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.

MMI

Required number. Modified Mercalli intensity (may be fractional).

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

Wald et al. (1999) MMI-to-PGA bounds used in ShakeMap ground-motion to intensity conversion. Inputs are interpreted exactly in the displayed units and the calculation returns the following fields without presentation rounding.

PGA lo (g)

Returned number in g. Lower PGA bound for this intensity bin.

PGA hi (g)

Returned number in g. Upper PGA bound.

Descriptor

Returned string. Qualitative shaking.

• 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.
• MMI must be at least 1.
• MMI must be no greater than 10.
• 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.

• ShakeMap Instrument Intensity PGV Converter

  Convert PGV to instrumental intensity with the Wald et al. (1999) ShakeMap relation MMI = 3.51·log₁₀(PGV in cm/s) + 2.35, clamped to the displayed MMI range 1–10.

• PGV PGA Correlation Approximation Calculator

  Approximate PGV from PGA by treating the motion as a sinusoid with effective period T_ref: PGV ≈ PGA·T_ref/(2π); this is a kinematic estimate, not an empirical ground-motion conversion.

• Arias Intensity Duration Proxy Calculator

  Arias intensity proxy I_a ≈ (π/2g)·∫a² dt approximated by PGA² times effective duration.