They aren't physical lines on the ocean floor unless I'm horribly misreading something. Google Earth is just pulling together whatever ocean depth data it can find for the oceans in order to build a 3D image (or at least 2D visual impression, I haven't dived into Google Earth for a while to have a look), and in this location there happens to be an average ocean depth reading crossed with much more accurate echosounding data, giving the impression of their being trenches in the ocean floor.
I need an analogy. Imagine a flat surface. To us, it looks pretty uniform, it's flat, it's smooth… stick it under a microscope and you'll see that it is far from flat, it's full of groves, channels, dents, whatever. Here, we have a smooth flat reading of the ocean floor, but under the microscope we have a more accurate depth reading - only we haven't put the entire ocean floor under the microscope, just a few paths across it.
