NEW AIRBORNE MAGNETIC AND GROUND GRAVITY DATA USED TO IMAGE FAULTS AND CONTACTS IN THE CRYSTALLINE BASEMENT OF NORTH-CENTRAL OKLAHOMA

One of the great challenges to estimating earthquake hazard in the central and eastern U.S. is that numerous larger earthquakes occur on subsurface faults that had not previously been mapped. This is particularly the case in Oklahoma, where fluid injection activity is believed to have caused thousands of earthquakes since 2009. While many of these earthquakes have formed sequences along linear tends, few of these sequences are aligned with mapped faults. This may be because most fault maps in the region are based on imaging and analysis of the sedimentary cover, whereas most of the earthquakes are occurring at depths within the underlying crystalline basement. Alternatively, perhaps new faults are being formed from existing joints and cracks in response to injection activity. Improved maps of basement faults are thus needed to address these different hypotheses.

To image faults at depths beneath the Oklahoma sedimentary cover (1-4 km for north-central Oklahoma), geophysical approaches are needed. New aeromagnetic survey data, collected August 2017, will provide a marked increase in resolution over existing public data flown with 5-10 km line spacing. Derivative maps of these data will highlight contrasts in basement rock magnetic properties representing contacts and faults (noting that the local sedimentary cover is poor in magnetic minerals). New and existing ground gravity station data (1.5-15 km spacing) provide constraints on variations on basement depth and lithology. Few existing wells penetrate the basement, but analyses sedimentary formation depths (e.g. the Viola Fm.) allow improvement upon existing maps of faults within the sedimentary cover. In particular, offsets in formation depths are aligned with existing fault maps, and in some areas they suggest improvement over the current mapped trace. Comparisons between magnetic, gravity, well and earthquake sequence data will assist mapping of optimally oriented faults that may pose significant seismic hazard, and will also contribute to a fuller understanding the mechanisms of fault rupture in this environment.