The Brawley Seismic Zone is a band of intense earthquake activity in the Imperial Valley, connecting the southern end of the San Andreas Fault near the Salton Sea to the northern end of the Imperial Fault. Rather than a single fault, it is a zone of extension where the crust is pulling apart along a young, landlocked spreading centre.
This zone behaves like a small continental version of a mid-ocean ridge, with thin, hot crust, high geothermal activity and swarms of small to moderate quakes. Numerous short faults step sideways here, transferring plate motion between the two great strike-slip systems.
The zone regularly produces earthquake swarms, including sequences in 1981 and 2012 with events up to around magnitude ~5.5. Its restless behaviour is closely watched because a large rupture here could potentially load and trigger the nearby southern San Andreas Fault.