Scenario of the 1996 volcanic tsunamis in Karymskoye Lake, Kamchatka, inferred from X-ray tomography of heavy minerals in tsunami deposits
Résumé
The concentration and distribution of heavy minerals in tsunami deposits is not random and mostly sourcedependent.
Heavy minerals may thus be good indicators of sediment provenance and tsunami flow dynamics.
The tsunamis generated by the 1996 phreato-magmatic eruption in Karymskoye Lake represent a relevant casestudy
because the provenance of the abundant heavy minerals found in the tsunami deposits is well constrained
(the on-going basaltic eruption itself). X-ray computed tomography (X-CT) of cores of tsunami sediments is used
to identify heavy minerals and characterise their source and spatial distribution in the tsunami deposit, and to
propose a scenario of the coupled eruption and tsunamis. An original combination of methods including X-CT,
SEM and XRF core scanner allows distinguishing subunits corresponding to pulses of sediments deposition and
associated inputs of heavy minerals, together with erosive contacts, laminations, and rip-up clasts of the
substratum. The structure of the tsunami deposits suggests that a major tsunami consisting of two main waves
inundated the coastal terrace up to 100 m inland on the eastern shore of the lake; a scenario that is consistent
with waves generated by experimental explosions. This largest tsunami might have occurred when underwater
explosions were at a critical water depth of 40 m (corresponding to a two-third submerged explosion in the 60 m
deep lake). However, more investigations are needed to better understand the critical conditions leading to a
tsunami during underwater eruptions.