Analysis of Magnitude-Spatial Trends of Induced Seismicity in Oklahoma, Usa

Abstract eng:
Small earthquakes are occurring with a high rate in Oklahoma, USA, and have been attributed to ongoing fluid injection into the crust from disposal wells in the state. The rate of mb>3 is observed to be about 2 per day, or 730 per year, which exceeds the rate in California. Seismicity is occurring in central and northern Oklahoma, although fluid injection wells are distributed throughout the state. The largest recent earthquake in the region of high seismicity is the Pawnee, Oklahoma M 5.8 earthquake on September 3, 2016, which caused damage to multiple structures. Analysis of the b-value of small earthquakes since February 2014 indicates that b-values vary in time, ranging from about 1.0 to 3.0. Also, the temporal occurrences of larger micro-earthquakes (mb>4.2) appear to follow a peak in b-values. Finally, four moderate earthquakes (M>4.9) have occurred at the edges of the region of micro-seismicity. These effects are consistent with pore pressure changes from fluid disposal wells extending to larger distances with time, affecting favorably-oriented faults in adjacent areas, and triggering earthquakes on those faults. These effects can be replicated with a simple model of crustal pore pressure changes caused by fluid injection into a permeable geologic unit, with faults oriented randomly but close to a direction favorable to rupture under crustal stress conditions with small pore pressure changes, and with an exponential distribution of fault areas leading to an exponential distribution of earthquake magnitudes. Since January 2016 the rates of fluid injection in disposal wells have been restricted by Oklahoma state officials, so occurrences and characteristics of micro-seismicity in the region likely will change.

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