Aeromagnetic Data Reveal Potential Seismogenic Basement Faults in the Induced Seismicity Setting of Oklahoma. Issue 12 (22nd June 2018)

• Record Type: Journal Article
• Authors: Shah, A. K.; Crain, K.
• Is Part Of: Geophysical research letters. Volume 45:Issue 12(2018)
• Page Start: 5948
• Page End: 5958
• Publication Date: 2018-06-22
• Subjects: earthquake hazards -- induced seismicity -- magnetic methods -- fault delineation -- Precambrian basement -- Oklahoma; Geophysics -- Periodicals; Planets -- Periodicals; Lunar geology -- Periodicals; 550
• DOI: 10.1029/2018GL077768
• Languages: English
• ISSNs: 0094-8276

Abstract

Abstract: New aeromagnetic survey data collected over north central Oklahoma image possible seismogenic faults in the crystalline basement. Linear earthquake sequences associated with induced seismicity suggest the reactivation of ancient basement faults, but few of these sequences are aligned with mapped faults. The new data show many earthquake sequences aligned with linear magnetic gradients or offsets between anomalies, while mapped faults, which mainly describe sedimentary cover, show limited correspondence with either. This strongly suggests significant structural differences between the crystalline basement and sedimentary cover. Furthermore, while the earthquakes are occurring on reactivated ancient faults, most of these faults have likely been inactive for millions of years. The magnetic data exhibit many gradient lineaments that are optimally oriented for fault slip, and the earthquake data suggest additional optimally oriented faults. Together these data suggest the presence of potentially numerous seismogenic faults throughout the region, which may contribute to high levels of induced seismicity. Plain Language Summary: Oklahoma has been the site of thousands of earthquakes associated with wastewater injection activity (i.e., induced seismicity ), but few of these earthquake sequences occur on mapped faults. This makes seismic hazard difficult to estimate. We used new airborne magnetic data to image rocks several miles beneath the surface where the earthquakes are occurring. These data reveal numerous possible faults, in the form of linear features in the magnetic data (called lineaments ), which had not previously been mapped. A number of these lineaments are aligned with earthquake sequences, suggesting they represent ancient faults that have been reactivated by fluid injection. Analyses of these lineaments shows that the deep rocks have a preferred orientation or grain direction, and that the grain direction is favorably oriented for fault slip under current background stresses. This may contribute to the high level of seismicity in response to wastewater injection. Interestingly, the possible faults imaged by the magnetics are different from those in existing fault maps. This is attributed to the magnetics representing older, deeper basement rocks and the fault maps representing mostly younger, shallower of the sedimentary cover. This variation probably came about from the different tectonic events that shaped the two layers. Key Points: New aeromagnetic data suggest previously unmapped basement faults or contacts that are aligned with earthquake sequences in Oklahoma Limited correlation with mapped faults suggests significant structural differences between the crystalline basement and sedimentary cover The basement‐dominant structural grain is aligned near the optimal orientation for fault slip, suggesting numerous seismogenic faults