Japan\'s domestic sales of new cars, trucks and buses slumped 15 percent in 2011 mainly due to the huge disruption sparked by March\'s quake and tsunami, industry groups said Thursday. Sales fell to 4.21 million vehicles last year, the Japan Automobile Dealers Association and the Japan Mini Vehicles Association said, after the March 11 natural disasters damaged factories and crippled supply chains, forcing the likes of Toyota to shutter plants and halt production. The industry was already suffering sluggish demand after state subsidies for the purchase of environmentally friendly vehicles ended in September 2010. But a breakdown of the figures revealed that things were starting to look up for the industry. In December alone, sales of new vehicles jumped 21 percent to 349,206 units, as the sector continues to rebound from the slump, the two industry groups said.