Coseismic and other short-term strain changes recorded with Sacks-Evertson strainmeters in a deep mine, South Africa

Abstract

Sacks-Evertson borehole dilatometers were installed at the ends of boreholes drilled into the sidewall of an experimental tunnel at a depth of 3.1 km in the ERPM gold mine near Johannesburg. In the following year coseismic strain changes ranging from 5 x lo-'' to values exceeding 5 x lo-(' were recorded for hundreds of mine tremors in the magnitude range -1 to 3.7 and at hypocentral distances of 5 0 m to about 2 km. Hypocentral coordinates and magnitudes were determined from seismograms recorded from an underground array of geophones. Amplitudes and polarities of the coseismic strain steps are generally in excellent agreement with theoretical expectations based on point-source dislocation theory; specifically, the strain steps are proportional to the seismic moment divided by the cube of hypocentral distance. At a strain level of 5 x lod9 or greater the tremors do not appear to be preceded by any short-term indications of instability even for tremors producing coseismic steps greater than 5 x and for which the strainmeters were within a source radius of the hypocentre. Continuous strain changes observed at the times when the mine excavation, at a distance of about 100 m, is extended are in good agreement with calculated changes based on the theory of elasticity. A similar calculation is consistent with post-seismic strain changes observed to follow some of the closer tremors. These post-seismic strains show a logarithmic dependence on time following the tremor and appear to be due to the interaction of a tremor with the adjacent mine excavation rather than to deformation within the actual seismic source region. Date Time bl 05 J u l y 77 1646 1.2 x lo-' 06 J u l y 07 J u l y OM J u l y 09 J u l y 12 J u l y 14 J u l y 16 J u l y

Detecting Earthquake-Related Anomalies of a Borehole Strain Network Based on Multi-Channel Singular Spectrum Analysis

“…According to previous studies [ 9 , 50 , 51 ], borehole strainmeters are capable of recording subsurface seismic strain steps with a high sampling rate. Thus, in our strain network detection, the recorded co-seismic strain changes may be one of the main sources for strongly connected networks.…”

“…Strain measurements play an essential role in analyzing geodynamic processes, such as those used to study slow earthquakes [ 1 , 2 ], volcanic activities [ 3 ], and preparation processes that occur prior to earthquakes [ 4 , 5 , 6 , 7 ]. In an effort to monitor the crustal activities associated with earthquakes, numerous geodetic monitoring systems have been established around the world, which provide an opportunity to investigate complex tectonic structures [ 8 , 9 , 10 , 11 ], short- and medium-term earthquake prediction [ 12 , 13 , 14 ], etc. The development of such monitoring systems has facilitated intensive studies on crustal deformations associated with earthquake preparation, their occurrence, and post-earthquake phases [ 9 , 15 , 16 , 17 , 18 ], such as those from the 2011 Tohoku earthquake in Japan [ 19 , 20 , 21 ] and the 2008 Wenchuan earthquake in China [ 22 , 23 ].…”

Extracting borehole strain precursors associated with the Lushan earthquake through principal component analysis

“…High-resolution records from borehole strainmeters allow for the detection of subtle strain variations. Borehole strain data have been used to research various phenomena, including slow earthquakes [Sacks et al, 1978[Sacks et al, , 1981Linde et al, 1996;Liu et al, 2009], seismic strain steps [McGarr et al, 1982;Asai et al, 2005;Qiu et al, 2003Qiu et al, , 2004Qiu et al, , 2015Barbour et al, 2012], and volcanic eruptions [Voight et al, 2006;Hautmann et al, 2013;Sturkell et al, 2013].…”

Small slow‐strain steps and their forerunners observed in gold mine in South Africa

“… McGarr et al [1982] installed three Sacks‐Evertson dilatometers at a depth of 3.1km in the ERPM gold mine in South Africa, and monitored rock‐mass behavior associated with earthquakes. The dilatometer recorded the strain changes associated with the earthquake, with a magnitude of 3.1 and at a distance of 150 m. However, the dynamic range of only ±5 × 10 −6 did not fully accommodate the change.…”