Seth owns a property on a small island with six other property owners. Recently, Seth built a fishing pier along the water’s edge; however, the pier is actually on his neighbor’s property, which Seth has no rights to. What might this be an example of?

Encroachment.

Explanation:

Encroachment of land occurs when a person violates his neighbors property right  through building a structure that extends beyond the boundary of his land and into another person's property.

However , this may be mutually agreed on by the two parties involved , and compensation given under a term called easement.

In a situation where the encroached property belongs to the government , this is referred to as structural encroachment and the government may destroy such structure without giving any compensation

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