$f1 = 1.45 + ([Mw] > 5.5 ? 0.31 : 1) * ([Mw] - 5.5);
$f2 = [dh:km] > 27.5 ? 27.5 * pow([dh:km] / 27.5, 0.7) : [dh:km];
$f3 = 0.0073 * ([Mw] > 5.5 ? pow(10, -0.3 * ([Mw] - 5.5)) : 1);
$amp = [
  "A" => 0.79,
  "B" => 0.79,
  "C" => 1.00,
  "D" => 1.35,
  "E" => 1.64
];
return pow(10, ($f1 - log10($f2) - $f3 * [dh:km] + log10($amp[[NEHRP]])))/9.81;

• Boatwright, J.; Bundock, H.; Luetgert, J.; Seekins, L.; Gee, L.; Lombard, P., "The dependence of PGA and PGV on distance and magnitude inferred from Northern California ShakeMap data", 2003
• Douglas, John , "Ground motion prediction equations 1964 - 2021", 2022

1. See Douglas (2022), page 138.
https://rapidn.jrc.ec.europa.eu/reference/152?__process=9880
2. See Boatwright et al. (2003), graph in Figure 7
https://rapidn.jrc.ec.europa.eu/reference/2?__data=4295
Validation:
M = 5.0, d = 10 km, NEHRP = "C" : PGA = 0.077 g
M = 6.0, d = 10 km, NEHRP = "C" : PGA = 0.364 g
M = 5.0, d = 100 km, NEHRP = "C" : PGA = 0.002 g