Submarine landslide as the source for the October 11, 1918 Mona Passage tsunami: Observations and modeling

The October 11, 1918 ML 7.5 earthquake in the Mona Passage between Hispaniola
and Puerto Rico generated a local tsunami that claimed approximately 100 lives
along the western coast of Puerto Rico. The area affected by this tsunami is
now significantly more populated. Newly acquired high-resolution bathymetry
and seismic reflection lines in the Mona Passage show a fresh submarine landslide
15 km northwest of Rinćon in northwestern Puerto Rico and in the vicinity of
the first published earthquake epicenter. The landslide area is approximately
76 km2 and probably displaced a total volume of 10 km3. The landslide's headscarp
is at a water depth of 1200 m, with the debris flow extending to a water depth
of 4200 m.

Submarine telegraph cables were reported cut by a landslide in this area
following the earthquake, further suggesting that the landslide was the result
of the October 11, 1918 earthquake. On the other hand, the location of the
previously suggested source of the 1918 tsunami, a normal fault along the east
wall of Mona Rift, does not show recent seafloor rupture. Using the extended,
weakly non-linear hydrodynamic equations implemented in the program COULWAVE,
we modeled the tsunami as generated by a landslide with a duration of 325 s
(corresponding to an average speed of ~ 27 m/s) and with the observed dimensions
and location. Calculated marigrams show a leading depression wave followed by a
maximum positive amplitude in agreement with the reported polarity, relative
amplitudes, and arrival times.

Our results suggest this newly-identified landslide, which was likely triggered
by the 1918 earthquake, was the primary cause of the October 11, 1918 tsunami
and not the earthquake itself. Results from this study should be useful to help
discern poorly constrained tsunami sources in other case studies.