  We report seismic signals on a desert playa caused by convective vortices and
dust devils. The long-period (10-100s) signatures, with tilts of ~10$^{-7}$
radians, are correlated with the presence of vortices, detected with nearby
sensors as sharp temporary pressure drops (0.2-1 mbar) and solar obscuration by
dust. We show that the shape and amplitude of the signals, manifesting
primarily as horizontal accelerations, can be modeled approximately with a
simple quasi-static point-load model of the negative pressure field associated
with the vortices acting on the ground as an elastic half space. We suggest the
load imposed by a dust devil of diameter D and core pressure {\Delta}Po is
~({\pi}/2){\Delta}PoD$^2$, or for a typical terrestrial devil of 5 m diameter
and 2 mbar, about the weight of a small car. The tilt depends on the inverse
square of distance, and on the elastic properties of the ground, and the large
signals we observe are in part due to the relatively soft playa sediment and
the shallow installation of the instrument. Ground tilt may be a particularly
sensitive means of detecting dust devils. The simple point-load model fails for
large dust devils at short ranges, but more elaborate models incorporating the
work of Sorrells (1971) may explain some of the more complex features in such
cases, taking the vortex winds and ground velocity into account. We discuss
some implications for the InSight mission to Mars.
