ABSTRACT On 30 November 2018, a sequence of seismicity including a felt (ML∼4.5) induced earthquake occurred ∼16 km southwest of Fort St. John, British Columbia. Using a local seismograph network around the epicentral region, we identified > 560 seismic events over a two-week period, incorporating two mainshock events within a 45 min time interval, both with ML>4.3. This seismicity occurred close in location and depth to ongoing hydraulic fracturing operations. Using previously unpublished data, our analysis suggests that events, including the largest mainshock, occurred at the interval of fluid injection, which is shallower than previously reported. The events showed a mix of reverse, oblique normal, and strike-slip mechanisms within a well-defined structural corridor that forms the southern margin of the Fort St. John graben. The two mainshock events reveal opposing mechanisms: one as a reverse (re)activation of a normal fault (ML 4.5) and the other an oblique normal mechanism (ML 4.3). Stress inversion and bootstrap analysis of 72 well-constrained focal mechanisms indicate that the maximum principal stress direction is horizontal, oriented in a north-northeast direction (3°–36°). However, the intermediate and minimum stress axes fluctuate between horizontal and vertical and are nearly equal in magnitude, indicating that both reverse and strike-slip regimes can occur in response to relatively small stress perturbations. Stress inversions using event subsets before and after the largest mainshock reveals an approximately 30° counter-clockwise coseismic rotation of the principal stress axes in the hypocentral region. Furthermore, the observed seismicity suggests that the largest mainshock event exceeded the calculated Mmax using models based on injected volumes, suggesting that it may be an example of runaway rupture. This has important implications for risk analysis, because small changes in the stress field may be induced through ongoing operations in this area, destabilizing different faults within a complex structural environment.
Research Article| May 01, 2018 Interevent Triggering in Microseismicity Induced by Hydraulic Fracturing Samira Maghsoudi; Samira Maghsoudi aComplexity Science Group, Department of Physics and Astronomy, University of Calgary, 2500 University Drive NW, Calgary, Alberta, Canada T2N 1N4, davidsen@phas.ucalgary.cacAlso at Department of Geoscience, University of Calgary, 2500 University Drive NW, Calgary, Alberta, Canada T2N 1N4. Search for other works by this author on: GSW Google Scholar Jordi Baró; Jordi Baró aComplexity Science Group, Department of Physics and Astronomy, University of Calgary, 2500 University Drive NW, Calgary, Alberta, Canada T2N 1N4, davidsen@phas.ucalgary.ca Search for other works by this author on: GSW Google Scholar Alana Kent; Alana Kent bDepartment of Geoscience, University of Calgary, 2500 University Drive NW, Calgary, Alberta, Canada T2N 1N4 Search for other works by this author on: GSW Google Scholar David Eaton; David Eaton bDepartment of Geoscience, University of Calgary, 2500 University Drive NW, Calgary, Alberta, Canada T2N 1N4 Search for other works by this author on: GSW Google Scholar Jörn Davidsen Jörn Davidsen aComplexity Science Group, Department of Physics and Astronomy, University of Calgary, 2500 University Drive NW, Calgary, Alberta, Canada T2N 1N4, davidsen@phas.ucalgary.ca Search for other works by this author on: GSW Google Scholar Bulletin of the Seismological Society of America (2018) 108 (3A): 1133–1146. https://doi.org/10.1785/0120170368 Article history first online: 01 May 2018 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn Email Tools Icon Tools Get Permissions Search Site Citation Samira Maghsoudi, Jordi Baró, Alana Kent, David Eaton, Jörn Davidsen; Interevent Triggering in Microseismicity Induced by Hydraulic Fracturing. Bulletin of the Seismological Society of America 2018;; 108 (3A): 1133–1146. doi: https://doi.org/10.1785/0120170368 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search nav search search input Search input auto suggest search filter All ContentBy SocietyBulletin of the Seismological Society of America Search Advanced Search Abstract Permeability enhancing treatments such as hydraulic fracturing (HF) induce microseismic events with typical magnitudes in the −3.0 to −0.5 range, although significantly larger induced earthquakes up to 4.7 in moment magnitude have been reported. Diffusion of pore pressure away from the hydraulic fracture system is thought to be a primary controlling mechanism. Understanding other primary or secondary triggering mechanisms during HF is expected to furnish new insights regarding stress, strength of faults, and rupture initiation and propagation. Using novel methods from statistical seismology, we present evidence for the existence of event–event triggering cascades in microseismicity induced by HF. Although background seismicity dominates, we find that these triggering cascades exhibit features that also characterize tectonic aftershock sequences such as the empirical Omori–Utsu relation and the productivity relation. This suggests that the underlying physical earthquake–earthquake triggering mechanisms are similar in both cases, as also observed for other earthquake swarms. The presence of triggering cascades is of direct relevance for optimizing the effectiveness of the stimulation. You do not currently have access to this article.
Using formulae for both tensile and shear sources, we investigate spectral characteristics of microearthquakes induced by hydraulic fracturing, with application to passive-seismic data recorded during a multistage treatment programme in western Canada. For small moment magnitudes (Mw < 0), reliable determination of corner frequency requires accurate knowledge of QP and QS, although spectral estimates of magnitude are relatively unaffected by uncertainty in seismic attenuation. Here, we estimate QP and QS using spectral ratios derived from perforation shots. Of the microseismic events analysed during the hydraulic-fracture treatment, 17 of 20 exhibit an S/P spectral ratio <5, which is consistent with tensile failure. In addition, four microseismic events are characterized by a modulating source spectrum containing quasi-periodic notches. We interpret this spectral character to reflect a complex rupture pattern that involves rapid (5−8 ms) opening and closing of tensile cracks. In general, for tensile rupture on a penny-shaped crack, our model predicts that source radius (a) is related to moment magnitude (Mw) and internal fluid pressure within the fracture (Pi) by a simple empirical scaling relation: log10(a) = [9 − log102]/3 + 0.5Mw − log10(Pi)/3.
SUMMARY During two stages of hydraulic fracturing monitored by 12 three-component geophones, we observe three groups of resonances in the frequency band 15-60 Hz. The fluid injected is a mixture of slurry, which consists mainly of water and solid particles (proppant), and supercritical nitrogen. Considering that the resonances are caused by source effects, we investigate the response of three models, namely periodic repetitive events, fluid-filled cracks and fluid-flow instabilities, and compare them with the resonance characteristics observed. The resonance frequencies last for several tens of minutes, and are stable, with narrow-band peaks, which is incompatible with the repetitive events model. The variations in frequency, amplitude and quality factor associated with these resonances are correlated with the pumping curves. One group of resonances is positively correlated with the slurry flow rate, which is in contradiction with the fluid-filled crack model, as the fracture length is expected to increase over time and the resonance frequencies to decrease. Another group of resonances is clearly correlated with the nitrogen injection rate, and the last group also seems to be correlated to the nitrogen injection rate. Resonance characteristics resulting from jet instabilities are consistent with most of these resonances. Resonance frequencies seem to follow pumping rates, either the slurry flow or the nitrogen injection rate, while quality factors and amplitudes seem to follow only the nitrogen injection rate. The different resonance characteristics could then provide complementary information on the cause of harmonic resonances, as well as the geomechanical behavior during fluid movements in reservoir and volcanoes.
Event detection is an essential component of microseismic data analysis. This process is typically carried out using a short- and long-term-average-ratio (STA/LTA) method, which is simple and computationally efficient but often yields inconsistent results for noisy data sets. We have aimed to optimize the performance of the STA/LTA method by testing different input forms of 3C waveform data and different characteristic functions (CFs), including a proposed [Formula: see text]-mean CF. These tests are evaluated using receiver operating characteristic (ROC) analysis and are compared based on synthetic and field data examples. Our analysis indicates that the STA/LTA method using a [Formula: see text]-mean CF improves the detection sensitivity and yields more robust event detection on noisy data sets than some previous approaches. In addition, microseismic events are detected efficiently on field data examples using the same detection threshold obtained from the ROC analysis on synthetic data examples. We recommend the use of the Youden index based on ROC analysis using a training subset, extracted from the continuous data, to further improve the detection threshold for field microseismic data.
