A large strike-slip earthquake occurred in the Caribbean Sea on 28 January 2020. We inverted teleseismic P-waveforms from the earthquake to construct a finite-fault model by a new method of inversion that simultaneously resolves the spatiotemporal evolution of fault geometry and slip. The model showed almost unilateral rupture propagation westward from the epicenter along a 300 km section of the Oriente transform fault with two episodes of rupture at speeds exceeding the local shear-wave velocity. Our modeling indicated that the 2020 Caribbean earthquake rupture encountered a bend in the fault system associated with a bathymetric feature near the source region. The geometric complexity of the fault system triggered multiple rupture episodes and a complex rupture evolution. Our analysis of the earthquake revealed complexity of rupture process and fault geometry previously unrecognized for an oceanic transform fault that was thought to be part of a simple linear transform fault system.

We developed a flexible finite-fault inversion method for teleseismic P waveforms to obtain a detailed rupture process of a complex multiple-fault earthquake. We estimate the distribution of potency-rate density tensors on an assumed model plane to clarify rupture evolution processes, including variations of fault geometry. We applied our method to the 23 January 2018 Gulf of Alaska earthquake by representing slip on a projected horizontal model plane at a depth of 33.6 km to fit the distribution of aftershocks occurring within one week of the mainshock. The obtained source model, which successfully explained the complex teleseismic P waveforms, shows that the 2018 earthquake ruptured a conjugate system of N-S and E-W faults. The spatiotemporal rupture evolution indicates irregular rupture behavior involving a multiple-shock sequence, which is likely associated with discontinuities in the fault geometry that originated from E-W sea-floor fracture zones and N-S plate-bending faults.