What is the term for the surface where two blocks of earth suddenly slip past one another?

The term for the surface where two blocks of earth suddenly slip past one another is known as a fault or fault plane. This geological feature is critical in understanding seismic activity and tectonic movements. When stress builds up along these faults due to the movement of tectonic plates, it can cause an earthquake when the energy is released. The fault plane is essentially the flat surface along which this movement occurs.

In geological terms, faults are classified into various types based on their movement: normal faults, reverse faults, and strike-slip faults, each corresponding to different stress conditions in the Earth's crust. Understanding faults is essential in the fields of geology and civil engineering, particularly for assessing earthquake risks and designing buildings and infrastructure accordingly.

The other terms listed do not accurately describe this phenomenon. A crevice refers to a narrow opening or gap, often formed in rocks or ice, but it does not pertain to the specific movement of tectonic plates. A seam typically refers to a line formed by joining two pieces of material, which is not related to geological processes. A trench is a deeper, elongated depression in the earth's surface but is associated with different geological formations, such as subduction zones, rather than the slipping movement of blocks along a fault.