The Earth’s outermost layer, the lithosphere, is broken into large pieces called tectonic plates. These plates are constantly in motion, moving at a rate of a few centimeters per year. This movement creates geological activity, with most earthquakes occurring at the boundaries where these plates meet. The type of interaction at these boundaries influences the earthquakes that occur.
Divergent Plate Boundaries
Divergent plate boundaries are where two tectonic plates move away from each other. This pulling-apart motion generates tensional stress within the Earth’s crust. As the plates separate, magma from the mantle rises to fill the gap, creating new crustal material, such as at mid-ocean ridges or continental rift zones.
Earthquakes at these boundaries occur as the crust stretches and thins, causing blocks of rock to break and slide past each other. This movement happens along normal faults, where one block of rock moves downward relative to the other. Earthquakes at divergent boundaries are shallow, less than 30 kilometers deep, and are less powerful than those at other boundary types.
Convergent Plate Boundaries
Convergent plate boundaries are where two tectonic plates move towards each other, resulting in compressional stress. This collision leads to powerful and deep earthquakes. The intense forces result in reverse or thrust faults, where one block of rock is pushed up and over another.
Oceanic-Oceanic Convergence
Oceanic-oceanic convergence involves one oceanic plate subducting beneath another. This forms deep ocean trenches and volcanic island arcs, generating earthquakes from shallow to very deep, sometimes over 700 kilometers into the mantle.
Oceanic-Continental Convergence
Oceanic-continental convergence occurs where a denser oceanic plate subducts beneath a lighter continental plate. This creates volcanic mountain ranges and produces earthquakes of varied depths, including some of the largest and most destructive.
Continental-Continental Convergence
Continental-continental convergence occurs when two continental plates collide. Neither plate subducts significantly due to buoyancy, leading to intense crumpling and uplift that forms vast mountain ranges. Widespread shallow to intermediate-depth earthquakes, which can be very powerful, are common.
Transform Plate Boundaries
Transform plate boundaries involve two tectonic plates sliding horizontally past each other. This side-by-side motion neither creates nor destroys crustal material. Instead, it generates significant shear stress along the boundary.
Friction causes rocks along the boundary to lock together, accumulating elastic energy. When this stored stress exceeds the frictional resistance, the rocks suddenly slip, releasing energy as seismic waves. This sudden movement occurs along strike-slip faults, where blocks of rock move sideways relative to each other. Earthquakes at transform boundaries are shallow, less than 30 kilometers deep, but they can be very powerful, with magnitudes reaching up to 8. These events frequently occur in long, linear zones, reflecting the extent of horizontal plate motion.