Condition of Occurrence of Large Man-Made Earthquakes in the Zone of Oil Production, Oklahoma
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ondition of Occurrence of Large Man-Made Earthquakes in the Zone of Oil Production, Oklahoma Inessa Vorobievaa, *, Peter Shebalina, c, **, and Clément Narteaub, *** aInstitute
of Earthquake Prediction Theory and Mathematical Geophysics, Moscow, 117997 Russia Université de Paris, Institut de Physique du Globe de Paris, CNRS, Paris, F-75005 France c Schmidt Institute of Physics of the Earth, Russian Academy of Sciences, Moscow, 123242 Russia *e-mail: [email protected] **e-mail: [email protected] ***e-mail: [email protected] b
Received June 22, 2019; revised July 3, 2019; accepted July 4, 2020
Abstract—Man-made seismicity is a response of the brittle crust to fluid injection at depth and to the subsequent increase in pore-pressure and stress field perturbations. In Oklahoma, where the sharp increase in earthquake rate correlates with injection operations, we show that the earthquake-size distribution can differ significantly on the volume of injected fluid. The size distribution of M < 3.5 earthquakes exhibits a near-constant slope b, while significant variation of b-values (from b ≈ 1 to b > 2) may be documented for larger magnitude ranges. This change shows statistically significant positive dependence on injection activity. In addition, largest events occur at the border of the injection area at some distance from massive injection, and in the periods of steady injection rate. These observations suggest that a deficit of large induced earthquakes under conditions of high injection rate can be accompanied by an overall increase of natural seismicity along pre-existing faults in the surrounding volume, where large events are more likely to be triggered over longer space-time scales. Keywords: man-made seismicity, triggered seismicity, Oklahoma, fluid injection, b-value, band-limited analysis DOI: 10.1134/S1069351320060130
INTRODUCTION Fluid injection at depth can produce an increase in earthquake activity under the combined effect of porepressure changes and stress field perturbations (Davis and Frohlich, 1993; Ellsworth, 2013; Rubinstein and Mahani, 2015; Adushkin and Turuntaev, 2015). First, the frictional strength of faults decreases significantly as the pore pressure in excess to lithostatic pressure reduces the effective normal stress and simplifies beginning of slip. This mechanism supposes a direct hydrological connection between injection wells and faults. Second, deep fluid injection can modify the stress field and the loading conditions in the surrounding rock volume, even in zones with no direct change in pore pressure. Physical mechanism of these earthquakes meets most of the characteristics of the seismicity associated with active tectonic zones. These two mechanisms can initiate seismic ruptures, which are commonly referred to as induced or triggered earthquakes (Adushkin and Turuntaev, 2015). These two types of events are difficult to distinguish from one another nearby injection wells and a major question is to determine if they have a statistical signature in earthquake catalogs.
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