Frequency‐dependent regional wave attenuation is estimated for continental paths to the NORESS array in Norway. Regional Lg and Pn spectra from 186 events at ranges between 200 and 1400 km and local magnitudes between 1.1 and 4.8 are inverted for both seismic moment and apparent attenuation. The Lg spectra were inverted between 1 and 7 Hz, and the Pn spectra were inverted between 1 and 15 Hz. The method uses both the spectral and spatial decay of observed signal amplitudes to separate source and path contributions. The assumptions include the geometric spreading rate and the source spectrum to be uniquely defined by its long‐period level. Most events considered have local magnitudes less than 3.0, so the source corner frequencies are near or beyond the upper limit of the inverted bandwidth. The Q results, particularly for Lg, are therefore not very sensitive to the details of our source parameterization. The inversion parameters are source moment (for each event), a constant relating corner frequency and moment for the entire data set, and two parameters describing a power law frequency dependence of Q in the region. For fixed source and spreading assumptions the inversion defines clear trade‐offs among model parameters. These trade‐offs are resolved by adding the constraint that the separately derived source parameters for Lg and Pn are consistent. The “preferred” estimates for the apparent attenuation are QLg(f) = 560f0.26 and QPn(f) = 325f0.48. These Q values correspond to assumed geometric spreading rates of r−0.5 for Lg and r−1.3 for Pn. For fixed Lg spreading, the Pn spreading rate is constrained by requiring that the relative Lg amplitude for earthquakes and explosions of the same moment be consistent with well‐supported results from previous empirical studies. The relationship between the inverted seismic moment values and local magnitude is generally consistent with values from near‐field studies. Since magnitude does not enter the inversion, this result lends considerable support to the derived Q models. Whatever the physical interpretation of the results, they certainly provide an accurate parameterization of observed amplitude spectra in this region. This is valuable for representing wave propagation in the region, and it provides important data for assessing the event monitoring capabilities of small regional networks.
This paper presents the results of a theoretical study of the sensitivity of Pn attenuation to the elastic and anelastic structure of the uppermost mantle for frequencies between 1 and 15 Hz and distances between 200 and 1000 km. Synthetic seismograms are computed using wavenumber integration for an elastic model consisting of a crastal layer over an upper mantle with a slight linear velocity gradient (approximately equal to the earth‐flattening transformation of a spherically symmetric homogeneous upper mantle). These are compared to synthetic seismograms for a crustal layer over a mantle halfspace (pure elastic head wave). We find that earth sphericity alone causes a significant departure in Pn attenuation from that of a canonical head wave (almost two orders of magnitude at 1000 km and 15 Hz), and that high frequencies attenuate less rapidly than low frequencies if the spherical earth velocity gradient is greater than or equal to zero. This implies that methods that assume frequency‐independent Pn geometric spreading will produce biased estimates of the anelasticity of the upper mantle. This is demonstrated using synthetic seismograms computed for an anelastic spherical earth model. These results show that the velocity structure must be carefully considered when relating observed Pn spectral amplitudes to the anelastic structure of the upper mantle.
The oceanic lithosphere is an extremely efficient waveguide for high‐frequency seismic energy. In particular, the propagation of the regional to teleseismic oceanic Pn and Sn phases is largely controlled by properties of the oceanic plates. The shallow velocity gradient in the sub‐Moho lithosphere results in a nearly linear travel time curve for these oceanic phases and an onset velocity near the material velocity of the uppermost mantle. The confinement of Pn/Sn to the lithosphere imposes a constraint on the maximum range that a normally refracted wave can be observed. The rapid disappearance of Sn and the discontinuous drop in Pn/Sn group velocity beyond a critical distance, dependent upon the local thickness of the lithosphere, are interpreted as a shadowing effect of the low Q asthenosphere. Wave number integration was used to compute complete synthetic seismograms for a model of oceanic lithosphere. The results were compared to data collected during the 1983 Ngendei Seismic Experiment in the southwest Pacific. The Pn/Sn coda is successfully modeled as a sum of leaky organ‐pipe modes in the sediment layer and oceanic water column. While scattering is present to some degree, it is not required to explain the long duration and complicated nature of the Pn/Sn wave trains. The presence of extremely high frequencies in Pn/Sn phases and the greater efficiency of Sn than Pn propagation are interpreted in terms of an absorption band rheology. A shorter high‐frequency relaxation time for P waves than for S waves results in a rheology with the property that Qα > Qβ at low frequency while Qβ > Qα at high frequency, consistent with the teleseismic Pn/Sn observations. The absorption band model is to viewed as only an approximation to the true frequency dependence of Q in the oceanic lithosphere for which analytic expressions for the material dispersion have been developed.
The detection threshold of a network including NORESS‐type arrays in the Soviet Union is estimated using the frequency‐dependent attenuation of Pn and Lg observed at NORESS to normalize the threshold of individual stations. The normalization begins with a parameterization of the magnitude and range dependence of the NORESS Pn and Lg spectra in terms of seismic moment and apparent attenuation. A relationship between these spectral amplitudes and the time‐domain amplitudes used in signal detection at NORESS is then determined. This allows an estimate of NORESS detection capability from the spectral parameterization and a prediction of the Pn detection capability for regions with different wave propagation characteristics. A comparable prediction is not possible for Lg because of dispersion and the non‐stationarity of pre‐Lg noise. Instead, an empirical relationship for Lg detectability based on observed temporal amplitudes is determined. To validate the results for NORESS, predictions based on our model are compared with observed detection statistics and with results obtained by Ringdal (1986), who compared detections at NORESS to bulletins produced by local seismic networks. The simulations represent the detection capability of a network of NORESS‐type arrays, 20 within and 13 outside the Soviet Union. The 90% ML threshold for detecting three Pn phases for events in the Soviet Union is estimated to be between 2.4 and 2.7 if the frequency‐dependent attenuation and noise throughout the network are the same as those observed at NORESS. This threshold is reduced by 0.1 ML if Pn Q is 50% higher in the Soviet Union than estimated for Scandinavia, and is increased by 0.2–0.3 ML if Pn Q is 50% lower. The ML thresholds are reduced by 0.2–0.3 if the detection criteria include Lg as well as Pn phases. While these simulations are carefully normalized by the actual detection capability of a prototype station for test ban treaty monitoring, observations of attenuation and noise inside the Soviet Union are needed to determine how they actually differ from those at NORESS. Also, it must be noted that the NORESS detection capability estimates (and therefore the simulations) are for average conditions and there are regular variations that must be considered when estimating treaty monitoring capability.
Ð S/P amplitude ratios have proven to be a valuable discriminant in support of monitoring a Comprehensive Nuclear Test Ban Treaty. Regional S and P phases attenuate at dierent rates and the attenuation can vary geographically. Therefore, calibration is needed to apply the S/P discriminant in new regions. Calibration includes application of frequency-dependent source and distance corrections for regional Pn, Pg, Sn, and Lg phases. JENKINS et al. (1998) developed Pn, Pg, Sn, and Lg amplitude models for nine geographic regions and two global composite models, stable and tectonic. They determined frequency-dependent source and attenuation corrections from a large data set obtained from the Prototype International Data Center (PIDC). We use their corrections to evaluate calibrated S/P discriminants.Our discrimination data set includes >1000 amplitude ratios from earthquakes, industrial explosions, chemical explosions, and nuclear explosions from Lop Nor, India and Pakistan. We ®nd that the calibrated S/P ratio is largest for earthquakes and smallest for the nuclear explosions, as expected. However, the discriminant is not universally valid. In particular, the S/P ratio for the Pakistan nuclear explosion fell within the normal range for the earthquakes. This event was recorded by only a few stations at far-regional distances and appears to have an anomalously high Sn amplitude. The industrial explosions overlap with the earthquake population, however the buried chemical explosions generally register lower S/P ratio than earthquakes.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.