New Zealand Journal of Geology and Geophysics abstracts
The 1968 Inangahua, New Zealand, and 1994 Northridge, California,
earthquakes: implications for northwest Nelson
ROBERT S. YEATS
Department of Geosciences
Oregon State University
Corvallis, OR 97331-5506, U.S.A.
Abstract The 1968 Inangahua earthquake ruptured a
west-dipping, largely blind reverse fault, here called the Rotokohu
Fault, that is close to the east-dipping Lyell Fault at the surface. Most
surface rupture was secondary. The Lyell Fault was reactivated west-side-up in
1968, in the opposite sense to its long-term displacement. The west-dipping
Inangahua Fault was also reactivated in the opposite sense to its long-term
offset of west-side-up; this rupture may have been related to folding. The
southern boundary of secondary surface rupture is a "cross-basin structure", a
lateral ramp connecting the blind Rotokohu Fault to a blind fault at the
western margin of the Grey-Inangahua Depression. Increase of Coulomb stress at
the lateral ramp in 1968 would shorten the time before failure of the
continuation of the Rotokohu Fault south of the lateral ramp. In its proximity
to another fault with opposite dip, the 1968 earthquake resembles the 1994
Northridge, California, earthquake, in which the blind source fault was beneath
another active fault dipping in the opposite direction, uplifting its hanging
wall and footwall. The blind, thick-skinned reverse fault on which the 1968
earthquake occurred is marked at the surface by a monocline with evidence of
late Quaternary folding and secondary flexural-slip faulting. Shortening rates
in northwest Nelson are higher in the Murchison Basin than they are farther
west, suggesting that the high seismicity from the White Creek Fault to
Westport may not be representative of long-term behaviour and earthquake
potential.
Keywords Rotokohu Fault; new structural name; earthquake
hazard; blind reverse faults; footwall uplift; lateral ramp; convergence rates;
northwest Nelson; Inangahua earthquake; Northridge earthquake
New Zealand Journal of Geology & Geophysics, 2000, Vol. 43:
587-599
0028-8306/00/4304-0587 $7.00/0 (c) The Royal Society of New Zealand
2000
PDF file of entire paper: medium quality (1236K); (scanned from paper original: notes about this process)
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