You might expect more earthquakes near the earth's surface since the rocks here are brittle, and those deeper are more ductile from increased temperature and pressure. San Andreas fault. Shallow-focus earthquakes are typical of those that occur at the boundary of the North American Plate, which is moving against the Pacific Plate. In California, the boundary between these two plates is known as the San Andreas fault. The two plates are tightly pressed against each other, and friction between the rocks along the fault prevents them from moving easily.
Stress continues to build along the entire fault as one plate attempts to move along the other.
The stressed rock, now released of the strain, snaps suddenly into new positions in the phenomenon known as elastic rebound. The rocks are displaced to new positions on either side of the fault, and the vibrations from the sudden movement are felt as an earthquake. The elastic rebound and movement tend to occur along short segments of the fault at different times rather than along long lengths. Thus, the resulting earthquake tends to be a localized phenomenon rather than a regional one. An earthquake is a quaking, shaking, vibrating, or upheaval of the ground, a result of the sudden release of energy that comes from stress on rock beneath the earth's surface.