Anelastic attenuation refers to the absorption of elastic energy associated with the seismic wave. It eventually causes the total disappearance of the seismic wave as it propagates and is an essential factor, especially in seismic hazard estimations. We have studied crustal and upper mantle attenuation of regional waveforms in eastern Canada. We found observational evidence of Lg blockage across the continental margin of Nova Scotia and estimated attenuation of Lg and Sn. Based on the observation of a strong trade-off between Sn and Lg amplitudes in the continental oceanic transition zone and their different attenuation characteristics, we concluded that in the continental margins, the common method of estimating the regional attenuation based on shear wave package, without taking into consideration the wave propagation, may bias the Q estimation. This is in agreement with recent observational evidence indicating that the magnitude of offshore events in eastern Canada is underestimated.
Zhao, L. F., and Mousavi, S. M., (2018). Lateral Variation of Crustal Lg Attenuation in Eastern North America, Scientific Report.
We perform Q Lg tomography for the northeastern part of North America. Vertical broadband seismograms of 473 crustal earthquakes recorded by 302 stations are processed to extract the Lg amplitude spectra. Tomographic inversions are independently conducted at 58 discrete frequencies distributed evenly in log space between 0.1 and 20.0 Hz. This relatively large dataset with good ray coverage allows us to image lateral variation of the crustal attenuation over the region. Obtained Q Lg maps at broadband and individual frequencies provide new insights into the crustal attenuation of the region and its relationship to geological structures and past tectonic activity in the area. The Q Lg shows more uniform values over the older, colder, and drier Canadian Shield, in contrast to higher variations in the younger margins. Results confirm the correlation of large-scale variations with crustal geological features in the area. Existence of low-velocity anomalies, thick sediments, volcanic rocks, and thin oceanic crust are potential sources of observed anomalies. The mean Q values are inversely correlated with average heat flow/generation for main geological provinces.
Mousavi, S. M., Cramer, C. H., Langston, C. A., (2014). Average QLg, QSn, and Observation of Lg Blockage in the Continental Margin of Nova Scotia, Canada, Journal of Geophysical Research.
The term “Lg blockage” refers to the sudden disappearance of the Lg phase along a particular propagation path which is commonly seen in continental-oceanic transition zones. In this paper, we present observational evidence of Lg blockage across the continental margin of Nova Scotia in eastern Canada. Regional Lg and Sn spectra from 91 events with epicentral distances between 100 and 1200 km and magnitudes between 2.5 and 4.7 are inverted simultaneously for the source spectrum, site amplification, and average attenuation. The vertical displacement spectra were estimated between 0.9 and 10.75 Hz. The assumptions include a fixed frequency-independent geometric spreading rate for Lg and a frequency-dependent spreading model for the Sn. Estimates for the apparent regional attenuations are QLg (f) = 615(±25) f0.35(±0.04) and QSn (f) = 404(±23) f0.45(±0.03). Results from this study provide an accurate parameterization of observed amplitude spectra and are valuable for representing wave propagation in the region. Based on the observation of a strong trade-off between Sn and Lg amplitudes which have different attenuation characteristics, we conclude any attenuation study based on measuring the amplitude of a package of several different phases, without taking into consideration the propagation characteristics of individual waveforms at the region of study, may bias the estimation of average regional Q.