PLANETARY TREMORS: Two Earthquakes (4.8 And 3.8 Magnitude) Rock Assam, India Within 2 Hours!

The Met department put the epicenter at Morigaon and Sonitpur.

April 17, 2015 - ASSAM, INDIA - Two earthquakes, one measuring 4.8 on the Richter scale and the other 3.8, rocked Assam in a span of about two hours in the early hours of Friday, forcing people to jump out of bed and run outdoors.

While the first tremor, which lasted about seven seconds, was felt at 3:35 AM, it was followed by smaller one at 5:50 AM.

The Met department website put the epicenter of the first earthquake at a place in Sonitpur district at 26.8 N Latitude and 92.7 E Longitude, and that of the second tremor in Morigaon district at 26.3 N Latitude and 92.4 E Longitude. The depth of both the tremors has been put at 10 km.

Geomorphologically located in earthquake zone V of the Indian subcontinent, the Northeastern region is marked as a highly seismic area.

With these, the Northeastern region along with adjoining Bangladesh, Myanmar and Andaman & Nicobar Islands have recorded as many as nine earthquakes in the current month.

The earlier seven earthquakes felt in the region were Andaman & Nicobar Islands (April 3, magnitude 4), Bangladesh (April 8, magnitude 4.5), Sonitpur, Assam (April 9, magnitude 3.1), West Khasi Hills, Meghalaya (April 9, magnitude 2.8), Andaman & Nicobar Islands (April 9, magnitude 5.1), Chandel, Manipur (April 12, magnitude 3.5) and Myanmar (April 15, magnitude 4.1). - Indian Express.

Tectonic Summary - Seismotectonics of the Himalaya and Vicinity

Seismicity in the Himalaya dominantly results from the continental collision of the India and Eurasia plates, which are converging at a relative rate of 40-50 mm/yr. Northward underthrusting of India beneath Eurasia generates numerous earthquakes and consequently makes this area one of the most seismically hazardous regions on Earth. The surface expression of the plate boundary is marked by the foothills of the north-south trending Sulaiman Range in the west, the Indo-Burmese Arc in the east and the east-west trending Himalaya Front in the north of India.

The India-Eurasia plate boundary is a diffuse boundary, which in the region near the north of India, lies within the limits of the Indus-Tsangpo (also called the Yarlung-Zangbo) Suture to the north and the Main Frontal Thrust to the south. The Indus-Tsangpo Suture Zone is located roughly 200 km north of the Himalaya Front and is defined by an exposed ophiolite chain along its southern margin. The narrow (less than 200km) Himalaya Front includes numerous east-west trending, parallel structures. This region has the highest rates of seismicity and largest earthquakes in the Himalaya region, caused mainly by movement on thrust faults. Examples of significant earthquakes, in this densely populated region, caused by reverse slip movement include the 1934 M8.1 Bihar, the 1905 M7.5 Kangra and the 2005 M7.6 Kashmir earthquakes. The latter two resulted in the highest death tolls for Himalaya earthquakes seen to date, together killing over 100,000 people and leaving millions homeless. The largest instrumentally recorded Himalaya earthquake occurred on 15th August 1950 in Assam, eastern India. This M8.6 right-lateral, strike-slip, earthquake was widely felt over a broad area of central Asia, causing extensive damage to villages in the epicentral region.

The Tibetan Plateau is situated north of the Himalaya, stretching approximately 1000km north-south and 2500km east-west, and is geologically and tectonically complex with several sutures which are hundreds of kilometer-long and generally trend east-west. The Tibetan Plateau is cut by a number of large (greater than 1000km) east-west trending, left-lateral, strike-slip faults, including the long Kunlun, Haiyuan, and the Altyn Tagh. Right-lateral, strike-slip faults (comparable in size to the left-lateral faults), in this region include the Karakorum, Red River, and Sagaing. Secondary north-south trending normal faults also cut the Tibetan Plateau. Thrust faults are found towards the north and south of the Tibetan Plateau. Collectively, these faults accommodate crustal shortening associated with the ongoing collision of the India and Eurasia plates, with thrust faults accommodating north south compression, and normal and strike-slip accommodating east-west extension.

Along the western margin of the Tibetan Plateau, in the vicinity of south-eastern Afghanistan and western Pakistan, the India plate translates obliquely relative to the Eurasia plate, resulting in a complex fold-and-thrust belt known as the Sulaiman Range. Faulting in this region includes strike-slip, reverse-slip and oblique-slip motion and often results in shallow, destructive earthquakes. The active, left-lateral, strike-slip Chaman fault is the fastest moving fault in the region. In 1505, a segment of the Chaman fault near Kabul, Afghanistan, ruptured causing widespread destruction. In the same region the more recent 30 May 1935, M7.6 Quetta earthquake, which occurred in the Sulaiman Range in Pakistan, killed between 30,000 and 60,000 people.

On the north-western side of the Tibetan Plateau, beneath the Pamir-Hindu Kush Mountains of northern Afghanistan, earthquakes occur at depths as great as 200 km as a result of remnant lithospheric subduction. The curved arc of deep earthquakes found in the Hindu Kush Pamir region indicates the presence of a lithospheric body at depth, thought to be remnants of a subducting slab. Cross-sections through the Hindu Kush region suggest a near vertical northerly-dipping subducting slab, whereas cross-sections through the nearby Pamir region to the east indicate a much shallower dipping, southerly subducting slab. Some models suggest the presence of two subduction zones; with the Indian plate being subducted beneath the Hindu Kush region and the Eurasian plate being subducted beneath the Pamir region. However, other models suggest that just one of the two plates is being subducted and that the slab has become contorted and overturned in places.

Shallow crustal earthquakes also occur in this region near the Main Pamir Thrust and other active Quaternary faults. The Main Pamir Thrust, north of the Pamir Mountains, is an active shortening structure. The northern portion of the Main Pamir Thrust produces many shallow earthquakes, whereas its western and eastern borders display a combination of thrust and strike-slip mechanisms. On the 18 February 1911, the M7.4 Sarez earthquake ruptured in the Central Pamir Mountains, killing numerous people and triggering a landside, which blocked the Murghab River.

Further north, the Tian Shan is a seismically active intra-continental mountain belt, which extends 2500 km in an ENE-WNW orientation north of the Tarim Basin. This belt is defined by numerous east-west trending thrust faults, creating a compressional basin and range landscape. It is generally thought that regional stresses associated with the collision of the India and Eurasia plates are responsible for faulting in the region. The region has had three major earthquakes (greater than M7.6) at the start of the 20th Century, including the 1902 Atushi earthquake, which killed an estimated 5,000 people. The range is cut through in the west by the 700-km-long, northwest-southeast striking, Talas-Ferghana active right-lateral, strike-slip fault system. Though the system has produced no major earthquakes in the last 250 years, paleo-seismic studies indicate that it has the potential to produce M7.0+ earthquakes and it is thought to represent a significant hazard.

The northern portion of the Tibetan Plateau itself is largely dominated by the motion on three large left-lateral, strike-slip fault systems; the Altyn Tagh, Kunlun and Haiyuan. The Altyn Tagh fault is the longest of these strike slip faults and it is thought to accommodate a significant portion of plate convergence. However, this system has not experienced significant historical earthquakes, though paleoseismic studies show evidence of prehistoric M7.0-8.0 events. Thrust faults link with the Altyn Tagh at its eastern and western termini. The Kunlun Fault, south of the Altyn Tagh, is seismically active, producing large earthquakes such as the 8th November 1997, M7.6 Manyi earthquake and the 14th November 2001, M7.8 Kokoxili earthquake. The Haiyuan Fault, in the far north-east, generated the 16 December 1920, M7.8 earthquake that killed approximately 200,000 people and the 22 May 1927 M7.6 earthquake that killed 40,912.

The Longmen Shan thrust belt, along the eastern margin of the Tibetan Plateau, is an important structural feature and forms a transitional zone between the complexly deformed Songpan-Garze Fold Belt and the relatively undeformed Sichuan Basin. On 12 May 2008, the thrust belt produced the reverse slip, M7.9 Wenchuan earthquake, killing over 87,000 people and causing billions of US dollars in damages and landslides which dammed several rivers and lakes.

Southeast of the Tibetan Plateau are the right-lateral, strike-slip Red River and the left-lateral, strike-slip Xiangshuihe-Xiaojiang fault systems. The Red River Fault experienced large scale, left-lateral ductile shear during the Tertiary period before changing to its present day right-lateral slip rate of approximately 5 mm/yr. This fault has produced several earthquakes greater than M6.0 including the 4 January 1970, M7.5 earthquake in Tonghai which killed over 10,000 people. Since the start of the 20th century, the Xiangshuihe-Xiaojiang Fault system has generated several M7.0+ earthquakes including the M7.5 Luhuo earthquake which ruptured on the 22 April 1973. Some studies suggest that due to the high slip rate on this fault, future large earthquakes are highly possible along the 65km stretch between Daofu and Qianning and the 135km stretch that runs through Kangding.

Shallow earthquakes within the Indo-Burmese Arc, predominantly occur on a combination of strike-slip and reverse faults, including the Sagaing, Kabaw and Dauki faults. Between 1930 and 1956, six M7.0+ earthquakes occurred near the right-lateral Sagaing Fault, resulting in severe damage in Myanmar including the generation of landslides, liquefaction and the loss of 610 lives. Deep earthquakes (200km) have also been known to occur in this region, these are thought to be due to the subduction of the eastwards dipping, India plate, though whether subduction is currently active is debated. Within the pre-instrumental period, the large Shillong earthquake occurred on the 12 June 1897, causing widespread destruction.

- USGS.

PLANETARY TREMORS: Strong Magnitude 6.0 Earthquake Hits The Bay Of Bengal - Tremors Felt In East India, Delhi, And Chennai! [MAPS+TECTONIC SUMMARY]

May 21, 2014 - BAY OF BENGAL & THE ANDAMAN SEA BASIN - Mild to moderate tremors were felt in northern and eastern India on Wednesday night.

USGS earthquake location map.

Tremors were felt in places like Kolkata, Bhubaneshwar, Ranchi, Gaya and Chennai.

In Chennai tremors were felt in parts of Nungambakkam, Porur and Triplicane, according to some reports.

Tremors were also felt in Delhi and NCR.

The epicentre of earthquake was in Bay of Bengal, 275 km South-East of Paradip Garh. Its magnitude was 6:0 on the Richter scale and depth was 10 km, as per India Meterological Department.

The tremors struck around 9:52 pm.

USGS earthquake shakemap intensity

The tremor was felt for a few seconds in many areas of Odisha, including state capital Bhubaneshwar but it was enough to trigger panic among people who immediately moved out of buildings, PTI reported.

Panic also gripped several parts of Odisha's Kendrapara district where residents felt the tremors for about 10 seconds.

No reports of damage to human life and property has been reported as yet.  - ZEE News.

Tectonic Setting and Seismotectonic History of the Andaman Sea Basin.

The Andaman Sea is a highly folded and spreading geosynclinal basin, about 650 km wide from east to west and about 1200 km long from north to south. Its total area is estimated to be 600,000 to 800,000 km2.

The Andamans and the Nicobars are a group of 349 islands - summits of a submarine mountain range situated on the western side of the basin, formed by tectonic interactions. The present configuration resulted about 26 million years ago. The islands are the boundary separating the Andaman Sea basin from the Indian Ocean. The Andaman group has a total of 325 islands, while the Nicobar group has 24 islands. Only 38 of these islands are inhabited.

Tectonic Setting - The Andaman Sea Basin, is a seismically active region at the southeastern end of the Alpine-Himalayan belt,. For millions of years the India tectonic plate has moved in a north/northeast direction, colliding with the Eurasian tectonic plate. The Indian plate's eastern boundary, along the Andaman and Nicobar islands and Northern Sumatra, is a diffuse zone of seismicity and deformation, characterized by extensive faulting and numerous large shallow and intermediate earthquakes.

The Burma microplate encompasses the northwest portion of the island of Sumatra, as well as the Andaman and Nicobar Islands. Further to the east of the Andaman and Nicobar islands, a divergent boundary separates the Burma plate from the Sunda plate.

Seismotectonic History - The seimotectonic history of the region is extensively covered in the scientific literature (Sinvhal et al.1978, Verma et al. 1978). More recent research documents the following regional tectonic evolution. Accordingly, an extensional feature developed along a leaky transform segment of the megashear zone - the Andaman fault - between the Indo-Australian domain and the Sunda-Indochina block (Uyeda and Kanamori, 1979; Taylor and Karner, 1983). This old shear zone acted as a western strike slip guide for the extrusion of the Indochina block about 50-20 My (Tapponnier et al., 1986) - and in response to the indentation of the Indian tectonic plate into Eurasian block.

Collision of Indochina with the Sunda and Australian blocks stopped this crustal extrusion process. Subsequently, the Andaman fault system - recently prolonged through the Sumatra zone (the Sumatra fault) - reactivated due to the lateral escape of the Sumatra forearc sliver plate and as a result of the oblique convergence and subduction with the Indo-Australian plate.

The Indian plate's oblique subduction beneath the Burmese Microplate has created the Andaman segment of the great Sunda Trench. The Andaman and Nicobar Islands are located within the tectonic sliver near the boundary of the Indian plate and the Burmese Microplate. Similarly the oblique subduction has created the north-south trending West Andaman fault - another strike-slip fault system in the Andaman Sea to the east of the island chain.

The Volcanic Arc - The subduction process has also formed a volcanic arc. There are two known volcanoes along this arc. The one in the North is known as the Barren Island Volcano - considered active as it has erupted within recent times. The other is known as the Narcondum volcano and is considered dormant.

Seismicity of the Region - Shallow and occasional intermediate-depth earthquakes delineate the subducted slab under the Andaman-Nicobar islands joining the seismicity trend of the Indo-Burman ranges. The active seismicity of the Andaman Sea Basin, has caused many minor and intermediate earthquakes, a few major events ,and only one known earthquake with magnitude greater than 8. According to the literature (Bapat 1982) from 1900 to 1980, a total of 348 earthquakes were recorded in the area bounded by 7.0 N to 22.0 N and 88.0 E to 100 E. - Dr. George P.C.

MONUMENTAL GEOLOGICAL UPHEAVAL: Sinkholes Are Opening Up All Over The Nicobar Islands!

May 04, 2014 - NICOBAR ISLANDS, INDIAN OCEAN -  While the United States Geological Survey (USGS) is sparing no effort to fill a rapidly widening sinkhole in Florida since Apr. 23, India's Geological Survey has closed its field station in the Andaman and Nicobar Islands where sinkholes have sprung up all over as an aftermath of the 2004 Asian tsunami.

A sinkhole is widening in Car Nicobar, but the authorities are clueless about its potential dangers. Malini Shankar/IPS

The administration in this popular tourist destination in the Bay of Bengal may be prepared for another tsunami. But it seems clueless about these holes in the ground that can sometimes cave in or lead to other geological events like hot springs, water spouts, natural gas emissions or even cracks in the subterranean magma chambers.

Islanders told IPS that sinkholes have appeared all over Nicobar. Whether that is also the case with the Andamans remains a matter of speculation as there is no official documentation of it, nor did the administration facilitate this writer's photo assignment to visit the geologically volatile islands.

IPS discovered and photographed sinkholes in three Nicobar Islands - Car Nicobar, Kamorta and Campbell Bay.

"Car Nicobar is full of sinkholes after the tsunami. Even though I grew up here, our parents are now petrified of us swimming near the beach," says Dr. Christina Rossetti, a local of Car Nicobar who works at a government-run hospital here.

Indian Air Force officers at Car Nicobar documented a water spout in April 2013 which shot up from a sinkhole to 1,000 metres in the sky over the Bay of Bengal.

Tsunami survivors in Car Nicobar also told IPS about water spouts that injured their eyes during the disaster.

Sinkholes can be either the cause or the consequence of quicksand, hot springs, geysers, natural gas emissions or water spouts. Initially the surface starts collapsing.

"Usually the depression goes on increasing in depth and it transits from depression to saucer to cup," Dr. Arun Bapat, formerly head of earthquake engineering research at the Central Water and Power Research Station in the western Indian city of Pune, told IPS.

"Sinkholes are usually formed in calcareous formation. The reduction or dissolution of calcium due to drainage or erosion or natural flow of water can cause sinkholes. Earthquakes are not the main cause of sinkholes. But it is possible that in calcareous rock, when a landslide has occurred during or immediately after earthquake, landslides could lead to sinkholes," says Bapat.

Sinkholes look deceptively benign, but anything from quicksand to natural gas could be hidden beneath, deceiving people and livestock who may innocently trample the surface and be swallowed into geysers or cavernous black holes in the ground.

Sinkholes, which range from a few centimetres to 600 metres in diameter, can appear in the aftermath of big seismic events.

Referring to Thailand, the 2005 United Nations Environment Programme report 'Rapid Assessment after Asian Tsunami' says: "Between the earthquake of 26 December 2004 and 24 January 2005, 25 sinkholes have been reported, an unprecedented frequency; 17 of them were reported in the six tsunami-affected provinces."

But no such assessment has been done for India's Andaman and Nicobar Islands.

IPS approached several authorities, including the National Geophysical Research Institute, the National Institute of Ocean Technology, the Indian National Centre for Ocean Information Services, the Geological Society of India, but drew a blank.

The secretary of the Disaster Management Authority for Andaman and Nicobar Islands (DMA) was on leave and the director of DMA did not answer calls.

Sakshi Mittal, deputy commissioner of Nicobar, said, "This has not been brought to my notice yet." Lt. Gen. A.K. Singh, Lieutenant-Governor of Andaman and Nicobar, the highest ranking official of the island territory, told IPS, "We have no idea about sinkholes, please complete your research and inform us."

This administration seems unaware of potential dangers even though the area is home to 350,000 people, 20,000 of whom are highly endangered indigenous people. Its picturesque locales drew 250,445 tourists in 2013.

Ambikaprasad Mallik, a scuba diving instructor in Havelock Island, told IPS, "If a series of sinkholes on the beach collapses at one go, the difference of levels in the water and land masses can create waves and even cause a small local tsunami."

Sinkholes occur in many parts of India and the world. They have accounted for the disappearance of human beings, livestock, rivers, buildings and vehicles.

"Sinkholes represent a hazard to property and human safety in a wide variety of geologic settings across the globe," says the USGS on one of its websites.

Florida in the U.S. is particularly prone to sinkholes, with one last year swallowing a 37-year-old man in his sleep. Another engulfed a forest in Louisiana.

Geologists fight shy of forecasting the precise cause and consequence of sinkholes.

Prof. Kusala Rajendran of the Centre for Earth Sciences at the Indian Institute of Science in Bangalore told IPS, "It is unlikely to lead to anything life threatening, but there may be signatures of deformation such as fissures. This might depend on the region. During seismic activity, land can go down soon after the earthquake. Sinkholes form much later. They develop gradually and are well expressed."

Bapat says, "The sinkholes recently formed in Andaman and Nicobar are probably due to the tsunami. Sometimes, due to geological formation and geometry in the coastal area, stationary waves are formed and this keeps the water vibrating in vertical direction."

USGS notes: "Sinkholes are dramatic because the land usually stays intact for a period of time until the underground spaces just get too big. In most cases, the subsidence rate of a sinkhole represents the most significant potential impact and risk to public safety."

Sinkholes on volcanic slopes like Krakatau in the Java Straits have triggered minor earthquakes. Barren Island, South Asia's only active volcano located in the Andamans, has been spewing lava since January 2010.

With no public transport available to Barren Island, this writer's request to the island administration to facilitate a photo shoot there and in other parts of Andaman district where mud volcanoes are expanding was not accepted. - IPS.

PLANETARY TREMORS: Global Seismic Uptick - Powerful Magnitude 6.7 Earthquake Strikes Off India's Nicobar Islands! [MAPS+ESTIMATES]

March 21, 2014 - INDIA - A powerful 6.7-magnitude earthquake struck off India's Nicobar Islands on Friday, the U.S. Geological Service initially reported at 13:41:07 UTC.

The quake hit at a relatively shallow depth of 6.2 miles  or 10 kilometres and was located at 7.769°N 94.325°.

USGS earthquake location map.

USGS earthquake shakemap intensity map.

The epicentre was about 68 miles southeast of Misha, Nicobar Islands, in the Indian Ocean, the agency said.

The tremor was later downgraded to a magnitude of 6.5.

An official at the provincial disaster management control room said there were no immediate reports of casualties or damage but the earthquake had been felt by islanders.

USGS earthquake uncertainty ratio map.

USGS earthquake population exposure map.

The official said there was no tsunami alert.

Neither NOAA's National Tsunami Warning Center or Pacific Tsunami Warning Center issued a tsunami warning, watch or advisory.

USGS earthquake estimates and losses.

USGS earthquake estimates and losses.

More details awaited on disaster impact. Watch this space for updates.

Andaman and Nicobar is made up of more than 500 mostly uninhabited islands.  The entire island chain is also susceptible to tsunamis both from large local quakes and also from massive distant shocks.

The map below, shows all historically recorded earthquakes in the Andaman & Nicobar Islands. Earthquakes having magnitudes greater than 4.0 since 1973 are also shown. Two prominent red triangles visible to the north-east of Port Blair, i.e. in the top half of the map, are the only volcanoes in India.

The northernmost of the two is the Barren Island Volcano. This volcano has erupted in recent times. The other volcano is called Narcondam and is dormant. The Andaman and Nicobar Islands are located near the boundary of the Indian plate and the Burmese Microplate.

ASC earthquake historical seismicity

The Andaman Trench marks this boundary and lies in the Bay of Bengal to the west of the archipelago. Another prominent feature is the north-south West Andaman fault which is strike-slip in nature and lies in the Andaman Sea, to the east of this island chain. The Andaman Sea, just like the Atlantic Ocean, is presently being widened by a tectonic process called "Sea Floor spreading".

This is taking place along undersea ridges on the seafloor. The Indian plate is diving beneath the Burmese Microplate along the Andaman Trench in a process known as "Subduction". Shallow and occasional intermediate-depth earthquakes delineate the subducted slab under the Andaman-Nicobar islands joining the seismicity trend of the Indo-Burman ranges.

However, it must be stated that proximity to faults does not necessarily translate into a higher hazard as compared to areas located further away, as damage from earthquakes depends on numerous factors such as subsurface geology as well as adherence to the building codes.

SOURCES: Reuters | ASC.

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.

USGS earthquake plate tectonics.

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.

DISASTER PREPAREDNESS: Malaysia Told to be Prepared For Tsunami - Resulting From the Unstable Tectonic Plates in Sumatra and the Nicobar Islands!

Malaysian Meteorological Department director-general Datuk Dr Yap Kok Seng said yesterday the public should brace themselves for the worst in a calm and orderly manner if and when a tsunami strikes. 

A member of the Malaysian Civil Defence Department giving aid to a ‘victim’ who ‘drowned’ during
a session at a tsunami preparedness training exercise in Batu Ferringhi yesterday.

"With the unstable tectonic plates in Sumatra and Nicobar islands, our level of preparedness should be at its highest so rescue efforts can be carried out smoothly. Those in Penang, Perlis, Kedah and Perak must be extra careful."  He said the public must follow orders given by disaster rescue teams and stay away from the shores when warned of an impending tsunami.  "One of the warnings we use for an impending tsunami is the fixed line alert system. It will automatically call all fixed line telephones within 3km from the sea shores," he said. 

The public would also be informed by phone of the assembly points in their respective areas.  Yap said this after presenting certificates to participants at a tsunami preparedness training in Batu Ferringhi here yesterday.  More than 300 people from the security forces, government agencies and students took part in a five-hour mock disaster rescue mission that included learning emergency first aid, evacuation assembly and crowd dispersal management.  Penang is the first state to conduct the training, jointly organised by the department and the state National Security Council.  The council's secretary Zakaria Ibrahim said similar training would soon be held in other states. - New Strait Times.