Geometry of earthquake sources — from double couples to beach balls
Drag sliders to change fault geometry. The lower-hemisphere Wulff projection updates in real time, showing compressional (dark) and dilatational (light) quadrants.
Click a fault type to explore its beach ball signature and tectonic setting.
Right-lateral · Transform margins
Left-lateral · Transform margins
Extensional · Rifts, MORs
Compressional · Subduction, collision
Upward first motion (compression) = filled circle; downward (dilatation) = open circle. The pattern constrains the nodal planes.
Maximum compressive stress. Bisects the compressional (dark) quadrants. Located 45° from both nodal planes.
Maximum tensile stress. Bisects the dilatational (white) quadrants. Perpendicular to the P-axis in the fault plane.
Lies along the intersection of the two nodal planes. Perpendicular to both P and T.
Anderson (1905, 1951): the Earth's free surface forces one principal stress to be vertical. The relative magnitude of the vertical stress determines which fault style is favoured.
Faults nucleate at θ = 45°+φ/2 from σ₃ (i.e. 45°−φ/2 from σ₁). Adjust internal friction angle φ to see how fault orientation changes for each tectonic regime.
Normal stress (σₙ) vs shear stress (τ) on all possible planes. Failure where the circle touches τ = C + μσₙ.
Add mechanisms consistent with the selected regime. As the ensemble grows, the accumulated pattern reveals the dominant stress configuration — the basis of formal stress tensor inversion.