Plenkers, Katrin (2010) On the characteristics of mining-induced seismicity with magnitudes -5 < Mw < -1. PhD thesis, Deutsches GeoForschungsZentrum.
Full text not available from this repository.Abstract
Seismology is often focused on the observation and analysis of large, destructive seismic events. The observation of small seismic ruptures (M < 1) inside the Earth's crust is technically challenging because it requires the recording of ground motions with high sensitivity and in a very high frequency range. The proper recording of seismic events with rupture areas on a centimeter scale, has not been realized in nature so far. Field seismology and laboratory fracture experiments are detached by the observational gap that prevents a direct comparison of results. The observation of small scale seismic ruptures is required in order to gain insights into the intrinsic parameters and characteristics of earthquake nucleation. For example, the continuation of earthquake self-similarity towards very small ruptures is heavily discussed, because it constrains the early stages of rupture generation. The Japanese-German Underground Acoustic Emission Research in South Africa (JAGUARS) project aims to close the observational gap by monitoring seismic events with rupture areas on a centimeter scale, named picoseismicity. The observation is realized monitoring a highly stressed rock volume inside the Mponeng Deep Gold Mine in Carletonville, South Africa. The sophisticated recording of frequencies f >1 kHz was realized using short-period accelerometers (sensitivity 50 Hz to 25 kHz) and acoustic emission (AE) sensors (sensitivity 700 Hz to 200 kHz). In total, more than 500,000 events were recorded during two years with the JAGUARS network. The main objective of this thesis is to obtain insights on the characteristics and internal properties of picoseismicity. Furthermore, we aim to improve the understanding of high frequency recording that is mandatory to close the observational gap. We analyze the data recorded within the JAGUARS project. We use the results to improve the JAGUARS network data recording. For the first step, we create an earthquake data catalog. We investigate the spatio-temporal distribution of picoseismicity and reveal that the data catalog contains several distinct seismicity patterns closely correlated to the ongoing mining work. The seismic patterns are in agreement with earlier observations of microseismicity in mines. Secondly we investigate the performance of the high-frequency recording network. We correlate the limitations of high-frequency (f >> 25 kHz) recording to damping effects introduced by larger source-receiver distances, engineered structures, e. g. tunnels and geological boundaries. Furthermore, we analyze the performance of AE sensors taking into account the sensor-rock coupling that is crucial in AE sensor recording. The analysis is important because any analysis on source properties requires full knowledge on the sensor response. We introduce an in-situ calibration method for AE sensors that is based on the recordings of the fully calibrated 3-component accelerometer. Finally, we analyze the frequency-magnitude distribution. For this purpose, we estimate moment magnitudes using both recordings from the 3-component accelerometer as well as from AE sensors. We link the spectral level of AE sensors to the absolute values of the accelerometer by regression analysis. The frequency-magnitude distribution estimated is in agreement with the Gutenberg-Richter distribution, and does neither present a bimodal distribution nor a minimum magnitude, as suggested in earlier studies. This indicates that the physical processes governing earthquake rupture are similar down to the smallest magnitudes observed. Differences in magnitude of completeness in different areas of the observational volume are discussed. For this analysis we optimize the probability-based magnitude of completeness method to make it applicable to complex, underground networks. We reveal that the magnitude of completeness in the JAGUARS network varies strongly in space, in agreement with the network sensitivity. The analysis of picoseismicity in this thesis significantly impacts the long-lasting discussion on earthquake self-similarity and therefore gives new insights into earthquake nucleation. Our results suggest self-similarity of seismic events down to magnitudes as small as M -4.5. We close the current observational gap, which allows for a detailed comparison of laboratory studies and field seismology in the future. The characteristics of picoseismicity recorded and analyzed in this study reveal no deviation from the characteristics of micro- and large-scale seismicity.
[error in script]| Item Type: | Thesis (PhD) |
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| Subjects: | Region > South Africa Inducing technology > Underground mining |
| Project: | Geo-INQUIRE > JAGUARS: Mining induced seismicity associated to gold mining |
