  Incommensurability plays a critical role in many strongly correlated systems.
In some cases, the origin of such exotic order can be theoretically understood
in the framework of 1d line-like topological excitations known as ``quantum
strings''. Here we study an extended transverse field Ising model on a
triangular lattice. Using the large scale quantum Monte Carlo simulations, we
find that the spatial anisotropy can stabilize an incommensurate phase out of
the commensurate clock order. Our results for the structure factor and the
string density exhibit a linear relationship between incommensurate ordering
wave vector and the density of quantum strings, which is reminiscent of hole
density in under-doped cuprate superconductors. When introducing the
next-nearest-neighbor interaction, we observe a quantum tricritical point out
of the incommensurate phase. After carefully analyzing the ground state
energies within different string topological sectors, we conclude that this
tricriticality is non-trivially caused by effective long-range inter-string
interactions with two competing terms following different decaying behaviors.
