$a1 = -1.296;
$a2 =  0.556;
$a3 = -1.582;
if ([Vs30:m/s] >= 800) {
  $S = 0;
}
else if ([Vs30:m/s] >= 360) {
  $S = 0.22;
}
else if ([Vs30:m/s] >= 180) {
  $S = 0.304;
}
else {
  $S = 0.332;
}
return pow(10 , $a1 + $a2 * [Mw] + $a3 * log10([dh:km]) + $S);

• Douglas, J., "Ground-motion prediction equations 1964-2010", 2011
• Cauzzi, C.; Faccioli, E., "Broadband (0.05 to 20 s) prediction of displacement response spectra based on worldwide digital records", 2008
• Cauzzi, C.; Faccioli, E.; Paolucci, R.; Villani, M., "Long-period ground motion evaluation from a large worldwide digital strong motion database", 2008
• Cauzzi, C. V., "Broadband empirical prediction of displacement response spectra based on worldwide digital records", 2008

See Douglas (2011), page 226.

Validation:

• M = 5.0, d = 10 km, Vs30 = 800 m/s (Site Category A, Eurocode 8): PGA = 0.79799468726798 m/s2
• M = 6.0, d = 10 km, Vs30 = 800 m/s (Site Category A, Eurocode 8): PGA = 2.87078058202470 m/s2
• M = 5.0, d = 100 km, Vs30 = 800 m/s (Site Category A, Eurocode 8): PGA = 0.02089296130854 m/s2