  Double neutron star (DNS) merger events are promosing candidates of short
Gamma-ray Burst (sGRB) progenitors as well as high-frequecy gravitational wave
(GW) emitters. On August 17, 2017, such a coinciding event was detected by both
the LIGO-Virgo gravitational wave detector network as GW170817 and Gamma-Ray
Monitor on board NASA's {\it Fermi} Space Telescope as GRB 170817A. Here we
show that the fluence and spectral peak energy of this sGRB fall into the lower
portion of the distributions of known sGRBs. Its peak isotropic luminosity is
abnormally low. The estimated event rate density above this luminosity is at
least $190^{+440}_{-160} {\rm Gpc^{-3} \ yr^{-1}}$, which is close to but still
below the DNS merger event rate density. This event likely originates from a
structured jet viewed from a large viewing angle. There are similar faint soft
GRBs in the {\it Fermi} archival data, a small fraction of which might belong
to this new population of nearby, low-luminosity sGRBs.
