Combine the tomatoes, ketchup, vinegar, garlic, cumin, jalapeno and 3/4 teaspoon salt in a 5-to-6-quart slow cooker. Add the steak, bell peppers and onion and toss to coat. Cover and cook on low, undisturbed, 8 hours.
Uncover and skim off any excess fat. Coarsely shred the meat with 2 forks, then stir in the olives and olive brine. Serve over rice.