We compare the spatiotemporal progression, geometry, and earthquake source characteristics of a zone of anomalous swarm seismicity between the Maacama and Bartlett Springs faults (California, United States) within the northern San Andreas fault system to both laboratory studies of fracture initiation and structural field observations of fault formation. The similarities between laboratory and field studies of incipient faulting and the earthquake swarms suggest that the seismic lineament on which the swarms occur is a fault in an early stage of development. Kinematic descriptions of faulting and models of fault system development suggest that the ability of existing faults to accommodate deformation across plate boundaries is governed by the length scales over which they accommodate stress. Many of the characteristics of juvenile fault zones, such as segmentation, geometric complexity, and the depth extent of faulting, act to reduce this length scale; this requires the reactivation of existing faults or production of accessory faults to accommodate deformation across the plate boundary.

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