$q1=4.31964;
$q2=-0.000175;
$q3=-2.40199;
$q4=0.19029;
$q5=2.14088;
$S=[Vs30:m/s]>800?0:0.09754;
$q7=-0.21015;
$q8=0.38884;
$F=[FM]=='S'?0:([FM]=='T'?1:0.5);
$q9=-2.29732;
$q10=448.8836;

return exp($q1 + $q2*[Mw] + ($q3 + $q4*[Mw])*log([dr:km] + exp($q5)) + $S + $q7*pow([Mw]- 6,2) + $q8*$F+ $q9/tanh([dr:km] + $q10));

• Gregor, N.; Silva, W.; Darragh, B., "Development of attenuation relations for peak particle velocity and displacement", 2002
• Douglas, J., "Ground-motion prediction equations 1964-2010", 2011

See Douglas (2011), page 133.