The seismicity witnessed on 22 March 2022 in parts of Dindigul, Tamilnadu again indicates the seismic alertness of the southern part of the Indian peninsula. So, an integrated study was carried out between the lineaments and the present episode and the historical seismicity of the region as well. It revealed the coincidence of the maximum seismic episodes of 54% − 88% with N15o-60oE / S15o-60oW oriented tectonic lineaments/faults and the lineaments/faults associated with geomorphic anomalies. Since, these are inferred to be the regional sinistral strike slip faults related to the ongoing post collision tectonic compression of the Indian plate, the lineaments in general and the lineaments of NE-SW group in particular warrant detailed study.
Research Article
Seismic vulnerability of southern parts of South India in the context of the recent earth tremors (22 March 2022) occurred in Dindigul region, Tamil Nadu, India
https://doi.org/10.21203/rs.3.rs-1653190/v1
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Earth tremors
Dindigul-Tamilnadu
active tectonics and seismicity
seismic vulnerability of NE-SW faults
In contrast to the high degree of seismicity of the northern parts of Indian peninsula covering the mighty Himalayan mountain ranges, Gujarat and the North-Eastern states of the Indian peninsula, the southern parts of the peninsula is comparatively less seismic and related earthquake. However, medium to low intensity seismic episodes have been recurrently occurring in different parts of South India (https://seismo.gov.in/data-portal, https://www.gsi.gov.in). However, the studies carried out in parts of South India, for example, the mapping of tectonic lineaments from the satellite multispectral data of the various global satellites and their integration with locales of seismicity have indicated that the majority of the seismic episodes fell along the lineaments and their close proximities (Anon 2000; Ganesh Raj et al 2001; Ramasamy et al 2009; Ramasamy et al 2013). The studies of Ganesh Raj et al (2001) and Ramasamy et al (2009) have also indicated that the seismic episodes fell dominantly along certain lineaments of preferred directions. Some of the studies have also indicated that the seismic episodes significantly fell along the lineament intersections of specific angles (Ramasamy et al 2009). Further, the studies of Ramasamy et al (2009) have also predicted the probable period/s of future occurrence of seismicity in parts of South India. However, no precise information is still available on the seismic vulnerability of the southern part of the Indian peninsula in general and the locales of seismicity in particular.
Under this scenario, recently, on the early morning of 22 March 2022 (https://seismo.gov.in/data-portal, https://www.gsi.gov.in) three back-to-back earth tremors of 1.5 to 1.6 magnitude have occurred in a time gap of 2–3 hours along the eastern fringes of the Western Ghats of Dindigul - Ottanchattiram area, Tamil Nadu (Box 1B, Fig. 1). On the above date of the event, these tremors were felt (1–3, 1B, Fig. 1) with thunderous noise in some villages like K.Keeranur and Kallimandayam (4&5, Fig. 1B) of Dindigul - Ottanchattiram area. In these villages, development of cracks in the walls of even the well plastered buildings, pealing of plasters and sinking of roofs and falling of tiles, rattling of windows and tossing / falling of utensils were also noted in some buildings (Fig. 1B; photographs − 1,2 &3). In addition, this area has also been witnessing frequent tremors in the past. So, an appraisal was made by conjunctively studying the locations of these epicentres / seismic episodes and the tremors / damages witnessed villages along with the lineaments and fracture valleys inferred from the satellite multispectral remote sensing data of the area. Such an appraisal indicated that these episodes/events seem to have occurred along the NE-SW to ENE-WSW lineaments and fracture valleys (6 & 7, Fig. 1B) found in the fringe regions of the Western Ghats of Dindigul area. This prompted for a deeper look on the phenomenon in an area of 2070 Sq.km. (Fig. 1) in Dindigul region using the seismicity of 22 March 2022 and the other historical seismic episodes of this region. The article briefly narrates some of the newer inferences made on the seismic vulnerability of the southern part of South India.
Photographs: 1,2 & 3 cracks in the plastered walls; 4 - cracked and deflected walls with the direction of deflection away from the building; 5 - sunken roof with deflected and fallen tiles.
In the present study, IRS − 1C satellite data was subjected to various digital image processing techniques using ENVI image processing software (Lillesand 1989). From these, the lineaments of tectonic origin were interpreted on the basis of various photo recognition elements like tonal and textural linearities and the linearities and the curvilinearities of the topography, hill escarpments, valleys, drainages and other similar linear anomalies (Lillesand 1989) (Fig. 1). Such tectonic lineament data was correlated with lineament maps prepared by the earlier workers (Ramasamy et al 2011 & 2013) and the integrated / updated tectonic lineament map was prepared using ArcGIS software (Fig. 1). Then, the locations of the recent Dindigul - Ottanchattiram seismic episodes (1–5, Fig. 1B) of 22 March 2022 and the historical seismicity data (Singh and Raghavan 1989; Anon 2000; Ramasamy et al 2019) were plotted over the updated tectonic lineament map using ArcGIS software (Fig. 1). This showed the falling of most of the seismic episodes along and in close proximities to the tectonic lineaments, indicating that these lineaments might have acted as pathways for the seismic energy release. Then, to further understand it in deeper, all the lineaments of the region were classified into 3 categories viz, (i) Tectonic lineaments having seismic episodes, (ii) Lineament intersections having seismic episodes and (iii) lineaments with geomorphic anomalies having seismic episodes (a-c, Fig. 1). Then, these three categories of lineaments were independently classified into 12 azimuthal groups of 15oeach from N90oW (270o in circular bearing) to N90oE (90o in circular bearing) as shown in Table − 1. This was followed by the counting of number of seismic episodes falling in each of the 12 azimuthal groups of these 3 categories of lineaments (Table − 1). Further, a bar diagram was also prepared (Fig. 2) showing the number of seismic episodes in different azimuthal frequencies of lineaments, to have precise appreciation on the relation between the azimuths of lineaments and the seismic episodes. Finally, from all these data, the relationship between the lineaments and the seismicity and therefrom the seismic vulnerability status of the area was brought out.
Table-1
From the above analyses, the following inferences were made on the seismic vulnerability of the study area.
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Amongst the total of nearly 85 seismic episodes observed in the area, 75 (88%) fell along the first category of lineaments (a, Fig. 1) namely ‘the tectonic lineaments’, 28 (33%) fell along the second category, namely ‘the lineament intersections’ (b, Fig. 1) and 46 (54%) fell along the third category of lineaments say ‘the lineaments with geomorphic anomalies’ (c, Fig. 1). From these, it can be broadly said that the first and the third categories of lineaments have got significant control over the seismicity of the area.
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Amongst the first category of the lineaments, say the ‘tectonic lineaments’ (a, Fig. 1), the lineaments falling in N15o- 60oE orientations had the maximum of 30 seismic episodes out of 48 of the NE-SW azimuthal frequency group (Table − 1). Whereas, in the case of lineament intersections (category 2; b, Fig. 1), only less number of seismic episodes fell along it. In this connection, the Shevroy - Chitteri - Kalrayan hills of Salem area, which falls in the study region is the best example for the prolific lineament intersections and least seismic episodes along them (Fig. 1C). Again, in the case of the relation between the number of seismic episodes and the azimuthal group of lineament intersections too, the number of episodes were equally distributed in all the12 azimuthal frequencies of lineaments (Fig. 2; Table-1). So, this convincingly shows that the lineament intersections do not have much contribution to the seismicity of the region. On the contrary, in the third category of lineaments (c, Fig. 1), that is the lineaments with geomorphic anomalies, 23 episodes out of 33 fell, again in the lineaments in the azimuthal frequency of N15o-60oE, similar to the first category of lineaments.
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These above observations show that, the seismicity of the present area are mostly confined to the NE-SW group of lineaments.
The above analyses between the lineaments and the seismic episodes show that the lineaments falling in the azimuthal range of N15-600E - S15-600W have intimate relation with seismicity. The NE-SW group of lineaments are regional sinistral faults related to the northerly directed compressive force, which has originally drifted the Indian plate towards northerly, caused its collision with Eurasian plate leading to the rise of the Mighty Himalayan mountains, the still prevalent northerly compressive force, its obstruction by The Himalayas and the assimilation of these compressive forces within the Indian plate in the form of series of East - West buckling and the spectrum of faults with dominant NE-SW aligned sinistral strike slip faults (Ramasamy 2006). It has also been observed that these NE-SW spectrum of lineaments/faults gradually take a swing towards west south westerly and caused a series of sinistral faults / enechelon faults (Fig. 1D) all along the West coast of Kerala and Karnataka (Ramasamy 1995). These sinistral faults further continue in the west in the Arabian sea and sinistrally shifted the Laccadive Islands (Ramasamy 1995), which again indicates its active tectonics and thus its seismic nature, as also confirmed by the seismic events in the Arabian sea along these faults (Fig. 1D). These sinistral movements along the NE-SW enechelon faults were also attributed to the north easterly directed compressive force due to the rise of the Carlsberg ridge which is located in the far south west of Laccadive and Maldives Islands (Ramasamy 1995). It is convincing because, the Carlsberg ridge was observed to be on the rise at a rate of 1–30 cm per year (Le Pichon 1968). The ongoing tectonic movements along these enechelon faults are also clear from the wave diffraction pattern seen along these enechelon faults in the Arabian Sea (Fig. 1D).
Further, these NE-SW to ENE-WSW lineaments observed to show a series of tectonic and geomorphic anomalies in parts of Tamil Nadu. Significant amongst them are, (i) the NE-SW trending sub parallel lineaments extending from Cumbum valley in the southwest to Pondicherry in the northeast displaying a hierarchy of tectonic and geomorphic anomalies (Ramasamy and Karthikeyan 1998) like, (i) Tectonic subsidence in Cumbum valley (Fig. 1E), (ii ) Sinistral shift of Kadavur ring structure (Fig. 1F), (iii) Mega eyed drainages formed by Cauvery and Coleroon in Tiruchirappalli region, indicating the probable land subsidence along the two NE - SW trending sub parallel faults that are bounding the eyed drainage on its either sides (Ramasamy and Kumanan 2000) and (iv) Sinistral shifting of Mio-Pliocene sandstone in Neyveli region in the north eastern end of the NE-SW trending Cumbum-Pondicherry sub parallel faults (Fig. 1G). All these (i – iv) indicate the ongoing grabening / land subsidence along Cumbum-Pondicherry, besides sinistral strike slip movements along these NE-SW faults (Ramasamy and Karthikeyan 1998).
These NE-SW lineaments again discretely display a large number of drainage anomalies like restricted floodplains and compressed meanders (Ramasamy and Saravanavel 2020), S - shaped drags in the compressed meanders in Coleroon river (Ramasamy 2006) and sinistral dragging of river island in between 1915 and 1990 AD in Coleroon river, north of Sirkazhi (Ramasamy and Ramesh 1999) in Cauvery delta. These NE-SW faults also show sinistral shifts of the coastal beds along the east coast of Tamilnadu, where the northerly hooking spits were developed due to the interface dynamics between the northerly moving littoral currents and the sinistral faults (Ramasamy 2006).
All these observations show that the NE-SW trending lineaments falling in N15°-60°E / S15°-60°W azimuthal frequency appear to be the active seismogenic corridors in the southern most parts of the Indian Peninsula covering the states of Kerala and Tamil Nadu. The present Dindigul - Ottanchattiram episode of 22 March 2022 also fell along these azimuthal group of lineaments confirming the above observations.
Hence, the southernmost parts of the Indian Peninsula warrant detailed studies, particularly the NE-SW spectrum of trending lineaments.
Acknowledgement
The authors record the deep sense of gratitude to the authorities of Bharathidasan University for having facilitated them to carry out the study.
Statement and Declaration
The authors declare that the above research work is original and has not been published elsewhere in any form or language.
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Photographs 1-5 are available in the Supplementary Files section
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Photographs: 1,2 & 3 cracks in the plastered walls; 4 - cracked and deflected walls with the direction of deflection away from the building; 5 - sunken roof with deflected and fallen tiles.
