Instability due to dike intrusion and pressurization of superheated groundwater from a reawakened Cumbre Vieja volcano destabilizes the western flank of La Palma and sends it into the Atlantic. This is the worst case scenario for La Palma or any other active volcano in the Canary Islands. Numerical modeling by Ward and Day (2001) quantifies the effects of the large scale, rapid failure of the La Palma along the inferred detachment zone on the west flank. The model of the landslide and subsequent tsunami provides a valuable glimpse at the Atlantic wide impact of such an initially localized event.
The Ward and Day (2001) model uses linear wave theory to model tsunami waveforms for uniform and non-uniform ocean depth. They specifically envision a landslide block 500 km3, 25 km long, 15 km wide, and 1400 m thick, moving at 100 m/s (Fig. 1). This generalization has its source in the observed volume of individual landslide debris fans in the ocean surrounding La Palma and adjacent Canary Islands.
Figure 1: The inferred extent of the landslide block on La Palma (Ward and Day, 2001).
The following animations by Ward and Day (2001) (click on the images) show the extent of the slide and amplitude and location of the tsunami wave-front as a function of time after the collapse occurs.
The displacement of the landslide material generates a 900 m high dome of water above the descending slide block after 2 minutes. From 10 to 20 minutes the neighboring Canary Islands have been inundated to several hundred meters height. The waves approach the Western Sahara over the next hour as waves 50-100 m stack together and sweep ashore (Ward and Day, 2001).
The brunt of the tsunami traverses the Atlantic Ocean basin, growing in width over the next 5 hours. The coast of western Europe receives 5-7 m waves. Meanwhile, 15-20 m waves strike northern South America while 10 m waves buffet Newfoundland. Along the shallow passive margin of Eastern North America, the tsunami builds to 20-25 m just prior to landfall around 8 hours. Within 11 hours, the Caribbean, North Atlantic, Gulf of Mexico, and Central America have been impacted by direct or diffracted tsunami waves (Ward and Day, 2001).
The scale of the simulated landslide and resulting tsunami must be kept in perspective. These images result from the absolute worst case scenario i.e. the complete detachment of the largest inferred size of the unstable block of La Palma’s volcano. Ward and Day (2001) note that a 250 km3 block, moving 50 m/s, generates a tsunami only 25-40% the size of worst-case. Although this landslide is half the magnitude and intensity, the resultant waves would still match the size and destructive capacity of the recent tsunami in Southeast Asia.
The effects of such a sudden event would be devastating, with the populations of the Canary Islands and the coastal areas of Africa decimated. In Europe and South America, warning time would be greater but likely still insufficient to evacuate the coastal areas. The most publicized effect of such a wave would be that on the Eastern seaboard of the United States. The tsunami would impact most coastal areas within 8 hours of La Palma’s collapse. Even with the maximum warning time, it is highly unlikely that all areas at risk for inundation could be sufficiently evacuated. In some cases, the best escape may be vertical, with high rises in New York, Boston, and other port cities used to escape the rising waters.