Construction defects of all sorts must be treated seriously. These are matters that can lead to you not being able to use the building without running the risk of someone being injured or harmed in some way. The fact of the matter here is that you didn't pay for something with defects. You paid for quality workmanship using appropriate materials.
Landslides are a risk for many homes in California, but most people aren't going to avoid homes where this is possible. There are some ways that you can protect your home, so speak to a professional about what options are present for your home. They should be able to give you an idea of how effective certain things that will be.