The yellow circles in the figure show earthquakes of similar magnitudes occurring over the 34-year period ending in 2008. 1, numerous M≥2.5 earthquakes have occurred throughout much of the central part of the state in the past 5 years ( 16). No state has experienced a more significant increase in seismicity in recent years than Oklahoma. Injection-related seismicity has been discussed in a variety of contexts in which large volumes of fluid have been, or might be, injected into subsurface formations ( 14, 15). The pressure change resulting from fluid injection simply triggers its release. Strain energy (or stress) on a fault accumulates over time as a natural geologic process. An earthquake on a critically stressed fault caused by fluid injection is referred to as a triggered earthquake when a relatively small perturbation triggers the release of already stored energy in an earthquake ( 9, 13, 14). Such faults are often referred to as critically stressed faults. Because an increase in pore pressure reduces the effective normal stress, it acts to unclamp a fault, potentially triggering the release of accumulated strain energy on a preexisting fault that is already close to failure ( 9). The effective normal stress resists fault slip by acting perpendicular to the fault, in a sense clamping the fault. Simply put, increased fluid pressure decreases the effective normal stress on a fault. The fact that increased pore pressure at depth resulting from fluid injection can trigger slip on preexisting, already-stressed faults is well documented ( 9– 13), and the mechanisms by which triggered fault slip occurs are generally well known ( 9). In general, produced water is either reinjected into the oil producing formation as part of water-flooding enhanced oil recovery (EOR) operations or disposed of in dedicated saltwater disposal (SWD) wells where it is usually injected into saline aquifers, sedimentary formations with relatively high porosity and permeability. Class II UIC wells are also used to inject “produced” water, saline water that was produced from water-bearing oil reservoirs. Because flow-back water is usually quite saline (and can contain other contaminants), it is often disposed of through injection into regulated class II underground injection control (UIC) wells ( 9). As noted by a number of authors ( 2– 7), some of this seismicity appears to be associated with increases in saltwater disposal that originates as “flow-back” water after multistage hydraulic fracturing operations ( 8). Use of the output of this website does not imply approval by the governing building code bodies responsible for building code approval and interpretation for the building site described by latitude/longitude location in the search results of this website.The number of small- to moderate-sized earthquakes in much of the central and eastern United States began to increase markedly around 2009 ( 1). Users of the information from this website assume all liability arising from such use. SEAOC / OSHPD do not intend that the use of this information replace the sound judgment of such competent professionals, having experience and knowledge in the field of practice, nor to substitute for the standard of care required of such professionals in interpreting and applying the results of the seismic data provided by this website. The material presented in this web application should not be used or relied upon for any specific application without competent examination and verification of its accuracy, suitability and applicability by engineers or other licensed professionals. While the information presented on this website is believed to be correct, SEAOC / OSHPD and its sponsors and contributors assume no responsibility or liability for its accuracy.
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