  Tectonic faults slip in various manners, ranging from ordinary earthquakes to
slow slip events to aseismic fault creep. The frequent occurrence of slow
earthquakes and their sensitivity to stress make them a promising probe of the
neighboring locked zone where megaquakes take place. This relationship,
however, remains poorly understood. We show that the Cascadia megathrust is
continuously broadcasting a tremor-like signal that precisely informs of fault
displacement rate throughout the slow slip cycle. We posit that this signal
provides indirect, real-time access to physical properties of the megathrust
and may ultimately reveal a connection between slow slip and megaquakes.
