29 Dec 2015, 8:30 am PST
A deep M=4.3 mainshock and shallow M=3.4 aftershock
A magnitude 4.3 shock struck this morning in Edmond, OK, shaking all of Oklahoma City and much of Tulsa. The M=4.3 quake was followed 2 hours later by a M=3.4 shock 1.5 km (1 mi) away, at a depth of 4 km (2.5 mi). During the preceding 24 hours, there were two unrelated M=3.4-3.5 quakes located 60 km (40 mi) to the north.
Slip on a left-lateral strike-slip fault is possible
The earthquake mechanism is strike-slip, with either right-lateral slip (like the San Andreas) on a northeast-striking plane, or left-lateral slip on a northwest-striking plane. A plausible alignment of recent quakes on the northwest-striking plane, as seen in the Temblor map below, makes this a more likely candidate.
Oklahoma has a higher rate of M>3 quakes than California
This earthquake was most likely triggered by enhanced oil and gas recovery in Oklahoma. The M≥3 seismicity rate has increased thirty-fold since intensive fracking and enhanced oil recovery began about a decade ago, as seen in the figure modified from Ellsworth (2013) below. As a result, Oklahoma dethroned California more than a year ago as the most seismically active state in the lower 48. Oil field operations are not causing quakes, but instead promoting them in time by reducing the friction on already-stressed natural faults.
Interestingly, the M=4.3 quake nucleated at a depth of 8 km (5 mi), well below the depth of waste fluid injection, suggesting that some fracking fluids and other sources of “produced water” are migrating to greater depths, probably within fault zones and other fractures. But the center of the fault slip (its “centroid”) lies at about 4 km (2.5 mi), suggesting that the fault ruptured vertically toward the Earth’s surface.
Ross Stein & Volkan Sevilgen Temblor
Data from USGS, Oklahoma Geological Survey, and Ellsworth (2013)
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