Broadband surface waves from large earthquakes can be rapidly processed to estimate seismic moment, faulting duration, and general slip distribution, supplying important information for hazard response efforts, including tsunami warnings. Deconvolution of surface-wave Green's functions removes propagation effects, yielding azimuthally varying effective source time functions. Even a single fault-perpendicular time function can provide robust first-order estimates of the temporal history of seismic radiation and associated slip distribution. Simplified two-dimensional finite-fault modeling using a few surface-wave time functions can reliably characterize the smooth component of the overall rupture process. These procedures are demonstrated using 13 Rayleigh wave observations for the 26 December 2004 Sumatra-Andaman earthquake, yielding satisfactory agreement with models based on more complete seismic data sets. Depending on source and station locations, stable slip estimates can be obtained within 15--60 minutes of the onset of a large earthquake.