I bought a house in Bay Area, CA 1 year ago with a pool. The concrete deck had small cracks that I didn't really pay attention to at first. In the past 2 months, the cracks have expanded significantly with smaller cracks appearing weekly. The worst area is in the close-up picture, where the outer concrete sunk 1/4 inch. The pool itself has no visible crack. I am new to owning a pool but realize this is a serious issue. I would appreciate any advice on 1. Cause of this; 2. How urgent is this; 3. How to proceed on repairs; 4. Estimated cost to repair?