Why do tectonic plates line under the sea?

Question

earthquakes

Answer 1

Because that was where the earths crust formed years ago.

Answer 2

They are all over the world but most of the continents are the above ground visible part of each major tectonic plate. A very few fractures run under a continent. Here's a picture:

Answer 3

I'm not exactly sure what you mean by "line under the sea".  

The basic point in plate tectonics is, that earth's surface is broken into seven large and many small moving plates, hence the name. These plates move relative to one another an average of a few Centimeter a year. Three types of movement are recognized at the boundaries between plates: convergent, divergent and transform-fault. This is a rather simple result of the the fact that earth is a sphere with more or less constant diameter. So if two plates move in different direction, they either collide or move away from each other. A special case is when they both move parallel to their boundary.

There are only two types of plates, continental plate, which is higher than sea level and oceanic plate, which is below sea level. The reason is simple: they are composed of different materials. Continental plate is thick and composed of sediments and volcanic rocks, oceanic crust is thinner and consist only of a certain kind of volcanic rocks. The plates  move on the viscous upper mantle. Oceanic crust is thinner and heavier.

Where there is a division between plates under the sea, we can have undersea earthquakes.