  Multiple lines of evidence indicate that the 2023 Mw 7.8 Turkey earthquake
started on a splay fault, then branched bilaterally onto the nearby East
Anatolian Fault (EAF). This rupture pattern includes one feature deemed
implausible, called backward rupture branching: rupture propagating from the
splay fault onto the SW EAF segment through a sharp corner (with an acute angle
between the two faults). To understand this feature, we perform 2.5-D dynamic
rupture simulations considering a large set of possible scenarios. We find that
both subshear and supershear ruptures on the splay fault can trigger bilateral
ruptures on the EAF, which themselves can be either subshear, supershear, or a
mixture of the two. In most cases, rupture on the SW segment of the EAF starts
after rupture onset on its NE segment: the SW rupture is triggered by the NE
rupture. Only when the EAF has initial stresses very close to failure, its SW
segment can be directly triggered by the initial splay-fault rupture, earlier
than the activation of the NE segment. These results advance our understanding
of the mechanisms of multi-segment rupture and the complexity of rupture
processes, paving the way for a more accurate assessment of earthquake hazards.
