Constraining shallow slip deficit with phase gradient data

SUMMARY Coseismic slip models for large (Mw > 7) strike-slip earthquakes present a variety of shallow slip deficit (SSD). Accurate estimate of SSD is difficult, and it has been suggested that SSD are to some degree associated with fault-zone characteristics, incompleteness of data coverage as well as simplified model assumptions. Furthermore, SSD can also be sensitive to the amount of model smoothness adopted. Since phase gradient are sensitive to the missing shallow slip from our simulated data, we performed a synthetic test and presented a case study of the 2019 Ridgecrest earthquake sequence to validate that phase gradient from radar interferometry could help reveal the actual SSD for kinematic slip models even without enough near-fault observation. Our results indicate that even in the presence of a greater degree of observational gaps, the phase gradient can still nearly substitute for near-fault observations in constraining the shallow slip. Lastly, we provide a preferred coseismic slip model constrained by all available observations including phase gradient, but with 4-km data gap near the fault trace. This model results in ∼35 per cent SSD for the Ridgecrest earthquakes, matching previous estimates that incorporate near-field data. Considering the phase gradient approach is a straightforward mathematical operation, this approach may be applicable to other types of earthquakes. Notably, due to the smaller amplitude and lower signal-to-noise ratio for the phase gradient data, one needs to carefully balance the trade-offs among weights of different data sets and model smoothness.