New Zealand Journal of Geology and Geophysics abstracts
The 2001 ML 6.2 Jackson Bay earthquake
sequence, South Island, New Zealand
Peter McGinty
Russell Robinson
Terry Webb
Institute of Geological & Nuclear Sciences Ltd
P.O. Box 30 368
Lower Hutt, New Zealand
p.mcginty@gns.cri.nz
Abstract Following the 2001 December 7 Jackson Bay
earthquake (ML 6.2, MW
5.8), a temporary network of five portable seismographs was
deployed in the region to record aftershock activity. Data recorded by
the temporary network and nearby New Zealand National Seismological
Network stations have been used to define a velocity model for the
region and station corrections for each recording station. The
locations of the best recorded aftershocks and the revised location of
the mainshock indicate that the Jackson Bay earthquake sequence
occurred 3–10 km to the east of the Alpine Fault, which is vertical in
this region. A fault plane solution obtained from body-wave modelling
suggests the mainshock was primarily a reverse event (rake = 103°)
centred at c. 4 km depth on a fault striking northeast–southwest
(48°) and probably dipping to the southeast (45°), which is
roughly consistent with the Harvard CMT solution for this earthquake.
However, on examination of the aftershock locations, such a fault plane
is not clear, nor is any other. The aftershocks are located mainly in
two clusters near each other at depths between 3 and 8 km and aligned
approximately north–south. Their positions are in accord with induced
stress considerations and the mainshock fault plane lying between the
clusters. Individual focal mechanisms for 33 aftershocks have a wide
range of solutions. As a group, however, their P and T axes are
reasonably well aligned and consistent with the background stress
regime in the region as determined by direct inversion of P-wave
polarities. The Jackson Bay earthquake was the third thrust earthquake
of magnitude >6 to occur just east of the Alpine Fault in a 7 yr
period. Consideration of the mechanics of this earthquake, and the
previous two, suggests that the regional stress is at a high level, in
accord with the long elapsed time since a large Alpine Fault event.
Although the area is small, the Jackson Bay mainshock induced a mainly
positive change in Coulomb Failure Stress (CFS) on the closest section
of the Alpine Fault, up to c. 0.7 MPa (7 bars).
Keywords Jackson Bay; New Zealand; earthquakes;
aftershocks; Alpine Fault; stress
G04015; Received 7 April 2004; accepted 17 December 2004; Online
publication date 3 June 2005
New Zealand Journal of Geology & Geophysics, 2005, Vol. 48:
315–324
0028–8306/05/4802–0315© The Royal Society of New Zealand 2005
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