  It was recently discovered that the time correlations of repeating fast radio
bursts (FRBs) are similar to earthquake aftershocks. Motivated by the
association between FRBs and magnetars, here we report correlation function
analyses in the time-energy space for the 563 periodic radio pulses and the 579
X-ray short bursts from the magnetar SGR 1935+2154, which is known to have
generated FRBs. Although radio pulses are concentrated near the fixed phase of
the rotational cycle, we find that when multiple pulses occur within a single
cycle, their correlation properties (aftershock production probability,
aftershock rate decaying in power of time, and more) are similar to those of
extragalactic FRBs and earthquakes. A possible interpretation is that the radio
pulses are produced by rupture of the neutron star crust, and the first pulse
within one cycle is triggered by external force periodically exerted on the
crust. The periodic external force may be from the interaction of the
magnetosphere with material ejected in an outburst. For X-ray bursts, we found
no significant correlation signal, though correlation on the same time scale as
radio pulses may be hidden due to the long event duration. The aftershock
similarity between the periodic radio pulsation and FRBs is surprising, given
that the two are energetically very different, and therefore the energy sources
would be different. This suggests that the essence of FRB-like phenomena is
starquakes, regardless of the energy source, and it is important to search for
FRB-like bursts from neutron stars with various properties or environments.
