strong earthquakes failed to trigger a tsunami

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STRONG EARTHQUAKE

No tsunami!! Its ok its ok…

 

  • Researchers at the Centre for Earth Sciences of the Indian Institute of Science (IISc) in Bangalore admit that the sequence of two surpris­ingly big Indian Ocean earthquakesof magnitude 8.6 and 8.2 separated by about two hours on April 11, 2011 took them by surprise because both did not occur on the plate boundary but on  the subducting oceanic plate. However, the fact that these massive earthquakes failed to trigger a tsunami doesn’t surprise them.
  • The quakes were located ap­proximately 100 km to the southwest of the major subduction zone that defines the plate boundary between the Indo-Australia and Sunda plates, offshore Sumatra. Along this plate boundary — which had sourced the 2004 great tsunamigenic earthquake ­the Indo-Australia plate moves north-northeast with respect to the Sunda plate at a velocity of approximately 52mm per year.
  • But researchers Kusala Rajendran and her team at IISc were not taken aback by the fact these earthquakes, despite their great size, failed to gener­ate a tsunami. Their earlier study of earthquakes in that region had shown the reason why the April 11 events did not produce a tsunami such as the one that followed the 9.1 quake on 26 December 2004.
<li>Neither of the two April 11 earth­quakes resulted from shifting one tectonic plate beneath another near the deep-sea trench. Instead, these earthquakes resulted from sideways motion on faults farther offshore within the subducting Indian plate and not at the plate boundary itself as in the case of the 2004 earthquake. So the region was saved from tsunami.</li>
<li>Rajendran said that the earlier study by her team had in fact ana­lysed the so-called 'strike-slip faulting earthquakes' that had taken place since 2004. She said the study "suggested that subducting plate off the Sumatra and Nicobar segments deforms in response to a generally northwest-southeast oriented compression, contrasting the general northeast directed motion that dominates the India-Eurasia plate.' Although rare, large strike-slip earth­quakes (where motion is horizontal), are not uncommon here, she said.</li>
<li>Thus, she said, while most earth­quakes on the plate boundary occur in response to NE-directed compression leading to thrust type earthquakes with vertical component of slip, "the earthquakes on the deforming oceanic plate occur in response to NW-oriented compression, with little or no vertical component of slip." In other words,</li>

 
EASTERN COAST OF INDIA COULD BE NEW ‘CRADLE’ OF EVOLVING EARTHQUAKE ZONES
 

  • v  Apart from the southern shores inherently being a Earthquake Zone; most recent, detailed studies of India’s eastern coast and beyond suggest large earthquakes may not be foreign to our eastern coasts either.
  • v  A 5.5 magnitude earthquake occurred off the Puducherry coast in September 2001. Three years later, small tremors lasting a few seconds each continued to rattle the region—the Sumatran-Andaman earthquake of December 2004 (magnitude 9.1) and subsequent earthquakes of March 28 (magnitude 8.3) and July 24, 2005 (magnitude 7.3). Neotectonic activity has been established in the coastal and offshore regions of Visakhapatnam, Vizianagaram, Ongole and Puducherry, writes K S R Murthy, a CSIR (Council for Scientific and
  • v  Industrial Research) Emeritus scientist with the National Institute of Oceonagraphy, Goa, in a paper titled “Structure and Tectonics of the Eastern Continental Margin of India”, which sums up decades of work involving the entire region. “It is also important to note that the Andaman Nicobar convergent margin is experiencing continuous aftershocks even today and the impact of these aftershocks is felt on the east coast of India,” Murthy continues.
  • v  So are these tremors because of events in the far east? Not necessarily.
  • v  The reasons lie in our own region, in our landmass and under our sea. The Indian subcontinent broke away from eastern Antarctica about 160 million years ago. The initial break-up is believed to have happened near the Krishna-Godavari basin where, if one looks at the map, one can notice an inward curve called a bight. Over the years, scientists studied this offshore area and found two, almost parallel, tectonic lineaments (geographically visible manifestations of underlying faults)—the Avanigadda cross trend and the Chintalapudi cross trend—extending far into the sea, perhaps reaching the Continent Ocean Boundary. In between these two lineaments was a large, rectangular sediment deposit approximately 60 km wide. What this appeared to imply is that these cross trends are older than the rest of the basin, making them “tectonically significant”. Similarly in the Mahanadi basin, in a 1.41 lakh square km area further north, the sea-bed is lined with a series of highs and depressions that run almost parallel to the coast with a shearing pattern, created by pressure strong enough to deform rocks and which may cause faults. These sea-bed landscapes were again tucked in between two major linea­ments—the Chilika Offshore Lineament and the Dhamara Offshore Lineament,
  • v  Far south where the Cauvery drains into the sea, the situation is similar. Two mega lineaments stretch from the western shore al­most parallel to 11 and 12 degree N latitudes. The 750 km Palghat-Cauvery Lineament is associated with
  • v  epicentre locations” To its north runs the 800 km Moyar-Bhavani-Atur lineament. Three faults were identified between these lineaments, all associated with earthquake epicenters. Two abruptly cut off at the southern lineament, suggesting “westward “highmasgnitucie earthquake, hearing”, One was associated with the 2001 Puducherry quake.

All this calls for new Zonation nomenclature for the nation as far as seismicity is concerned.
Even the national Capital that was long held to be located on a stable Aravalli craton is feeling the peripherals shock of the Himalayan
Earth
 
EARTHQUAKE SANS TSUNAMI
DISTANCE FROM EPICENTRE PROTECTS INDIAN CITIES
 
 

  • If the amount of energy released during an earthquake is measured in terms of magnitude, its effects on the earth’s surface are expressed is in ter measured o
  • felt by people. The one widely used today is the Modified Mercalli (MM) intensity scale. ci ties
  • The intensity this earthquake tremor was felt indoors by many people in several  (MM intensity scale III). But reports of cracks developing in buildings increase the intensity scale to IV. Intensity of an earthquake varies from place to place. It will be the maximum at the epicentre and will keep reducing with distance from the epicentre.
  • Acceleration, on the other hand, is a measure of shaking of the ground at a particular
  • point due to an earthquake.
  • The seismically active zone off the west coast of Indonesia is about 1,500 km away
  • from the east coast of India. The acceleration value will be very low at a place which is at this distance from the epicentre. Acceleration falls drastically beyond 200 km from the epicenter and thus the reason for Indian cities saved by such strong shake.
  • quake is measured in terms of magnitude, its effects on the earth’s
  • surface are expressed in terms of intensity. Unlike magnitude that is determined using an instru ment, intensity is measured only by effects felt by people. The one widely used today is the Modified Mercalli (MM) intensity scale.

 
The intensity this earthquake tremor was felt indoors by many people
 
 

  • in several cities (MM intensity scale III). But reports of cracks developing in buildings increase the intensity scale to IV. Intensity of an earthquake varies from place to place. It will be the maximum at the epicentre and will keep reducing with distance from the epicentre. Acceleration, on the other hand, is a measure of shaking of the ground at a particular point due to an earthquake.
  • The seismically active zone off the west coast of Indonesia is about 1,500 km away from the east coast of India. The acceleration value will be very low at a place which is at this distance from the epicentre. Acceleration falls drastically beyond 200 km from the epicenter and thus the reason for Indian cities saved by such strong shake.
  • Plate tectonics makes the whole discourse of any ‘stable’ and ‘stag­nant’ information about internal crust dynamics meaningless. Appreciating these new changes and imbibing the same for disaster management can be the only way which can minimize destruction of life and property by another earthquake. Yes! this time ‘we’
  • may not be that lucky.

 
 
v  “the oceanic plate off the Sumatra and Nicobar segments of the plate boundary behaves as a chip of the India-Australia plate, with its NW-directed plate motion.” Although the earthquake sources within the trench and those on the subducting plate are not too far separated, the lack of vertical component explains the near absence of tsunami. The implication of such stress partitioning on deformation models of the subductiori processes is an important question that needs to be resolved as far as scientific research in the field is concerned so that a better regime can be evolved which can lead us further to better forecasting.
 
 
Points to remember
 

  • Besides the subduction zone

(where the Indian plate dives under the Burmese plate) that lies to the west of Indonesia, the Indian Ocean has a diffused seis­micity zone. This zone lies to the west of the subduction zone. The cast-west trending zone extends from 80 degree East longitude to around 93 degree East longitude (where the April 11, 2012 quake occurred). The length of the zone is around 1,300 km.

  • There have been seven major

earthquakes between 6 and 7.5 magnitude in this region during the last 25 years. The 8.6 magni­tude quake of April 11 and the aftershock of 8.2 magnitude should now be added to this list.
The April 11 quake was not in the subduction zone but to the west of it. It was basically an intra-plate earthquake said the Chief Scien­tist at the National Geophysical Research Institute (NGRI), Hy­derabad. An earthquake is called intra-plate when it occurs within the plate and not at the edges of two plates.
Since it is an intra-plate earth­quake, no subduction takes place and the fracturing causes a strike-slip fault — where one edge of the plate grinds past the other horizontally. The strike-slip fault has an east-west direction. This fault could have formed as
 

  1. v  a result of westward transfer of stress from the subduction zone. The April 11 earthquake was about 100 km west of the subduc­tion zone. It is within the Indo­Australian plate. The movement of this plate (at a rate of 52 mm per year in a NNE direction) in the diffused zone of seismicity is governed by mid-ocean ridges in the Indian Ocean. There are two ridges there — Southeast Indian Ocean ridge and the South Indian Ocean ridge.
  2. v  There is a belief that a new bound­ary is developing in the Indo­Australian plate along the diffused zone of seismicity. Some scientists speculate that this may become the new boundary between the Indian plate and the Australian plate.
  3. v  In the last 100 years of earthquake data, the 15-year-period between 1950 and 1965 witnessed a clus­tering of large quakes of magni­tude greater than 8.5. A similar clustering of large-magnitude earthquakes is now being observed since the December 2004 Sumatra earthquake. Starting from Decem­ber 2004, there have been about six quakes of magnitude greater than 8.5 globally. It is following the same pattern of 1950-1965. The chances of another high-magnitude quake occurring at some place cannot be ruled out if this clustering is indeed hap­pening.
  4. v  Distance from epicentre protects Indian cities: If the amount of energy released during an earth-

 

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