POSTULATED MASSIVE SLOPE FAILURES OF ISLAND STRATOVOLCANOES ON LA
PALMA, CANARY ISLANDS,
AND ON THE ISLAND OF HAWAII
Massive flank failures of island stratovolcanoes are extremely rare phenomena and none have
occurred within recorded history.
Recent numerical modeling studies, forecasting mega tsunami
generation from postulated, massive slope failures
Cumbre Vieja in La Palma,
Canary Islands, and
Kilauea, in Hawaii, have been based on incorrect assumptions of volcanic island slope instability, source
dimensions, speed of failure and tsunami coupling mechanisms.
Incorrect input parameters and
treatment of wave energy propagation and dispersion, have led to overestimates of tsunami far field
Inappropriate media attention and publicity to such probabilistic results have created
unnecessary anxiety that mega tsunamis may be imminent and may devastate
coastlines at locations distant from the source - in both the Atlantic and Pacific Oceans.
The present study examines the assumptions and input parameters used by probabilistic numerical
models and evaluates the threat of mega tsunami generation from flank failures of island stratovolcanoes.
Based on geologic evidence and historic events, it concludes that massive flank collapses of
Vieja or Kilauea volcanoes are extremely unlikely to occur in the near geologic future.
The flanks of
these island stratovolcanoes will continue to slip aseismically, as in the past.
Sudden slope failures can
be expected to occur along faults paralleling rift zones,
but these will occur in phases, over a period of
time, and not necessarily as single, sudden, large-scale, massive collapses.
Most of the failures will occur
in the upper flanks of the volcanoes, above and below sea level, rather than at the basal decollement
region on the ocean floor.
The sudden flank failures of the volcanoes of Mauna Loa and Kilauea in
1868 and 1975 and the resulting earthquakes generated only destructive local tsunamis with insignificant
far field effects.
Caldera collapses and large slope failures
associated with volcanic explosions of
Krakatau in 1883 and of Santorin in 1490 B.C., generated catastrophic local tsunamis, but no waves of
significance at distant locations.
Mega tsunami generation, even from the larger slope failures of island
stratovolcanoes, is extremely unlikely to occur.
Greater source dimensions and longer wave periods are
required to generate tsunamis that can have significant, far field effects.
The threat of mega tsunami
generation from massive flank failures of island stratovolcanoes has been greatly overstated.
Science of Tsunami Hazards, Volume 20, Number 5, page 251 - 277 (2002