Earthightis : Creaking Hills and Wobbling Valleys
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Earthightis: Creaking Hills and Wobbling Valleys – K. R. Y. Simha, Indian Institute of Science, Bangalore (E: [email protected]) Earthightis continues with earlier Earth Day themes published in the Journal of Geological Society of India: Earthmetic and Earthmia written especially for school and college students and teachers to promote field observation skills for deeper geological interpretation. Obliquely subducting plates and junctions embrittle overlying mountain ranges. Arthritis refers to minerals depleted brittle bones and joints in the human body with inflammation (google Arthritis Foundation 2020). This word earthightis is coined for highlighting the inexorable crackling, creaking and rumbling sounds accompanying uninterrupted seismic landscaping activity overlying obliquely subducting plate corners and junctions. Counting and correlating these continuous bursts of acoustic emission (AE) with the progressive accumulation of damage offers remote sensing and damage control of brittle bone, rock or wood. Shillong and surroundings furnish a spectacular history of seismic events discussed passionately and documented continuously for centuries. The Geological Survey of India Memoir on 1897 Assam earthquake studded with tables and photographs is a 557 page masterpiece by Oldham, then an assistant superintendent with the Survey. Oldham begins the introductory narrative lamenting, “…an area of 150,000 square miles had been laid in ruins…. the hills rent and cast down in land slips, and the plains fissured and riddled with vents, from which sand and water poured out in most astounding quantities; and ten minutes had not elapsed from the time when Shillong was laid in ruins..”. A recent 2019 Geological Society, London, Lyell special issue on Himalayan tectonics featuring Billham’s comprehensive summary chronicled the effort of groups led by England, Molnar, Iyengar, Rajendran, Srivatsava along with a reference to the fracture expert Rice. A 2020 article led by Mallick speculated on Shillong obduction from subduction to its south augmented by obstruction at north and east plates. Other relevant contributions as introduced vividly in books by Valdiya and Fowler include a 7843 seismic events catalog spanning 1411-1999 led by Iyengar, while directing CBRI and hazard assessment modeling pertaining to the states of Tripura and Mizoram led by Seetharam (presently directing IIT Guwahati). Recent workers are increasingly preoccupied with forecasting catastrophic mega earthquake scenarios (Richter magnitude M > 9). Fortunately, such extremely rare mega events having occurred once or twice in the recorded history suggest that continental subduction and tectonics overall are gradually trending downwards to a more predictable state of affairs like global weather and circulation. Should this conjecture come true, then the plates will be gently moving up and down carrying their cargo of continents like a feedback controlled system of escalators, elevators and conveyor belts in deep underground mines or mega shopping malls! Subduction a
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