Discussion
Multiple datasets and results derived from various inversion methods
show that the 2021 Maduo earthquake ruptured simultaneously on the two
branches of the fault with a branching angle of ~20°.
However, numerical simulations suggest that simultaneous rupturing on
both fault branches is usually difficult where the branching angle is
narrow, due to strong stress interactions between faults [Aochi
and Fukuyama , 2002; Kame et al. , 2003]. The dynamic stress
interaction (or dynamic rupture weakening) plays an important role
during earthquake ruptures. Given the stress shadow imposed by the
rupture on the other fault branch, rupture through a fault bifurcation
that has a narrow branching angle is most likely be simultaneous.
Suppose that one of the fault branches ruptures earlier; then the
Coulomb stress change on the other fault segment will prohibit its
rupture. Only when the ruptures propagate simultaneously through the two
fault branches (e.g., Fig.5c), back and forth slip of the fault segments
could be avoided. Because the two sides of the wedge block have opposite
slip directions (Fig.5c), the final dislocation of this block is smaller
than that to the north and south of the block (e.g., Fig.1a inset).
Numerical simulations also suggest that a simultaneous rupture is
promoted when the friction coefficients are low [Aochi et al. ,
2002]. However, we observe more high-frequency radiation from the
bifurcated fault branches, which indicates rougher fault friction. More
high-frequency energy released on the branched faults could also be an
indication of more energy dissipation, which could eventually lead to
the termination of the rupture. Also note, that the bifurcated fault
branches are located within slightly higher mountain ranges, suggesting
the rupture could have propagated from one geological unit to another
[Ren et al. , 2022], which could also be a reason for
different fault friction. The rupture speed of the Maduo earthquake was
quite stable around 2.5 km/s, which is in contrast with supershear
rupture speed derived from the other BP analysis [Yue et al. ,
2022; Zhang et al. , 2022]. The primarily reason for this
discrepancy could be that we applied travel time path calibrations (see
more details in [Zeng et al. , 2022]) but the other BP studies
did not. The rupture did not slow down at the bends and step-overs. This
is because all bends/step-overs are releasing bends/step-overs,
therefore normal stress decreased when rupture propagated through them,
and facilitating the rupture pass-through. Fault bifurcations with
narrow branching angle are quite common for strike-slip earthquakes
(Fig.S16). The number of fault segments and fault segment length of the
Maduo earthquake, including the bifurcated branches, fit well with
statistics from [Klinger , 2010]. We therefore suggest that
such simultaneous ruptures of the bifurcated fault branches could be
common during strike-slip earthquakes. Dynamic simulations have to take
into consideration complex fault geometry, specific stress conditions,
and frictional heterogeneities to properly estimate the relationship
between rupture through multiple fault segments.