  Most earthquake ruptures propagate at speeds below the shear wave velocity
within the crust, but in some rare cases, ruptures reach supershear speeds. The
physics underlying the transition of natural subshear earthquakes to supershear
ones is currently not fully understood. Most observational studies of
supershear earthquakes have focused on determining which fault segments sustain
fully-grown supershear ruptures. Experimentally cross-validated numerical
models have identified some of the key ingredients required to trigger a
transition to supershear speed. However, the conditions for such a transition
in nature are still unclear, including the precise location of this transition.
In this work, we provide theoretical and numerical insights to identify the
precise location of such a transition in nature. We use fracture mechanics
arguments with multiple numerical models to identify the signature of
supershear transition in coseismic off-fault damage. We then cross-validate
this signature with high-resolution observations of fault zone width and early
aftershock distributions. We confirm that the location of the transition from
subshear to supershear speed is characterized by a decrease in the width of the
coseismic off-fault damage zone. We thus help refine the precise location of
such a transition for natural supershear earthquakes.
