Full Waveform Ambient Noise Tomography for the Northern Mississippi Embayment

We use seismic ambient noise data recorded by broadband stations around the northern Mississippi Embayment to develop a three‐dimensional shear wave velocity model with full waveform inversion. Empirical Green's functions at periods between 8 and 40s are extracted using a data processing flow based on the continuous wavelet transform. Synthetic waveforms are calculated with an isotropic model through a Graphics Processor Unit‐enabled, collocated finite‐difference code. Starting from the Central United States Velocity Model, the shear wave velocity is iteratively updated with sensitivity kernels constructed using the adjoint method. Several mid‐crustal velocity variations are related to major geological features including the Mississippi Valley graben (MVG), the Ouachita thrust belt, and the Missouri batholith. An intrusion is imaged in northwestern Alabama, coincident with a previously unexplained gravity high. A major change in mid‐crustal velocity occurs across the MVG; much higher velocity crust is present southeast of the graben than northwest. The high velocities are attributed to numerous igneous intrusions, possibly related to formation of the Granite‐Rhyolite province. A strength contrast produced by the change in mid‐crustal velocities may have facilitated formation of the younger, shallower MVG, as external stresses became tensional during Iapetus rifting. The rift pillow is interpreted as the deeper expression of the high velocity crust. Low velocity crust is present below southern Missouri starting at a depth of roughly 20 km. The boundary between the low velocity crust and higher velocity crust to the south and east is sharp and coincides with the Nd‐line. Plain Language Summary We use seismic waves generated by random noise to investigate the velocity of rocks located down to 25 km in the crust below the central United States. Our velocity model is the first well resolved three‐dimensional velocity model of the mid‐crust and reveals several interesting features. We find low velocities below extended portions of the crust such as the Mississippi Valley graben (MVG) (Reelfoot rift). We find very high velocity crust southeast of the MVG that extends throughout the velocity model. The high velocity crust may have formed more than a billion years ago. Differences in the strength of the crust established by the presence of the high velocity rocks may have influenced the location of the younger MVG and the present‐day location of the New Madrid seis