  Photoluminescence polarization is experimentally studied for samples with
(In,Ga)As/GaAs selfassembled quantum dots in transverse magnetic field (Hanle
effect) under slow modulation of the excitation light polarization from
fractions of Hz to tens of kHz. The polarization reflects the evolution of
strongly coupled electron-nuclear spin system in the quantum dots. Strong
modification of the Hanle curves under variation of the modulation period is
attributed to the peculiarities of the spin dynamics of quadrupole nuclei,
which states are split due to deformation of the crystal lattice in the quantum
dots. Analysis of the Hanle curves is fulfilled in the framework of a
phenomenological model considering a separate dynamics of a nuclear field BNd
determined by the +/- 12 nuclear spin states and of a nuclear field BNq
determined by the split-off states +/- 3/2, +/- 5/2, etc. It is found that the
characteristic relaxation time for the nuclear field BNd is of order of 0.5 s,
while the relaxation of the field BNq is faster by three orders of magnitude.
