Strike-Slip

Plates of crust that make up the outermost shell of Europa or Earth do not only move apart (as discussed in the previous chapter) or together (to be discussed in the next chapter). They also can slide past one another—a phenomenon I describe in some detail in this chapter. On Earth, for example, the coastal area of southern California scrapes northward past the rest of North America along the San Andreas Fault. This sort of “shear displacement,” or sliding movement along cracks, is very common on Europa.

In geological parlance, the shearing of one plate past one another is called “strike-slip” displacement. “Strike” refers to the direction of a fault across the landscape. A pilot following along a fault line would call this direction the “heading.” So “strike-slip” means that the displacement ( the “slip”) is along the strike. Strike-slip motion is parallel to the strike of the crack. In contrast, when a crack dilates, the displacement is perpendicular to the strike.