@article{KwiatekMartinezGarzonPlenkersetal.2018,
  author    = {Kwiatek, Grzegorz and Martinez-Garzon, Patricia and Plenkers, K. and Leonhardt, Maria and Zang, Arno and von Specht, Sebastian and Dresen, Georg and Bohnhoff, Marco},
  title     = {Insights into complex subdecimeter fracturing processes occurring during a water injection experiment at depth in Aspo Hard Rock Laboratory, Sweden},
  series = {Journal of geophysical research : Solid earth},
  volume    = {123},
  journal   = {Journal of geophysical research : Solid earth},
  number    = {8},
  publisher = {American Geophysical Union},
  address   = {Washington},
  issn      = {2169-9313},
  doi       = {10.1029/2017JB014715},
  pages     = {6616 -- 6635},
  year      = {2018},
  abstract  = {We investigate the source characteristics of picoseismicity (M-w < -2) recorded during a hydraulic fracturing in situ experiment performed in the underground Aspo Hard Rock Laboratory, Sweden. The experiment consisted of six stimulations driven by three different water injection schemes and was performed inside a 28-m-long, horizontal borehole located at 410-m depth. The fracturing processes were monitored with a variety of seismic networks including broadband seismometers, geophones, high-frequency accelerometers, and acoustic emission sensors thereby covering a wide frequency band between 0.01 and 100,000Hz. Here we study the high-frequency signals with dominant frequencies exceeding 1000 Hz. The combined seismic network allowed for detection and detailed analysis of 196 small-scale seismic events with moment magnitudes M-W < -3.5 (source sizes of decimeter scale) that occurred solely during the stimulations and shortly after. The double-difference relocated hypocenter catalog as well as source parameters were used to study the physical characteristics of the induced seismicity and then compared to the stimulation parameters. We observe a spatiotemporal migration of the picoseismic events away and toward the injection intervals in direct correlation with changes in the hydraulic energy (product of fluid injection pressure and injection rate). We find that the total radiated seismic energy is extremely low with respect to the product of injected fluid volume and pressure (hydraulic energy). The radiated seismic energy correlates well with the hydraulic energy rate. The obtained fault plane solutions for particularly well-characterized events signify the reactivation of preexisting rock defects under influence of increased pore fluid pressure on fault plane orientations in good correspondence with the local stress field orientation.},
  language  = {en}
}
@article{KwiatekMartinezGarzonDresenetal.2015,
  author    = {Kwiatek, Grzegorz and Martinez-Garzon, Patricia and Dresen, Georg and Bohnhoff, Marco and Sone, Hiroki and Hartline, Craig},
  title     = {Effects of long-term fluid injection on induced seismicity parameters and maximum magnitude in northwestern part of The Geysers geothermal field},
  series = {Journal of geophysical research : Solid earth},
  volume    = {120},
  journal   = {Journal of geophysical research : Solid earth},
  number    = {10},
  publisher = {American Geophysical Union},
  address   = {Washington},
  issn      = {2169-9313},
  doi       = {10.1002/2015JB012362},
  pages     = {7085 -- 7101},
  year      = {2015},
  abstract  = {The long-term temporal and spatial changes in statistical, source, and stress characteristics of one cluster of induced seismicity recorded at The Geysers geothermal field (U.S.) are analyzed in relation to the field operations, fluid migration, and constraints on the maximum likely magnitude. Two injection wells, Prati-9 and Prati-29, located in the northwestern part of the field and their associated seismicity composed of 1776 events recorded throughout a 7year period were analyzed. The seismicity catalog was relocated, and the source characteristics including focal mechanisms and static source parameters were refined using first-motion polarity, spectral fitting, and mesh spectral ratio analysis techniques. The source characteristics together with statistical parameters (b value) and cluster dynamics were used to investigate and understand the details of fluid migration scheme in the vicinity of injection wells. The observed temporal, spatial, and source characteristics were clearly attributed to fluid injection and fluid migration toward greater depths, involving increasing pore pressure in the reservoir. The seasonal changes of injection rates were found to directly impact the shape and spatial extent of the seismic cloud. A tendency of larger seismic events to occur closer to injection wells and a correlation between the spatial extent of the seismic cloud and source sizes of the largest events was observed suggesting geometrical constraints on the maximum likely magnitude and its correlation to the average injection rate and volume of fluids present in the reservoir.},
  language  = {en}
}