Long-Term Records of Tsunamis
Contributors:
Ben Horton
Brady Rhodes
Martitia Tuttle
Harvey Kelsey
Lisa Doner
Alan Nelson
Marco Cisternas
I. Introduction
Damaging tsunamis are rare but costly. Consequently, studies of historical tsunamis—as
recorded in written and video archives or instrumental sea-level data—are limited in what they can
tell us about the scale and complexity of past tsunamis. Long-term (hundreds to thousands of years)
records of tsunamis, as recorded by the sedimentary deposits they leave behind, help us understand
tsunami processes by greatly expanding the range of tsunamis available for study. Understanding
potentially catastrophic coastal processes, such as tsunamis, is key to assessing and mitigating the
undesirable effects of both catastrophic and non-catastrophic processes on the ecology,
geomorphology, and rapidly expanding human populations of coasts. Well preserved sequences of
tsunami deposits can be used to estimate recurrence intervals of tsunami source events, including
earthquakes, landslides, or volcanic eruptions, and to model source processes on meaningful
timescales, for example, the long-term behavior of plate-boundary fault systems. Ancient tsunami
deposits reveal the scale of catastrophic events such as meteor or asteroid impacts, for which there is
no modern analog. By promoting cross-disciplinary collaboration in fields as diverse as biology,
geomorphology, geophysics, marine geology, sedimentology, seismology, and human geography,
research on long-term tsunami records may lead to unanticipated advances in multiple fields.
Because study of long-term records improves our understanding of modern tsunami hazards, longterm tsunami research contributes to mitigation efforts around the world.
II. Previous Studies Using Long-Term Records
Tsunamis
The literature on tsunamis and tsunami deposits has rapidly expanded over the past decade (e.g.
Tappin, 2004; Pelinovsky & Tinti, 2005) and the Indian Ocean tsunami of December 2004 will
dramatically accentuate this trend. Inferring the character and scale of past tsunami source
mechanisms (e.g., sea-floor faulting, mass movement) and the impact of tsunamis on former coasts
is an iterative process involving modelling and field geology that begins with the identification of
tsunami deposits. Such studies require integration of terrestrial and marine sedimentary records,
observations of modern tsunamis, paleoenvironmental reconstructions, and computer simulations
(Edwards, in press).
Modern Observations: Although tsunamis deposit beds of sediment that are anomalous in many
settings, such as beds of marine sand within sequences of terrestrial peat or freshwater lake sediment
(e.g. Dawson et al., 1991; Bondevik et al., 1997a, b,), rarely are the lithology and other
characteristics of the beds unique. For example, storm surges may deposit marine sediment many
meters above normal tidal levels (Dawson & Shi, 2000). Comparative studies examining storm and
tsunami deposits are particularly valuable in this respect (e.g. Nanayama et al., 2000), as is
documentation of the processes and impacts of sediment erosion and deposition during modern
tsunamis (e.g. Maramai et al., 2005a, b). A recent example is the study by Goff et al. (2004) in New
Zealand, which compares sediment deposited by a 15th century tsunami and a large storm that
occurred in 2002. The two types of deposits show clear differences in their sedimentology??, bed
continuity, and inland extent. Similarly, a study comparing tsunami deposits in southern
Newfoundland resulting from the 1929 Grand Banks earthquake with deposits resulting from
submarine slides and storms in New England during the 1991 Halloween storm found that the
deposits differed in their landscape positions and sedimentary characteristics?? (Tuttle et al.,
2004). On the basis of these and similar differences described in the literature for other tsunami and
storm deposits, Tuttle et al. (2004) outlined preliminary criteria for distinguishing the two types of
deposits. More detailed studies of tsunami deposits and their look-alikes will aid development of
diagnostic criteria for the identification of prehistoric tsunami deposits (e.g. Goff et al., 2001; Smith
et al., 2004; Bondevik et al., 2005; Williams et al., 2005). Replace vague terms above with
specific sediment characteristics.
Models of processes capable of transporting coarse gravely sediment inland are much less
developed for rocky coastlines than for sandy coasts (Felton & Crook, 2003). Consequently,
interpretations of potential tsunami deposits on rocky coasts are more inferential (e.g. McMurtry et
al., 2004; Scheffers, 2004). New data suggests that storm waves may transport larger clasts than
previously thought (Edwards, in press). Recent studies of modern coarse storm deposits suggest
previous modeling of the height of wave required to lift blocks of a given mass (e.g. Nott, 2003)
overestimates wave height (e.g. Mastronuzzi & Sanso, 2004). For example, Williams & Hall (2004)
graphically illustrate the power of storm waves in their description of boulders weighing several tons
found as much as 50 m above sea level on cliff-tops along the Atlantic Irish coast. Such
reassessments of the power of storm waves are prompting reinterpretation of some proposed tsunami
deposits.
Paleoenvironmental reconstructions: Investigations of past tsunamis can be used to develop and
test models of tsunami generation and runup, and identify regions at greatest risk from future
tsunamis (Edwards, in press). The number, timing, and height of tsunami waves reflect the source
mechanism. For example,for fault-related tsunamis runup height rarely exceeds twice the fault slip
(Okal & Synolakis, 2004). Consequently, maximum water levels of 25-30 m recorded in Sumatra
(Stein & Okal, 2005) suggest the recent Indian Ocean tsunami was generated by fault slip of 12-15
m. Mass movements have the potential to produce even higher waves, but tsunamis produced by
submarine slides and slumps are particularly difficult to model (Pelinovsky & Tinti, 2005).
Modeling is further complicated when large earthquakes with sea-floor displacement on faults also
trigger mass movements.
Smith et al. (2004) comprehensively review evidence for the Storegga Slide tsunami from 32
coastal sites in northern Britain. At most sites, evidence consists of widespread but anomalous sand
beds containing marine microfossils that date from ca. 8000 cal. yr BP. Sedimentological
evidence?? from these sites confirms that the beds were deposited by more than one wave,
supporting Bondevik et al.’s (2003) interpretation of multiple waves from the characteristics of
tsunami deposits in lakes? on the Shetland Isles. Tooley & Smith (2005) show that tsunami deposits
can be identified even within a sequence of high-energy deposits; they describe two fining-up
sequences in coarse sand and gravel from eastern? Scotland.
Reconstructing runup height from a tsunami deposit requires information on the maximum
elevation attained by tsunami waves, and the elevation of sea level at the time of the tsunami. Runup
heights based on the inland extent of anomalous sand beds are minimum estimates because water
levels exceed the elevation of deposited sediment (Dawson, 1999; Dawson & Shi, 2000; Tuttle et al,
2004). In many studies, the precision of inundation heights of past tsunami is further limited by
estimates of former sea-level at the time of deposition based on geophysical models of glacialisostatic adjustment of the crust (e.g. Bondevik et al., 2003). In an interesting development, Smith et
al. (2004) use the tsunami deposits as a time horizon. By locating the inland limit of intertidal
sediment capped by tsunami deposits, they reconstruct the shoreline position at the time of the
tsunami, and from the position identify subsequent patterns of coastal retreat. Smith et al. (2004)
attribute the variable pattern of runup height to differences in wave erosion, and to differences in tide
level at the time of the tsunami.
Kelsey et al.’s (2005) study of Bradley Lake, on the southern Oregon coast, revealed a 7000-yrlong record of local plate-boundary earthquakes and accompanying tsunamis on the Cascadia
subduction zone. At least 12, and probably 13, tsunamis deposited landward-thinning sheets of sand ,
derived from nearshore, beach, and dune environments to the west, in the lake. Kelsey et al. (2005)
calculate that the tsunamis rose at least 5–8 m above sea level, and that the cumulative duration of
each tsunami was at least 10 min. Between 4600 and 2800 cal yr B.P., tsunamis occurred at the
average frequency of ~3–4 every 1000 yr. Then, starting ~2800 cal yr B.P., there was a 930–1260 yr
interval with no tsunamis. That gap was followed by a ~1000 yr period with 4 tsunamis. In the last
millennium, a 670–750 yr gap preceded the A.D. 1700 earthquake and tsunami. Kelsey et al. (2005)
suggest that the A.D. 1700 earthquake may be the first of a new cluster of plate-boundary
earthquakes and accompanying tsunamis.
Determining the frequency of tsunamis through dating tsunami deposits is complicated by the
difficulties in developing tsunami histories from sedimentary sequences (Edwards, 2003, in press;
e.g.,). With a few notable exceptions (e.g., Satake et al., 2003; Ollerhead et al., 2001; Clague et al.,),
dating of tsunami deposits relies on 14C ages, which typically have errors of at least many decades
and commonly hundreds of years (Nelson et al., 1996; Atwater and Hemphill-Haley, 1997; Witter et
al., 2003; Kelsey et al., 2005). For example, the uncertainty in the time of the Storegga Slide
tsunami in the North Sea is still hundreds of years despite many tens of 14C ages due to dating errors
and other problems such as erosion of slide deposits. Fish skeletons and plant macrofossils preserved
within Storegga slide deposits suggest that the tsunami occurred in late autumn (Bondevik et al.,
1997; Dawson & Smith, 2000). A related problem is that many radiometric 14C ages on bulk,
organic-rich sediment are commonly younger than the time of sediment deposition (Nelson, 1992;
Bondevik et al., 2003, in press). Nevertheless, even though the times of individual tsunamis may be
uncertain, errors in radiocarbon dating are rarely a significant source of uncertainty in determining
the average recurrence of tsunamis from long tsunami records.
Dating errors are of much greater concern when the ages of tsunami deposits are used as a
diagnostic characteristic when correlating deposits to show their tsunami origin. For example,
Williams et al. (2005) attempt to fingerprint the sources of nine muddy sand beds preserved within a
tidal marsh in Washington State, USA by comparing bed ages with ages for tsunami deposits at
other sites in the region. The authors conclude that four to six of the beds are probably tsunami
deposits, but that the tsunamis probably had various sources, such as mass movements and distant as
well as and nearby earthquakes. Correlation of deposits laid down by tsunamis accompanying plateboundary earthquakes is complicated by the fact that the frequency of high tsunamis on many plateboundary coasts is of similar magnitude to the errors on typical tsunami deposit ages (Nelson, 1992;
Williams et al., 2005).
Model simulation and marine data: Computer models are used to simulate patterns of wave
propagation, height, and other characteristics of tsunamis from different source mechanisms
(McMurty et al., 2004; Fryer et al., 2004; Løvholt et al., 2005). For example, Bondevik et al. (2005)
compare the extensive set of geologic runup data from Norway, Scotland and the Shetland Islands,
with numerical simulations of the Storegga slide. Their best-fit model suggests that sea levels along
the Norwegian coast fell by 20 m during the first 30 minutes following the slide. The simulation also
predicts the generation of multiple waves, matching inferences about tsunami characteristics based
on sedimentological data. In an alternative application, Okal (2005) uses a model to choose between
two sources for the 1906 Pacific-wide tsunami. Although the two tsunamis occurred within 30
minutes of each other, the modeled tsunami matches only the far-field characteristics of the tsunami
generated in Chile.
Additional information on tsunami sources is provided by marine seismic data. The location,
extent and architecture of the Storegga slide is now well-constrained by geophysical surveys
(Haflidason et al., 2004). The rest of this paragraph needs a better home. It is not about marine
seismic data. The triggering mechanism is still under investigation, and Bryn et al. (2005) suggest a
strong earthquake is a likely cause. This may have been facilitated by excess porewater pressure in
the sediments brought about by high rates of deposition. Solheim et al. (2005) suggest that
immediately following deglaciation, rapid sedimentation coupled with glacio-isostatic seismicity
could produce conditions favourable for slope failure. They report seven large pre-Holocene slides in
the area that appear to form a complex of failures related to the glacial-interglacial cycle. The recent
discovery on the Shetland Islands of two tsunami-like deposits post-dating the Storegga Slide
suggests that this instability persists throughout the Holocene (Bondevik et al., in press). Hutton &
Syvitskli (2004) model sediment failures under changing sea levels, and note that whilst most occur
during sea level falls or lowstands, the largest volume failures are associated with rising sea levels
and highstands.
In fact, the nature of coastal sedimentation is increasingly being tied with tsunami risk. Using a
catalogue of historic tsunamis in the Pacific Ocean, Gusiakov (2005) shows that sedimentation has a
strong control on the likelihood that an earthquake will generate a tsunami. In this paper, written
almost a year before the Indian Ocean tsunami of 2004, the extremely high efficiency of earthquakes
in the western part of Indonesia is highlighted. In a recent paper, Syvitski et al. (2005) use a model to
examine how the flux of sediment into the global coastal ocean has changed due to human activities.
One of their results is that, in contrast to many other parts of the globe, Indonesian rivers now deliver
much more sediment to their coastal waters than before. This research highlights the need for a
greater understanding of land-ocean sediment fluxes and processes operating on the shelf (Long,
2003). I had a hard time understanding this paragraph and why it is important for this report.
It needs to be deleted or completely rewritten.
Tsunami deposits and earthquake-related land-level and sea-level changes
A major factor in the long-term preservation of tsunami deposits is the direction and rate of
vertical movements of shoreline areas that reflect crustal deformation during cycles of tectonic strain
accumulation and their release during earthquakes, particularly near plate-boundary faults (Clague,
1997). Tsunami deposits are best preserved where coasts subside (and relative sea level rises) during
large or great earthquakes because the deposits are rapidly buried by intertidal sediment (e.g.
Atwater, 1987; Atwater & Hemphill-Haley, 1997). Where either coseismic or postseismic uplift
exceed net subsidence or where long-term regional sea level falls (e.g., Atwater et al., 1992;
Cisternas et al., in press), shorelines will rise relative to the sea and tsunami deposits may only be
preserved in special environments such as lakes (Bondevik et al., 1997; Kelsey et al., 2005).
Because such shoreline movements are complex and the characteristics of tsunami deposits are
rarely unique, a thorough understanding of the stratigraphic context of deposits in host sediment
sequences is required for the correct identification of most tsunami deposits. Thus, studies of
relative sea-level changes recorded by sequences in which tsunami deposits are found go hand-inhand with detailed analyses of tsunami-laid beds. The most detailed and precise records of sea-level
changes have been developed through quantitative reconstructions employing microfossil
assemblages in host sediment (e.g., Shennan et al., 1999; Sawai et al. 2004a; Hawkes et al., 2005).
For example, Zong et al. (2003) and Hamilton & Shennan (2005; in press) use diatoms and pollen to
examine relative sea-level changes in response to the 1964 earthquake at two sites in Cook Inlet,
Alaska. Zong et al.’s (2003) data, with a maximum precision of ± 0.06 m, indicate pre-seismic
subsidence of around 0.15 m at both locations. Elsewhere Hayward et al. (2004) use a combination
of foraminifera and diatoms to identify sudden elevation changes in three Holocene sedimentary
sequences from Ohiwa Harbour, New Zealand. Their records indicate potential coseismic subsidence
of around 2 m, although the reconstructions are of lower precision than the transfer functions used in
North America. In Japan, Sawai et al. (2004b) use a diatom-based transfer function to reconstruct
relative sea-level changes in eastern Hokkaido about the time of a large earthquake and tsunami in
the 17th century. Detailed analyses above and below the tsunami sand bed indicate gradual preseismic subsidence followed by post-seismic uplift. Intriguingly, this post-seismic uplift appears to
have persisted for several decades rather than occurring abruptly.
The history of land- and sea-level changes at sites of long-term tsunami records bear directly on
the question of thresholds of preservation, both elevation above sea level and distance of
preservation inland from the coast. Because relative sea-level changes with time, the relative sealevel history at a site must be known to address questions of inundation extent and height. We
recommend that any investigation of long-term records should address changes in relative sea level
at that site for the time period of interest.
III. Direction of Future Tsunami Deposit Research
Over the next several years, research on long-term tsunamis records will likely focus on three
interrelated areas: 1. Field studies will aim at finding new long-term records along unexplored coast
lines or more complete records along coasts with known tsunami deposits; 2. New analytical
methods will be developed to calculate or estimate water velocity, flow characteristics, and water
depth from the characteristics of tsunami-laid sediment, to improve dating of tsunami deposits, and
to more accurately reconstruct environments from stratigraphic sequences deposited by tsunamis;
and 3. New methods will be developed to interpret the spatial and temporal distribution of tsunamis
in terms of source areas, mechanisms, and recurrence of tsunamis.
Finding New Long-Term records of Tsunamis
The 26 December 2004, Indian Ocean Tsunami affected a large part of the Indian Ocean basin
that, with the exception of Indonesia, has no historical record of damaging tsunamis nor any
documented prehistoric sedimentary record of tsunamis. This event highlights our incomplete
knowledge of the history of tsunamis world-wide, including their frequency and the variety of
potential tsunami sources. The 26 December catastrophe provides an opportunity to broaden and
deepen our knowledge of tsunamis by investigating new long-term records of tsunami deposits.
Below we recommend topics for funding whose investigation would improve the archive of longterm tsunami records. Projects might be small, short exploratory projects or large, lengthy ventures.
The most effective projects will involve researchers in ways that cross disciplines and cultural
boundaries.
Tectonic Environments: The most compelling need is for more long-term records from coasts
most vulnerable to tsunami inundation, such as Indonesia and other western Pacific coasts. We stress
that long-term tsunami records are commonly most easily interpreted and dated where studied in
conjunction with long-term records of coseismic and interseismic crustal movement. Damaging
tsunamis are most frequent on the coasts of active strike-slip or convergent continental margins, but,
as illustrated in December 2004, tsunami inundation can be equally catastrophic on the coasts of
passive continental margins. Because passive margins are tectonically stable, long-term preservation
of tsunami deposits depends on the persistence of coastal lakes or lagoons, gradual relative sea-level
rise, or gradual coastal progradation. For example, rapid fluvial erosion and sedimentation in the
Mediterranean region turned ancient lagoons and harbors into small lakes capable of recording
tsunamis. The wealth of written and archaeological records in the region could provide unusually
precise ages for tsunamis recorded in sediment cores from such sites.
Few long-term tsunami records comparable to those from Scandinavia (isolation basin studies)
and the west coast of North America (stratigraphic studies of intertidal wetlands) are available from
Japan, Alaska, and Chile, and none are available from coasts of the Indian Ocean and most passive
continental margins. The highest priorities for long-term tsunami records are from the northern and
western Pacific Rim and around the Indian Ocean, but well preserved long-term tsunami records that
can be dated should be investigated on any coasts with significant human populations.
Correlating Records: Another value of long-term tsunami records is that they most likely can be
correlated among coastal sites. If tsunami deposit records are correlated in conjunction with the
correlation of related abrupt sea level changes, these records can be used to approximate the
tsunamigenic source, be it local earthquake, trans-oceanic earthquake, landslide or (a lesser
possibility) bolide impact. In addition to correlating lithostratigraphy, correlation relies on both
dating (mainly 14C methods) and use of biostratigraphic analysis (diatoms, forams) to document the
abruptness of, and magnitude of, relative sea level change. I don’t really understand the point of
this paragraph. Much of it seems redundant with other sections. Delete?
Pre-Holocene Records: We contend that it would be unusual for a continuous tsunami record to
go back for more than about 6,000 or 7,000 years before present because it was at that time that the
rate of eustatic sea level rise started to decelerate, allowing coastal landforms such as sand barriers to
be come stable and develop in response to slowly changing relative sea level. However, older
tsunami deposits doubtless remain to be discovered. Building on recent and future advances in
understanding of the Holocene record of tsunamis, new efforts at find pre-Holocene tsunami deposits
should be encouraged. Such projects should be aimed at extracting meaningful oceanographic or
tectonic information from these older deposits. I agree with Harvey’s comments on this
paragraph and don’t feel it adds much to the report. The content of the first sentence might be
moved elsewhere.
Depositional Environments: Most tsunami-deposit research has been done in temperate-climate
zones where depositional settings, typically intertidal wetlands in estuaries, are comparatively well
understood. In the wake of the December 2004 tsunami in Indonesia, more research on long-term
tsunami depositional settings should be done in tropical and semi tropical settings. Such research
will confront issues such as how well do mangrove forests preserve tsunami deposits, what
taphonomic changes occur in tsunami deposits that accumulate in tropical climates, and what kinds
of sites conditions are necessary for best preserving tsunami deposits in the tropics and semi-tropics?
Another issue is the degree to which tsunami deposits are preserved offshore of coasts subject to
inundation by high tsunamis. For the December 2004 tsunami, the numerous observations of
tsunami-transported sediment moving seaward during return flow begs the question can tsunami
deposits be studied in the offshore sedimentary record? This seems unlikely on high or moderate
energy coastlines but may be possible in large lagoons and embayments.
Distinguishing Tsunami Deposits: Although not a new research direction, distinguishing
tsunami from storm deposits in long-term records remains an important aspect of tsunami-deposit
research. Because this is a second order problem at this point, proposals on this topic should have
clear cut objectives or testable hypotheses that rely on previously-documented sites with records of
deposition by both mechanisms.
Exploration and Sampling Techniques: We encourage research that improves upon, or
develops, new techniques to find and sample long tsunami deposit records. We temper this directive,
however, by pointing out that finding a depositional site with an extensive, well preserved record is
much more important than using a fancy new technique at a site with a poorly preserved record. For
instance, investigation of outcrops and hand-dug soil pits at promising sites is likely to be far more
valuable than using multi-thousand dollar equipment at poor sites. Nevertheless, relatively
sophisticated coring devises, with multi-centimeter diameter cores, are needed to adequately sample
and date deep long-term tsunami records.
Remote sensing can help to identify promising coastal sites along hundreds of kilometers of
remote, unmapped coastline, but knowledgeable coastal geologists need to guide such efforts. To
identify sites, careful review of existing topographical maps and available aerial photograph and
satellite imagery should precede field investigations, selection of key sites, and sophisticated
sampling. No technique for finding new sites, no matter how sophisticated, will substitute for the use
of basic geomorphic and stratigraphic expertise and knowledge of coastal and tsunami depositional
processes.
Methodologies for Analyzing Long-Term Records
Prior research on tsunami deposits has shown that analytical techniques in geochemistry,
sedimentology, paleontology, and geochronology are effective means of studying long-term tsunami
records. Use of multiple methods helps form a coherent interpretation of past tsunamis; no single
techniqueis commonly effective at demonstrating a tsunami origin. The multidisciplinary approach
also best addresses questions about spatial and temporal extent of tsunamis, and helps in developing
recurrence probabilities for tsunamis of varying height.
Typical techniques for describing tsunami deposits and distinguishing them from storm deposits
include sudden changes in oxidation state, geochemical signatures of provenance, carbon content,
sediment size, sedimentary facies characteristics, fossil assemblages, and fossil preservation
(broken/unbroken). In addition to the use of geochronological methods for dating tsunami deposits,
characteristic patterns in the distribution of ages, such as hiatuses and/or deposits containing a
mixture of older and younger fossils, helps in identifying and correlating tsunami deposits.
Because tsunami deposits occur offshore as well as onshore, tsunami deposit research offers an
unusual opportunity for collaborations by coastal and oceanographic researchers. We encourage
such collaborative proposals, particularly those that propose use of methods outside of their usual
applications to address issues facing tsunami researchers. For example, stable isotope analyses on
offshore cores might be used to demonstrate a terrestrial source for the organic carbon (from C/N
ratios) and inorganic carbonates (from 87Sr/86Sr) in tsunami deposits. Likewise, because hurricanes
are accompanied by large freshwater fluxes to the continental shelves and such fluxes do not occur
during tsunamis, 18O analyses of offshore sediment cores might distinguish the two types of
deposits.
Because lakes often provide the capability of very high (temporal) resolution sampling, the shortterm nature of the tsunamis can be more easily detected in long-term lake records than in offshore
records. Some methods routinely used by limnologists might be effectively applied to tsunami
deposits, such as the short-lived effects of salinity changes on ostracods, diatoms, and aquatic plants
during and after inundation.
Historical archives from regions with extended written history have provided useful data about
tsunami activity and run-up (i.e., Ottoman Turkey). Archaeological records have likewise
contributed to our understanding of tsunami recurrence and societal impact. Besides these, the recent
video and photographic material from the December 2004 tsunami has become an important visual
archive for both modelers and field scientists.
Interpretation Long-Term Records
Recurrence probability: Long-term tsunami records allow determination of the recurrence of
tsunamis that are high enough to leave deposits onshore and offshore. Recurrence is defined by the
mean, variation, and distribution of recurrence intervals between tsunamis and is useful for
evaluating the history of tsunami source faults and for assessing the probability of future tsunamis.
Probability uncertainties are affected by the length of the record as well as by uncertainties in the
dating of individual tsunamis (McCalpin, 1996). Although earthquake recurrence has been
described in terms of various frequency distributions (Poisson, lognormal, Weibull, and bimodal),
much remains to be learned about which frequency distributions best describes tsunami recurrence
on different coasts.
Interpreting Tsunami-Generating Mechanisms: Before assessing tsunami recurrence, long-term
tsunami records must be identified and interpreted in terms of source area (e.g., transoceanic,
regional, local) and mechanisms (e.g., landsliding, faulting, volcanic eruption, meteorite impact).
Empirical data from modern and historic tsunamis whose locations and source mechanisms are
known suggest that similar-age tsunami deposits found over thousands of kilometers of coastline,
possibly including deposits on more than one continent, are probably the result of a transoceanic
tsunami; whereas, more limited geographical distributions of deposits reflect a regional or even a
local (e.g., landslide generated) tsunami. Overlapping inundation zones, where two tsunamis
deposited sediment in the same place but at slightly different times, might also occur where multiple
fault segments or earthquake-triggered landslides generated multiple tsunamis. On the basis of
source modeling, a narrow but peaked coastal distribution of tsunami deposits would indicate a
landslide source; whereas a broader and less peaked distribution would reflect a fault source. Other
types of deposits resulting from ground shaking, volcanic eruptions, and even meteor impacts can
help to interpret the long-term tsunami record. We advocate case studies of modern and historic
tsunamis designed to test this and related hypotheses.
Combining Long-Term Records with Modern Observations: Much knowledge about processes
involved in sediment erosion, transport, and deposition, impacts on coastal environments and
geomorphology, and scale of different types of events can be learned from observations of modern
tsunamis. This type of information also helps to interpret the record of paleotsunamis. In turn, longterm records provide information about the relative importance of tsunamis on the stratigraphic,
environmental, and geomorphic development of a region over time. Frequent large events involving
severe erosion and transport of large quantities of sediment or significant land-level changes will
have a larger impact than less frequent or less severe events.
The correlation of tsunami deposits at isolated locations over large areas is a critical step in
tsunami-deposit studies. This task is made more difficult by uncertainties associated with commonly
used dating techniques such as radiocarbon and optically stimulated luminescence. New approaches
to analyzing age data such as stratigraphic ordering of calibrated radiocarbon age distributions and
summing of probability density functions of dates have helped to narrow uncertainties associated
with estimates of event timing (e.g., Biasi et al., 2002; Weldon et al., 2005). Nevertheless, dating
uncertainties can lead to non-unique interpretations of the data. Advances in existing dating
techniques, development of new techniques, and further improvements in data analysis could greatly
improve confidence in estimates of event timing and recurrence intervals as well as correlation of
tsunami deposits.
Combining Geologic and Historical Records: Regions with long historical records can provide
information useful for interpreting the geologic record of tsunamis in regions with short historical
records. For example, the 2000?-year-long historical record of Japan provided critical information
that helped to establish the timing and magnitude of the 1700 A.D. Cascadia mega-thrust earthquake
(Satake et al., 2003). In addition, historical records are likely to contain additional information
regarding historical tsunamis. For example, a recent effort to glean new information about the
effects of the transoceanic tsunami resulting from the1755 Lisbon, Portugal earthquake on the
islands of the Lesser Antilles in the Caribbean has already yielded results (Ruffman, 2005). As
demonstrated by the Japanese and Caribbean studies, historical records are likely to contain
information about tsunamis that could make an important contribution to future tsunami studies.
Ground-Truthing Prehistoric Tsunamis Models: The geologic record can provide the
opportunity to ground-truth models of tsunamis that are unrepresented in the modern record.
Unrepresented tsunamis may include rare events resulting from mega-thrust earthquakes,
earthquake-triggered landslides, volcanic eruptions or collapse, and meteorite impacts. The
characteristics (e.g., thickness, grain-size distribution) and distribution of tsunami deposits can help
to determine if models are realistic and also to constrain models. For example, modeling of a
tsunami generated by collapse of the flank of the Cumbra Vieja volcano in the Canary Islands off the
western coast of Africa predicted a transoceanic tsunami with 10-25 m waves reaching the eastern
coast of the United States (Ward and Day, 2001). More recently, it has been argued that the size of
the landslide blocks and thus the waves reaching the U.S. shores may be smaller than first estimated.
Although this is a low probability event (perhaps occurring once every 1,000-10,000 years), the
proposed mega-tsunami has captivated the imagination of many, especially the media. If such an
event occurred during the Holocene, it probably left a signature along the coasts of northwestern
Africa, southwestern Europe, islands of the Lesser Antilles, and southeastern U.S. A simple test of
the model would be to search coastal areas on both sides of the Atlantic for an unusual and
synchronous tsunami deposit that matched the age of the most recent volcanic collapse in the Canary
Islands.
Ecological, Economic and Societal Impacts of Tsunamis: Many coastal areas (i.e., shore-zone)
experience dramatic increases in levels of flooding, accelerated erosion, loss of wetlands and lowlying terrestrial ecosystems, and seawater intrusion into freshwater sources as a result of a tsunami.
Prediction of shoreline retreat and land loss rates and ecological recovery is critical to the planning
of future coastal zone management strategies, and assessing ecological impacts due to habitat
changes and loss.
Rates of shoreline recession vary dramatically alongshore and are a function of shoreline type,
geometry and composition, geographic location, size and shape of the associated coastal water body,
coastal vegetation, and water level, maximum run-up and lateral extent of the tsunami. In addition,
the function of many shore zones may be significantly altered by ecological state changes (the
transformation of one ecosystem class to another, e.g., wetlands to open water) forced by the
inundation of the tsunami. Ultimately, to manage wisely our coastal estuarine resources and
maximize human utilization, long term solutions of estuarine shoreline-erosion problems must be in
harmony with the dynamics of the total coastal system (i.e., shoreline and shore-zone). By evaluating
the modern shoreline dynamics and developing a predictive tool of habitat modification in response
to tsunamis of different size, we can provide coastal managers with products and knowledge needed
for proactive management and a basis for regulatory protection.
Evaluating the processes driving ecological change and subsequent recovery can be achieved
because wetland and estuarine zones exhibit strong environmental gradients since they encompass
the transition from terrestrial to marine conditions. As a consequence of this, numerous indicators
(e.g. lithological, biological, such as inter-tidal organisms, and geochemical) tend to occur in
distinctive vertical zones, reflecting physical variables such as the duration of sub-aerial exposure
(related to water level) and salinity. Knowledge of modern coastal environment prior to a tsunami
can be applied to sediments recovered in an area affected by a tsunami to provide quantitative
estimates through time of water level (and storm surge height), salinity, pH, substrate and vegetation
cover.
Linking Onshore and Offshore Tsunami Records: Tsunami deposits constitute marker horizons
in the stratigraphic record that are essential isochronous and hence have the potential to serve as the
basis for time-stratigraphic correlation between sediments formed on land and lakes and on the seabottom. In recent years, considerable advances have been made in the recognition, extraction and
identification to source of tsunami deposits from a range of sedimentary contexts. However
correlation on the basis of tsunami deposits must meet the following criteria: first, a lengthy
stratigraphic record must be available; second, evidence of change in the stratigraphic record must
be clear and unequivocal; third, sedimentation must be continuous; and fourth, correlations will be
more secure if a time-frame can be established.
Identification of Tsunami Precursors: Microfossil data collected from the Cascadia subduction
zone and Alaska indicate that relative sea level may have risen in the years and decades immediately
prior to several late Holocene earthquakes and subsequent tsunamis. These observations suggest
that pre-seismic relative sea-level rise may be an early warning of an imminent plate boundary
earthquake. Comments on this work include the suggestion that ENSO-type processes, shakinginduced lowering of the marsh surface and associated sediment compaction prior to coseismic
subsidence, or to the effects of bioturbation and sediment mixing could explain the changes
observed. These alternative hypotheses can be tested through detailed analysis of the other events
sampled within Cascadia, Alaska and elsewhere. If pre-seismic signals occur in these events then
this would be a strong case against the other explanations.
The identification of these subtle pre-seismic changes raises the specter that acceleration in the rate
of relative sea-level rise may represent a precursor of an imminent plate boundary earthquake. We
do not yet know the pattern of these movements and how they relate to the spatial pattern of
displacement during the subsequent earthquake. Because of the potential of identifying a precursor,
we must place particular attention in the microfossil analysis on the period immediately prior to each
burial event. Thus, sampling interval for pollen and diatom analyses across these contacts will
typically be less than 1cm.
By looking at complete earthquake deformation cycles we introduce a timescale dimension to
test current seismic models that is much longer than just depending on direct observations that by
definition are limited to timescales of years or, at best, decades. Current research, including GPS
measurements, seismicity observations and model development, is producing much debate about
changes in strain accumulation, both spatially and through time in active subduction zones.
Significant elements of the arguments for seismic quiescence and the role of aseismic slip in
relieving the build-up of crustal strain currently rely on measurements that cover less than a decade
and the up-scaling of a laboratory-derived friction law to produce a numerical simulation model.
These have significant limitations. Leveling and GPS data cover only a fragment of the post-seismic
and interseismic and the duration of the observation records are inadequate to test the hypothesis that
the locking of the plate interface varies with time.
Identification of interseismic and pre-seismic relative sea-level change leading up to former
earthquakes can be applied to current and developing seismic models. These models include
elements such as seismic quiescence and aseismic slip and develop from theory but with
observations over a limited timescale, typically a few years to a decade. Field evidence of former
earthquakes and tsunamis will provide an independent test of these models using a much longer
timescale of decades to centuries. One particular hypothesis to test is whether a change from relative
sea-level fall during interseismic strain accumulation to relative sea-level rise reflects that significant
quasi-stable sliding occurs prior to a great earthquake.
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