ALBERT

Beschreibung: 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.

Beschreibung: We present the software package hybridMT which allows performing seismic moment tensor inversion and refinement, optimized for earthquake data recorded by regional-to-local seismic networks as well as for acoustic emission activity. The provided software package is designed predominantly for use in MATLAB (see Data and Resources )/shell environments. The algorithm uses first P -wave amplitudes to invert for unconstrained full, deviatoric, and double-couple constrained moment tensors. Uncertainty assessment is performed by bootstrap resampling. The moment tensor inversion may be performed directly in the shell environment (by a dedicated command-line tool) or conveniently through the MATLAB interface (m-functions). In addition to moment tensor inversion, we also provide the MATLAB implementation of the hybrid moment tensor technique. This methodology increases the quality of calculated seismic moment tensors from events forming a spatial cluster by assessing and correcting for poorly known path and site effects. We tested hybridMT on synthetic datasets, acoustic emission data recorded during laboratory rock deformation experiments, and induced seismicity data from a geothermal reservoir. The package is supplemented with extensive documentation, tutorials, and a dedicated website. HybridMT is freely available and distributed under General Public License.

Beschreibung: Seismic moment tensors can provide information on the size and orientation of fractures producing acoustic emissions (AEs) and on the stress conditions in the sample. The moment tensor inversion of AEs is, however, a demanding procedure requiring carefully calibrated sensors and accurate knowledge of the velocity model. In field observations, the velocity model is usually isotropic and time independent. In laboratory experiments, the velocity is often anisotropic and time dependent and attenuation might be significant due to opening or closure of microcracks in the sample during loading. In this paper, we study the sensitivity of the moment tensor inversion to anisotropy of P -wave velocities and attenuation. We show that retrieved moment tensors critically depend on anisotropy and attenuation and their neglect can lead to misinterpretations of the source mechanisms. The accuracy of the inversion also depends on the fracturing mode of AEs: tensile events are more sensitive to P -wave anisotropy and attenuation than shear events. We show that geometry of faulting in anisotropic rocks should be studied using the source tensors, since the P - and T -axes of the moment tensors are affected by velocity anisotropy and deviate from the true orientation of faulting. The stronger the anisotropy is, the larger the deviations are. Finally, we prove that the moment tensor inversion applied to a large dataset of AEs can be utilized to provide information on the attenuation parameters of the rock sample. The method is capable of measuring anisotropic attenuation in the sample and allows for detection of dilatant cracking according to the stress regime.

Beschreibung: 〈span〉〈div〉Abstract〈/div〉Over 700 induced seismic events recorded between June 2009 and March 2015 at different parts of The Geysers geothermal field, California, are used to estimate local 〈span〉S〈/span〉‐wave coda quality factors (QC). Recorded by the 31‐station short‐period Berkeley‐Geysers seismic network, the events have duration magnitudes 1〈MD〈3, depth ranges of 1 and 4 km, and epicentral distance ranges of 0.7–19 km. We apply the coda analysis technique of 〈a href="https://pubs.geoscienceworld.org/bssa#rf27"〉Phillips (1985)〈/a〉 to find QC. Using a sequence of overlapping time windows, the average power spectral density of the coda is calculated. We extract the signal amplitudes at fixed octave‐width frequency bands, measure their decay with time and fit for QC(f) estimates with associated uncertainties. We investigate the sensitivity of the QC results to different input parameters, including lapse time, magnitude range, moving window width, total coda length, and seismic‐sensor components. The choice of quality criteria—signal‐to‐noise ratio and coda Q uncertainties (2σ(QC(f)))—are found to be most sensitive factors. Testing different window lengths and lapse times results in relatively small variations of QC. The final, highest quality mean coda Q (QC‾(f)) estimates are further tested in the context of their spatiotemporal behavior in the reservoir. We found that distance and azimuthal dependence of QC‾(f) are related to the observed crack‐induced reservoir anisotropy, lithological, and structural features. At the northwestern The Geysers about 50% larger QC‾(〉40  Hz) estimates are obtained compared with the southwest. In contrast, geothermal production rate variations, analyzed for a tight cluster in the northwest, do not influence the QC(f) estimates. Moreover, we compare the QC(f) results with previous estimates of direct 〈span〉S〈/span〉‐wave quality factors (QD). A match for QC results at 7 Hz center frequency with QD estimates is observed. However, QC estimates show lower scattering and thus a higher stability.〈/span〉

Beschreibung: Estimation of the recording completeness of seismic catalogs recorded with small networks in a heterogeneous observation volume, for example, in mines, is difficult. Local heterogeneities have a strong influence on the wave path and attenuation and must be taken into account. In order to analyze the spatially varying completeness of such catalogs in three dimensions, we present a new approach based on the probability-based magnitude of completeness (PMC) method of Schorlemmer and Woessner (2008). We demonstrate that the traditional approach of Schorlemmer and Woessner (2008) is insufficient in very complex and heterogeneous settings. To account for this problem, we extend the PMC method, taking into account the direction of incoming seismic waves. This allows us to analyze the influence of small heterogeneities on the recording completeness in high resolution. We compare the results with results obtained by a traditional Gutenberg–Richter frequency-magnitude analysis for the JAGUARS catalog: The Japanese German Underground Acoustic Emission Research in South Africa (JAGUARS) project recorded approximately 500,000 seismic events with magnitudes -5

Beschreibung: 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

Beschreibung: 〈span〉〈div〉Abstract〈/div〉Over 700 induced seismic events recorded between June 2009 and March 2015 at different parts of The Geysers geothermal field, California, are used to estimate local 〈span〉S〈/span〉‐wave coda quality factors (QC). Recorded by the 31‐station short‐period Berkeley‐Geysers seismic network, the events have duration magnitudes 1〈MD〈3, depth ranges of 1 and 4 km, and epicentral distance ranges of 0.7–19 km. We apply the coda analysis technique of 〈a href="https://pubs.geoscienceworld.org/bssa#rf27"〉Phillips (1985)〈/a〉 to find QC. Using a sequence of overlapping time windows, the average power spectral density of the coda is calculated. We extract the signal amplitudes at fixed octave‐width frequency bands, measure their decay with time and fit for QC(f) estimates with associated uncertainties. We investigate the sensitivity of the QC results to different input parameters, including lapse time, magnitude range, moving window width, total coda length, and seismic‐sensor components. The choice of quality criteria—signal‐to‐noise ratio and coda Q uncertainties (2σ(QC(f)))—are found to be most sensitive factors. Testing different window lengths and lapse times results in relatively small variations of QC. The final, highest quality mean coda Q (QC‾(f)) estimates are further tested in the context of their spatiotemporal behavior in the reservoir. We found that distance and azimuthal dependence of QC‾(f) are related to the observed crack‐induced reservoir anisotropy, lithological, and structural features. At the northwestern The Geysers about 50% larger QC‾(〉40  Hz) estimates are obtained compared with the southwest. In contrast, geothermal production rate variations, analyzed for a tight cluster in the northwest, do not influence the QC(f) estimates. Moreover, we compare the QC(f) results with previous estimates of direct 〈span〉S〈/span〉‐wave quality factors (QD). A match for QC results at 7 Hz center frequency with QD estimates is observed. However, QC estimates show lower scattering and thus a higher stability.〈/span〉

Beschreibung: 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.

Beschreibung: 〈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〉