tsunami geology – what do we know? what do we want to...
TRANSCRIPT
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TSUNAMI GEOLOGY –What do we know?
What do we want to know?
Jody BourgeoisEarth & Space Sciences
University of Washington
Notes for the NSF Workshop 26-28 December 2006
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TSUNAMI GEOLOGY –What do we know?
What do we want to know?
Jody BourgeoisEarth & Space Sciences
University of Washington
Credit to Hig[Bretwood Higman}
Credit especially to Hig, and also many other current and former graduate students and colleagues
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Digital Globe
Jantang, Aceh 500 m
Since 26 December 2004, we know a lot more about tsunami erosion and deposition. Moreover, the community of scientists working on these problems had expanded rapidly. Satellite overlay by Bretwood Higman.
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We also have many still and video images of the 26 December 2004 tsunami. This is the tsunami arrival on the coast of Thailand. Note the bore front, and the long, relatively flat, unidirectionally flowing water behind. The flow is fully turbulent and sediment charged.
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Higman, UW
I asked Hig to make this diagram to educate the public as to a fundamental difference between tsunamis and wind waves – the amplitude might be similar, but the tsunami will continue to flood the land for 10s of minutes; retreat commonly can take longer than flooding.
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Nicaragua 1992 tsunami effects, le PopoyoBourgeois, 1993, in Higman and Bourgeois, in press
As per this morning’s discussion, here are some typical tsunami-flooded profiles in Nicaragua; in this locality the tsunami height was typically 4-6 m above sea level. Most flow indicators are landward, and evidence is that much of the tsunami floodwater drained in specific channels, rather than back across the same surface of flooding.
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Nicaragua 1992 tsunami effects, le PopoyoBourgeois, 1993, in Higman and Bourgeois, in press
To match the following set of slides, I’ve just flipped the profiles so that the tsunami would be approaching from the left.
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Tsunami approaching the shore: The tsunami may approach the beach as a breaking wave. Sometimes the water will recede before this wave.
Higman graphic
Here is a cartoon view of a tsunami attacking a coastal site like Nicaragua [and many, many other examples since documented]. As the wave approaches, there is some drawdown [in most cases]; the tsunami front is highly turbulent, and we can expect that it is fully charged with sediment before it reaches vegetated surfaces, and decelerates across a profile.
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Tsunami arrives on shore: The tsunami will surge inland. Because the tsunami wave is very long, the water remains high after the wave arrives.
Higman graphic
Most [but not all] videos of tsunamis show a breaking front. In the case of Nicaragua, eyewitnesses decribed both the white water and the NOISE at the wave approached.
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Tsunami does damage: The tsunami will flow violently inland, carrying whatever lies in its path.
Higman graphic
Vegetation is a roughness element that can decelerate the flow. As illustrated yesterday by Dr. Shuto, fast flows will destroy trees and other objects [like houses].
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Tsunami pauses: The water may stop flowing before returning to the sea.
Higman graphic
Eyewitnesses commonly describe a cessation of flow and slow lowering of water, rather than retreat as “violent” as the wave attack. Of course there are many variations depending on initial wave characteristics as well as local bathymetry and topography. A sedimentologist looking for evidence of this stage might look for finer-grained sediment deposition, such as a mud cap.
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Water withdraws: Floating debris are pulled back into the ocean.
Higman graphic
The water retreats between waves, though some eyewitnesses describe second or later waves coming in on top of flooded surfaces.
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More tsunami waves are likely: Usually there will be multiple tsunami waves. Some of these later waves can be larger than the first wave.
T = 20 - 60 minutesHigman graphic
Tsunamis are wave trains. In Nicaragua, there was only one large wave that flooded the land, but especially in the biggest tsunami cases such as Chile and Sumatra, multiple waves flood the land. The period is tens of minutes, much closer in order of magnitude to tides than to wind waves.
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Possible [partial] analogues?
•Tidal bores•Dam breaks•Turbidity currents •Pyroclastic flows
Hig man graphic
Fully developed boundary layerHigh turbulence at head
Quasi-steady uniform flow?
Higman
So—what can we learn from other processes that start suddenly and have time periods of 10s of minutes to hours? First, expect the initial wave to be erosional [turbulent head], followed commonly by rapid deposition, most likely from suspended-load conditions. Result—graded beds and parallel laminated beds. Bed forms [indicators of bed load transport] [exhibited as ripple cross-lamination or other cross bedding] are rare.
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Spring 1987
Brian Atwater,USGS at UW
June 2005
Mary Ann Reinhart
Coast of Washington State
Let’s go back to earlier studies, example from the coast of the northwestern United State. 20 years ago, the questions were much more basic – in our own case in the U.S. Pacific Northwest, many people didn’t “believe” in the Cascadia subduction zone existed or produced large earthquakes and tsunamis, and numbers of scientists didn’t “believe” that tsunamis left deposits.
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April 1987 – Willapa Bay, Washington State coast
Mary Ann Reinhart
BASIC QUESTIONS:
Is it a tsunami deposit?What was the source?How big was the tsunami?
[height, velocity]How many waves?Are there other, older deposits?What is frequency of deposits?
These were the kinds of questions we were asking 20 years ago.
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1957 tsunami from Aleutians
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Oahu, Hawaii
NOAA slide set
It was surprising to us that tsunami scientists resisted the idea that tsunamis leave deposits [in part it was argued because they are solitons] because there were several pieces of evidence of such deposits from the 1946, 1957, and 1960 [but not specifically talking about “tsunami deposits”].
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Copalis deposit
Copalis River bank, Washington State coast
grassy field c. 300 years ago
tsunami deposit
Hypothesis:mudflat deposits in years after tsunami
One approach: study by analogueand case histories
On the coast of SW Washington, a thin sand layer was present above a marsh peat [i.e., deposited on a vegetated surface], overlain by tidal mudflat deposits [probably indicating subsidence]. Brian Atwater developed a hypothesis that this record represented co-seismic subsidence from a subduction-zone earthquake, followed by tsunami flooding and deposition. He asked us to examine the details of the sedimentology. We proposed a sediment-transport analysis, but reviewers, who were skeptical overall in any case, urged us to study modern analogues, so we went to Chile.
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1960 southern ChileMw 9.5
max runup 25 mHilo runup 10 m
Field work 1989
analogue?
This coastline of south-central Chile has similarities to the coast of Washington State. For example, the vegetation is similar. At this locality, the village of Mehuinat the mouth of the Rio Lingue, the mouth of the river was severely eroded by the tsunami, which overtopped the rocky island in this view. It washed away the fishermen’s village, which would have been to the left of the rock. 40 people died here, but many survived by running to local hillsides [because there was local knowledge of past tsunamis].
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1960 tsunami deposits
rock
The tsunami spread beach and river sediments over farmers’ fields.
This aerial photo shows the erosion near the river mouth, with areas of tsunami-sand deposition shaded in tan. We interviewed eyewitnesses, andwithout prompting, they described the post-tsunami surface (of their former fields) as covered with “arena” (Spanish for sand). Just one example [red dot] from a locality similar to Washington coast tideland examples.
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In marsh
stove-pipecorer
1989 Chilean-American field crew, Rio Lingue marshJB 1989
We and our Chilean assistants and collleagues improvised with a stove-pipe for coring, because the coring equipment we brought didn’t work.
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Fine deposits
May 1960 farmer’s land
22 May 1960 tsunami deposit
post-1960 marsh mudRio LingueChile
splitstove-pipecore1989
TSUNAMI-DEPOSIT “TRINITY”
Here is a split-open core, showing the three layers typical of a post-tsunami event: the former surface, the tsunami deposit, and more sediment accumulated above. This shoreline subsided during the earthquake, so what was a farmer’s field is now a marshy tideland.
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Copalis deposit
Copalis River bank, Washington State coast
grassy field c. 300 years ago
tsunami deposit
“Confirmation” by analogue:mudflat deposits in years after tsunami
So here is that very similar example from the coast of Washington State. The age of this tsunami deposit was originally determined by radiocarbon dating to be about 300 years ago. Later precision radiocarbon dating and tree-ring work narrowed the time down to the period of about 1690-1710, and later even more accurately to about 1700.
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April 1987
Mary Ann Reinhart
BASIC QUESTIONS:
Is it a tsunami deposit?What was the source?How big was the tsunami?
height, velocityHow many waves?Are there other, older deposits?What is frequency of deposits?
Despite the satisfying [but not rigorously testable] hypothesis confirmation, we were left with many questions. In particular, how big was the tsunami, and from that could we tell how big the earthquake was?
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Reinhart (unpubl. M.S.)
Map of tsunami depositsin SW Washington
Sediment transport model (1989-1990)
Mary Ann Reinhart conducted detailed analyses of some deposits in Willapa Bay and concluded that 1) the flooding in the interior of the bay was on the order of 2 m across vegetated tide flats and 2) the calcualted velocities of this flooding precluded storm surge.
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AtwaterMusumi-Rokkaku
SatakeTsujiUeda
Yamaguchi2005
USGS & UW Press
This earthquake and tsunami are now considered “historic” because records of its arrival have been found in Japan. Estimates of the earthquake magnitude are about Mw 9.0; tsunami propagation and runup models are in progress.
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Paleotsunami Deposits
Other DepositsTsunami Deposits
Tsunami Flows
Differentiation
Common Process
Source
Constraints
Propagation
Taphonomy
SedimentModeling Model
Validation
JUNE 2005 NSF Tsunami Deposit Workshop
“Matrix of knowledge”
So that’s one case history of a study that started 20 years ago. In the 1990s, a number of tsunamis occurred [starting with Nicaragua 1992], and geologists began to work with other tsunami scientists to fill out the matrix shown here. We geologists start at the paleotsunami deposit end, and work toward the source.
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Earthquake
AsteroidVolcano
Landslide
Higman graphic
Research question: Can we use tsunami geology to help distinguish tsunami sources?
With regard to source, here’s a very basic question, not easily answered.
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Cretaceous-Tertiary boundary in Texas
65 million years agoglobal mass extinction
Unusual, coarse-grained layer
in North AmericanGulf Coast region
But here is another example of using sedimentary geology to answer a tsunami question. Actually, this unusual layer was decribed in the literature as a turbidite, a storm deposit, or a sequence boundary. I suspected tsunami and proposed to go example the deposit.
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Brazos River area, Texas
Darting Minnow Creek
Cottonmouth Creek JB/87
Here is my sketch of the unusual layer at the boundary. Above and below this coarse, sandy layer are marine shelf or slope mudstones. They don’t crop out well. We asked the question, “If this layer was deposited in about 100 m, as the mudstone suggests, how could such large rocks be moved, as shown in this coarse layer?” This treatment used initiation of transport of large clasts from lift and drag [because they are much larger than the bed material].
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Shear stress necessary to transport largest clastsrequired wave 50-100 m high in 100 m water depth
Bourgeois, Hansen, Wiberg & Kauffman 1988
Here was our analysis, published in Science in 1988.
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Outstanding problem:
The Chicxulub impactwas in shallow water.
How can we model such a tsunami?
There is little doubt now that a large (about 10-km diameter) body hit the Yucatan Peninsula 65 million years ago. One of the interesting aspects of the tsunami problem that remains for me is that the impact was in shallow water, and you can’t make a big wave out of shallow water. So maybe a lot of the tsunami deposits in this region come from impact-earthquake-triggered landslides. It’s estimated that this impact produced an earthquake on the scale of about magnitude 12, much larger than any subduction-zone earthquake could be.
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How do we evaluate “outrageous” claims for tsunami depositsand tsunami geomorphology?
“chevrons”better known as parabolic dunes
Impact-tsunami science is a frontier field with more speculation and dubious science than in more-established fields. For example, it’s been suggested that chevrons such as these in Australia and others in Madagascar were produced by mega-tsunamis.
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White Sands, New Mexico
Idaho, Snake River plain
eastern Washington State
These are well studied and understood eolian bed forms
However, parabolic dunes are common in many eolian settings, including MANY cases far from the sea. Moreover, the examples in Australia and Madagascar are strongly parallel to prevailing winds, the bedforms showing no evidence of bathymetric or topographic steering, as would be expected even for a mega-tsunami.
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How are large boulders transported by storms and by tsunamis?
Can we tell which is which?
Scheffers, 2005 NSF Workshop
Another field with lots of controversy and speculation is the interpretation of large clasts in coastal settings. At the 2005 workshop, this area of study produced the most heated debates, with not very productive resolution. Storms move large clasts; tsunamis move large clasts. A frontier area.
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What is the offshore record of tsunamis?
Another frontier area is the nature of tsunami deposits offshore. Geologists are very interested in this question, but engineers are not, particularly. It is hard to design appropriate field programs, and to get them funded, given the logistics of offshore work. Geologists have claimed that certain features in sedimentary rocks millions to billions of years old may be the product of tsunami action in the offshore region. My argument, for one—most tsunamis are much less effective than big storms, once you get out beyond the nearshore region.
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Paleotsunami Deposits
Other DepositsTsunami Deposits
Tsunami Flows
Differentiation
Common Process
Source
Constraints
Propagation
Taphonomy
SedimentModeling Model
Validation
JUNE 2005 NSF Workshopon Tsunami Deposits[or Tsunami Geology]
One of our highest priorities, and one where there is ongoing progress, is the distinction of tsunami deposits from other possibly similar deposits, or deposits found in similar settings. Note that while we want to distinguish tsunami deposits from these other deposits, we also note we can learn by features that different processes have in common.
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Storm with storm surge
Short waves dissipaterapidly carrying sedimentshort distatnces
Flow reversesoften
Steep front withenhanced turbulence
Erosion steepens beach
Erosionfocused at top ofbeachSteep fronted tsunami
Ordinary conditions
Research priority: Distinguish storm deposits
from tsunami deposits
Higman
The storm here is a little lurid, but the basic message is clear—storm surges and tsunamis may have similar amplitudes, but they have distinct characteristics that should be recorded in the sediments they leave behind.
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Storm with storm surge
Short waves dissipaterapidly carrying sedimentshort distatnces
Flow reversesoften
Steep front withenhanced turbulence
Erosion steepens beach
Erosionfocused at top ofbeachSteep fronted tsunami
Ordinary conditions
Research priority: Distinguish storm deposits
from tsunami deposits
Bedload, prolonged reworking
Suspended load, rapid deposition
Higman
For example…
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Katrina effects, Deer Island, off Biloxi, Mississippi [Eipert, in progress]
Higman, 2005
In case you don’t believe storm surges can rival tsunamis in amplitude… But this storm surge left primarily a cross-stratifield, wedge-shaped sand deposit of limited local extent.
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Hig man graphic
Higman, in progress
Research priority: To quantify the relationship between tsunami behaviorand the geologic recordof deposition and erosion
Back to the “holy grail” of tsunami sedimentology.
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Example of a site survey, Sri Lanka2005
Data collected includes:
•Topographic profile•Tsunami elevations,
inundation, runup•Tsunami flow depth•Tsunami deposit
documentation &samples
DamageEyewitness accounts
former soil surface
tsunami sand deposit
Many detailed surveys of tsunami deposits have resulted in a plethora of information. Note the deposit here is parallel laminated, indicating deposition from suspended load [or sheet flow]. We also brought some peels of these kinds of deposits for your examination at the workshop.
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TsunamiFlow Speed
Inverse Modeling of Tsunami Flow Speed from Tsunami Deposits – Jaffe et al.
Rousecalculation
Tsunami deposit Sediment concentration in water column necessary to produce deposit
U* [Uf]
If the tsunami drops it sediment rapidly [as seems indicated here by a graded bed—this case from Papua New Guinea], after development of a quasi-steady uniform flow, one can back-calculate flow speeds using the Rouse equation and the grain size distirbution of the deposit.
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Atwater and Moore, 1992
Andy Moore, UW Ph.D.
An alternative method, especially if the sediment is deposited across a vegetated surface, as in this case [a tsunami deposit about 1000 years old near Seattle], is to use a simple trajectory, or advection model.
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Characteristic <u> = 4 m/sCharacteristic depth = 1.4 m
Moore, M.S. thesis & 2001 workshopMoore and Mohrig (1995 abstr.)
This model examines the farthest a relatively coarse grain gets across the surface, assuming it started near the top of the flow. This method solves for a depth-velocity product, so another method must to be used in conjunction, if solving for each.
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Calculations from Moore’s M.S. analysis were used to calibrate Koshimura and Mofjeld’s tsunami model
Koshimura & Mofjeld
Though relatively simple [if not crude], the results from this analysis have helped calibrate runup models from this 1000-year old tsunami from a Seattle fault earthquake.
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models
(Willapa Bay sedimentationand erosion model)
Lesser & Gelfenbaum
Study of presence, absence, distribution, and character of tsunami deposits can
1) Help ground-truth models2) Provide calibration for
models3) Provide data where models
are difficult to run.
Blue -- erosionRed -- deposition
These different kinds of models are simple, but we must start with simple! In any case, the detailed mapping and description of these deposits is an important component of paleotsunami studies, which on the coast of Washington state, is all the information we have [i.e. no historic local cases].
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Research priorities:•Develop and comparetsunami sedimentation models
[discussions later today]•Conduct laboratory experiments•Continue field studies --
develop protocols for field and lab [e.g., grain size] analyses
•Continue careful description of sedimentary structures and textures
Much remains to be done.
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Kelsey et al., 2004Kelsey et al. 2004
Importance of longer-term records
I didn’t have time, but here I’ve included a few slides about longer-term records of tsunami deposits, going back thousands of years, that is, over many earthquake cycles. Such studies [example above] have been key to working out recurrent intervals for Cascadia earthquakes, a necessary component for risk assessment.
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Tsunami deposits tell us about larger, prehistoric tsunamis, even in regions with large historic tsunamis.
HOKKAIDO
Nanayama, Atwater, Satake and others
Also, this slide shows that even in areas where there is a historic record of tsunamis [though not very long on Hokkaido] a prehistoric tsunami deposit shows that a 13th
century tsunami was larger than any historically documented tsunami on the Tokachi-Oki coast of Hokkaido.
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Applications of tsunami geology studies
PaleoseismologyNeotectonics – plate tectonicsArchaeologyCoastal geomorphologySeismic & tsunami hazardsEducation & outreach
Tsunami geology studies have many applications. I’ll give an example of a “pure science” application of tsunami-deposit studies to a plate tectonics problem.
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Tsunamis in understandingtectonic behavior
FUNDINGU.S. National Science Foundation
Russian Foundation for Basic Research
This is part of a large, multiyear, multidisciplinary study of coastal morphotectonicsand paleoseismology of the coasts of Kamchatka and the Kuril Islands. This very sparsely populated coast has a superb record of tsunami deposits, with age control from dated volcanic ash layers. In this region, even historical tsunamis are poorly documented.
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LARGE Polar with plate boundaries
OKH
BER
Can we quantify the rate of convergence?
The example I’ve chosen to talk about is the region north of the known, activesubduction zone. Some scientists assign BER and OKH to the North America plate, but others have proposed that the Bering and Okhotsk plates are moving independent of North America. Accumulating data, including our work, supports the latter model. So—the question is, can we reconstruct tsunamigenic earthquakes to help quantify the rate of plate convergence here?
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in Bourgeois et al. (2006)courtesy of
Kevin Mackey &Kaz Fujita
recentearthquakes
11 Nov 1969
Compared to the subduction zone to the south, this region is relatively quiet seismically, but there have been a number of historical, large earthquakes, including a very recent onshore one [March 2006]. Here we treat the tsunamigenic thrust earthquake of 11 Nov 1969.
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KAMCHATKA LANDMw 7.7 tsunamigenic earthquake
Green diamonds are field sites; I’ll show just one example of our work, from Stolbovaia [Sb].
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Stolbovaya field site
peat & volcanic ash
Bering Sea coast
Some idea of the field area, along the southern Bering Sea coast
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Stolbovaya siteProfile 1, trench 104
1969 Ozernoi tsunami deposit ->1964 Shiveluch volcanic ash ->
paleo-tsunami deposit ->
paleo-tsunami deposit ->
Shiveluch c. 1650 A.D. ->
Ksudach caldera c. 250 A.D. ->
One of about 50 excavations, showing a coast sand layer above soil bearing a 1964 volcanic ash layer. We interpret this as the 1969 tsunami deposit.
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The 1969 tsunami runup at this profile was about 5 meters.
The tsunami that deposited this layer generated runup of about 5 meters.
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Maximum elevation of 1969? tsunami deposits
Compared to a tsunami model ofpropagation
Southern deposits cannot be explained by 1969
Martin, Weiss, et al.,in progress
max deposit elevation (m)
Plotting all the young deposits along this coast and modeling the 1969 tsunami, we found we couldn’t explain the southern high runups. Ongoing modeling indicates that these are probably from a 1971 tsunami that’s been barely documented. [Both these tsunamis, though, have tide gage records on Kamchatka and in Hilo, e.g. –they did happen!
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Going back in time, we have 14 profiles~50 excavations with records >3000 years.
Bourgeois et al. 2006
Our excavations show that there are many other prehistoric tsunami deposits in this region.
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Tsunami frequencyper 1000 years
This is an early attempt to quantify tsunami recurrence based on the deposits [compared to another Kamchatka site along the subduction zone]. More complete results are in Bourgeois et al., 2006 Bulletin of the Geological Society of America. Note that the recurrence at Stolbovaya is greater than for Cascadia, e.g., The dropoff in recurrence is probably an effect of preservation.
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Surface erosion
Bioturbation, weathering, infiltration of fines
Liquefaction
Loss ofrecords byerosion
Higman graphic
[aside] Research priority: Understand “taphonomy”
of tsunami deposits[taphonomy = post-life history]
That brings up another research priority—we need to understand and maybe even quantify preservation factors for tsunami deposits.
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Tsunami deposits support a version of thistsunami model, which inverts to fault deformation with
~3 m horizontal shortening
Back to the 1969 tsunami, our models to fit the tsunami –deposit data give about 3 [2-4] m horizontal shortening.
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1969 Ozernoi earthquake modeled by tsunami data & deposits indicates c. 3 m horizontal shortening
Recurrence intervals for such tsunamis (from deposits) indicates shortening
of ~15 mm/yr over last 4000 yr
Using the deposit recurrent interval, we get a [horizontal] convergence rate of 15 mm/yr; this is inconsistent with a single North American plate model and consistent with motion of the Okhotsk and/or Bering plates. This photo shows uplifted marine terrace at Cape Ozernoi (Kamchatka Peninsula), looking south. The rocks in the water are the modern marine terrace. These terraces and a 1969 earthquake at Cape Ozernoi and its tsunami provide evidence for a plate boundary east of this Cape. Photo 2003 J. Bourgeois
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Long-Term Record Challenges
GeochronologyCorrelationStatistics
How many observationsare sufficient?
What is the preservation factor?
Here are some of the outstanding challenges for studies of long-term records of tsunami deposits [you can see more of this on my powerpoint from the NSF tsunami deposits workshop]
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Going beyondtsunami recurrence
to earthquake recurrence
As already noted, we want to use tsunami deposits, amongst other applications, for reconstructing earthquakes and their recurrence.
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Going beyondtsunami recurrence
to earthquake recurrence
and onward to paleotsunami and earthquake magnitudes
And even farther along, we want to invert these studies to determine paleotsunami and earthquake magnitudes.
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Challenges
PaleogeographyPaleobathymetryPaleotopography
and onward to paleotsunami and earthquake magnitudes
The farther back you go in time, the more challenging the problems – think about how many factors there are in modeling a modern tsunami. Then, realize that to reconstruct the past, we need to reconstruct all the factors you might need.
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Paleotsunami Deposits
Other DepositsTsunami Deposits
Tsunami Flows
Differentiation
Common Process
Source
Constraints
Propagation
Taphonomy
SedimentModeling Model
Validation
JUNE 2005 NSF Workshopon Tsunami Deposits[or Tsunami Geology]
Each of these arrows represents paths of study where there remain important challenges. We would like your imput on what aspects of this web you find most important, any ideas you have for approaches, and interest in collaborating with sedimentologists.
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Tsunami geology—a lot of digging
Thank you!
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WE CAN USE (HISTORIC AND PRE-HISTORIC) TSUNAMI DEPOSITS AND TSUNAMI GEOMORPHOLOGY:
and where prehistoric tsunamis are larger; To help produce probabilistic hazard maps; To calculate tsunami recurrene intervals (typically centuries long); To understand tsunami behavior; To calibrate, test and enhance tsunami runup modeling; and To educate the public.
To reconstruct tsunamis in Earth history; To document tsunamis and tsunami hazard where unknown historically
This is just an extra slide with material from the poster, which you also can find at the tsunami deposits website.