Birth of a fault: Connecting the Kern County and Walker Pass, California, earthquakes

A band of seismicity transects the southern Sierra Nevada range between the northeastern end of the site of the 1952 M_W (moment magnitude) 7.3 Kern County earthquake and the site of the 1946 M_W 6.1 Walker Pass earthquake. Relocated earthquakes in this band, which lacks a surface expression, better delineate the northeast-trending seismic lineament and resolve complex structure near the Walker Pass mainshock. Left-lateral earthquake focal planes are rotated counterclockwise from the strike of the seismic lineament, consistent with slip on shear fractures such as those observed in the early stages of fault development in laboratory experiments. We interpret this seismic lineament as a previously unrecognized, incipient, currently blind, strike-slip fault, a unique example of a newly forming structure.