Plate boundaries experience distinct stress regimes: divergent boundaries have extension (lowest stress vertical); transform boundaries have strike-slip (highest stress vertical); convergent boundaries have compression. Focal mechanisms of earthquakes reveal stress orientation and help identify which type of plate boundary exists.
Use focal mechanisms to infer stress orientations. Compare stress tensors to plate motion vectors and boundary geometry.
From your study of stress and strain in rock deformation, you know that stress has three principal axes — the maximum, intermediate, and minimum compressive stresses (σ₁, σ₂, σ₃) — and that the orientation of these axes determines what kind of faulting occurs. At plate boundaries, the type of relative motion between plates dictates the stress regime, and earthquakes along these boundaries broadcast that information through their focal mechanisms.
At divergent boundaries (mid-ocean ridges, continental rifts), plates pull apart. The dominant stress is extensional: the minimum compressive stress (σ₃) is horizontal and perpendicular to the ridge axis, while the maximum stress (σ₁) is vertical — the weight of the overlying rock. This produces normal faulting, where blocks drop down along steeply dipping fault planes. On a focal mechanism diagram (the "beachball" pattern derived from seismic first motions), normal faults appear with the tension axis (T) horizontal, confirming that the boundary is being stretched apart.
At convergent boundaries (subduction zones, collision belts), the situation reverses. Plates push together, making the maximum compressive stress (σ₁) horizontal and directed toward the overriding plate, while the minimum stress (σ₃) is vertical. This produces reverse and thrust faulting, where rock is pushed up and over itself along low-angle fault planes. Focal mechanisms at subduction zones show compression axes aligned with the direction of plate convergence. The stress field can be complex — the subducting slab pulls downward under its own weight (slab pull), while the overriding plate is compressed — but the net effect is shortening and crustal thickening.
Transform boundaries are the third case. Here, plates slide horizontally past each other, and the stress field is dominated by shear. The maximum and minimum compressive stresses are both horizontal, oriented at 45° to the fault trace, while the intermediate stress (σ₂) is vertical. This produces strike-slip faulting — pure horizontal displacement with no significant vertical motion. The San Andreas Fault is the textbook example: focal mechanisms along it consistently show horizontal T and P axes rotated ~45° from the fault strike, confirming lateral shear.
The practical power of this framework is that stress orientation at any point on Earth's surface can be inferred from earthquake focal mechanisms, and that information feeds directly into hazard assessment. A region showing consistent thrust-fault mechanisms is accumulating compressive strain and may be building toward a large earthquake. A region showing normal-fault mechanisms is extending and thinning. By mapping focal mechanisms across a plate boundary zone, geologists can identify not only the boundary type but also where stress is concentrated, where strain is partitioned across multiple faults, and where the next major rupture is most likely to occur.