Lateral Variations in Teleseismic Attenuation of the Conterminous U.S. and New Insights Derived From Its Relationship to Mantle Seismic Velocity

• Published in: Journal of geophysical research. Solid earth Vol. 128; no. 12
• Main Authors: Bezada, M. J., Byrnes, J. S., Zhu, Z., Lee, H.
• Format: Journal Article
• Language: English
• Published: Washington Blackwell Publishing Ltd 01-12-2023
• Subjects: Anomalies; Broadband; Calculators; Grain size; Heterogeneity; Inversions; Lithosphere; Moisture content; North America; P-waves; Physical properties; Rheological properties; Rheology; S waves; Seismic activity; Seismic attenuation; Seismic velocities; Seismic waves; Shear wave velocities; Tectonics; Tiles; Variation; Velocity; Water content; Wave attenuation; Wave velocity; United States > US
• ISSN: 2169-9313; 2169-9356
• DOI: 10.1029/2023JB027299

Much of our knowledge of the North American lithosphere comes from imaging seismic velocities. Additional constraints on the subsurface can be gained by studying seismic attenuation, which has different sensitivity to physical properties. We produce a model of lateral variations in attenuation across the conterminous U.S. by analyzing data recorded by the EarthScope Transportable Array. We divide the study area into 12 overlapping tiles and differential attenuation is measured in each tile independently; and twice for four of the tiles. Measurements are combined into a smooth map using a set of linear inversions. Comparing results for adjacent tiles and for repeated tiles shows that the imaged features are robust. The final map shows generally higher attenuation west of the Rocky Mountain Front than east of it, with significant small length scale variations superimposed on that broad pattern. In general, there is a strong anticorrelation between differential attenuation and shear wave velocities at depths of 80–250 km. However, a given change in velocity may correspond to a large or small change in attenuation, depending on the area; suggesting that different physical mechanisms are operating. In the western and south‐central U.S., as well as the Appalachians, velocity variations are large compared to attenuation changes, while the opposite is true in the north‐central and southeastern U.S. Calculations with the Very Broadband Rheology calculator show that these results are consistent with the main source of heterogeneity being temperature and melt fraction in the former regions and grain size variability in the latter ones.
Plain Language Summary
Seismic waves in the mantle propagate at lower speeds when temperatures are higher, rocks have higher water content, and small amounts of melt are present. These conditions also affect how much energy the wave loses as it passes through, which we call seismic attenuation. In this study we produce a map of seismic attenuation for the conterminous United States. We find that, in most places, where seismic velocities are low, attenuation is high, and vice versa. This is what we expect. Interestingly, the size of the change in attenuation that corresponds to a given change in velocity varies by region. In places with thicker lithosphere and without recent tectonic activity attenuation anomalies are large compared to velocity anomalies, the opposite is true in places with thin lithosphere and recent tectonic activity. Considering the results of lab experiments on velocity and attenuation, this suggests that in the regions with thick lithosphere and without recent tectonic activity the main cause of the anomalies is changes in the size of the mineral grains in the mantle, whereas in the regions with thin lithosphere and recent tectonic activity the main cause of the anomalies is changes in temperature and the amount of melt.
Key Points
Teleseismic P attenuation is largely anti‐correlated with upper mantle seismic velocity
Significant lateral variations in attenuation are observed in the tectonically quiescent regions
The ratio of attenuation to velocity anomaly amplitudes may be indicative of mantle conditions