What the number actually describes
The approximate 4.5 ± 1 mm/yr value used in regional studies describes relative plate motion across a broad zone. It is a tectonic vector, not a ruler placed perpendicular to the narrowest part of the Strait.
Deformation is distributed
The Gibraltar Arc, Alboran Sea, Betics, Rif and Gulf of Cádiz form a system of faults, crustal blocks, strike-slip motion, compression, extension and deep processes. Motion is absorbed away from the channel and in directions that do not directly reduce its width.
Why dividing 14 km by 4.5 mm/yr is invalid
The arithmetic yields a number, but the physical model is false. It assumes all convergence is concentrated at the narrowest line, acts perpendicular to the coasts, remains constant for millions of years and is not distributed through faults or internal deformation. None of those assumptions is established.
Earthquakes are not a closure speedometer
Seismicity records stress release on active structures. Counts of small earthquakes also depend on instrumental coverage, magnitude thresholds, revisions and local sequences. There is no valid conversion from “earthquakes per month” to “millimetres of closure”.
The supported conclusion
The region is tectonically active and deserves seismic and geodetic monitoring. There is no measured direct closure rate for the channel and no forecast that it will disappear within decades or centuries.