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The Alabama, U.S.A., seismic event and strata collapse of May 7, 1986

Pure and Applied Geophysics PAGEOPH

By:
and
DOI: 10.1007/BF00874517

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Abstract

On May 7, 1986, the residents of Tuscaloosa, Alabama, felt a seismic event of local magnitude 3.6 that occurred at the same time as a rock burst and roof collapse in an active longwall coal mine. Visual inspection of the seismograms reveals a deficiency in energy at frequencies above 20 Hz compared to tectonic earthquakes or surface blasts. The predominance of energy below 5 Hz may explain reports of body wave magnitudes (mb) greater than 4.2. Also, 1.0 Hz surface waves were more strongly excited than body waves and may explain local felt effects more typically associated with greater epicentral distances. All recorded first motions were dilatational. The concentration of stations in the northern hemisphere allows reverse motion on an east-trending near-vertical plane or strike-slip motion on northwest or southeast trending planes. The reverse focal mechanism is preferred, because the area of roof collapse and the area of active longwall mining are located between two east-striking loose vertical fracture zones. The characteristics of the seismic event suggest that it might have been sudden shear failure resulting from accumulated strain energy in overlying strata behind an active longwall. Although an alternate interpretation of the focal mechanism as an implosion or shear failure in the strata above previously mined out areas is also allowed by the first motion data, this alternate intepretation is not supported by geological data. ?? 1989 Birkha??user Verlag.

Additional Publication Details

Publication type:
Article
Publication Subtype:
Journal Article
Title:
The Alabama, U.S.A., seismic event and strata collapse of May 7, 1986
Series title:
Pure and Applied Geophysics PAGEOPH
DOI:
10.1007/BF00874517
Volume
129
Issue:
3-4
Year Published:
1989
Language:
English
Publisher location:
Birkha??user-Verlag
Larger Work Type:
Article
Larger Work Subtype:
Journal Article
Larger Work Title:
Pure and Applied Geophysics PAGEOPH
First page:
415
Last page:
421
Number of Pages:
7