- Ground-motion model is:
where y is in cm∕s2, b1 = 1.59±0.69, b2 = 0.25±0.03, b3 = -0.79±0.12, α = 0 and σ = 0.25 for horizontal
PGA and b1 = 1.38 ± 1.89, b2 = 0.24 ± 0.09, b3 = -0.78 ± 0.25 and σ = 0.25 for vertical PGA.
- Use two site classes:
- Includes alluvium and moraine deposits of varying thicknesses and characteristics.
- Includes limestone, dolomite, flysch and cemented conglomerates, even if heavily fractured, overlain
by not more than 4–5m of alluvium.
Use published and unpublished material for classification.
- Focal depths between 6 and 11km.
- Use data from Friuli 1976 mainshock and subsequent earthquakes from four networks including temporary
stations (ENEL, CNEN, IZIIS and CEA/DSN). Data from ENEL, CNEN and IZIIS from RFT-250 and
SMA-1 instruments and data from CEA/DSN from short-period seismographs. Some records not available
in digital form so used reported PGAs.
- Almost all records from free-field stations.
- 58 PGAs from rhypo ≤ 20km.
- 13cm∕s2 ≤ PGA ≤ 515cm∕s with 93% above 30cm∕s2.
- Best-recorded earthquake (mainshock) contributed 24 PGAs.
- One station contributed 17 PGAs.
- Also regresses just using data from mainshock.
- α is either 0 or 25 in regression. Prefer results with α = 0 because smaller standard errors in b3.
- Statistical tests show b2 and b3 are significantly different than 0.
- Also present coefficients for rock-like stations only and soil stations only. Find that effect of selection by
site class does not greatly affect coefficients.
- Process a smaller set of records available in digitized form (76 horizontal components) using high-pass filter
(cut-off and roll-off of 0.4–0.8Hz) based on digitization noise. Note difficulty in standard processing due to
high-frequency content and short durations. Use sampling rate of 100Hz. Find that corrected horizontal
PGAs are on average 6% lower than uncorrected PGAs and 15% show difference larger than 10%. For
vertical PGAs average difference is 12%. Develop equations based on this subset (for horizontal PGA
b1 = 1.51±0.77, b2 = 0.24±0.04, b3 = 0.70±0.21 and σ = 0.24). Note similarity to results for uncorrected
- Also derive equation using only 39 PGAs from rhypo ≤ 20km and note weak magnitude and distance
dependence. Compare to data from shallow soil sites at Forgaria-Cornino and Breginj and note that local
site conditions can significantly modify bedrock motions even at close distances.