Focal Mechanism Calculator

Professional Notes

1) Source meaning

A focal mechanism summarizes the earthquake source radiation pattern. For many crustal earthquakes, a double-couple model is an effective approximation for rapid interpretation.

2) Geometry controls radiation

Strike, dip, and rake encode the plane orientation and slip direction. Small changes can rotate nodal lines and alter quadrant patterns that first-motion data constrain.

3) M0 provides scale

M0 scales tensor amplitudes. If Mw is available, the calculator estimates M0 using a standard logarithmic relationship so your tensor values match catalog magnitudes.

4) P, T, B axes

P and T axes represent maximum compressive and tensile directions of the source, and B is intermediate. They support quick classification into normal, reverse, or strike-slip regimes.

5) Auxiliary plane choice

The two nodal planes are indistinguishable from the radiation pattern alone. Use mapped faults, aftershock alignment, or geodetic slip models to select the actual rupture plane.

6) Convention awareness

Different software may use NED or ENU, and rake sign conventions can differ. Validate your workflow with a known example before processing large catalogs.

7) Practical QA

Check angle ranges, confirm symmetry, and ensure results are stable under small input changes. If values jump, revisit the angle convention and unit scaling.

8) Reporting workflow

Use the exports to attach computed planes, axes, and tensor components to event notes and reports, keeping assumptions and scaling transparent for reviewers.

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FAQs

1) Why are there two planes?

Radiation constraints yield two nodal planes. Additional geological evidence is required to identify which plane is the true fault plane.

2) Can I run without Mw or M0?

Yes. If you provide only angles, the calculator uses M0=1 so you still get normalized tensor components, axes, and the auxiliary plane.

3) What if my rake convention differs?

If your reference uses an alternate rake sign, your tensor will rotate accordingly. Test your convention using a published example and compare quadrants and axes.

4) Do P and T equal tectonic stress?

Not exactly. They describe the source radiation pattern. They can correlate with regional stress but local fault geometry and pre-existing weaknesses influence them.

5) Why is dip limited to 90 degrees?

Dip is usually reported as an acute angle. Equivalent plane orientations beyond 90 degrees can be represented by rotating strike 180 degrees and using the acute dip.

6) Why does the auxiliary plane look odd?

The auxiliary plane can have very different strike and rake. That is expected because it reproduces the same radiation quadrants even if it is not the rupture plane.

7) Does this include isotropic or CLVD terms?

No. This is a pure double-couple calculator. Full moment tensor decompositions require inversion outputs that include isotropic and CLVD components.