In seismology, the rupture mechanism of an earthquake, a glacier
stick-slip and a landslide is not directly observed, but inferred from
surface measurements. In contrast, laboratory experiments can illuminate
near field effects, which reflect the rupture mechanism but are highly
attenuated in the case of real-world surface data. We directly image the
elastic wave-field of a nucleating rupture non-invasively in its
near-field with ultrasound speckle correlation. Our imaging yields the
particle velocity of the full shear wave field at the source location
and inside the 3D frictional body. We experimentally show that a strong
bimaterial contrast, as encountered in environmental seismology, yields
a unidirectional or linear force mechanism for pre-rupture microslips
and decelerating supershear ruptures. A weak contrast, characteristic
for earthquakes, generates a double-couple source mechanism for
sub-Rayleigh ruptures, sometimes preceded by slow deformation at the
interface. This deformation is reproduced by the near field of a
unidirectional force.