Many ruptures across many scales
The period of seismic quiescence in Southern California was rudely interrupted by the Ridgecrest earthquake sequence in July 2019. Ross et al. mapped the slip sequences during the magnitude 6.4 and 7.1 earthquakes that shook the region. They found that ruptures of a few larger, but many smaller, faults occurred during both earthquakes. The Ridgecrest sequence calls for rethinking seismic hazard, as multifault ruptures are not usually considered when assessing seismic risk.
Science, this issue p. 346
A nearly 20-year hiatus in major seismic activity in southern California ended on 4 July 2019 with a sequence of intersecting earthquakes near the city of Ridgecrest, California. This sequence included a foreshock with a moment magnitude (Mw) of 6.4 followed by a Mw 7.1 mainshock nearly 34 hours later. Geodetic, seismic, and seismicity data provided an integrative view of this sequence, which ruptured an unmapped multiscale network of interlaced orthogonal faults. This complex fault geometry persists over the entire seismogenic depth range. The rupture of the mainshock terminated only a few kilometers from the major regional Garlock fault, triggering shallow creep and a substantial earthquake swarm. The repeated occurrence of multifault ruptures, as revealed by modern instrumentation and analysis techniques, poses a formidable challenge in quantifying regional seismic hazards.