Here's why:
* Transform boundaries: These boundaries involve plates sliding horizontally past each other. The friction between the plates creates stress that builds up and is released through earthquakes. Because the plates are sliding horizontally, the earthquakes tend to occur relatively close to the surface, resulting in shallow foci.
Here's why the other boundaries are not as associated with shallow-focus earthquakes:
* Convergent boundaries: These boundaries involve plates colliding. The collision can result in both shallow and deep-focus earthquakes. The deep-focus earthquakes are caused by the sinking of one plate beneath the other (subduction).
* Divergent boundaries: These boundaries involve plates moving apart. While these boundaries can also have earthquakes, they are generally less frequent and less powerful than those at convergent or transform boundaries. The earthquakes are often shallow-focus.
Examples of shallow-focus earthquakes at transform boundaries:
* The San Andreas Fault in California, USA
* The North Anatolian Fault in Turkey
Important note: While transform boundaries primarily have shallow-focus earthquakes, some can also have intermediate-depth earthquakes (between 70 and 300 km) due to complexities in the faulting.
