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Small slow‐strain steps and their forerunners observed in gold mine in South Africa
Author(s) -
Naoi Makoto,
Ogasawara Hiroshi,
Takeuchi Junichi,
Yamamoto Akihito,
Shimoda Naoyuki,
Morishita Ken,
Ishii Hiroshi,
Nakao Shigeru,
van Aswegen Gerrie,
Mendecki Aleksander J.,
Lenegan Patrick,
EbrahimTrollope Rookshana,
Iio Yoshihisa
Publication year - 2006
Publication title -
geophysical research letters
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.007
H-Index - 273
eISSN - 1944-8007
pISSN - 0094-8276
DOI - 10.1029/2006gl026507
Subject(s) - borehole , geology , seismology , strain (injury) , fault (geology) , geotechnical engineering , medicine
The Research Group for Semi‐controlled Earthquake‐generation Experiments in South African deep gold mines (SeeSA) has continuously monitored strain changes with a resolution of 24 bit 25 Hz at the Bambanani mine near Welkom. An Ishii borehole strainmeter was installed at a depth of 2.4 km near the potential M ∼ 3 earthquake source area. Instantaneous strain steps of ∼10 −4 strains associated with two M2 events were observed within a length of seismic fault. These steps were followed by significant post‐seismic creep‐like drift, but not preceded by forerunners. Analysis of the continuous 25 Hz data reveals many smaller steps with much longer durations (100 ms ∼ 100 s) than seen in normal earthquakes (−1 < M < 2) with source durations of 1 ms∼50 ms. Some of the especially slow steps were preceded by accelerations in strain, the maximum being as large as one‐third of the step.