TY - JOUR A1 - Kwiatek, Grzegorz A1 - Martinez-Garzon, Patricia A1 - Dresen, Georg A1 - Bohnhoff, Marco A1 - Sone, Hiroki A1 - Hartline, Craig T1 - Effects of long-term fluid injection on induced seismicity parameters and maximum magnitude in northwestern part of The Geysers geothermal field JF - Journal of geophysical research : Solid earth N2 - 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. KW - fluid-induced seismicity KW - maximum magnitude KW - reservoir characterization KW - source parameters KW - passive seismic monitoring Y1 - 2015 U6 - https://doi.org/10.1002/2015JB012362 SN - 2169-9313 SN - 2169-9356 VL - 120 IS - 10 SP - 7085 EP - 7101 PB - American Geophysical Union CY - Washington ER - TY - JOUR A1 - Ziegler, Moritz O. A1 - Reiter, Karsten A1 - Heidbach, Oliver A1 - Zang, Arno A1 - Kwiatek, Grzegorz A1 - Stromeyer, Dietrich A1 - Dahm, Torsten A1 - Dresen, Georg A1 - Hofmann, Gerhard T1 - Mining-Induced Stress Transfer and Its Relation to a 1.9 Seismic Event in an Ultra-deep South African Gold Mine JF - Pure and applied geophysics N2 - On 27 December 2007, a 1.9 seismic event occurred within a dyke in the deep-level Mponeng Gold Mine, South Africa. From the seismological network of the mine and the one from the Japanese-German Underground Acoustic Emission Research in South Africa (JAGUARS) group, the hypocentral depth (3,509 m), focal mechanism and aftershock location were estimated. Since no mining activity took place in the days before the event, dynamic triggering due to blasting can be ruled out as the cause. To investigate the hypothesis that stress transfer, due to excavation of the gold reef, induced the event, we set up a small-scale high-resolution three-dimensional (3D) geomechanical numerical model. The model consisted of the four different rock units present in the mine: quartzite (footwall), hard lava (hanging wall), conglomerate (gold reef) and diorite (dykes). The numerical solution was computed using a finite-element method with a discretised mesh of approximately elements. The initial stress state of the model is in agreement with in situ data from a neighbouring mine, and the step-wise excavation was simulated by mass removal from the gold reef. The resulting 3D stress tensor and its changes due to mining were analysed based on the Coulomb failure stress changes on the fault plane of the event. The results show that the seismic event was induced regardless of how the Coulomb failure stress changes were calculated and of the uncertainties in the fault plane solution. We also used the model to assess the seismic hazard due to the excavation towards the dyke. The resulting curve of stress changes shows a significant increase in the last in front of the dyke, indicating that small changes in the mining progress towards the dyke have a substantial impact on the stress transfer. KW - Induced seismicity KW - static stress change KW - deep-level mining KW - tabular mining KW - Coulomb failure stress KW - 3D geomechanical numerical model Y1 - 2015 U6 - https://doi.org/10.1007/s00024-015-1033-x SN - 0033-4553 SN - 1420-9136 VL - 172 IS - 10 SP - 2557 EP - 2570 PB - Springer CY - Basel ER -