Legal Question in Real Estate Law in California

I assume you mean when Property X is higher than Property Y, causing the dirt of X to move onto Y, either gradually or in a sudden landslide or collapse.

This can be researched on Google to some extent by using search terms "lateral support" and "subjacent support" - the following Wikipedia stub article is a starter:

http://en.wikipedia.org/wiki/Lateral_and_subjacent_support

In California, I believe the following principles apply:

1. So long as both parcels are left in their natural state, with little or no development or use, neither owner has an obligation to the other; whatever nature wants to happen is OK.

2. When the parcels are developed and used, both owners are expected to behave reasonably with respect to the property rights of the other; X can't bulldoze excess dirt onto Y, and Y can't excavate trenches so close to the property line that X's property collapses. Both neighbors need to avoid disturbance of the other's land through their activities on their own land, or through neglectful maintenance of their own properties.

3. Building and excavation permits must be obtained when required, and their limitations observed.

4. In general, the uphill neighbor is considered to have a greater responsibility because the force of gravity will magnify the results of his unreasonable activity or neglect of maintenance.

5. When there are known local conditions such as the tendency of a particular soil to crumble and collapse when loaded, or earhquake faults, a property owner may be liable for his failure to take these into consideration in the design and construction of improvements.