Seismotectonics of the New Guinea Region and Vicinity
The Australia-Pacific plate boundary is over 4000 km long on the
northern margin, from the Sunda (Java) trench in the west to the
Solomon Islands in the east. The eastern section is over 2300 km
long, extending west from northeast of the Australian continent
and the Coral Sea until it intersects the east coast of Papua New
Guinea. The boundary is dominated by the general northward subduction
of the Australia plate.
Along the South Solomon trench, the Australia plate converges with
the Pacific plate at a rate of approximately 95 mm/yr towards the
east-northeast. Seismicity along the trench is dominantly related
to subduction tectonics and large earthquakes are common: there have
been 13 M7.5+ earthquakes recorded since 1900. On April 1, 2007, a
M8.1 interplate megathrust earthquake occurred at the western end of
the trench, generating a tsunami and killing at least 40 people. This
was the third M8.1 megathrust event associated with this subduction
zone in the past century; the other two occurred in 1939 and 1977.
Further east at the New Britain trench, the relative motions of several
microplates surrounding the Australia-Pacific boundary, including
north-south oriented seafloor spreading in the Woodlark Basin south
of the Solomon Islands, maintain the general northward subduction of
Australia-affiliated lithosphere beneath Pacific-affiliated lithosphere.
Most of the large and great earthquakes east of New Guinea are related
to this subduction; such earthquakes are particularly concentrated at
the cusp of the trench south of New Ireland. 33 M7.5+ earthquakes have
been recorded since 1900, including three shallow thrust fault M8.1
events in 1906, 1919, and 2007.
The western end of the Australia-Pacific plate boundary is perhaps the
most complex portion of this boundary, extending 2000 km from Indonesia
and the Banda Sea to eastern New Guinea. The boundary is dominantly
convergent along an arc-continent collision segment spanning the width
of New Guinea, but the regions near the edges of the impinging Australia
continental margin also include relatively short segments of extensional,
strike-slip and convergent deformation. The dominant convergence is
accommodated by shortening and uplift across a 250-350 km-wide band of
northern New Guinea, as well as by slow southward-verging subduction of
the Pacific plate north of New Guinea at the New Guinea trench. Here,
the Australia-Pacific plate relative velocity is approximately 110 mm/yr
towards the northeast, leading to the 2-8 mm/yr uplift of the New Guinea
Highlands.
Whereas the northern band of deformation is relatively diffuse east of the
Indonesia-Papua New Guinea border, in western New Guinea there are at least
two small (<100,000 km²) blocks of relatively undeformed lithosphere. The
westernmost of these is the Birds Head Peninsula microplate in Indonesia's
West Papua province, bounded on the south by the Seram trench. The Seram
trench was originally interpreted as an extreme bend in the Sunda subduction
zone, but is now thought to represent a southward-verging subduction zone
between Birds Head and the Banda Sea.
There have been 22 M7.5+ earthquakes recorded in the New Guinea region since
1900. The dominant earthquake mechanisms are thrust and strike slip, associated
with the arc-continent collision and the relative motions between numerous
local microplates. The largest earthquake in the region was a M8.2 shallow
thrust fault event in the northern Papua province of Indonesia that killed
166 people in 1996.
The western portion of the northern Australia plate boundary extends
approximately 4800 km from New Guinea to Sumatra and primarily separates
Australia from the Eurasia plate, including the Sunda block. This portion
is dominantly convergent and includes subduction at the Sunda (Java) trench,
and a young arc-continent collision.
In the east, this boundary extends from the Kai Islands to Sumba along the
Timor trough, offset from the Sunda trench by 250 km south of Sumba.
Contrary to earlier tectonic models in which this trough was interpreted
as a subduction feature continuous with the Sunda subduction zone, it is
now thought to represent a subsiding deformational feature related to the
collision of the Australia plate continental margin and the volcanic arc
of the Eurasia plate, initiating in the last 5-8 Myr. Before collision began,
the Sunda subduction zone extended eastward to at least the Kai Islands,
evidenced by the presence of a northward-dipping zone of seismicity beneath
Timor Leste. A more detailed examination of the seismic zone along it's
eastern segment reveals a gap in intermediate depth seismicity under Timor
and seismic mechanisms that indicate an eastward propagating tear in the
descending slab as the negatively buoyant oceanic lithosphere detaches from
positively buoyant continental lithosphere. On the surface, GPS measurements
indicate that the region around Timor is currently no longer connected to the
Eurasia plate, but instead is moving at nearly the same velocity as the
Australia plate, another consequence of collision.
