Once the volcanoes of northern Snow Mountains erupted, the magma upwelled to heat the water of the natural underground reservoirs, and then raised the water pressure and triggered the collapse, which created a big landslide. The chain effect took place; whole an enormous volume of earth rolled and fell that made the smooth coastline to form the beautiful concave arc.
From the east flank of Snow Mountains to the original coastline of Ilan, the area is about 800 km2. After volcano erupted and collapsed that lowered the average elevation about 600 m; therefore, estimating the volume of earth about 480 km3 dropped into the Pacific Ocean that caused a mega-tsunami.
The original coastline of northeast corner of Taiwan was pushed and raised by Eurasia plate and Philippine marine plate. So, it should spread from the coast of Hua-Tung straightly, through the east of Turtle Island, until reaches Santiago, and presents with the precipitous rocky coast.