The intention is that the parameters _s and _t are being calculated in each timestep amidst the 30-second earthquake excitation.

; --- s-t ---

def _s_t

local pnt = zone_head

loop while pnt # null

global _s = abs(z_sig1(pnt)+z_sig3(pnt))/2

z_extra (pnt, 1) = _s

global _t = abs(z_sig1(pnt)-z_sig3(pnt))/2

z_extra (pnt, 2) = _t

pnt = z_next(pnt)

end_loop

end

@_s_t

so age 30