Either click on the plus icon next to the "Zoom Control" slider bar to zoom in by a factor of two, or
Click to the right of the slider on the slider bar to possibly zoom in further. Each notch on the slider bar is considered a factor of two.
or
Alternately, if you have javascript enabled, you may click and drag on the map to create a rectangle.
As soon as you release the mouse button, the map should zoom to the rectangle extents.
This can be done at any time on the smaller index map, or on the larger main map when the "Recenter" button has been selected.