Calculator Form
Example Data Table
| Method | Sample Inputs | Estimated MMI | Roman Class | Typical Interpretation |
|---|---|---|---|---|
| Direct PGA | 0.12 g, Rock, No Basin | 6.1 | VI | Strong shaking with moderate nonstructural concern. |
| Direct PGV | 12 cm/s, Stiff Soil | 6.9 | VII | Very strong shaking and elevated damage screening. |
| Combined | 0.18 g and 18 cm/s, Soft Soil | 7.6 | VIII | Severe local shaking with urgent inspection priority. |
| Magnitude + Distance | M6.8, 25 km, 12 km depth, Soft Soil | 7.1 | VII | Very strong shaking from screening attenuation inputs. |
These rows are illustrative examples for layout and interpretation.
Formula Used
This tool uses screening-style seismic intensity equations for rapid engineering comparison. It is best for preliminary review, not final structural design.
1) Direct PGA Equation
MMI = 2.20 + 3.30 × log10(PGA + 0.10)
PGA is converted to cm/s² before evaluation. Site, basin, and topographic multipliers adjust local shaking demand.
2) Direct PGV Equation
MMI = 3.40 + 3.10 × log10(PGV + 0.05)
PGV is converted to cm/s before evaluation. The same local modifiers are applied for consistency.
3) Combined Equation
MMIcombined = 0.55 × MMIPGA + 0.45 × MMIPGV
This option blends acceleration and velocity effects for a stronger screening estimate when both measurements exist.
4) Magnitude and Distance Screening Equation
PGAg = 10(0.30M − 1.10log10(Reff) − 0.0015D − 1.35) × Fsite × Fbasin × Ftopo × Ffault
Reff = √(R² + D²) + 10
M is moment magnitude, R is epicentral distance, D is focal depth, and the multipliers represent local amplification and source style adjustments.
5) Facility Concern Index
Concern Index = min[100, (MMI / 12) × 100 × Facility Factor]
This score helps compare urgency across occupancy types such as hospitals, bridges, schools, and homes.
How to Use This Calculator
- Select the calculation method that matches your available seismic data.
- Enter PGA, PGV, or magnitude-distance values as needed.
- Choose site condition, basin effect, and topographic amplification.
- Select the facility type for engineering response prioritization.
- Click Calculate Intensity to show the result above the form.
- Review numeric MMI, Roman class, likely effects, and concern index.
- Use the CSV and PDF buttons to export the latest computed result.
FAQs
1) What is the difference between magnitude and intensity?
Earthquake magnitude measures released energy at the source. Intensity estimates the shaking and observed effects at a location. This calculator focuses on location-specific shaking severity.
2) Why use both PGA and PGV?
PGA often tracks short-period force demand, while PGV can reflect longer-duration motion and potential damage trends. Using both usually improves screening consistency.
3) Can this replace structural design checks?
No. It provides rapid screening estimates. Final design or post-event decisions should rely on site-specific hazard studies, instrument records, and qualified structural engineering review.
4) How does soil affect intensity?
Soft soil can amplify shaking, lengthen motion, and raise local intensity estimates. Hard rock usually transmits lower amplification for the same source motion.
5) When should I use the magnitude-distance method?
Magnitude and distance give a useful first-pass estimate when instrument data is unavailable. Direct PGA or PGV inputs are usually better when measured values exist.
6) Why are Roman numerals shown?
Roman numerals convert the numeric intensity estimate into conventional Modified Mercalli style classes, making reports easier to compare and communicate.
7) What do the export buttons save?
The export tools capture the latest computed result. Recalculate after changing inputs so the downloaded file matches your current scenario.
8) Does the calculator support multiple units?
Yes. The form accepts metric and common seismic units for PGA and PGV. Internal conversions standardize values before calculating intensity.
Engineering Note
Use this page for rapid screening, early comparisons, and reporting support. For life-safety decisions, use site-specific spectra, code checks, recorded motions, and qualified seismic review.