Alfaro et al. (1990)

Ground-motion model for near field is: $1 log(A) = a1 + a2Ms - log(r2 + a23)2$
where A is in g, a₁ = -1.116, a₂ = 0.312, a₃ = 7.9 and σ = 0.21.
Ground-motion model for far field is:
$2 21 log(A) = b1 + b2Ms + b3 log(r + b4)2$
where A is in g, b₁ = -1.638, b₂ = 0.438, b₃ = -1.181, b₄ = 70.0 and σ = 0.21.
Separate crustal and subduction data because of differences in travel path and stress conditions:
1.
Near field
2.
Far field, 20 records from San Salvador, 20 earthquakes, 4.2 ≤ M_s ≤ 7.2, depths between 36 and 94km, 31 ≤ r ≤ 298km.