Introduction to the Special Section on the 2019 Ridgecrest, California, Earthquake Sequence

Following the 16 October 1999 M_w 7.1 Hector Mine earthquake, no earthquake larger than magnitude 6.0 occurred in southern California for nearly 20 yr. That lull came to an end with the occurrence of the 4 July 2019 M_w 6.4 earthquake at 22:40 UTC (10:33 a.m. local time). Located in a relatively sparsely populated part of the state, with most of its rupture within the China Lake Naval Air Weapons Station (NAWSCL) adjacent to the civilian town of Ridgecrest (population: 26,000), the earthquake was felt widely across most of southern California. Immediate response efforts were mounted to investigate the rupture and assess damage (Stewart et al., 2019) and to calculate aftershock probabilities (see Hardebeck, 2020; also see Data and Resources). The earthquake occurred in a region historically characterized by higher than average aftershock productivity (Hardebeck et al., 2019), leading to a higher‐than‐average probabilities for both large aftershocks and subsequent larger events. The latter odds, although still small, were borne out with the occurrence of 6 July M_w 7.1 earthquake at 03:19:53 UTC (8:19:53 p.m. local time on 5 July) ∼34 hr after the initial M_w 6.4. Arguably, the M_w 7.1 could be considered an aftershock, but, by convention, this earthquake is regarded as the mainshock, whereas the initial M_w 6.4 is considered a foreshock