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.

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.