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.

M

Required number. Moment magnitude.

R (km)

Required number in km. Distance.

R₀ (km)

Optional number in km. Reference distance for saturation.

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

Simplified GMPE ground-motion attenuation with log-distance spreading. Inputs are interpreted exactly in the displayed units and the calculation returns the following fields without presentation rounding.

PGA (g)

Returned number in g. Peak ground acceleration estimate.

• 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.
• R 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.

• Felt Radius Heuristic Log Scaling Calculator

  User-parameterized log-linear felt-radius screen R = R₀·10^(kM). The default R₀=0.1 km and k=0.5 are illustrative and must be calibrated for regional attenuation, depth, and reporting conditions.

• Geometrical Spreading Body Wave Calculator

  Body-wave geometrical spreading amplitude decays as 1/r relative to a reference distance.

• Intensity PGA Wald 1999 Table Lookup

  Wald et al. (1999) MMI-to-PGA bounds used in ShakeMap ground-motion to intensity conversion.