The
M 6.5 August 16, 2013 earthquake south of Blenheim, New Zealand,
occurred as the result of strike-slip faulting on or near the plate
boundary between the Pacific and Australia plates. At the latitude of
this event, the Pacific plate moves towards the WSW with respect to
Australia at a rate of approximately 41 mm/yr. Preliminary faulting
mechanisms for the earthquake suggest it is related to either NE-SW
right-lateral strike-slip motion (consistent with plate boundary
oriented deformation), or NW-SE left-lateral strike-slip motion.
This
region of New Zealand has hosted a number of small-moderate sized
earthquakes in recent weeks, including a M 6.5 earthquake approximately
40 km east of the August 16 event in the Cook Straight, on July 21,
2013. The July 21 event was preceded by several M 5.3-5.8 events and was
followed by a dozen or more aftershocks between M 4.5-5.0, delineating
shallow upper plate structures aligned NE-SW, and some deeper
subduction-related activity, mostly offshore of the north coast of New
Zealand’s South Island.
In contrast to the earlier events, the
August 16 earthquake is on land, near the eastern end of the complex
Marlborough Fault System. The event is located approximately 10 km
southeast of the Awatere Fault in the vicinity of Lake Grassmere. The
Marlborough Fault system is characterized by a series of NE striking
right-lateral strike slip faults that have dismembered the northern
South Island into a series of crustal blocks that are being transported
to the northeast. Although there is no specific mapped surface fault
that can be linked to the August 16 event at this time, the NE trending
fault plane is similarly oriented to the Awatere and Clarence faults of
the Marlborough system. In 1966 a M 5.8 earthquake (interpreted to have
occurred offshore to the NE of the August 16 event) was widely felt in
this area, causing surface deformation of the main railroad line in the
region. That event is interpreted to have occurred on a blind structure
sub-parallel to the Awatere fault to its northeast.
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.
Large earthquakes are common along the
North New Hebrides trench and have mechanisms associated with
subduction tectonics, though occasional strike slip earthquakes occur
near the subduction of the D'Entrecasteaux ridge. Within the subduction
zone 34 M7.5+ earthquakes have been recorded since 1900. On October 7,
2009, a large interplate thrust fault earthquake (M7.6) in the northern
North New Hebrides subduction zone was followed 15 minutes later by an
even larger interplate event (M7.8) 60 km to the north. It is likely
that the first event triggered the second of the so-called earthquake
"doublet".
More information on regional seismicity and tectonics
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