagating through the structurally complex Appalachian Province, contaminated by the high-frequency coda of Sn waves. The Lg contamination over the shield paths becomes severe starting at 14 Hz, twice the frequency above which the Lg signal propagating over the Appalachian Province becomes completely dominated by non-Lg arrivals.

Attenuation of Lg waves in the Canadian Shield

Abstract An empirical model that relates Fourier amplitude density of ground acceleration (FA) of Lg waves of Canadian Shield earthquakes to magnitude and distance has been calibrated at discrete frequencies. Magnitude ( m bLg ) ranges from 2.8 to 5.2, epicentral distance ( R ) from 70 to 900 km, and frequency ( f ) from 0.6 to 20 Hz. The data base consists of the vertical component of Lg waves as recorded in digital format by the Eastern Canada Telemetered Network. A linear regression is applied to the logarithm of the following model: FA = ke bm bLg e −γR R −0.5 , where k = (2πf) 2 c and c is a constant coefficient (related to the prototype Fourier amplitude density of ground displacement), b reflects the magnitude dependence, γ is the anelastic attenuation coefficient, and 0.5 is the geometric spreading factor of surface waves. The magnitude coefficient, b , varies from a maximum of 3.0 at 1 Hz to 1.8 at 20 Hz, a trend indicative of the decrease in corner frequency with increasing magnitude. The anelastic attenuation coefficient can be represented by a frequency dependence of the form γ = 0.001 f 0.8 (km −1 ), and the associated quality factor, Q , by Q = 900 f 0.2 . Empirical relations for FA predict spectral trends that are generally within a factor of two of the mean trend of actual spectra. Since Lg waves are the primary contributor to moderate ground motion (and felt sensations) in the magnitude and distance range specified above, empirical relations derived for FA are of relevance not only to seismologists, but also to earthquake engineers as well .

Frequency-dependent attenuation in the crust beneath Central France fromLgwaves: Data analysis and numerical modeling

AbstractThe quality factor of shear waves in the crust beneath Central France is evaluated by the combined use of data analysis and theoretical modeling. The measurement is made from Lg wave records of 18 earthquakes at 22 short-period vertical seismic stations of the French seismic network. The geometrical attenuation of Lg waves has been calculated through a numerical simulation in a perfectly elastic model. The station responses (including site effects) have been evaluated from Lg waves and from records of the seismic noise. Q has been calculated for a set of 11 frequencies between 0.5 and 10 Hz and has been found to be in the formQ = 290 * f 0.52where f is the frequency. The numerical simulation leads to the interpretation of Q of Lg waves as the mean value of Q within the crust. Theoretical and experimental decay of energy with distance are in very good agreement. The Lg waves present a different sensitivity to a low Q zone whether this zone is associated with the sediments or with scattering in the lower crust. The lower crust beneath central France appears from our experimental results to be associated with high values of Q.

Lg-wave propagation and crustal structure differences near Denmark and the North Sea

Summary. Recordings of regional earthquakes near Denmark and the North Sea have been collected in an attempt to find the Lg-wave propagation properties in the area. Propagation paths of lengths between 300 and 1100km are investigated. The amplitudes of the Lg-waves and the other seismic phases in the seismograms are compared. The propagation paths are classified according to whether the Lg-wave propagation compared to P- and S-wave propagation is good, intermediate or poor. It is noted that there is a distinct regional pattern in the Lg-wave attenuation. The Lg-wave propagation is poor across the North Sea. The North Sea is surrounded by areas of good, continental Lg-wave propagation except towards the south-east in the Netherlands and western Germany. An area in Skagerrak is characterized by good, but slow propagation of a surface wave similar to Lg. Since this is exactly the area of a cluster of earthquakes in Skagerrak, it is suggested that the two phenomena are caused by the same structural inhomogeneities in the area, just off the coast of north-western Denmark. The Lg-wave propagation is as good in the northern part of Denmark as it is in the Fennoscandian shield. The extent to which the propagation is poor across the North Sea is dependent on the frequency of the Lg-waves observed with different seismographs. The Lg-waves are more attenuated, relative to the other seismic phases, the higher the frequency is between 0.25 and 2 Hz. As the structural differences between the Fennoscandian bedrock area and the Norwegian-Danish basin with 8–10 km of sediments has no effect on the Lg-wave propagation, it is suggested that the structural anomalies causing the poor Lg-wave propagation across the North Sea are deep crustal anomalies, possibly significant variations in crustal thickness. It is likely that such deep crustal differences are located along the most prominent upper crustal differences, in the sedimentary grabens in the middle of the North Sea. The poor propagation across the North Sea is similar to that observed across a sedimentary basin in eastern Greenland, which also developed mainly before seafloor spreading separated Greenland from Europe. The frequency-dependent propagation of the Lg-waves is explainable in terms of surface wave propagation across structural boundaries as well as in terms of anelastic attenuation.

Theoretical study of the excitation, spectral characteristics, and geometrical attenuation of regional seismic phases

AbstractWe present a theoretical study of the generation and geometrical attenuation of regional crustal phases. We do this through the computation of seismograms in the epicentral distance range from 60 to 500 km. The geometrical attenuation of Lg waves with epicentral distance is of the form r−0.83. Pg wave amplitudes display a much stronger decay of the form r−1.5. The spectral density of the crustal transfer function for Pg waves is relatively flat for frequencies between 0.1 and 5 Hz while Lg wave spectra strongly fall off beyond 2 to 3 Hz.The excitation of Pg wave is insensitive to the depth of the source within the crust while the Lg amplitude is about 50 per cent higher for a source in the upper and middle crust than in the lower crust. The amplitudes of these two phases drastically decrease when the source is below the Moho. These results illustrate the important role of wave guide played by the crust for the propagation of Lg and Pg.We find that the geometrical attenuation of Pg and Lg waves is independent of source mechanisms. In the case of an explosion, the excitation of Pg is insensitive to the source depth. The Lg wave amplitude is small in comparison to Pg and Rayleigh waves and depends on the closeness of the source to an interface or to the free surface.

Seismicity and observations of Lg wave attenuation in Greenland

Earthquakes of local magnitudes between 2 1 2 and 5 1 2 have occurred in Greenland in the last few decades. Most of the earthquakes are located near the coasts. Some of the earthquake zones are connected to known tectonic features in and near Greenland. The magnitude determinations are based on Lg wave amplitudes and are calibrated using the m b determinations of the International Seismological Centre. The attenuation of the Lg waves differs from region to region. Along some paths crossing younger geologic features the attenuation is large compared to that in the paths of the old crystalline shield rocks. The Lg wave propagation is influenced by the Greenland ice cap. All the long paths go through the interior of Greenland with its 2–3 km ice cap thickness and they exhibit relatively large Lg wave attenuation.

On the attenuation ofLgwaves in western and central Asia and their use as a discriminant between earthquakes and explosions

abstractData from earthquakes and explosions in the western and central parts of the Soviet Union are used to determine the values of the coefficient of anelastic attenuation of 1-sec period Lg waves from the Russian uplands region and for the region from the Caspian Sea to the southern perimeter of the Soviet Union. For the former region, the value is approximately 0.15 deg−1, and for the latter 0.35 deg−1.Using the empirical Veith and Clawson (1972) curve for the amplitude of P waves at regional distances and theoretical curves for the attenuation of higher mode surface waves for Lg, curves are constructed which show the variation of the amplitude ratio of Lg to P with epicentral distance, with the coefficient of anelastic attenuation as a parameter. When data from explosions and earthquakes in the Soviet Union are compared with the calculated curves, the amplitude ratios of Lg and P on the average are larger for earthquakes than explosions. However, there is much overlap, so that this ratio by itself does not appear to be a useful discriminant between earthquakes and explosions.

A time-domain study of the attenuation of 10-Hz waves in the New Madrid seismic zone

AbstractFrom a time-domain study, the amplitude of 10-Hz P waves in the New Madrid seismic zone was found to fall off as the inverse 1.4 power of the epicentral distance. The amplitude of 10-Hz Lg waves was found to decay as for an Airy phase with a coefficient of anelastic attenuation of 0.006 km-1. In almost all cases the Lg motion was found to be larger than that of P, even at epicentral distances of 5 km and less. Thus design earthquake motions need to be concerned with Lg waves, whose coda also have a longer duration than those of the P waves.The apparent Q for the 10-Hz Lg waves is 1500, identical to that found previously by Nuttli (1973) for higher mode 1-Hz Lg waves. The attenuation of 10-Hz Lg waves is sufficiently small that one must pay attention to their damage potential at distances as large as a few hundred kilometers.

Scaling northeastern United States/southeastern Canadian earthquakes by their Lg waves

abstract Displacement spectral densities and spatial attenuation of Lg waves from four earthquakes were studied in detail to determine the applicability of Nuttli's (1973) Lg-magnitude formulas to northeastern North American earthquakes. It was found that the 1-sec-period Lg wave from these events was attenuated at a slightly greater rate than has been observed in the central United States. However, comparison of the A/T observations and the Lg spectral analysis results of the two regions demonstrated that it was permissible to use Nuttli's Lg-magnitude formulas for scaling northeastern earthquakes if the amplitude observations are restricted to within 20° of the earthquake's epicenter. This result establishes the basis for a unified mb magnitude scale between central and northeastern North America.