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ICMS amplification factors

The amplification factors quantify in a synthetic way how much the site amplifies seismic shaking with respect to the reference bedrock. They are the main output of a Level 3 seismic microzonation per the Italian Indirizzi e Criteri per la Microzonazione Sismica (ICMS 2008/2018, Department of Civil Protection).

The 3 factors

ICMS requires three factors computed over specific period intervals:

Factor Interval Meaning
FA 0.1 – 0.5 s Short periods — stiff structures (buildings up to 5 storeys)
FH 0.4 – 0.8 s Medium-long periods — 5-10 storey buildings
FT all Full spectrum (a summary scalar)

Each factor is the ratio of the integral of the site response spectrum (RSL output) and the integral of the NTC bedrock spectrum (cat. A) over the specific period interval:

\[ F = \frac{\int_{T_1}^{T_2} \text{PSA}_{\text{site}}(T) \, dT}{\int_{T_1}^{T_2} \text{PSA}_{\text{bedrock}}(T) \, dT} \]

(simplified formulation — the ICMS version uses integration over discrete PSA values at the step specified by the standard).

Interpretation

  • F = 1.0 → the site behaves like a standard bedrock (no amplification)
  • F = 1.0–1.5 → modest amplification, common
  • F = 1.5–2.0 → significant amplification, sensitive site
  • F > 2.0 → very high amplification, problematic soil (sedimentary basins, thick soft soils)

The F values are the main input for municipal seismic microzonation maps and for urban planning decisions (unstable zones, stable amplified zones, stable non-amplified zones).

Which factor to use for what

For building design

It depends on the fundamental period of the building T₁:

  • Low-rise buildings (1-3 storeys, T₁ ≈ 0.1-0.3 s): use FA
  • Medium-rise buildings (4-7 storeys, T₁ ≈ 0.4-0.7 s): use FH
  • Tall buildings (10+ storeys, T₁ > 1 s): use the RSL spectrum directly, because standard ICMS does not cover long periods

For municipal microzonation

You produce FA maps and FH maps overlaid on the municipality's geological cartography. Areas with the same F range are homogeneous seismic micro-zones.

For vulnerability studies

FT (single scalar) is the fastest way to compare different stratigraphy-input combinations.

ICMS Level 3 procedure in practice

  1. Identify the representative geological sections of the municipality (typically 3-10 sections)
  2. Define the stratigraphy along each section (Vs from MASW + geognostic boreholes)
  3. Choose 7 accelerograms spectrum-compatible with the area's NTC
  4. For each significant stratigraphic column, run RSL III in multi-input mode with the 7 accelerograms
  5. Compute FA, FH, FT as the mean over the 7 accelerograms
  6. Create the thematic maps by overlaying the F values on the micro-areas
  7. Publish the microzonation report following the ICMS structure

RSL III includes automatic computation of FA, FH, FT — see the Results tab → Amplification factors pane.

RSL III output for ICMS

The Word report automatically includes:

  • Table of the 3 factors for each input accelerogram
  • Statistics (mean, standard deviation) in multi-input mode
  • Mean PSA plot + max/min envelope
  • Comparison with the standard NTC spectrum (cat. A) → reference line

For a complete microzonation project, also export:

  • PSA spectrum CSV point-by-point (for GIS cartography)
  • γ, a, σ vs depth profiles (for the technical report)

Reference standards

  • ICMS 2008/2018 — Indirizzi e Criteri per la Microzonazione Sismica, Italian Department of Civil Protection (DPC)
  • NTC 2018 — D.M. 17/01/2018, chapter 7.11.3 (seismic action)
  • EC8 — EN 1998-5 — Eurocode 8, Annex F (local effects)
  • OPCM 3274/2003 and 3519/2006 — establishment of mandatory seismic microzonation in Italian municipalities

See also

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