The black-and-white circles on earthquake maps are called focal mechanisms. Their familiar nickname—“beach balls”—comes from their segmented appearance. They summarize the orientation of faulting and the first motion recorded by seismic stations.
A focal mechanism is not a photograph of a fault, nor does the circle show the physical shape of an earthquake. It is a lower-hemisphere projection: a compact way to display how seismic waves left the source in different directions.
Three patterns to recognize
Real mechanisms vary with fault angle and oblique movement, but three broad families provide a useful starting point.
Normal faulting
A pale center commonly indicates extension: the crust is being pulled apart.
Reverse or thrust
A dark center commonly indicates compression: crustal blocks are being pushed together.
Strike-slip
A four-quadrant pattern generally signals mostly horizontal motion along a steep fault.
Always check the legend. Most diagrams shade compressional quadrants, but conventions can vary. Color may also encode depth or another map variable rather than the first-motion polarity itself.
A practical reading sequence
- Identify the overall family. Look first for a light center, dark center or four-lobed pattern.
- Check orientation. Treat the top of the symbol as north unless the figure states otherwise. The dividing curves, called nodal planes, show possible plane orientations.
- Examine the curves. A line close to a diameter represents a steep plane; a strongly curved line nearer the rim represents a shallower plane.
- Use the map context. Compare the candidate orientations with mapped faults, aftershock patterns and regional tectonics.
Why are there two possible fault planes?
A standard focal mechanism contains two nodal planes. One may be the plane on which rupture occurred; the other is an auxiliary plane produced by the mathematics of the seismic-radiation pattern. The beach ball alone usually cannot distinguish them.
Analysts resolve this ambiguity with outside evidence. A mapped fault trace, the distribution of aftershocks, finite-fault modeling, surface rupture or regional tectonic structure may identify the more plausible plane.
Normal and reverse examples
Normal faults often dip at moderate angles. A typical normal-fault mechanism can therefore offer two oppositely dipping candidate planes. Reverse and thrust mechanisms may show one shallow and one steep candidate. In a well-understood subduction setting, the shallow plane may fit the plate boundary—but that conclusion still requires supporting evidence.
Strike-slip examples
Strike-slip mechanisms commonly present two steep, near-vertical candidates crossing at a high angle. One solution may represent left-lateral motion on one trend; the alternative represents right-lateral motion on the other. The mapped fault orientation often determines which interpretation is appropriate.
Real earthquakes are often oblique
Natural faults rarely conform perfectly to textbook categories. A mechanism may combine reverse and strike-slip movement, or normal and strike-slip movement. These mixed patterns reflect oblique motion rather than a flawed diagram.
Magnitude and depth are also separate pieces of information. Symbol size may represent magnitude, while color may show depth, but those visual choices belong to the map designer. Read the legend before comparing symbols.
What causes the earthquake represented?
Tectonic plates move slowly, while friction can hold sections of a fault in place. Stress builds until part of the fault slips, releasing energy as seismic waves. The hypocenter is the point within Earth where rupture begins; the epicenter is the point on the surface directly above it.
Magnitude estimates the size of an earthquake from seismic observations. Modern reporting commonly uses moment magnitude for larger events because it relates to fault area, average slip and rock rigidity. A focal mechanism answers a different question: the dominant geometry and style of motion at the source.