Continuous GPS and seismic data from northern California show that slow earthquakes periodically rupture the Gorda-North America plate interface within southern Cascadia. On average, these creep events have occurred every 10.9 ¿ 1.2 months since at least 1998. Appearing as week-long GPS extensional transients that reverse secular forearc contraction, the data show a recurrence interval 22% shorter than slow events recognized to the north. Seismic tremor here accompanies the GPS reversals, correlated across as many as 5 northern California seismometers. Tremor occurs sporadically throughout the year, but increases in duration and intensity by a factor of about 10 simultaneous with the GPS reversals. Beneath west-central Oregon, three reversals are also apparent, but more stations are needed to confirm sporadic slip on the plate interface here. Together, these measurements suggest that slow earthquakes likely occur throughout the Cascadia subduction zone and add further evidence for the role of fault-fluid migration in controlling transient slow-slip events here.