Slip, Tripped, and Faulted: Earthquake risk assessment can be improved if we were able to quantify the recurrence and magnitude of slip events. Until recently though, a lack of sophisticated seismometers has forced us to rely on anecdotal evidence from those who survived major earthquakes or to look for clues in the landscape. Zielke et al. (p. 1119 , published online 21 January; see the Perspective by Scharer ) analyzed high-resolution images of the San Andreas Fault in southern California. The data showed that major surface ruptures, such as the 1857 Fort Tejon earthquake, resulted from slips of only about 5 meters; much less than previously thought. In a study that lends support to this discovery, Grant Ludwig et al. (p. 1117 , published online 21 January; see the Perspective by Scharer ) suggest from analysis of the geomorphic features of this region that several smaller earthquakes have occurred during recent centuries rather than infrequent but larger movements. The Perspective by Scharer (p. 1089 ) discusses how paleoseismological studies like these may be valuable for feeding data into earthquake prediction.

Nowcasting is a term originating from economics and finance. It refers to the process of determining the uncertain state of the economy or markets at the current time by indirect means. We apply this idea to seismically active regions, where the goal is to determine the current state of the fault system and its current level of progress through the earthquake cycle. In our implementation of this idea, we use the global catalog of earthquakes, using “small” earthquakes to determine the level of hazard from “large” earthquakes in the region. Our method does not involve any model other than the idea of an earthquake cycle. Rather, we define a specific region and a specific large earthquake magnitude of interest, ensuring that we have enough data to span at least ~20 or more large earthquake cycles in the region. We then compute the earthquake potential score (EPS) which is defined as the cumulative probability distribution P(n < n(t)) for the current count n(t) for the small earthquakes in the region. From the count of small earthquakes since the last large earthquake, we determine the value of EPS = P(n < n(t)). EPS is therefore the current level of hazard and assigns a number between 0% and 100% to every region so defined, thus providing a unique measure. Physically, the EPS corresponds to an estimate of the level of progress through the earthquake cycle in the defined region at the current time.

This paper describes the methods used to estimate positions, velocities, breaks, and seasonal terms from daily Global Navigation Satellite System (GNSS) measurements. Break detection and outlier removal have been automated so that decades of daily measurements from thousands of stations can be processed in a few hours. New measurements are added, and parameters are updated every week. Model parameters allow separation of interseismic, annual, coseismic, and postseismic signals. Tools available through GeoGateway (http://geo-gateway.org) allow rapid visualization and analysis of these terms for results that can be subsetted in time or space. Results show highly variable and nonlinear motion for GPS stations in southern California. The variable motion is related to seasonal motions, distributed tectonic motion, earthquakes, and postseismic motions that can continue for years. In some areas results suggest that additional processes are responsible for the observed motions. In general, following earthquakes, stations return to their long‐term motions after 2–3 years, though some exceptions occur. The use of the tools shows nonlinear motion in the Salton Trough of southern California related to the 2010 M7.2 El Mayor‐Cucapah earthquake, 2012 Brawley earthquake swarm, and a creep event on the Superstition Hills fault in 2017.

Slip, Tripped, and Faulted: Earthquake risk assessment can be improved if we were able to quantify the recurrence and magnitude of slip events. Until recently though, a lack of sophisticated seismometers has forced us to rely on anecdotal evidence from those who survived major earthquakes or to look for clues in the landscape. Zielke et al. (p. 1119 , published online 21 January; see the Perspective by Scharer ) analyzed high-resolution images of the San Andreas Fault in southern California. The data showed that major surface ruptures, such as the 1857 Fort Tejon earthquake, resulted from slips of only about 5 meters; much less than previously thought. In a study that lends support to this discovery, Grant Ludwig et al. (p. 1117 , published online 21 January; see the Perspective by Scharer ) suggest from analysis of the geomorphic features of this region that several smaller earthquakes have occurred during recent centuries rather than infrequent but larger movements. The Perspective by Scharer (p. 1089 ) discusses how paleoseismological studies like these may be valuable for feeding data into earthquake prediction.