In 1969, Muawia Barazangi and James Dorman published the locations of all earthquakes which occurred from 1961 to 1967. Most of the earthquakes are confined to narrow belts and these belts define the boundaries of the plates. The interiors of the plates themselves are largely free of large earthquakes, that is, they are aseismic. Seismic Zones Plate tectonics confirm that there are four types of seismic zones. The first follows the line of mid-ocean ridges.
Activity is low, and it occurs at very shallow depths.
The point is that the lithosphere is very thin and weak at these boundaries, so the strain cannot build up enough to cause large earthquakes. Associated with this type of seismicity is the volcanic activity along the axis of the ridges The second type of earthquake associated with plate tectonics is the shallow-focus event unaccompanied by volcanic activity. The San Andreas fault is a good example. In these faults, two mature plates are scraping by one another. The friction between the plates can be so great that very large strains can build up before they are periodically relieved by large earthquakes.