Variations In Crustal Thickness And Seismic Anisotropy In The Northeastern United States From Receiver Function Analyses
The northeastern region of the United States has a long and complex geologic history, which has influenced and is recorded in its crustal composition and structure. Constraining the thickness and layering of the crust using seismic waves can yield information on crustal composition and history of deformation. Conversions of P to S waves across impedance contrasts provide information on layering beneath a seismometer, and can be analyzed using the receiver function method. We analyzed Ps receiver functions from 33 stations across the northeast region centered around Chester gneiss dome in Vermont. We found that the depth to Moho varies from 28 km to 47 km and the crust is thicker at the boundary between the provinces; the Grenville Province and the Appalachian mountains.
We used harmonic decomposition of receiver functions from each station to constrain dipping layers as well as the presence of crustal seismic anisotropy – the dependence of seismic wave speed on polarization and propagation direction. Observations of seismic anisotropy can help constrain the structure and composition of the middle and lower crust by complementing the information coming from isotropic Vp and Vs. Because seismic anisotropy can be affected by several factors such as oriented cracks and mineral preferred orientations, it is important to investigate the relationship between these factors and the behavior of seismic anisotropy. To do so, we carried out forward modeling of the observed Ps receiver function harmonic decompositions using the Raysum code (Fredricksen and Bostock, 1999). We were able to fit most isotropic structures and not dipping layers or an anisotropic layer. Even though, we did not have success in best fitting dipping layers or anisotropic layers to our forward models, we believe, our model is pretty useful in identifying the presence of such structures beneath a seismic station.