Here's why:

* Plate Boundaries: Earthquakes are primarily caused by the movement of tectonic plates. The edges of these plates, where they interact, are called plate boundaries. These interactions are where the most significant stress builds up, leading to earthquakes.

* Shallow Focus: Shallow focus earthquakes occur at depths less than 70 kilometers (43 miles). These are the most common type of earthquake, and they are most frequently found along plate boundaries where the plates are actively interacting.

Types of Plate Boundaries and Earthquake Depth:

* Convergent Boundaries: Where plates collide (like the Pacific Ring of Fire). These often produce deep-focus earthquakes, but also shallow ones.

* Divergent Boundaries: Where plates move apart (like mid-ocean ridges). These mostly generate shallow focus earthquakes.

* Transform Boundaries: Where plates slide past each other (like the San Andreas Fault). These are known for producing shallow focus earthquakes.

Important Note: While most shallow focus earthquakes are found along plate boundaries, there are some exceptions. Intraplate earthquakes can occur within the interior of a tectonic plate. These are less common but can still be significant.