The 28 January 2020, Mw7.7, Cayman Trough / Oriente Fault, Supershear Earthquake Rupture

The largest magnitude strike-slip event of the instrumental seismology era along the northern Caribbean plate boundary, with a moment magnitude of 7.7, occurred on 28 January 2020 on the Oriente transform fault, along the northern edge of the Cayman Trough, west of Cuba. We use local, regional, and global seismic waveforms and coseismic geodetic offsets, to produce high-resolution rupture models for both the low-frequency (~ 0.02 Hz) and high-frequency (~ 1 Hz) components of the rupture using a finite fault kinematic inversion and back-projection imaging, respectively. We document a rupture that propagated predominantly unilaterally westward, with an initial phase at subshear speed for 20–-30~s and over 40 to 50~km, followed by an acceleration to supershear speed that persisted all the way to the western end of the rupture, for 40~s and over about 200~km. Supershear rupture speed is consistent with strong motion observations of low ground acceleration levels in the near-field of the fault and low aftershock production in numbers and moment release. The rupture followed a very linear, unsegmented portion of the Oriente fault that had not experienced significant seismic activity for at least a century. Observational evidence and models indicate that the 28 January 2020, Mw7.7 earthquake, supershear over most of its length, had a smooth rupture process along a simple linear fault segment where earthquake nucleation is infrequent and interseismic locking depth shallow, two characteristics that may explain this unusually large magnitude supershear event.