I would stop riding that bike. The original crack along the weld line has already opened up, and a secondary stress crack has formed extending at an angle from the weld to the bottom of the tube. That whole section of tube is already fatigued from flexing repeatedly.
Too late to re-weld along the crack line. It would be welding fatigued metal to other fatigued metal. The correct fix would be to strip the paint, cut out the entire chainstay, grind the ends smooth, and fabricate a new chainstay to weld in. Then heat treat and repaint.
Are you the original owner or did you buy it used? If you are the original owner Trek may provide a replacement frame if you are covered under their warranty.