Abstract

On 9 January 2014, a magnitude Mw 5.1 earthquake occurred along the Bahamas–Cuba suture at the north coast of Cuba, revealing a surprising seismic hazard source for southern Florida where it was widely felt. Because of its location, the event and its aftershocks were recorded only at far distances, resulting in high‐detection thresholds and low location accuracy. We use regional seismograms to determine source mechanisms and relative locations of the seven largest events in the sequence. Aftershock relocations relative to the mainshock, obtained by cross‐correlating Rayleigh waves, indicate a tight event cluster. Aligning low‐pass‐filtered (T>10  s) vertical seismograms from the closest site, station 061Z in southern Florida, on the P‐wave arrival reveals a surprising flip of the surface‐wave polarity for some aftershocks relative to the mainshock. Regional moment tensor inversion confirms that the mechanisms changed. Consistent with the Global Centroid Moment Tensor solution, we find an approximately east–west‐trending normal‐faulting mechanism for the main event and one aftershock. Three aftershocks indicate approximately east–west‐trending reverse faulting with P and T axes flipped relative to the normal‐faulting events. Within uncertainties, depths of all events are indistinguishable and indicate shallow faulting (10  km). One intriguing possible interpretation is that both families ruptured the same fault with reverse mechanisms compensating for dynamic overshooting. However, activity could instead be slightly separated with reverse mechanisms below extension or with slip on conjugate faults. The shallow source depths along the suture indicate that larger and potentially tsunamigenic earthquakes could occur offshore of northern Cuba, presenting a possible hazard locally and to Florida and the Bahamas.

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