TY - JOUR A1 - Zang, Arno A1 - Stephansson, Ove A1 - Stenberg, Leif A1 - Plenkers, Katrin A1 - von Specht, Sebastian A1 - Milkereit, Claus A1 - Schill, Eva A1 - Kwiatek, Grzegorz A1 - Dresen, Georg A1 - Zimmermann, Günter A1 - Dahm, Torsten A1 - Weber, Michael T1 - Hydraulic fracture monitoring in hard rock at 410 m depth with an advanced fluid-injection protocol and extensive sensor array JF - Geophysical journal international N2 - In this paper, an underground experiment at the Aspo Hard Rock Laboratory (HRL) is described. Main goal is optimizing geothermal heat exchange in crystalline rock mass at depth by multistage hydraulic fracturing with minimal impact on the environment, that is, seismic events. For this, three arrays with acoustic emission, microseismicity and electromagnetic sensors are installed mapping hydraulic fracture initiation and growth. Fractures are driven by three different water injection schemes (continuous, progressive and pulse pressurization). After a brief review of hydraulic fracture operations in crystalline rock mass at mine scale, the site geology and the stress conditions at Aspo HRL are described. Then, the continuous, single-flow rate and alternative, multiple-flow rate fracture breakdown tests in a horizontal borehole at depth level 410 m are described together with the monitoring networks and sensitivity. Monitoring results include the primary catalogue of acoustic emission hypocentres obtained from four hydraulic fractures with the in situ trigger and localizing network. The continuous versus alternative water injection schemes are discussed in terms of the fracture breakdown pressure, the fracture pattern from impression packer result and the monitoring at the arrays. An example of multistage hydraulic fracturing with several phases of opening and closing of fracture walls is evaluated using data from acoustic emissions, seismic broad-band recordings and electromagnetic signal response. Based on our limited amount of in situ tests (six) and evaluation of three tests in Avro granodiorite, in the multiple-flow rate test with progressively increasing target pressure, the acoustic emission activity starts at a later stage in the fracturing process compared to the conventional fracturing case with continuous water injection. In tendency, also the total number and magnitude of acoustic events are found to be smaller in the progressive treatment with frequent phases of depressurization. KW - Geomechanics KW - Fracture and flow KW - Broad-band seismometers Y1 - 2016 SN - 0956-540X SN - 1365-246X VL - 208 SP - 790 EP - 813 PB - Oxford Univ. Press CY - Oxford ER - TY - JOUR A1 - Socquet, Anne A1 - Valdes, Jesus Pina A1 - Jara, Jorge A1 - Cotton, Fabrice Pierre A1 - Walpersdorf, Andrea A1 - Cotte, Nathalie A1 - von Specht, Sebastian A1 - Ortega-Culaciati, Francisco A1 - Carrizo, Daniel A1 - Norabuena, Edmundo T1 - An 8month slow slip event triggers progressive nucleation of the 2014 Chile megathrust JF - Geophysical research letters N2 - The mechanisms leading to large earthquakes are poorly understood and documented. Here we characterize the long-term precursory phase of the 1 April 2014 M(w)8.1 North Chile megathrust. We show that a group of coastal GPS stations accelerated westward 8months before the main shock, corresponding to a M(w)6.5 slow slip event on the subduction interface, 80% of which was aseismic. Concurrent interface foreshocks underwent a diminution of their radiation at high frequency, as shown by the temporal evolution of Fourier spectra and residuals with respect to ground motions predicted by recent subduction models. Such ground motions change suggests that in response to the slow sliding of the subduction interface, seismic ruptures are progressively becoming smoother and/or slower. The gradual propagation of seismic ruptures beyond seismic asperities into surrounding metastable areas could explain these observations and might be the precursory mechanism eventually leading to the main shock. KW - seismology KW - GPS KW - subduction KW - precursor Y1 - 2017 U6 - https://doi.org/10.1002/2017GL073023 SN - 0094-8276 SN - 1944-8007 VL - 44 SP - 4046 EP - 4053 PB - American Geophysical Union CY - Washington ER -