  Capable of reaching similar magnitudes to large megathrust earthquakes
($M_w>7$), slow slip events play a major role in accommodating tectonic motion
on plate boundaries. These slip transients are the slow release of built-up
tectonic stress that are geodetically imaged as a predominantly aseismic
rupture, which is smooth in both time and space. We demonstrate here that large
slow slip events are in fact a cluster of short-duration slow transients. Using
a dense catalog of low-frequency earthquakes as a guide, we investigate the
$M_w7.5$ slow slip event that occurred in 2006 along the subduction interface
40~km beneath Guerrero, Mexico. We show that while the long-period surface
displacement as recorded by GPS suggests a six month duration, motion in the
direction of tectonic release only sporadically occurs over 55 days and its
surface signature is attenuated by rapid relocking of the plate interface.These
results demonstrate that our current conceptual model of slow and continuous
rupture is an artifact of low-resolution geodetic observations of a
superposition of small, clustered slip events. Our proposed description of slow
slip as a cluster of slow transients implies that we systematically
overestimate the duration $T$ and underestimate the moment magnitude $M$ of
large slow slip events.
