In a large saucepan, heat oil over medium heat. Add onion, cover, and cook until soft, about 8 minutes. Add garlic and cook 1 minute more. Stir in broth, tomatoes, and salsa, increase heat, and bring to a boil. Reduce heat and simmer 5 minutes.
Stir in drained corn, shrimp, and avocado, and heat through. Stir in cilantro and crushed tortilla chips, and serve immediately. Garnish with lime and green onion, if desired.