Porites coral grows in annual rings, much like a tree.
It prefers to be just below the sea surface.
When the coral head reaches the sea surface, the portion exposed to air dies, while the submerged portion continues to grow outwards, increasing the coral's width.
If the land on which the coral sits subsides, any live portions of the submerged coral will grow upward, reaching toward the surface. If an earthquake occurs and the land suddenly uplifts, exposed portions will die, while live submerged portions will grow wider by adding rings to the sides.
In graph, black triangles indicate coral growing upward toward the sea surface; red circles indiate coral is at or above the sea surface and so can only grow outward.
Credit: Danny Natawidjaja, Kerry Sieh, Tectonics Observatory