Geology topics

Margaret S. Boettcher

Publications and source records attributed to Margaret S. Boettcher.

4 recordsLinked to original sources

USGS2015

Development of the Global Earthquake Model’s neotectonic fault database

The Global Earthquake Model (GEM) aims to develop uniform, openly available, standards, datasets and tools for worldwide seismic risk assessment through global collaboration, transparent communication and adapting state-of-the-art science. GEM Faulted Earth (GFE) is one of GEM’s global hazard module projects. This paper describes GFE’s development of a modern neotectonic fault database and a unique graphical interface for the compilation of new fault data. A key design principle is that of an electronic field notebook for capturing observations a geologist would make about a fault. The database is designed to accommodate abundant as well as sparse fault observations. It features two layers, one for capturing neotectonic faults and fold observations, and the other to calculate potential earthquake fault sources from the observations. In order to test the flexibility of the database structure and to start a global compilation, five preexisting databases have been uploaded to the first layer and two to the second. In addition, the GFE project has characterised the world’s approximately 55,000 km of subduction interfaces in a globally consistent manner as a basis for generating earthquake event sets for inclusion in earthquake hazard and risk modelling. Following the subduction interface fault schema and including the trace attributes of the GFE database schema, the 2500-km-long frontal thrust fault system of the Himalaya has also been characterised. We propose the database structure to be used widely, so that neotectonic fault data can make a more complete and beneficial contribution to seismic hazard and risk characterisation globally.

Natural Hazards↗

USGS2015

Moment tensors and other source parameters of mining‐induced earthquakes in TauTona Mine, South Africa

Induced seismicity exhibits diverse source mechanisms that are often difficult to constrain for small events. Here, we use data from the in‐mine seismic network, the Natural Earthquake Laboratory in South African Mines network, and a temporary Program for the Array Seismic Studies of the Continental Lithosphere deployment in TauTona Mine, South Africa, to determine full moment tensors of 100 mining‐induced earthquakes in the magnitude range −2.7< M w <2.5. Ground displacement derived from velocity and acceleration data show clear near‐field effects, indicating that the lowest frequencies are well resolved. Phase amplitudes of between 11 and 77 picks per event were inverted to obtain the six independent moment tensor components. The quality of each moment tensor solution is quantified using (1) the misfit between observed and synthetic waveforms, (2) bootstrap resampling to estimate uncertainties, and (3) the F ‐test to determine the need for including an isotropic component with an extra degree of freedom in the solution. The results indicate 82% of the events have well‐constrained solutions, and 45% of the well‐constrained events require an isotropic source term. Throughout the magnitude range, both deviatoric and implosive mechanisms are observed, with implosive ratios of volume change to shear deformation (ΔV/Σ τ ) of −1.03 to −0.15. Two explosive events are observed at M w −0.5 and −0.2, withΔV/Σ τ =0.15 and 0.51, respectively. For the largest events, we determine maximum slip and apparent stress ( τ a ) and find values consistent with those of natural tectonic earthquakes, with 0.1≤ τ a ≤9.2 MPa. Our results support previous speculation on the nature of isotropic components of mining‐induced earthquakes, in which events of all sizes begin as shear failure that may intersect a void (tunnel or stope) and cause collapse, whereas only small events result in explosive sources.

TauTona Mine↗

USGS2009

Extension of Gutenberg‐Richter distribution to MW −1.3, no lower limit in sight

With twelve years of seismic data from TauTona Gold Mine, South Africa, we show that mining‐induced earthquakes follow the Gutenberg‐Richter relation with no scale break down to the completeness level of the catalog, at moment magnitude M W −1.3. Events recorded during relatively quiet hours in 2006 indicate that catalog detection limitations, not earthquake source physics, controlled the previously reported minimum magnitude in this mine. Within the Natural Earthquake Laboratory in South African Mines (NELSAM) experiment's dense seismic array, earthquakes that exhibit shear failure at magnitudes as small as M W −3.9 are observed, but we find no evidence that M W −3.9 represents the minimum magnitude. In contrast to previous work, our results imply small nucleation zones and that earthquake processes in the mine can readily be scaled to those in either laboratory experiments or natural faults.

TauTona Mine↗

USGS2009

A deployment of broadband seismic stations in two deep gold mines, South Africa

In-mine seismic networks throughout the TauTona and Mponeng gold mines provide precise locations and seismic source parameters of earthquakes. They also support small-scale experimental projects, including NELSAM (Natural Earthquake Laboratory in South African Mines), which is intended to record, at close hand, seismic rupture of a geologic fault that traverses the project region near the deepest part of TauTona. To resolve some questions regarding the in-mine and NELSAM networks, we deployed four portable broadband seismic stations at deep sites within TauTona and Mponeng for one week during September 2007 and recorded ground acceleration. Moderately large earthquakes within our temporary network were recorded with sufficiently high signal-to-noise that we were able to integrate the acceleration to ground velocity and displacement, from which moment tensors could be determined. We resolved the questions concerning the NELSAM and in-mine networks by using these moment tensors to calculate synthetic seismograms at various network recording sites for comparison with the ground motion recorded at the same locations. We also used the peak velocity of the S wave pulse, corrected for attenuation with distance, to estimate the maximum slip within the rupture zone of an earthquake. We then combined the maximum slip and seismic moment with results from laboratory friction experiments to estimate maximum slip rates within the same high-slip patches of the rupture zone. For the four largest earthquakes recorded within our network, all with magnitudes near 2, these inferred maximum slips range from 4 to 27 mm and the corresponding maximum slip rates range from 1 to 6 m/s. These results, in conjunction with information from previous ground motion studies, indicate that underground support should be capable of withstanding peak ground velocities of at least 5 m/s.

Mponeng gold mine, TauTona gold mine↗