July 06, 2013 - INDONESIA - A strong 6.4-magnitude quake struck off the Indonesian island of Sumatra on Saturday, the US Geological Survey said, but no tsunami warning was issued.
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USGS earthquake location. |
The quake struck at a shallow depth of just 23kms (14 miles), off the
west coast of the vast island and 154 kilometers southwest of
Sungaipenuh and was later downgraded to a 6.0 event, said the USGS.
Suharjono,
an official from the local meteorology, climatology and geophysics
agency who goes by one name, said there was no threat of a tsunami.
The
earthquake came just days after a strong inland tremor in Aceh
province, on the northern tip of Sumatra, which killed dozens of people
and left thousands homeless.
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USGS earthquake shakemap intensity. |
Indonesia sits on the Pacific “Ring of Fire” where tectonic plates collide, causing frequent seismic and volcanic activity. -
Hindustan Times.
Tectonic Summary - Seismotectonics of the Sumatra Region
The plate boundary southwest of Sumatra is part of a long tectonic
collision zone that extends over 8000 km from Papua in the east to the
Himalayan front in the west. The Sumatra-Andaman portion of the
collision zone forms a subduction zone megathrust plate boundary, the
Sunda-Java trench, which accommodates convergence between the
Indo-Australia and Sunda plates. This convergence is responsible for the
intense seismicity and volcanism in Sumatra. The Sumatra Fault, a major
transform structure that bisects Sumatra, accommodates the
northwest-increasing lateral component of relative plate motion.
Relative plate motion between the Indo-Australia and Sunda plates is
rapid, decreasing from roughly 63 mm/year near the southern tip of
Sumatra (Australia relative to Sunda) to 44 mm/year north of Andaman
Islands (India relative to Sunda) and rotating counterclockwise to the
northwest, so that relative motion near Jakarta is nearly trench-normal
but becomes nearly trench-parallel near Myanmar. As a result of the
rotation in relative motion along the strike of the arc and the
interaction of multiple tectonic plates, several interrelated tectonic
elements compose the Sumatra-Andaman plate boundary. Most strain
accumulation and release occurs along the Sunda megathrust of the main
subduction zone, where lithosphere of the subducting Indo-Australia
plate is in contact with the overlying Sunda plate down to a depth of 60
km. Strain release associated with deformation within the subducting
slab is evidenced by deeper earthquakes that extend to depths of less
than 300 km on Sumatra and 150 km or less along the Andaman Islands. The
increasingly oblique convergence between these two plates moving
northwest along the arc is accommodated by crustal seismicity along a
series of transform and normal faults. East of the Andaman Islands,
back- arc spreading in the Andaman Sea produces a zone of distributed
normal and strike-slip faulting. Similar to the Sumatran Fault, the
Sagaing Fault near Myanmar also accommodates the strike-slip component
of oblique plate motion. Plate-boundary related deformation is also not
restricted to the subduction zone and overriding plate: the subducting
Indo-Australian plate actually comprises two somewhat independent plates
(India and Australia), with small amounts of motion relative to one
another, that are joined along a broad, actively-deforming region
producing seismicity up to several hundred kilometers west of the
trench. This deformation is exemplified by the recent April 2012
earthquake sequence, which includes the April 11 M 8.6 and M 8.2
strike-slip events and their subsequent aftershocks.
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USGS plate tectonics for the region. |
Paleoseismic studies using coral reefs as a proxy for relative land
level changes associated with earthquake displacement suggest that the
Sunda arc has repeatedly ruptured during relatively large events in the
past, with records extending as far back as the 10th century. In
northern Simeulue Island, the southern terminus of the 2004 megathrust
earthquake rupture area, a cluster of megathrust earthquakes occurred
over a 56 year period between A.D. 1390 and 1455, resulting in uplift
substantially greater than that caused by the 2004 event. Studies that
look at large sheeted deposits of sand on land interpreted as the
transport of debris from a tsunami wave also indicate that this region
has experienced significant tsunamis in the past centuries, albeit
infrequently.
Prior to 2004, the most recent megathrust
earthquakes along the Sumatran-Andaman plate boundary were in 1797 (M
8.7-8.9), 1833 (M 8.9-9.1) and 1861 (M8.5). Since 2004, much of the
Sunda megathrust between the northern Andaman Islands and Enggano
Island, a distance of more than 2,000 km, has ruptured in a series of
large subduction zone earthquakes - most rupturing the plate boundary
south of Banda Aceh. The great M 9.1 earthquake of December 26, 2004,
which produced a devastating tsunami, ruptured much of the boundary
between Myanmar and Simeulue Island offshore Banda Aceh. Immediately to
the south of the great 2004 earthquake, the M 8.6 Nias Island earthquake
of March 28, 2005 ruptured a 400-km section between Simeulue and the
Batu Islands. Farther south in the Mentawai islands, two earthquakes on
September 12, 2007 of M 8.5 and M 7.9 occurred in the southern portion
of the estimated 1797 and 1833 ruptures zone, which extends from
approximately Enggano Island to the northern portion of Siberut Island.
Smaller earthquakes have also been locally important: a M 7.6 rupture
within the subducting plate caused considerable damage in Padang in
2009, and a M 7.8 rupture on October 25, 2010 occurred on the shallow
portion of the megathrust to the west of the Mentawai Islands, and
caused a substantial tsunami on the west coast of those islands.
In addition to the current seismic hazards along this portion of the
Sunda arc, this region is also recognized as having one of the highest
volcanic hazards in the world. One of the most dramatic eruptions in
human history was the Krakatau eruption on August 26-27, 1883, a volcano
just to the southeast of the island of Sumatra, which resulted in over
35, 000 casualties.
Subduction and seismicity along the plate
boundary adjacent to Java is fundamentally different from that of the
Sumatran-Andaman section. Relative motion along the Java arc is
trench-normal (approximately 65-70 mm/year) and does not exhibit the
same strain partitioning and back-arc strike- slip faulting that are
observed along the Sumatra margin. Neither has the Java subduction zone
hosted similar large magnitude megathrust events to those of its
neighbor, at least in documented history. Although this region is not as
seismically active as the Sumatra region, the Java arc has hosted low
to intermediate-magnitude extensional earthquakes and deep-focus
(300-700 km) events and exhibits a similar if not higher volcanic
hazard. This arc has also hosted two large, shallow tsunami earthquakes
in the recent past which resulted in high tsunami run-ups along the
southern Java coast. -
USGS.