Imaging crustal structure using local earthquake and ambient noise tomography in North Texas

North Texas, including the Dallas-Fort Worth metroplex, has experienced an increase in seismicity rate since 2008 and these events have been linked, to varying degrees of certainty, to wastewater injection in the Ordovician Ellenburger Formation, deposited on top of the crystalline basement. We focus here on local earthquake and ambient noise tomography approaches to imaging the subsurface velocity heterogeneity associated with the larger and better-recorded North Texas sequence: the Azle-Reno, the Irving-Dallas, and the M4 Venus earthquake sequences. The 2013-2014 Azle-Reno sequence generated 27 felt earthquakes (M2.1-3.6) between November 2013 and January 2014, although earthquake rates at lower magnitude levels (M0-2) during the same time window were highly variable. The most recent set of Azle-Reno earthquakes occurred in December 2015, following a nearby M3.0 event near Haslet. SMU and collaborators currently operate 30 seismic stations in the basin. In addition, SMU and Nodal Seismic collaborated to deploy a dense network of 10 Hz single-component (vertical) sensors over the Azle-Reno earthquakes source zone. Ten days of continuous data were acquired between February 25 and March 8, 2014. We present results of double-difference local earthquake tomography and ambient noise tomography using the local network and the nodal datasets. Ambient noise tomography allows us to obtain higher resolution near surface structure and group velocity maps to complement the Vp, Vs and Vp/Vs images provided by the local earthquake dataset. Local earthquake tomography results for the Irving-Dallas and Venus (Johnson County) seismicity will also be presented. The Fort Worth basin deepens from west-to-east, and we hypothesize that large NE-SW trending basement faults, some of which have been reactivated since 2008, compartmentalize the subsurface. Exploring similarities and differences in velocity heterogeneities in the Ellenburger formation, and the crystalline basement using tomography approaches may yield insights into understanding the physical mechanisms leading to these earthquakes.