Tamil Nadu Weatherman Special – Earthquake with Epicenter in Tamil Nadu near Chengam

After 3 years, a less intensity M3.0 earthquake with epiccentre in Tamil Nadu was witnessed near Chengam in Tiruvannamalai district yesterday (11.04.2021) night, Source – National Center for Seismology, Government of India. It was an Intra plate quake.
I want to use this event to talk more about the earthquakes, Tsunami, historical earthquakes in Tamil Nadu and many rare stats.
I. Major Plates in the World
Movement of the plates and their collision causes earthquakes. Of course, this is true in the larger perspective and the earthquake occurrences in Indian plate boundaries like Himalayan belts, Kashmir, Gujarat, North East India and Andaman Islands can be explained. But as far as South India and Tamil Nadu is concerned, all the earthquakes have not occurred in plate boundaries because we are not in fault lines, but only within a plate and hence it does not mean earthquakes will occur only at fault lines, sometimes earthquakes happen within plates its called Intra plate earthquakes. Very rarely they are strong quakes. But yes major and frequent earthquakes occur only in the fault lines where two plates collide. The major plates in the world is posted below and you can wonder why Himalayas, Kashmir, Gujarat, North East and Andaman get frequent earthquakes.
2. Indian Plate and surrounding Minor Plates
The Indian plate has some minor plates and no wonder the Andaman – Indonesia belt gets some daily earthquakes due to a small micro plate called the Burma Plate at one point three plates meet near Indonesia.
3. Intra Plate quakes
An intraplate earthquake occurs in the interior of a tectonic plate. Intraplate earthquakes are relatively rare. Interplate earthquakes, which occur at plate boundaries, are more common. Nonetheless, very large intraplate earthquakes can inflict heavy damage, particularly because such areas are not accustomed to earthquakes and buildings are usually not seismically retrofitted. Examples of damaging intraplate earthquakes are the devastating Gujarat earthquake in Bhuj in 2001 and 1993 Lattur earthquake in Maharashtra. The strong Intraplate Earthquakes like 2001 and 1993 are very very rare, though lesser intensity quakes occur occasionally. Here is the seismic zone map of India and you can see most of Zone 4 and 5 high risk areas falls along the fault lines.
4. Biggest Earthquakes in Last 200 years in Indian Region
The Indian subcontinent has a history of earthquakes. The reason for the intensity and high frequency of earthquakes is the Indian plate driving into Asia at a rate of approximately 49 mm/year. The following are the list of major earthquakes which have occurred in India.
5. Tsunami and what type of Earthquake causes it ?
Tsunami can be generated when the sea floor abruptly deforms and vertically displaces the overlying water. A tsunami can be generated when thrust faults associated with convergent or destructive plate boundaries move abruptly, resulting in water displacement, owing to the vertical component of movement involved. Tsunamis have a small amplitude (wave height) offshore, and a very long wavelength (often hundreds of kilometres long, whereas normal ocean waves have a wavelength of only 30 or 40 metres), which is why they generally pass unnoticed at sea, forming only a slight swell usually about 300 millimetres (12 in) above the normal sea surface. They grow in height when they reach shallower water, in a wave shoaling process described below. A tsunami can occur in any tidal state and even at low tide can still inundate coastal areas.
5.1. Types of earthquakes and which type causes Tsunami
Tsunamis usually are associated with dip-slip type earthquakes rather than with strike-slip type earthquakes. The earthquakes in the Andaman region are associated mainly with strike-slip type of faulting. It is believed that these events probably occur along the north-south trending West Andaman fault. Upto the 26 December 2004, the earthquake of 26 June 1941 had been the strongest ever recorded in the Andaman and Nicobar Islands, in generating a destructive Tsunami. The 1941 event was the last great earthquake in the Andaman and Nicobar Islands. The 1881 Nicobar Islands earthquake was the only other event of comparable magnitude.
(i). Strike-slip faults indicate rocks are sliding past each other horizontally, with little to no vertical movement. Both the San Andreas and Anatolian Faults are strike-slip. These rarely dont generate Tsunamis.
(ii). Dip-slip Normal faults create space. Two blocks of crust pull apart, stretching the crust into a valley. The Basin and Range Province in North America and the East African Rift Zone are two well-known regions where normal faults are spreading apart Earth’s crust.  These Dip slip types are the ones which generate Tsunami when they are in very high magnitude when happens in shallow depth.
(iii). Dip-slip Reverse faults, also called thrust faults, slide one block of crust on top of another. These faults are commonly found in collisions zones, where tectonic plates push up mountain ranges such as the Himalayas and the Rocky Mountains.
6. Historic Tsunamis in India in last 250 years
Tsunami was new news to me in 2004. Our Geography books did not teach us anything about Tsunami prior to 2004 event. I have compiled some of the tsunami events which affected the Indian coast in the last 250 years. Based on these statistical and historical information, it can be concluded that most of the earthquakes in the Andaman Sea Basin do not usually generate significant tsunamis with the possible reason for the low number of tsunamis is that most of the earthquakes in the Andaman Basin are mainly associated with strike-slip type of faulting.
However, Earthquakes with magnitude 8.0 or greater (such at the 1941 and 2004 events) associated with “dip-slip” types of vertical crustal displacements along thrust faults have the potential of generating very destructive tsunamis in the entire Bay of Bengal Region the Andaman Sea and the Indian Ocean. It can be observed that once in every 60 years statistically, there has been Tsunami in the India Coast.
7. List of earthquakes in last 200 years with Epicenter in Tamil Nadu – Rare List
The list of earthquakes which has happened in Tamil Nadu in the last two centuries is given below. Only epicenter in Tamil Nadu is considered. The seismic zones and the places of earthquakes in Tamil Nadu is given below. As you can see only one quake managed to cross 6.0 scale that was in coimbatore in 1900.  I had to spend weeks, refer many authors journals to prepare this list such that there is no duplication. As you can see there is an earthquake in Tamil Nadu after a gap of 3 years.
In Tamil Nadu, the Seismic Prone Lineaments were mapped by Dr.Ganapathy Pattukandan and we can see why small earthquakes always happen in the belt of Vellore-Dharmapuri-Salem-Tirupattur and Thiruvannamalai.
8. Chennai Tremors in last 200 years from far away earthquakes
The tremors are sometime felt in Chennai from far away powerful earthquakes and i still remember in the last ones in 2012 and 2009 when we ran out from our offices at T.Nagar. The Epicenter will be 1000s of kms away but still due to the magnitude we feel the shakes (tremors).
9. Magnitude
A measure of the strength of an earthquake or strain energy released by it, as determined by seismographic observations. The amplitude on a seismogram, the magnitude and the energy released are related through a loglinear Relationship, which was originally defined by Charles Richter in 1935. An increase of one unit of magnitude (for example, from 4.6 to 5.6) represents a 10-fold increase in wave amplitude on a seismogram or approximately a 30-fold increase in the energy released. In other words, a magnitude 6.7 earthquake releases over 900 times (30 times 30) the energy of a 4.7 earthquake – or it takes about 900 M:4.7 earthquakes to equal the energy released in a single 6.7 earthquake! This is an open-ended scale and hence there is no beginning or end to this scale. However, rock mechanics seems to preclude earthquakes smaller than about -1 or larger than about 9.5. An earthquake of magnitude -1.0 releases about 900 times less energy than a M:1.0 quake. Except in special circumstances, earthquakes below M:2.5 are not generally felt by humans. Depending upon the range of magnitude, epicentral distance and the type of seismic waves considered in the computation, there are several magnitude scales in use as: Local magnitude (Ml or ML), commonly referred to as “Richter magnitude”, Surface-wave magnitude (Ms), Body-wave magnitude (mb), and Moment magnitude (Mw). The first three magnitude scales Ml, Ms and mb make use of amplitudes and time periods of seismic wave and suffer from the saturation effect. They have some or other limitation with regard to their applicability uniformly to all magnitude ranges, epicentral distances and focal depths. To avoid the saturation effect and standardize the magnitude scales, a magnitude scale based on seismic moment (Mo) was proposed by Kanamori (1977). The moment magnitude (Mw) scale is estimated using the formula, Mw=(log Mo –16)/1.5, where Mo, is the seismic moment in dyne-cm. Since seismic moment is a measure of strain energy released from the entire rupture surface, a magnitude scale based on seismic moment most accurately describes the size of large earthquakes. Since Mo does not saturate, so also Mw. The moment magnitude scale is the most preferred magnitude scale in case of large earthquakes –  Source National Centre for Seismology.

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