New Zealand Journal of Geology and Geophysics abstracts
Rupturing of the Awatere Fault during the 1848 October 16 Marlborough
earthquake, New Zealand: historical and present day evidence
RODNEY GRAPES
TIMOTHY LITTLE
School of Earth Sciences
Victoria University of Wellington
P.O. Box 600
Wellington, New Zealand
GAYE DOWNES
Institute of Geological & Nuclear Sciences
P.O. Box 30368
Lower Hutt, New Zealand
Abstract Evidence from newspaper reports, diaries and
journals, related first-hand information, an 1854 survey map of the lower part
of the Awatere Valley, and reports by geologists between 1856 and 1890,
indicates that surface rupturing occurred on the Awatere Fault during the 1848
October 16 earthquake and not on the Wairau Fault as previously inferred. The
rupture was initially described as a "fissure", "crack", and later as a "rent",
and although it extended for c. 105 km (from the coast to Barefell
Pass), it was not termed a fault because displacement of the land surface or
strata across the rupture could not be determined. The coincidence of the 1848
earthquake "rent" and the Awatere Fault was first demonstrated by Alexander
McKay in 1885.
Present day evidence of the 1848 Awatere Fault rupture is indicated by a
depression between 0.6-1.5 m wide and c. 0.3 m deep that has the
appearance of an infilled fissure similar to that described in early reports.
The smallest and freshest displacements along the fault (15 displacements; 12
localities, most within c. 20 km of the coast) range between 4 and
8 m dextral with both the southeast and northwest sides variably upthrown
between 3.5 and 0.4 m. Dextral and vertical displacement appears to
decrease towards the coast, consistent with a low rate of late Quaternary slip
(<1.4 mm/yr) and the inferred termination of the fault in Cook Strait.
The 1848 rupture apparently bypassed the ENE-striking Molesworth section of the
Awatere Fault, which implies that the intersection (near Molesworth Station)
between this strand and the eastern section of the fault that reaches the coast
may be an important mechanical boundary controlling the location of earthquake
rupture segments along the Awatere Fault.
Reassessment of felt intensities (based on contemporary descriptions), minimum
fault rupture length (105 km), and average dextral displacement
(6 m), indicates a magnitude for the 1848 earthquake of M
7.4-7.5.
Keywords: 1848 earthquake; Awatere Fault; historical data;
dextral and vertical displacement; earthquake magnitude
New Zealand Journal of Geology & Geophysics, 1998, Vol. 41:
387-399
0028-8306/98/4104-0387 $7.00/0 (c) The Royal Society of New Zealand
1998
PDF file of entire paper: medium quality (2527K); (scanned from paper original: notes about this process)
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