NASA - National Aeronautics and Space Administration

Shallow creep events provide opportunities to understand the mechanical properties and behavior of faults. However, due to physical limitations observing creep events, the precise spatio‐temporal evolution of slip during creep events is not well understood. In 2023, the Superstition Hills and Imperial faults in California each experienced centimeter‐scale slip events that were captured in unprecedented detail by satellite radar, sub‐ daily Global Navigation Satellite Systems, and creepmeters. In both cases, the slip propagated along the fault over 2–3 weeks. The Superstition Hills event propagated bilaterally away from its initiation point at average velocities of ∼9 km/day, but propagation velocities were locally much higher. The ruptures were consistent with slip from tens of meters to ∼2 km depths. These slowly propagating events reveal that the shallow crust of the Imperial Valley does not obey purely velocity‐strengthening or velocity‐weakening rate‐and‐state friction, but instead requires the consideration of fault heterogeneity or fault‐frictional behaviors such as dilatant strengthening. Plain Language Summary Faults that slip in a slow, aseismic process called creep present an opportunity to understand the frictional behavior of fault systems. In the spring of 2023, two fault systems in southern California experienced large slip events that were recorded in high resolution by ground‐based and space‐based measurements from GPS and satellite radar. The slip began spontaneously on both the Superstition Hills and Imperial faults and slowly propagated to other parts of each fault. The average slip propagation speed ranged from 0.4 to 9 km per day. Interestingly, this velocity is very similar to propagation velocities observed in subduction zones around the world and is approximately the speed of a sloth or a snail. Future work may help us understand what physical properties, such as confining stress, frictional strength, fluid pressure, and fluid diffusivity, control the propagation velocity of a slow slip event.