First not all earthquakes trigger tsunamis. The 6.4 magnitude deadly earthquake south of Ponce Puerto Rico crushed buildings but fortunately didn’t trigger a tsunami.
In ideal conditions, a tsunami could radiate out from San Juan and reach Jacksonville between 4-5 hours based on NOAA Tsunami Travel Time Maps.
Magnitudes below 6.5 are very unlikely to trigger a tsunami. Earthquakes over 7.6 might produce destructive tsunamis, especially near the epicenter if it happens in a shallow marine area.
The reason why strong earthquakes don’t always result in large waves is due to the way seafloor disruption displaces the water.
Vertical shifting thrust earthquakes (as opposed to sideways strike slip) are more likely to generate tsunamis, but small tsunamis have occurred in a few cases from large over magnitude 8 strike-slip earthquakes.
Over the past several weeks, hundreds of small earthquakes have occurred in the Puerto Rico region, beginning in earnest with a M 4.7 earthquake late on December 28.
The magnitude 6.4 earthquake was widely felt. According to ShakeMap, strong to very strong shaking occurred across parts of Southern Puerto Rico closest to the event and moderate shaking occurred across the rest of the island.
The proximity of these events to Puerto Rico, and their shallow depth, mean that dozens of these events have been felt on land, though with the exception of the latest two earthquakes, the M 6.4 and the M 5.8, none are likely to have caused significant damage.
The January 6 and 7, 2020, M 5.8 and M 6.4 earthquakes offshore of southwest Puerto Rico occurred as the result of oblique strike slip faulting at shallow depth.
Tectonics in Puerto Rico are dominated by the convergence between the North America and Caribbean plates, with the island being squeezed between the two. To the north of Puerto Rico, North America subducts beneath the Caribbean plate along the Puerto Rico trench.