3-D S-Wave Velocity Structure and Poisson's Ratio of the Crust in Oklahoma from Stacking and Inversion of Local Earthquake Waveforms

Abstract:

Induced seismicity in Oklahoma along with the extensive seismic networks installed for earthquake monitoring can be considered as a unique large-scale seismic experiment. We use the rich catalog of local earthquake waveforms to obtain a S-wave crustal velocity model for Oklahoma. In combination with the recently established P-wave velocity model, our study addresses the processes that led to the Mesoproterozoic lithospheric expansion of Laurentia and provides insights on crustal structures such as the Midcontinent Rift.

We use data recorded at 10 seismic networks across Oklahoma spanning a period of 2010 to 2017, resulting in a total selection of 27, 582 events for this study. Traditional local earthquake tomography applied to local and sparse networks is not well suited for imaging of the deeper crust due to challenges in phase correlation at large offsets. Therefore, we use an alternative approach to focus on the deeper crust which recently has been successfully applied to P-waves. Our processing workflow includes band pass filtering, conversion to envelope, and STA/LTA detection. Furthermore, the data are common-mid-point sorted and stacked in offset bins to obtain stacked gathers which are representative of local 1D velocity-depth functions. This approach results in a significant increase in S/N ratio which allows for picking of Sg 1-D travel time curves for larger offset ranges. The 1-D travel time curves are then inverted and combined to obtain a 3-D S-wave crustal velocity model.

We combine the Sg velocity model with the Pg velocity model to obtain Poisson’s ratio. Our results are integrated with regional gravity and magnetic data, and we discuss the geologic implications of our model in terms of crustal evolution and the proposed extension of the Midcontinent Rift in Oklahoma.