Submarine mass failures as tsunami sources: their climate control
BY D. R. TAPPIN
British Geological Survey
Nottingham NG
GG
UK
Recent research on submarine mass failures (SMFs) shows that they are a source of hazardous tsunamis, with the tsunami magnitude mainly dependent on water depth of failure, SMF volume and failure mechanism, cohesive slump or fragmental landslide. A major control on the mechanism of SMFs is the sediment type, together with its post-depositional alteration. The type of sediment, fine- or coarse-grained, its rate of deposition together with post-depositional processes may all be influenced by climate. Post-depositional processes, termed sediment 'preconditioning', are known to promote instability and failure. Climate may also control the triggering of SMFs, for example through earthquake loading or cyclic loading from storm waves or tides. Instantaneous triggering by other mechanisms such as fluid overpressuring and hydrate instability is controversial, but is here considered unlikely. However, these mechanisms are known to promote sediment instability. SMFs occur in numerous environments, including the open continental shelf, submarine canyon/fan systems, fjords, active river deltas and convergent margins. In all these environments there is a latitudinal variation in the scale of SMFs. The database is limited, but the greatest climate influence appears to be in high latitudes where glacial/interglacial cyclicity has considerable control on sedimentation, preconditioning and triggering. Consideration of the different types of SMFs in the context of their climate controls provides additional insight into their potential hazard in sourcing tsunamis. For example, in the Atlantic, where SMFs are common, the tsunami hazard under the present-day climate may not be as great as their common occurrence suggests. One contribution of 15 to a Theme Issue 'Climate forcing of geological and geomorphological hazards'.
1. Introduction
Tsunamis, especially destructive ones, are mainly (approx. 7080%) caused by
earthquakes. However, they can also be sourced by failure of sediment and
rock both on land and at the seabed. Most of these sediment/rock failures
are in submarine sediments or from volcanic lateral collapse. Historical records
of destructive tsunamis caused by lateral collapse include eighteenth century
examples from Japan, such as Oshima-Oshima in 1741 (Satake & Kato 2001;
Satake 2007) and Unzen in 1792 (Siebert et al. 1987), as well as the AD 1888
collapse of Ritter Island in Papua New Guinea (Johnson 1987; Ward & Day 2003).
Such historical ocean-entering landslides are recognized from eyewitness accounts.
In contrast, prehistorical lateral collapses, such as those in Hawaii and the Canary
Islands, are identified from geological evidence (e.g. Moore et al. 1994; Urgeles
et al. 1997). This difference in identification is significant in terms of hazard
recognition. Submarine seabed failures, termed here submarine mass failures
(SMFs), are less easily recognized than those onland, and there are few historical
accounts of these events. Thus SMFs may have been underestimated as a tsunami
source. In fact, until recently, they were discounted as a cause of destructive
tsunamis (e.g. Jiang & LeBlond 1994; LeBlond & Jones 1995). Tsunamis such
as the Grand Banks in 1929 (e.g. Heezen et al. 1954; Piper & Asku 1987) and
those associated with the Good Friday 1964 earthquake in Alaska, at Seward and
Valdez (e.g. Lee et al. 2003), might have flagged the hazard but they did not. It
was not until 1998, when a submarine slump caused the devastating tsunami at
Sissano Lagoon in Papua New Guinea, in which 2200 people died, that the threat
from submarine landslides was fully realized (Tappin et al. 1999, 2001, 2008).
Since 1998 there have been major advances in understanding SMFs
as sources of tsunamis. Applying new mapping methodologies, such as
multibeam bathymetry, the numerous architectures of SMFs have been identified.
Additionally, there has been an improved understanding of the mechanisms of
SMFs, including their formation and triggering. Based on the new knowledge,
advanced parametric mathematical models of SMFs have been developed that
form the basis of more realistic tsunami wave propagation and run-up (Tappin
et al. 2008). Thus SMFs and their potential to generate hazardous tsunamis are
now much better understood. An aspect of this improved understanding is the
influence of climate on both their formation and triggering. It is, therefore, the
objective of this paper to present an overview of the tsunami hazard from SMFs
in the context of their climate control(s). Specifically to consider (i) the temporal
evidence on how climate change may relate to SMFs, (ii) how climate change
may influence the stability of submarine sediments leading to mass failure, (iii)
whether climate can control the triggering of SMFs, and (iv) how climate, and
associated sea-level change, may influence the preservation potential of tsunami
sediments derived from SMFs and thereby our pote (...truncated)