Shakingmap does not match the seismograms (PGA)

Hi everyone,

Recently I found a strange problem with specfem3D’s output.
I slice a profile across the shaking map to obtain the PGA PGV and PGD values,
and compare with seismogram array’s result (I calculate PGA PGV PGD from waveforms).
Then I found the PGA does not match (shaking map PGA larger than seismogram’s PGA). But PGV and PGD are fitting well.
This un-match situation only occurs when I use a source that has half duration 0.6s.
For a source that has half duration 0.05s, then the shaking map all fits with seismograms. (I use an isotropic source)

Does anyone know why the shaking map (PGA) is different from the seismogram’s PGA?
(ps:1.PGV and PGD are correct; 2. only happen when using long half duration)

This problem really makes me very troubled.
I will be grateful for any help you can provide.