  Aftershocks of the 2012 Off-Coast of Sumatra Earthquake Sequence exhibit a
complex and diffuse spatial distribution. The first-order complexity in
aftershock distribution is clear and well beyond the influence of typical
earthquake location uncertainty. The sequence included rupture of multiple
fault segments, spatially separated. We use surface-wave based relative
centroid locations to examine whether, at the small scale, the distribution of
the aftershocks was influenced by location errors. Surface-wave based relative
location has delineated precise oceanic transform fault earthquake locations in
multiple regions. However, the relocated aftershocks off the coast of Sumatra
seldom align along simple linear trends that are compatible with the
corresponding fault strikes as estimated for the GCMT catalog. The relocation
of roughly 60 moderate-earthquake epicentroids suggests that the faulting
involved in the 2012 earthquake aftershock sequence included strain release
along many short fault segments. Statistical analysis and temporal variations
of aftershocks show a typical decay of the aftershocks but a relatively low
number of aftershocks, as is common for intraplate oceanic earthquakes. Coulomb
stress calculations indicate that most of the moderate-magnitude aftershocks
are compatible with stress changes predicted by the large-event slip models.
The patterns in the aftershocks suggest that the formation of the boundary and
eventual localization of deformation between the Indian and Australian plate is
a complicated process.
