Use this seismology calculation to reproduce a stated convention, screen observations, or compare scenarios whose units, coordinate conventions, catalog selection, and model assumptions are known.

I₀

Required number. Epicentral intensity.

I_out

Required number. Outer contour intensity.

I_in

Required number. Inner contour intensity.

h (km)

Required number in km. Focal depth.

The cited overview of Seismic intensity scales supplies background for the terminology and domain context used by this tool.^[1]

Felt area of the annulus between two intensity contours using the Kövesligethy decay. Inputs are interpreted exactly in the displayed units and the calculation returns the following fields without presentation rounding.

A (km²)

Returned number in km². Area between the two isoseismals.

r_out (km)

Returned number in km. Outer contour radius.

r_in (km)

Returned number in km. Inner contour radius.

• Earth structure, source geometry, path effects, site response, instrument response, catalog completeness, uncertainty, and the fitted range can dominate the accuracy of simplified seismic calculations.
• h must be at least 0.
• Use finite inputs in the displayed units, preserve source measurements and assumptions, and independently verify consequential decisions.

For analysis or hazard decisions, retain uncertainty and provenance and compare the result with waveform processing, calibrated regional models, and guidance from qualified seismologists or the responsible agency.

• Kovesligethy Intensity Decay Isoseismal Calculator

  Kövesligethy isoseismal radius: I(r) = I₀ − 3·log₁₀(√(r²+h²)/h), inverted for a target intensity.

• RMS Velocity Dix Equation Interval Velocity Calculator

  Dix equation interval velocity between two horizons from RMS velocities and zero-offset times.

• Aftershock Zone Length Utsu Scaling Calculator

  Utsu aftershock-zone scaling: log₁₀L(km) = 0.5·M − 1.8, with equivalent area.