California's major faults are smooth at the depths where earthquakes occur

The precise location of earthquakes shows faults with smooth, flat or arched surfaces, on scales ranging from a few hundred metres to a few tens of kilometres, at the seismogenic depth. This regularity can play a crucial role in the genesis of major earthquakes, and can transform our understanding of the physics of rupture and seismic risk.

The physical behaviour of faults, and the resulting seismic risks, depend very much on whether they are rough or smooth at the depth at which energy is released during earthquakes. At this depth of around 4-15 km in California, the location of earthquakes has suggested that faults are irregular at scales greater than a kilometre. In addition, the faults mapped on the surface are also generally complex, with offsets at all scales. This leads to the assumption that major faults are very rough at depth, so the rupture of a major earthquake would be like trying to slide two egg cartons along their bumpy sides.

The authors, including a CNRS-INSU researcher, apply a new earthquake location procedure to large earthquake sequences and microseismicity along strike-slip faults in California. This multi-scale method makes it possible to correct for certain distortion effects and the relocation of earthquakes reveals that fault surfaces are smooth at depth, flat or arched on scales ranging from a few hundred metres to a few tens of kilometres. Scientists are therefore demonstrating that seismic rupture is more like egg cartons sliding on their smooth sides, and this has obvious consequences. The presence at depth of smooth surfaces on several scales in major strike-slip fault zones can influence the initiation, rupture, direction and cessation of seismic ruptures, and these smooth faults may even be necessary for large earthquakes to occur. These results can help map the seismic hazard and reinforce recent work on surface ruptures. This work shows that surface ruptures largely reflect shallow and often complex secondary deformation, rather than seismically active slip surfaces at depth.