Ⓔ The January 2014 Northern Cuba Earthquake Sequence: Unusual Location and Unexpected Source Mechanism Variability by Jochen Braunmiller, Glenn Thompson, and Stephen R. McNutt Abstract On 9 January 2014, a magnitude M w 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 deter- mine source mechanisms and relative locations of the seven largest events in the sequence. Aftershock relocations relative to the mainshock, obtained by cross-corre- lating 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 change. Consistent with the Global Centroid Moment Tensor solution, we find an approximately east–west-trending normal-fault- ing 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 indistin- guishable and indicate shallow faulting (≤ 10 km). One intriguing possible interpre- tation 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 tsu- namigenic earthquakes could occur offshore of northern Cuba, presenting a possible hazard locally and to Florida and the Bahamas. Supplemental Content: Documentation of weak-motion observations across the Florida carbonate platform from the main event, synthetic relative relocation results illustrating the effect of source mechanism difference on location results, and source parameters for four additional earthquakes in northern Cuba. Introduction A magnitude M w 5.1 earthquake occurred on 9 January 2014 near the Bahamas–Cuba suture zone along the northern coast of Cuba (Fig. 1). Northern Cuba is seismically rela- tively quiet, and the main event was felt in Florida as far north as Tampa and Orlando (U.S. Geological Survey [USGS], 2014), about 600 km north of the epicenter. The event thus revealed a surprising seismic hazard source for Cuba, southern Florida, and the Bahamas. Because of its moderate size, the earthquake caused no reported damage. The main event and its aftershocks are intraplate earth- quakes and occurred in an unusual location. Tectonically, Cuba has been part of the North American plate since the Middle Eocene (e.g., Iturralde-Vinent, 1994; Pindell and Kennan, 2009; Giunta and Orioli, 2011) when the collision of the Greater Antilles oceanic island arc with the continental Bahamas platform shut down subduction. It also caused a plate boundary reconfiguration, successively transferring parts of the Yucatan basin and Cuba, including the Cretaceous and early Tertiary Cuban arcs, from the Caribbean to the North American plate (e.g., Rosencrantz, 1990; Mann et al., 1995). The current plate boundary in the northwestern Caribbean is characterized by roughly east–west-trending left-lateral strike-slip faults that separate the North American plate along the Swan Island fault from the Caribbean plate west of the short Cayman spreading center and along the Oriente fault zone from the Gonave 12 13 1 Bulletin of the Seismological Society of America, Vol. , No. , pp. –, , doi: 10.1785/0120180272