Crustal Seismic Attenuation of Western Canada from Lg Q Inversions

Southwestern Canada has experienced major tectonic activities involving craton assembly and Cordilleran orogenesis. These processes have left crustal imprints that are observable from the propagation times, waveforms and amplitudes of seismic waves. So far, the majority of the crustal models of this region has been determined from elastic seismic wave-speeds, whereas anelasticity remains poorly understood. In this study, we investigate the lateral variations of crustal attenuation structure by measuring 1 Hz Lg Q (Q0) values using a standard two-station method. Fourier spectral amplitudes are measured from Lg waves windowed between 3.0 and 3.5 km/s. We utilize 148 regional earthquakes and 561 broadband stations from various regional seismic networks. The Q0 values are measured between ~130000 station pairs, resulting in a dense raypath coverage. In general, the Canadian Cordillera exhibits slightly higher Q0 values (~500) than the Alberta Basin (~300), which could potentially reflect the presence of a thick sedimentary cover in the latter region. The boundary between the two crustal regimes lies beneath the Rocky Mountains, where elevated Q0 values are observed. Farther north, we identify extremely low Q0 values (~100) beneath the Buffalo Head Terrane, a Proterozoic domain with a proposed history of Paleoproterozoic post-collisional magmatism and kimberlite intrusion. On the other hand, our data set suggests high Q0 values in the Archean-aged Medicine Hat Block in southern Alberta, which is concordant with its proposed Archean origin. Overall, Lg wave attenuation offers new constraints on the temperature and composition of the continental crust/lithosphere beneath the western margin of North America.