Whenever the compression force exceeds a particular limit tremors are felt. The subterranean magma is also being pushed out and spread out on the ocean floor.
In the above diagram, the arrow marks show the direction of stress release when the mid Indian Ocean compression zone receives a jolt from the southern tectonic plate. The immediate result would be a tsunami which can cause tremendous damage to southern tip of India. Part of it enters the Gulf of Mannar where damages are restricted by promontory like formations. Setu Karai and Rameswaram would have escaped damage, but experienced a denuding of silt in the region of the underwater land connection between India and Lanka. Rama made use of this opportunity and quickly raised the region with solid stuff.
All this is attributed to Rama and his astra, for Rama as Time and Kalapurusha is the Primary cause of all events.
At a time Rama was waiting to cross the sea at Setu, Agastya was preparing himself to head the revival of Tamil Sangam by engaging in a 12 year long penance at the receptacle of Kaveri.