4.79 Abrahamson and Silva (1997)

• Ground-motion model is⁴:
  
  where is expected peak acceleration on rock as predicted by the attenuation equation with S = 0.
• Response parameter is acceleration for unspecified⁵ damping.
• Use two site categories:

  S = 0

  Rock: rock (V _s > 600m∕s), very thin soil (< 5m) over rock or soil 5 to 20m thick over rock.

  S = 1

  Deep soil: deep soil in narrow canyon (soil > 20m thick and canyon < 2km wide) or deep soil in broad canyon (soil > 20m thick and canyon > 2km wide).

• All records reprocessed using common procedure. Interpolated to 400 samples/sec, low-pass filtering with corner frequency selected for each record based on visual examination of Fourier amplitude spectrum, instrument corrected, decimated to 100 to 200 samples/sec depending on low-pass corner frequency, baseline correction using 0 to 10 degree polynomial, high-pass filtered based on integrated displacements.
• Only use response spectral data within frequency band 1.25f_h to 0.8f_l to avoid effects of filter roll-off. Hence number of records used for regression at each period varies, minimum number is less than 100 records for 0.01s.
• Well distributed dataset in terms of magnitude and distance.
• Supplement data with records from Gazli, Friuli, Tabas, Taiwan, Nahanni and Spitak.
• Consider source mechanism: reverse ⇒ F = 1, reverse/oblique ⇒ F = 0.5, others (strike-slip and normal) ⇒ F = 0).
• Consider hanging wall effect: if over hanging wall HW = 1, otherwise HW = 0.
• Note that interpretation of c₄ is not clear for their distance measure but yields better fit.
• Model nonlinear soil response by f₅.
• Model uncertainty as magnitude dependent.
• Fix some coefficients to be independent of period so that response spectral values vary smoothly with distance, magnitude and period.
• Smooth coefficients using piecewise continuous linear fits on log period axis. For highly correlated coefficients, smooth one coefficient and re-estimate other coefficients.