High-resolution Fault Zone Structure of the 2011 Mw 5.7 Prague, Oklahoma Earthquake Sequence

The 2011 Mw 5.7 Prague, Oklahoma earthquake sequence occurred along the structurally complex Wilzetta fault system near Prague, Oklahoma. The sequence included three Mw>4.8 earthquakes and thousands of aftershocks. Previous studies suggest that fluid injection from nearby wells may have triggered the foreshocks, which then led to cascading failure of the whole sequence (Keranen et al., 2013, Sumy et al., 2014). However, it remains unclear what is the relationship between the crustal structures and the locations of induced earthquakes and what are the characteristics of the complex fault system. In this study, we apply local earthquake tomography to image high-resolution Vp, Vs, and Vp/Vs ratio structures in the epicentral area of the Prague sequence. The inversion uses P and S wave travel-times from 1508 relocated earthquakes recorded by 24 stations within 20 km of the fault zone. The inverted fine-scale upper crustal velocity models clearly reveal strong correlations between an anomalous low-velocity zone and the spatial distribution of the earthquakes. The low-velocity zone, most prominent in the basement at depths between ~4 km and ~8 km, is likely associated with the Meeker-Prague fault (MPF), previously identified and named based on seismicity alignment, that hosted the Mw 5.7 mainshock and the majority of aftershocks. Moreover, anomalously low Vp/Vs ratios (~1.6) are observed in the low-velocity zone around the earthquake hypocenters, suggesting the presence of water-filled cracks in the seismogenic zone, which is presumably attributed to wastewater injection from nearby wells. We are currently working on relocating all earthquakes based on waveform correlation and the new 3D tomography model, as well as measuring local Vp/Vs ratio using differential times from similar event clusters. The integrated study will provide further insights on fault zone properties, possible interaction between faults in the crystalline basement and the overlaying sedimentary layer.