Seismotectonics of the Eastern Margin of the Australia Plate
The eastern margin of the Australia plate is one of the most sesimically active
areas of the world due to high rates of convergence between the Australia and
Pacific plates. In the region of New Zealand, the 3000 km long Australia-Pacific
plate boundary extends from south of Macquarie Island to the southern Kermadec
Island chain. It includes an oceanic transform (the Macquarie Ridge), two
oppositely verging subduction zones (Puysegur and Hikurangi), and a transpressive
continental transform, the Alpine Fault through South Island, New Zealand.
Since 1900 there have been 15 M7.5+ earthquakes recorded near New
Zealand. Nine of these, and the four largest, occurred along or near the Macquarie
Ridge, including the 1989 M8.2 event on the ridge itself, and the 2004 M8.1 event
200 km to the west of the plate boundary, reflecting intraplate deformation.
The largest recorded earthquake in New Zealand itself was the 1931 M7.8 Hawke's
Bay earthquake, which killed 256 people. The last M7.5+ earthquake along the
Alpine Fault was 170 years ago; studies of the faults' strain accumulation suggest
that similar events are likely to occur again.
North of New Zealand, the Australia-Pacific boundary stretches east of Tonga and
Fiji to 250 km south of Samoa. For 2,200 km the trench is approximately linear,
and includes two segments where old (>120 Myr) Pacific oceanic lithosphere rapidly
subducts westward (Kermadec and Tonga). At the northern end of the Tonga trench,
the boundary curves sharply westward and changes along a 700 km-long segment from
trench-normal subduction, to oblique subduction, to a left lateral transform-like
structure.
Australia-Pacific convergence rates increase northward from 60 mm/yr at the southern
Kermadec trench to 90 mm/yr at the northern Tonga trench; however, significant back
arc extension (or equivalently, slab rollback) causes the consumption rate of
subducting Pacific lithosphere to be much faster. The spreading rate in the Havre
trough, west of the Kermadec trench, increases northward from 8 to 20 mm/yr. The
southern tip of this spreading center is propagating into the North Island of New
Zealand, rifting it apart. In the southern Lau Basin, west of the Tonga trench, the
spreading rate increases northward from 60 to 90 mm/yr, and in the northern Lau
Basin, multiple spreading centers result in an extension rate as high as 160 mm/yr.
The overall subduction velocity of the Pacific plate is the vector sum of
Australia-Pacific velocity and back arc spreading velocity: thus it increases northward
along the Kermadec trench from 70 to 100 mm/yr, and along the Tonga trench from 150
to 240 mm/yr.
The Kermadec-Tonga subduction zone generates many large earthquakes on the interface
between the descending Pacific and overriding Australia plates, within the two plates
themselves and, less frequently, near the outer rise of the Pacific plate east of the
trench. Since 1900, 40 M7.5+ earthquakes have been recorded, mostly north of 30°S.
However, it is unclear whether any of the few historic M8+ events that have occurred
close to the plate boundary were underthrusting events on the plate interface, or were
intraplate earthquakes. On September 29, 2009, one of the largest normal fault (outer rise)
earthquakes ever recorded (M8.1) occurred south of Samoa, 40 km east of the Tonga trench,
generating a tsunami that killed at least 180 people.
Across the North Fiji Basin and to the west of the Vanuatu Islands, the Australia plate
again subducts eastwards beneath the Pacific, at the North New Hebrides trench. At the
southern end of this trench, east of the Loyalty Islands, the plate boundary curves east
into an oceanic transform-like structure analogous to the one north of Tonga.
Australia-Pacific convergence rates increase northward from 80 to 90 mm/yr along the
North New Hebrides trench, but the Australia plate consumption rate is increased by
extension in the back arc and in the North Fiji Basin. Back arc spreading occurs at a
rate of 50 mm/yr along most of the subduction zone, except near ~15°S, where the
D'Entrecasteaux ridge intersects the trench and causes localized compression of 50
mm/yr in the back arc. Therefore, the Australia plate subduction velocity ranges
from 120 mm/yr at the southern end of the North New Hebrides trench, to 40 mm/yr
at the D'Entrecasteaux ridge-trench intersection, to 170 mm/yr at the northern
end of the trench.
