The 2012 Indian Ocean earthquakes were magnitude 8.6 and 8.2 Mw undersea
earthquakes that struck near the Indonesian province of Aceh on 11 April
at 15:38 local time. Initially, authorities feared that the initial
earthquake would cause a tsunami and warnings were issued across the Indian
Ocean; however, these warnings were subsequently cancelled. The earthquake
was the 13th strongest earthquake since 1900, an unusually strong intraplate
earthquake, and the largest strike-slip earthquake ever recorded.
Tectonic setting The 2012 earthquake's epicenter was
located within the Indo-Australian Plate, which is divided into two sub- or
proto-plates: the Indian, and Australian. At their boundary, the
Indian and Australian Plates converge at 11 mm per year in a NNW–SSE direction.
This convergence is accommodated by a broad zone of diffuse deformation. As
part of that intraplate deformation, north–south trending fracture zones have
been reactivated from the Ninety East Ridge as far east as 97°E.
The Indo–Australian Plate was formed after the amalgamation of the Indian
Plate and the Australian Plate some 45 million years ago. However, there is a
relative movement between the Indian Plate and the Australian Plate. A
process which would eventually split the Indo-Australian Plate in two probably
have started 8 to 10 millions ago and is still taking place. Because of the
deformation, fracture zones have been reactivated. The 2012 Indian Ocean
earthquake is associated to the reactivation of the NNE-striking sea
floor fabric. Earthquake
The magnitude 8.6 earthquake occurred about 610 km southwest of Banda Aceh,
Indonesia at 08:38 UTC on 11 April 2012. The earthquake occurred at a depth of
22.9 km, which is considered relatively shallow according to the scale used by
the United States Geological Survey. The three minute quake was originally
reported as a magnitude 8.9, but was later downgraded to 8.6. It was felt as
far away as Malaysia, the Maldives and in India. The earthquake was caused by a
strike-slip motion. The earthquake and the largest aftershock had a fault
displacement of 21.3 m. The strike-slip nature of the earthquake meant that the
movement displaced relatively little seawater and was less likely to cause a
tsunami. Both the initial earthquake and the
magnitude 8.2 aftershock were classified as strike-slip earthquakes, meaning that
the crust on either side shifted against each other horizontally, rather than
vertically. Since 2006 there have been three other earthquakes in the area with
a similar faulting style. All these earthquakes were consistent with either
left-lateral slip on SSW–NNE orientated strike-slip faults, or right-lateral
slip on WNW–ESE orientated strike-slip faults, both compatible with the
direction of convergence. A back projection analysis of data collected by
Hi-net, an observation network in Japan, found a complex pattern of four
conjugate faults. There was a strong correlation between the fault rupture
pattern and the distribution of the aftershocks.
These earthquakes have a complex rupture process. The rupture of these
earthquakes occurred on multiple, almost orthogonal faults. This is rare in a
single earthquake. This earthquake had an overall relatively slow rupture
speed, although the speed was above the S-wave velocity in some fault segments.
The rupture may have affected the crust and the upper mantle according to the
kinematic source inversion. The rupture was caused by ductile shear heating
instability which is different from frictional failure and operates between
600–800 °C, which corresponds to about the depths of 40–60 km. The
serpentinization of oceanic lithosphere can lead to a low friction coefficient,
but the reaction is possible only up to 400–500 °C, which corresponds to the
depth of about 25 km. A single dynamic weakening mechanism which can work over
the whole range of slip of this earthquake is still to be identified.
= Damage= Four people in their 60s and 70s in
Banda Aceh, and a 39-year-old man in Lhokseumawe died from heart attacks or
shock. Injuries were reported in Aceh Singkil, including a child who was
critically injured by a falling tree. The quake prompted people in Indonesia,
Thailand and India to leave their homes and offices in fear of tsunamis. People
headed for higher ground in parts of Indonesia and Malaysia. In Aceh, where
31,000 people were killed in the 2004 Indian Ocean earthquake and tsunami,
people were reported weeping. Some people used cars and motorcycles to
reach higher ground. Patients were reportedly wheeled out of hospitals,
some with drips attached to their arms. One hotel guest was slightly injured
when he jumped out of his window. The earthquake was felt over a large
area, including Indonesia, the Maldives, Sri Lanka, India, Nepal, Bhutan, China,
Bangladesh, Burma, Thailand, Malaysia, Laos, Cambodia, Singapore, and Vietnam.
The quake was felt across the eastern coast of India including Chennai,
Bangalore, Kochi, Bhubaneswar, Vizag and Kolkata. The Metro Rail services were
suspended in Kolkata, and passengers were asked to leave stations. In
peninsular Malaysia, shaking was felt in Penang and Kuala Lumpur. The tremor was
felt in Colombo, Sri Lanka, where people in some high-rise buildings were
evacuated. In China, the earthquake was felt in Tibet. However, it was not
reported to be felt in places closer to the epicenter such as Yunnan and
Guangxi. = Aftershocks=
A magnitude 8.2 aftershock struck at a depth of 16.4 kilometres about 430 km
southwest of Banda Aceh at 10:43 UTC, two hours after the initial earthquake.
Many aftershocks with magnitude readings between 5.0 to 6.0 were recorded for
several hours after the initial earthquake which hit the west coast of
northern Sumatra. Since the initial magnitude 8.6 earthquake, there have
been 111 aftershocks over magnitude 4.0 according to USGS, including a magnitude
6.2 on April 15, 2012. Tsunami warnings
Three hours after the initial quake it was still unclear whether a tsunami had
been generated or if a tsunami had done any significant damage. A geophysicist
from the Pacific Tsunami Warning Center stated that a gauge closest to the
epicenter recorded a wave peak of about 1 m. The Pacific Tsunami Warning Center
said that at 10:14 UTC that a tsunami was generated and may have already
caused some coastal destruction. A seismologist from the British Geological
Survey said that the "tearing earthquake" displaced relatively little
water and was therefore unlikely to cause a significant tsunami. Indonesian
authorities also issued a statement saying that the likelihood of a tsunami
was low. Tsunami warnings were issued in the
following countries: Indonesia, Australia, Cambodia, Malaysia,
Singapore, Thailand, Myanmar, Bangladesh, India, Sri Lanka, Maldives,
Pakistan, Iran, United Arab Emirates, Oman, Yemen, Somalia, Kenya, Tanzania,
Seychelles, Mozambique, Mauritius, Comoros, Madagascar, South Africa and
the dependencies of Diego Garcia as part of the British Indian Ocean Territories,
Crozet Islands, Réunion and Kerguelen Islands. Projected tsunami landfall
times in GMT were: 8:38 for Indonesia, 11:38 for Sri Lanka, 14:38 for the
Seychelles, 17:38 for Madagascar and 20:38 for South Africa.
= India= After the earthquake, the Indian
National Centre for Ocean Information Services issued a high-level tsunami
warning for the Andaman and Nicobar Islands. Authorities told civilians to
move away from the coast and towards higher ground. Areas subject to warnings
included Tamil Nadu, Andhra Pradesh and the Union Territory of Andaman and
Nicobar Islands. Prabkhakar Rao of the disaster control room in Port Blair said
that there could be tsunamis as high as 1.5 m at Port Blair and 3.9 m at
Campbell Bay. In Kolkata, underground Metro Railways
services were stopped and tall office buildings and shopping malls were
evacuated. Navy warships were put on high alert. The Indian Air Force
dispatched two C-130s and one Il-76 aircraft to the Andaman islands.
= Sri Lanka= The Ministry of Disaster Management and
Human Rights authorised an evacuation order at 9:08 UTC advising people living
near coastal regions to move to higher ground. The initial quake was not
expected to affect Sri Lanka, but aftershocks were being monitored. Deputy
Director M. D. Dayananda said that a tsunami could hit Trincomalee.
As part of a risk mitigation measure, the Ceylon Electricity Board
disconnected power supplies to coastal areas and railway control rooms. Coastal
train services were suspended. The Road Development Authority removed toll
charges from the Southern Sri Lanka Distributor to aid the evacuation.
Coastal bus services were temporarily re-routed.
= Thailand= Six Andaman coast provinces urged people
to go to high ground and stay away from areas that could be affected. Phuket
International Airport was closed. The alerts caused panic as people fled
buildings and made for high ground. During the initial earthquake and for
more than two hours after none of Thailand's free television stations
reported the earthquake or tsunami warnings, preferring to continue a live
broadcast of a royal family ceremony, despite the National Broadcasting and
Telecommunications Commission's urgent order directing the emergency
information to be broadcast instead without delay. This caused criticism on
the internet, although it was muted by a lèse majesté law. The Television Pool of
Thailand defended their actions, saying that running texts about the warnings
were displayed onscreen during the broadcast.
Tsunami Small 0.5 m waves, within regular tide
limits, were reported in the Campbell Bay area of Great Nicobar Island, and 10
cm waves were reported in Thailand. The Indonesian Agency for Meteorology,
Climatology and Geophysics said three small tsunamis struck the Aceh coast.
The highest was 80 cm, at Meulaboh. After the sea was seen receding about 10
metres at Simeulue, a 1 m wave was observed. Simeulue is an island of
fishing villages 150 km off the west coast of Sumatra.
References Sources
External links Media related to 2012 Indian Ocean
earthquake at Wikimedia Commons Satellite View at Wikimapia.org
Preliminary source parameters obtained by automatic analysis - 201211 08:38 Off
West Coast of Northern Sumatra – International seismic network, NIED,
Japan After major earthquake, silence: Dynamic
stressing of a global system of faults results in rare seismic silence –
Science Daily Rare great earthquake in April triggers
large aftershocks all over the globe – Science Daily