ALBERT

Description: We investigate spatiotemporal variations of the crustal stress field orientation along the rupture zones of the 1999 August Izmit M w 7.4 and November Düzce M w 7.1 earthquakes at the North Anatolian Fault zone (NAFZ) in NW Turkey. Our primary focus is to elaborate on the relation between the state of the crustal stress field and distinct seismotectonic features as well as variations of coseismic slip within the seismogenic layer of the crust. To achieve this, we compile an extensive data base of hypocentres and first-motion polarities including a newly derived local hypocentre catalogue extending from 2 yr prior (1997) to 2 yr after (2001) the Izmit and Düzce main shocks. This combined data set allows studying spatial and temporal variations of stress field orientation along distinct fault segments for the pre- and post-seimic phase of the two large earthquakes in detail. Furthermore, the occurrence of two M  〉 7 earthquakes in rapid succession gives the unique opportunity to analyse the 87-d-long ‘inter-seismic phase’ between them. We use the MOTSI (first MOTion polarity Stress Inversion) procedure directly inverting first-motion polarities to study the stress field evolution of nine distinct segments. In particular, this allows to determine the stress tensor also for the pre- and post-seismic phases when no stable single-event focal mechanisms can be determined. We observe significantly different stress field orientations along the combined 200-km-long rupture in accordance with lateral variations of coseismic slip and seismotectonic setting. Distinct vertical linear segments of the NAFZ show either pure-strike slip behaviour or transtensional and normal faulting if located near pull-apart basins. Pull-apart structures such as the Akyazi and Düzce basins show a predominant normal faulting behaviour along the NAFZ and reflect clearly different characteristic from neighbouring strike-slip segments. Substantial lateral stress field heterogeneity following the two main shocks is observed that declines with time towards the post-seismic period that rather reflects the regional right-lateral strike-slip stress field.

Description: Calcite is one of the most ubiquitous minerals in the Earth’s crust and is mostly present as calcite or the slightly denser polymorph aragonite. In addition five different phases of CaCO 3 (calcite II–VI), which display similar structural features as calcite, have been observed with increasing pressure in different experiments by several authors. Experimentally, the CaCO 3 -III and CaCO 3 -IIIb polymorphs have recently been observed by Merlini et al. (2012) applying pressures between 2.5–15 GPa on natural samples of calcite using single-crystal synchrotron X-ray diffraction. Here we report an occurrence of metastable authigenic CaCO 3 -III and CaCO 3 -IIIb nanocrystals for the first time in nature. Using transmission electron microscopy, idiomorphic, 50–150 nm sized crystals were observed within several meters from the surface in quaternary loess deposits in Central Asia. Nanocrystals contain higher surface energy per volume compared to coarse-grained materials due to their larger surface area. The internal pressure of a solid, P S , is at equilibrium with the surface stress, which increases with decreasing particle size. We estimated internal pressures inside the observed nanocrystals between 2.54–4.06 GPa, assuming spherical crystals with 1 nm diameter and specific surface energies, between 1.27–2.03 J/m 2 ( Forbes et al. 2011 ).

Description: We observe void growth and coalescence into cavity-bearing shear bands during deformation of wet synthetic anorthite aggregates containing 〈3 vol. % silica-enriched melt. Samples were deformed in the Newtonian creep regime to high strain during torsion experiments at 1100°C and 400 MPa confining pressure. Localized cavity-bearing shear bands show an S–C'-geometry: the bands (C') are oriented at about 30° to the compression direction of the imposed simple shear and the internal foliation (S) of the bands is rotated towards the horizontal external shear plane. Cavity-bearing shear bands started to nucleate in the sample periphery above a shear strain threshold of 2. Quartz crystallized from the water-saturated SiO 2 -rich melt within large cavities inside these bands, which requires that the melt is decompressed by 〉200 MPa during their formation. The dynamically evolving cavities are sites of locally reduced pressure that collect the melt distributed in the adjacent matrix. Therefore, cavitation damage under ductile conditions may result in the development of an efficient melt channelling system controlling SiO 2 -rich melt flow in the lower crust. Electron backscatter diffraction analysis shows that the quartz inside the cavity bands has a crystallographic preferred orientation (CPO). The development of the CPO is explained by the preferred dissolution of crystals oriented with the rhombohedra and trigonal dipyramids orthogonal to the compression direction and by preferential growth of crystals aligned with their 〈0001〉 axis in the extension direction of the externally applied simple shear deformation.

Description: We investigate the source parameters of picoseismic and nanoseismic events (Mw〉-4.1) recorded with a high-sensitivity seismic network at the Mponeng gold mine in South Africa to gain new insights into the scaling of small seismic events. The Japanese–German Underground Acoustic Emission Research in South Africa (JAGUARS) network, composed of one three-component (3C) accelerometer (sensitivity 50 Hz to 25 kHz) and 8 acoustic emission (AE) sensors, (sensitivity 1 kHz to 180 kHz) is located at a depth of 3268 m and covers the limited volume of approximately 300×300×300 m. The AE sensors are calibrated with respect to the 3C accelerometer in the frequency band 400 Hz–17 kHz. The waveform data of two datasets are analyzed; (1) the aftershock sequence of an Mw 1.9 event that occurred approximately 30 m from our network, and (2) the postblasting activity recorded during working days, located at a distance 〉90 m from the network at the exploitation level. For the analysis we applied spectral fitting and spectral ratio methods. The calculated values of Mw range from -0.8 down to -4.1 with corner frequencies 0.8 kHz–13.6 kHz (source sizes from 8 cm to 1.3 m). We observe static stress drops ranging from 1 MPa to 10.0 MPa with apparent stresses of 0.01 MPa–1.00 MPa. Stress drops are independent of the moment, suggesting self-similar behavior in the analyzed magnitude range -4.1

Description: Solid-solid mineral reaction rates are influenced by the microfabrics of reactant phases and concurrent deformation. To investigate this interplay in carbonate systems, we performed annealing and deformation experiments on polycrystalline and single-crystal calcite and magnesite, forming dolomite (Dol) and magnesio-calcite (Mg-Cal). At a fixed temperature of T = 750 °C and confining pressure of P = 400 MPa, samples were either annealed for 29 h, or deformed in triaxial compression or torsion for 18 h using a Paterson-type gas deformation apparatus. At the contact interface of the starting reactants, Dol reaction rims and polycrystalline Mg-Cal layers were formed. The widths of the layers were in the ranges 4–117 and 30–147 μm, respectively, depending on the microstructure of starting materials and experimental conditions. Annealing experiments with polycrystalline reactants in contact with each other resulted in a ~22-fold increase in Dol rim thickness compared to a contact between two single crystals and a larger Mg-Cal layer width by a factor of 5 (cf. Helpa et al. 2014 ). This suggests that the microstructure of magnesite controls migration of the reaction front. For polycrystalline starting materials, axial stress accelerated Mg-Cal growth rates but not Dol growth rates. Highly strained torsion samples showed Dol formation along grain boundaries in Mg-Cal as well as in the polycrystalline calcite reactant. A reduction of Dol rim thickness between polycrystalline reactants deformed in torsion is possibly caused by concurrent grain coarsening of polycrystalline magnesite. Dol and Mg-Cal growth kinetics between single crystals were unaffected by the addition of ~0.3 wt% water. The experiments demonstrate that Dol reaction kinetics strongly correlate with magnesite reactant grain sizes, while Mg-Cal growth depends on the calcite reactant grain sizes. The dolomite-forming mineral reaction kinetics are not significantly affected by concurrent deformation. In contrast, deformation enhances Mg-Cal formation, especially at small calcite grain sizes that promote efficient grain boundary diffusion. Therefore, the fastest reactions forming Dol and Mg-Cal in nature are expected to occur in very fine-grained reactants. Concurrent deformation may drastically enhance reaction kinetics if grain size reduction of the reactants occurs by, for example, cataclasis or dynamic recrystallization.

Description: Variations in fault structure, for example, surface roughness and deformation zone width, influence the location and dynamics of large earthquakes as well as the distribution of small seismic events. In nature, changes in fault roughness and seismicity characteristics can rarely be studied simultaneously, so that little is known about their interaction and evolution. Here, we investigate the connection between fault structure and near-fault distributions of seismic events over series of stick-slip cycles in the laboratory. We conducted a set of experiments on rough faults that developed from incipient fracture surfaces. We monitored stress and seismic activity which occurred in the form of acoustic emissions (AEs). We determined AE density distributions as a function of fault normal distance based on high-accuracy hypocentre locations during subsequent interslip periods. The characteristics of these distributions were closely connected to different structural units of the faults, that is, the fault core, off-fault and background damage zone. The core deformation zone was characterized by consistently high seismic activity, whereas the off-fault damage zone displayed a power-law decay of seismic activity with increasing distance from the fault core. The exponents of the power-law-distributed off-fault activity increased with successive stick-slip events so that later interslip periods showed a more rapid spatial decay of seismic activity from the fault. The increase in exponents was strongest during the first one to three interslip periods and reached approximately constant values thereafter. The relatively rapid spatial decay of AE events during later interslip periods is likely an expression of decreasing fault zone complexity and roughness. Our results indicate a close relationship between fault structure, stress and seismic off-fault activity. A more extensive mapping of seismic off-fault activity-decay has the potential to significantly advance the understanding of fault zone properties including variations in fault roughness and stress.

Description: The motion along upper crustal faults in response to tectonic loading is controlled by both loading stresses and surface properties, for example, roughness. Fault roughness influences earthquake slip distributions, stress-drops and possible transitions from stable to unstable sliding which is connected to the radiation of seismic energy. The relationship between fault roughness and seismic event distributions is insufficiently understood, in particular, the underlying mechanisms of off-fault seismicity creation in the proximity of rough faults are debated. Here, we investigate the connection between roughness and acoustic emission (AE) density with increasing fault-normal distance during loading of surfaces with pre-defined roughness. We test the influence of fault roughness and normal stress variations on the characteristics of AE off-fault distributions. To this end, two sets of experiments were conducted: one to investigate the influence of initial surface roughness at constant confining pressure, and the other to investigate the influence of fault-normal stresses at constant roughness. Our experiments reveal a power-law decay of AE density with distance from the slip surface. The power-law exponents are sensitive to both fault roughness and normal stress variations so that larger normal stresses and increased roughness lead to slower AE density decay with fault-normal distance. This emphasizes that both roughness and stress have to be considered when trying to understand microseismic event distributions in the proximity of fault zones. Our results are largely in agreement with theoretical studies and observations of across-fault seismicity distributions in California suggesting a connection between off-fault seismicity and fault roughness over a wide range of scales. Seismicity analysis including a possible mapping between off-fault activity exponents, fault stresses and roughness, can be an important tool in understanding the mechanics of faults and their seismic hazard potential.

Description: Local rotations of the stress field might serve as an indicator to characterize the physical status of individual fault segments during the seismic cycle. In this study we focus on the pre-, 2-month aftershock- and post-seismic phase of the 1999 M w 7.4 Izmit earthquake in northwestern Turkey. Using a compilation of focal mechanism data we investigate spatiotemporal changes of the stress field orientations and find distinct variations along individual fault segments. Whereas the regional stress field prior to the Izmit earthquake and following the 2-month aftershock sequence reflects a stable strike-slip regime, the early aftershock period is dominated by EW-extension below the Akyazi Basin. During the 2-month aftershock period we find significant changes from strike-slip to normal-faulting during the main shock following by a systematic backrotation to the pre-main shock stress regime. This backrotation commences first in the Akyazi Plain hosting a co-seismic slip deficit of ≤3 m and propagates then further to the east towards the Karadere and Düzce faults where the Düzce M w 7.1 main shock nucleated 87 d later. Our results confirm that spatiotemporal stress field rotations are a useful indicator for variations of the seismotectonic setting during the seismic cycle.

Description: In this paper, an underground experiment at the Äspö 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 Äspö 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 Ävrö 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.

Description: 〈p〉We show that near–real-time seismic monitoring of fluid injection allowed control of induced earthquakes during the stimulation of a 6.1-km-deep geothermal well near Helsinki, Finland. A total of 18,160 m〈sup〉3〈/sup〉 of fresh water was pumped into crystalline rocks over 49 days in June to July 2018. Seismic monitoring was performed with a 24-station borehole seismometer network. Using near–real-time information on induced-earthquake rates, locations, magnitudes, and evolution of seismic and hydraulic energy, pumping was either stopped or varied—in the latter case, between well-head pressures of 60 and 90 MPa and flow rates of 400 and 800 liters/min. This procedure avoided the nucleation of a project-stopping magnitude 〈i〉M〈/i〉〈sub〉W〈/sub〉 2.0 induced earthquake, a limit set by local authorities. Our results suggest a possible physics-based approach to controlling stimulation-induced seismicity in geothermal projects.〈/p〉