Major California faults are smooth across multiple scales at seismogenic depth

Surface traces of earthquake faults are complex and segmented on multiple scales. At seismogenic depth the detailed geometry of faults and earthquake rupture is mainly constrained by earthquake locations. Standard earthquake locations are usually too diffuse to constrain multi-scale fault geometry, while differential-timing relocation mainly improves finest scale precision. NLL-SSST-coherence, an enhanced, absolute-timing earthquake location procedure, iteratively generates traveltime corrections to improve multi-scale precision and uses waveform similarity to improve fine-scale precision. Here we apply NLL-SSST-coherence to large-earthquake sequences and background seismicity along strike-slip faults in California. Our relocated seismicity at seismogenic depth along major fault segments and around large-earthquake ruptures often defines smooth, planar or arcuate, near-vertical surfaces across the sub-km to 10’s of km scales. These results show that multi-scale smooth fault segments are characteristic of major, strike-slip fault zones and may be essential to large earthquake rupture. Our results suggest that smoothness and curvature of faults influences earthquake initiation, rupture, rupture direction and arrest, and can define earthquake hazard. The results corroborate that surface traces of strike-slip fault zones reflect complex, shallow deformation and not directly simpler, main slip surfaces at depth, and support use of planar or smoothly curved faults for modeling primary earthquake rupture.