Reading a fault from its shaking
The claim is that the shaking recorded far from an earthquake carries enough information to reconstruct, after the fact, exactly how the fault at its source moved — not just that it slipped, but in which direction and along which orientation. This reconstruction is called a focal mechanism, or fault-plane solution, and it is built from the pattern of first ground motions recorded at seismic stations scattered around the world. Historically this meant simply noting whether the first arriving compressional wave at each station pushed the ground up or pulled it down; the modern version replaces that manual reading with an automated analysis of the full waveform, though the underlying logic — reading the radiation pattern of the earthquake’s energy release — is the same.
Push, pull, and the beach ball
The result is conventionally drawn as a lower-hemisphere stereographic projection, popularly called a beach ball diagram, built by plotting each station’s reading according to its azimuth from the earthquake and the angle at which the wave left the source. Stations recording upward first motion are marked as filled, downward motion as hollow, and weak or ambiguous arrivals as a cross. Two perpendicular great circles, called the nodal planes, divide the sphere into shaded and unshaded quadrants representing compression and tension, and three axes derived from the pattern — labelled P for the direction of maximum stress, T for minimum stress, and N for the intermediate direction — summarise the geometry of the slip in a form that can be compared across many earthquakes at once.
Three faults, three signatures
What holds up well is the correspondence between the diagram’s shape and the underlying fault geometry: a strike-slip fault, where the two sides move horizontally past each other, produces a visibly different four-quadrant pattern from a normal fault, where extension lets one block drop, or a reverse or thrust fault, where compression pushes one block up over the other. Because each of these mechanisms leaves a distinct signature on the beach ball, a single diagram lets a seismologist classify an earthquake’s faulting style without ever having seen the fault itself, working purely from how the ground moved at stations that may be thousands of kilometres away.
What the pattern can’t tell you
The method has real limits built into its geometry. A focal mechanism’s two nodal planes are, by construction, equally consistent with the recorded first motions, so the solution alone cannot say which of the two represents the actual fault and which is a mathematical mirror image — that ambiguity has to be resolved using other evidence, such as the pattern of aftershocks or known local geology. The diagram also describes the style and orientation of slip, not the earthquake’s size, its depth considered on its own, or the full extent of the ruptured surface, all of which require separate measurements to pin down alongside the focal mechanism itself.
Settling an argument about the seafloor
The clearest demonstration of the method’s value came from applying it to earthquakes along the ocean floor. Focal mechanisms on oceanic transform faults matched the specific pattern of motion that plate tectonic theory predicted for seafloor spreading, helping settle what had been a live question about how the seafloor moves. Applied to earthquakes deep inside subducting slabs, the same method produced a more surprising result: it showed that deep earthquake zones in some subducting slabs sit under compressional stress while others sit under tensional stress, rather than all behaving the same way, a distinction that reshaped how subduction zones are understood mechanically.
A short, sturdy piece of method
This is a compact, mechanical piece of science rather than a dramatic one, and it rewards a reader mainly by making a familiar image — the shaded and white beach ball attached to earthquake reports — actually legible. It is worth the short time it takes if the goal is understanding what that diagram encodes and why it mattered to a real theoretical dispute about plate tectonics; it is less rewarding for someone hoping for narrative or human drama, since the interest here sits entirely in the geometry and the inference, not in any single dramatic event or personality.