A sneaker wave, also known as a sleeper wave, or in Australia as a king wave, is a disproportionately large coastal wave that can sometimes appear in a wave train without warning.
Terminology
The
term "sneaker wave" is popular rather than scientific, derived from the
observation that such a wave can "sneak up" on an unwary beachgoer.
There is no scientific coverage of the phenomenon as a distinct sort of
wave with respect to height or predictability as there is on other
extreme wave events such as tsunamis or rogue waves,
and little or no scientific evidence has been gathered to identify,
describe, or define sneaker waves. Although the term "rogue wave" —
meaning an unusually tall or steep wave in mid-ocean — is sometimes used
as a synonym for "sneaker wave," one American oceanographer
distinguishes "rogue waves" as occurring on the ocean and "sneaker
waves" as occurring at the shore, while the National Oceanic and Atmospheric Administration
loosely defines rogue waves as offshore waves that are at least twice
the height of surrounding waves and sneaker waves as waves near shore
that are unexpectedly and significantly larger than other waves reaching
shore at the time. Scientists do not yet understand what causes sneaker waves, and their relationship to rogue waves, if any, has not been established.
In a 2018 paper, Oregon State University
researchers wrote that sneaker waves form in offshore storms that
transfer wind energy to the ocean surface. The resulting waves then
arrive along a coastline during periods of calm weather, and the greater
amount of energy they contain compared to the regular waves that
preceded them causes them to travel far higher up the shore than the
other waves. As of 2021, the National Weather Service in the United States viewed ocean conditions along the United States West Coast
as favorable for sneaker waves when an offshore storm generates waves
with a particularly long period — perhaps longer than 15 seconds —
between swells, allowing the swells to build considerable force before
reaching shore, where they might appear either as conventional large
waves or as sneaker waves.
Characteristics
Sneaker waves appear suddenly on a coastline and without warning;
generally, it is not obvious that they are larger than other waves
until they break and suddenly surge up a beach. A sneaker wave can occur
following a period of 10 to 20 minutes of gentle, lapping waves. Upon arriving, a sneaker wave can surge more than 150 feet (50 m) beyond the foam line, rushing up a beach with great force.
In addition to containing a large volume of rapidly surging water, a
sneaker wave also tends to carry a large amount of sand and gravel with
it. It can be strong enough to break over rocks and float or roll large,
waterlogged logs lying on the beach weighing several hundred pounds, moving them up the beach during the landward surge and then back down toward the ocean as the wave retreats. Sneaker waves appear to be more common along steep coastlines than in areas with broader, more gently sloped beaches.
Hazards
The
unpredictability of sneaker waves and their tendency to arrive suddenly
after lengthy periods of gentle, lapping waves makes it easy for them
to surprise unwary or inexperienced beachgoers; because they are much larger than preceding waves, sneaker waves can catch inattentive swimmers, waders, and other people on beaches and ocean jetties
and wash them into the sea. The force of a sneaker wave's surge and the
large volume of water rushing far up a beach is enough to suddenly
submerge people thigh- or waist-deep, knock them off their feet, and
drag them into the ocean or trap them against rocks.
Many coastlines more prone to sneaker waves lie in colder parts of the
world where beachgoers tend to wear heavier clothing; the amount of sand and gravel in a sneaker wave can quickly fill such clothing and footwear such as boots with sediment that weighs a person down as he or she is swept up a beach and then back into the sea, increasing the chances of drowning.
Floating and rolling logs in a sneaker wave also pose a danger, as they
can badly injure people as well as pin people down when they come to
rest, and it can be difficult or impossible to move such a log before a
person pinned by it drowns as later waves arrive and fill the person's
lungs with water and sediment.
Geographic distribution
Sneaker waves are mainly referred to in warnings and reports of incidents for the coasts of Central and NorthernCalifornia (including the San FranciscoBay Area's beaches, especially Ocean Beach, Baker Beach, and those that face the Pacific Ocean, e.g. from Big Sur to the California–Oregon border), Oregon, and Washington in the WesternUnited States.Sneaker waves also occur on the coast of British Columbia in WesternCanada, especially the province's southern coast, because they commonly occur on the west coast of Vancouver Island (including Tofino, Ucluelet, and Cape Scott Provincial Park). Sneaker waves are common on the southern coast of Iceland,
and warning signs were erected at Reynisfjara and Kirkjufjara beaches,
following three unrelated tourist deaths at those beaches over several
years, the third of them in January 2017. In Australia, where they are known as "king waves," sneaker waves occur especially in Western Australia and Tasmania, where they can be a hazard for rock fishermen.
Along much of the United States West Coast, sneaker waves kill more people than all other weather hazards combined. In Oregon, 21 deaths were attributed to sneaker waves from 1990 through March 2021, most of the deaths occurring between October and April, although sneaker waves also occurred at other times of year.
A sneaker wave incident gained worldwide media attention when two
large waves suddenly and unexpectedly struck a crowd watching the Mavericks surfing competition at Mavericks in Princeton-by-the-Sea, California, on February 13, 2010, breaking over a seawall onto a narrow beach and injuring at least 13 people. The incident was caught on film.
In March 2014, a massive wave struck Roi-Namur in Kwajalein Atoll in the Marshall Islands on an otherwise calm, sunny day, penetrating well inland, flooding parts of the island and swamping coastal roads.
On September 18, 2023, a sneaker wave smashed into a beachside restaurant at Marina Beach near Southbroom, South Africa, injuring seven people. One restaurant patron was swept out to sea but rescued by lifeguards. The wave was filmed.
Rio de Janeiro's Barra de Tijuca beach in Brazil experiences sneaker waves, known locally as ressaca waves. It also is a steep beach and a December 2023 news film shows the whole beach being cleared by a sneaker wave.
On 20 January 2024, one or more sneaker or rogue waves struck the United States Army′s Ronald Reagan Ballistic Missile Defense Test Site on Roi-Namur in Kwajalein Atoll in the Marshall Islands,
breaking down the doors of a dining hall, knocking several people off
their feet, moderately to severely damaging the dining hall, the
Outrigger Bar and Grill, the chapel, and the Tradewinds Theater, and
leaving parts of the island, including the automotive complex,
underwater. The flooding of the dining hall was filmed. The wave or
waves penetrated 300 feet (100 m) inland and probably were between 29
and 40 feet (8.8 and 12.2 m) tall amid a significant wave height of 10
feet (3 m) to 15 feet (5 m).
Seventh wave
In many parts of the world, local folklore predicts that out of a certain number of waves, one will be much larger than the rest. "Every seventh wave" or "every ninth wave"
are examples of such common beliefs that have wide circulation and have
entered popular culture through music, literature, and art. These ideas have some scientific merit, due to the occurrence of wave groups at sea,
but there is no explicit evidence for this specific phenomenon, or that
these wave groups are related to sneaker waves. The saying is likely
derived more from a cultural fascination with certain numbers, and it may also be designed to educate shore-dwellers about the necessity of remaining vigilant when near the ocean.
Subjective response to alcohol (SR) refers to an individual's unique experience of the pharmacological effects of alcohol and is a putative risk factor for the development of alcoholism.
Subjective effects include both stimulating experiences typically
occurring during the beginning of a drinking episode as breath alcohol
content (BAC) rises and sedative effects, which are more prevalent later
in a drinking episode as BAC wanes.
The combined influence of hedonic and aversive subjective experiences
over the course of a drinking session are strong predictors of alcohol
consumption and drinking consequences. There is also mounting evidence for consideration of SR as an endophenotype
with some studies suggesting that it accounts for a significant
proportion of genetic risk for the development of alcohol use disorder.
Theoretical models
Low Level of Response Model
The
Low Level of Response Model proposes that individuals who are less
sensitive to the effects of alcohol are at greater risk for developing
alcohol use disorder. One explanation for this phenomenon is that the
experiences of elevated intoxication constitutes a feedback mechanism,
which prompts drinking cessation. Low-level responders need to consume
more alcohol than high responders to achieve a similar level of
intoxication and experience the aversive effects of alcohol;
consequently, these individuals must consume more alcohol to trigger the
negative feedback loop.
Escalating alcohol consumption may ultimately contribute to the
development of tolerance, which further dampens sensitivity to alcohol's
unpleasant effects. Notably, there is no population-level demarcation
separating low from high responders and so level of response is
arbitrarily defined (generally in terciles) within a given sample.
Early studies compared SR in individuals (mostly males) with (FH+) and without (FH-) a family history of alcohol dependence
in order to demonstrate that individual differences in SR could be
considered genetically linked determinants of alcohol use disorder.
Non-placebo controlled studies conducted by Schuckit and colleagues
found that FH+ males experienced less of the aversive effects of alcohol
as compared to FH- males matched on key demographic and body mass
variables.
Furthermore, FH+ young males and their fathers showed similar SR after
reaching peak BAC, suggesting that SR is a heritable risk factor for the
development of alcohol use disorder.
Schuckit's placebo-controlled studies generally reported lower SR among
FH+, as compared to FH-, subjects along declining BAC, with differences
more evident among men than women. Additional studies found that FH+
subjects who experienced low-level of response were more than 4 times as
likely to meet criteria for alcohol use disorder at 10-year follow-up
as compared to FH- subjects who reported the same SR pattern.
Subsequent follow-up studies conducted primarily by Schuckit's group
established that low-level of response is a genetically linked risk
factor for alcohol use disorder, which is not better explained by robust
confounding factors such as age of first drink, current alcohol use and impulsivity.
A 1992 meta-analysis further buttressed the Low Level of Response Model
by reporting that sons of alcoholics exhibited lower responses to
alcohol on both the ascending and descending limbs of the BAC curve.
Importantly, differences in SR by family history were significant only
in the alcohol condition and not the placebo condition, suggesting that
SRs observed in the alcohol condition could be attributed to the
pharmacological effects of alcohol, rather than to a confounding factor.
A 2011 meta-analysis revealed that FH+ individuals reported lower SR in
comparison to FH- individuals across both limbs of intoxication,
consistent with the Low Level of Response Model.
These findings were more robust along the descending limb of the BAC
curve where sedative effects of alcohol are more prevalent and among
males who comprised the overwhelming majority of participants in early
SR studies.
Critics noted that studies supporting the Low Level of Response
Model only accounted for the negatively valenced sedative effects of
alcohol and that while decreased sensitivity to aversive effects of
alcohol would likely lead to increased drinking frequency and severity,
the subjective effects of alcohol are, in actuality, quite varied.
For example, it is widely accepted that the rewarding properties of
alcohol are reinforcing. Yet, according to the Low Level of Response
Model, reduced sensitivity to these rewarding effects is an indicator of
problematic drinking. To that end, critics have noted that in
Schuckit's seminal SR study,
FH+ males experienced more "energy" than FH- males along rising BAC,
suggesting that heightened sensitivity to the stimulating effects of
alcohol may convey risk for developing alcohol problems.
Another study found that FH+ subjects reported experiencing less
intoxication than FH- subjects in response to a placebo drink,
indicating that alcohol expectancies may account for differences in risk
more so than SR.
That is, individuals at greatest risk for developing alcohol use
disorder may expect alcohol to be more enjoyable and less aversive than
low-risk individuals.
Differentiator Model
The Differentiator Model
is based on the widely accepted notion that alcohol's effects are
biphasic. That is, the stimulating effects of alcohol (i.e., euphoria,
sociality, energy) are more prevalent as BAC rises (i.e., ascending
limb), while alcohol's sedative effects (i.e., relaxation, nausea,
headaches) are experienced most strongly as BAC falls (i.e., descending
limb). The Differentiator Model proposes that individuals at greatest
risk for developing alcohol use disorder (or those who already meet
criteria for alcohol use disorder) are more sensitive to the stimulating
effects of alcohol on the ascending limb of intoxication and less
sensitive to the sedative effects on the descending limb.
Additionally, the combination of heightened rewards and diminished
consequences over the course of a drinking episode increases motivation
to consume alcohol, leading to longer and more frequent drinking
episodes. Repeated engagement in these risky drinking occasions may
ultimately contribute to the development of alcohol use disorder.
Support for the Differentiator Model is mixed, perhaps reflecting
the paucity of studies designed to test the model itself or record SR
over both limbs of intoxication.
Both oral and intravenous alcohol administration studies reported that
FH+ subjects experienced elevated sensitivity to stimulating effects of
alcohol along rising BAC, without the corresponding attenuation of
sedative effects as BAC fell.
A study using an intravenous alcohol clamping method (participants were
titrated to a BAC of .06 g/dl whereupon alcohol was infused to maintain
a stable BAC for the duration of the study) reported that FH+ subjects
experienced heightened stimulation on the ascending limb of
intoxication, consistent with the Differentiator Model. However, FH+ subjects reported decreased stimulation during clamping, indicative of acute tolerance.
Findings by King and colleagues, which are largely corroborated
by a recent meta-analysis, suggest that the Differentiator Model best
characterizes heavy drinkers at risk for developing alcohol use
disorder.
Specifically, over the course of several studies, heavy drinkers
reported greater positive SR on the ascending limb of intoxication and
lower negative SR on the descending limb, in relation to light drinkers.
Increased sensitivity to alcohol's stimulating effects along rising BAC
and muted sensitivity to alcohol's sedative effects along waning BAC
were subsequently predictive of future increases in binge drinking,
blackouts, hangovers and alcohol use disorder symptomatology.
Measurement
The
Subjective High Assessment Scale (SHAS) captures sensations frequently
associated with intoxication such as "clumsy," "dizzy," "drunk" and
"high" and was administered extensively in early SR studies.
The SHAS is typically administered as a visual analog scale, allowing
subjects to rate the extent to which they experienced each symptom
during a given experiment.
Critics of the measure argue that it primarily captures the sedative
effects of alcohol while omitting many of alcohol's stimulating
properties.
The Biphasic Effects of Alcohol Scale (BAES) assesses 7 stimulating
(elated, energized, excited stimulated, talkative, up, vigorous) and 7
sedative (difficulty concentrating, down, heavy head, inactive, sedated,
slow thoughts, sluggish) effects of alcohol along an 11-point scale. Studies supporting the Differentiator Model have almost universally used the BAES, rather than the SHAS, as a measure of SR. Critics of the BAES assert that it does not adequately capture positive sedative effects. The Subjective Effects of Alcohol Scale (SEAS)
was published in 2013 to address this apparent limitation by
referencing positive and negative stimulating and sedative effects; to
date, this scale has not been widely used in alcohol challenge studies.
Genetic moderators
Most genetic studies in addiction research focus on the genetic determinants of diagnostic phenotypes
such as alcohol use disorder. However, because the causes of alcohol
use disorder are so numerous and varied, researchers have turned their
attention to endophenotypes, or distinct, genetically linked phenotypes
associated with a broad disorder. Endophenotypes are especially useful
in addictions research because they are more closely linked to genetic
variations than the broad disorder. Therefore, investigators have
explored the effects of genetic variation in the endogenous opioid system and the GABAergic system on SR.
Alcohol activates endogenous opioid receptors, potentiating
dopamine release which increases the rewarding effects of alcohol. To
that end, the A118G single nucleotide polymorphism (SNP) of the
mu-opioid receptor gene (OPRM1), has garnered much interest as a potential moderator of SR.
Numerous laboratory studies have demonstrated that G-allele carriers
experience the stimulating, hedonic effects of alcohol more strongly
than A homozygotes.
However, a study of non-treatment seeking participants with alcohol
dependence found that A homozygotes experienced more stimulation than G
carriers, and a study of heavy drinkers reported no differences in SR
between OPRM1 genotype. These mixed findings may stem from
differences in alcohol use severity among samples, as the allostatic
model of addiction contends that individuals shift from reward to relief
drinking as alcohol use disorder progresses.
Thus, it is possible that social drinkers and individuals with mild
alcohol use disorder may experience the hedonic effects of alcohol as
most salient while individuals with more severe alcohol use disorder may
consume alcohol for its negative reinforcing properties (i.e., to
reduce withdrawal symptoms). The use of retrospective, instead of real
time, self-reports of SR as well as differences in ethnicities of
samples may further contribute to discrepancies in studies exploring the
effects of the OPRM1 gene and SR. Taken together, the literature pertaining to the expression of SR by OPRM1 genotype suggests that the A118G SNP of the OPRM1 gene is associated with enhanced sensitivity to the stimulating, but not sedative, effects of alcohol.
Expression of the DAT1 dopamine transporter gene has also
been shown to predict severity of alcohol use disorder symptoms with a
recent study linking simultaneous carriers of the OPRM1 G-allele and DAT1 A10 allele homozygotes to pleasurable subjective effects along rising BAC.
Alcohol researchers have also evaluated the role of
gamma-Aminobutyric acid (GABA) receptors as moderators of SR. Most
investigation has focused on genes coding for GABAA receptors, which are involved in dopamine release. Some studies have linked GABRA2 and GABRG1 genes to reductions in the experience of positive and negative subjective effects.
Clinical implications
Because
SR is such a strong predictor of future alcohol consumption and
problems, medication development has focused on drugs which either
reduce the pleasant or increase the unpleasant effects of alcohol.
Naltrexone,
an opioid receptor antagonist, is frequently prescribed to patients
suffering from alcohol use disorder, with moderate effectiveness.
Studies have demonstrated that naltrexone reduces the stimulating and
heightens the aversive sedative effects of alcohol in individuals
at-risk for alcohol use disorder, contributing to decreases in
self-reported subjective high and liking of alcohol.
Only one study has reported on the effects of naltrexone on SR in a
sample of participants with alcohol dependence: naltrexone, in
comparison to a placebo, attenuated subjective stimulation within 10
minutes of administration of a moderate dose of alcohol, but not
thereafter.
Laboratory studies have shown that OPRM1 genotype
moderates the subjective effects of naltrexone in social and heavy
drinkers, such that G carriers reported reduced sensitivity to the
stimulating effects of alcohol.
Moreover, a placebo-controlled study of heavy drinkers of East Asian
descent demonstrated that G carriers experienced greater sensitivity to
alcohol's aversive effects as compared to A homozygotes.
There is limited evidence suggesting that quetiapine and varenicline increase the aversive effects of alcohol.
The New Madrid fault system was responsible for the 1811–1812 New Madrid earthquakes and has the potential to produce large earthquakes in the future. Since 1812, frequent smaller earthquakes have been recorded in the area.
The geological structure of Reelfoot Rift, USGS, 1996
The faults
responsible for the NMSZ are embedded in a subsurface geological
feature known as the Reelfoot Rift, which likely formed during the Cambrian Period. The Reelfoot Rift was first described by Ervin and McGinnis (1975) and believed to be of late Precambrian age. The rift failed to split the North American continent, but it has remained as an aulacogen (a scar or zone of weakness) deep underground.
This relative weakness is important, as it would allow the
relatively small east–west compressive forces associated with the
continuing westward continental drift of the North American plate
to reactivate old faults around New Madrid, making the area unusually
prone to earthquakes in spite of it being far from the nearest tectonic
plate boundary.
Since other ancient rifts are known to occur in North America,
but not all are associated with modern earthquakes, other processes
could be at work to locally increase mechanical stress on the New Madrid
faults. Some form of heating in the lithosphere
below the area has been suggested to be making deep rocks more plastic,
which would concentrate compressive stress in the shallower subsurface
area where the faulting occurs.
History
Earthquakes in the New Madrid and Wabash Valley seismic zones from 1974 to 2002, with magnitudes larger than 2.5
The zone had four of the largest earthquakes in recorded North American history, with moment magnitudes
estimated to be as large as 7 or greater, all occurring within a
3-month period between December 1811 and February 1812. Many of the
published accounts describe the cumulative effects of all the
earthquakes, known as the New Madrid Sequence, so finding the individual
effects of each quake can be difficult. Magnitude estimates and
epicenters are based on interpretations of historical accounts and may
vary.
Prehistoric earthquakes
As
uplift rates associated with large New Madrid earthquakes could not
have occurred continuously over geological timescales without
dramatically altering the local topography, studies have concluded that
the seismic activity there cannot have gone on for longer than 64,000
years, making the New Madrid seismic zone a young feature, or that
earthquakes and the associated uplift migrate around the area over time,
or that the NMSZ has short periods of activity interspersed with long
periods of dormancy.
Archaeological studies have found from studies of sand blows
and soil horizons that previous series of very large earthquakes have
occurred in the NMSZ in recent prehistory. Based on artifacts found
buried by sand blow deposits and from carbon-14 studies, previous large
earthquakes like those of 1811–12 appear to have happened around AD 1450
and 900, as well as around AD 300. Evidence has also been found for an apparent series of large earthquakes around 2350 BC.
About 80 kilometres (50 mi) southwest of the presently defined
NMSZ, but close enough to be associated with the Reelfoot Rift, near Marianna, Arkansas, two sets of liquefaction
features indicative of large earthquakes have been tentatively
identified and dated to 3500 and 4800 BC. These features were
interpreted to have been caused by groups of large earthquakes timed
closely together.
Dendrochronology (tree ring) studies conducted on the oldest bald cypress trees growing in Reelfoot Lake
found evidence of the 1811–12 series in the form of fractures followed
by rapid growth after their inundation, whereas cores taken from old
bald cypress trees in the St. Francis sunklands showed slowed growth in
the half century that followed 1812. These were interpreted as clear
signals of the 1811–12 earthquake series in tree rings. As the tree ring
record in Reelfoot Lake and the St. Francis sunk lands extend back to
1682 and 1321, respectively, Van Arsdale et al. interpret the lack of
similar signals elsewhere in the chronology as evidence against large
New Madrid earthquakes between those years and 1811.
December 25, 1699
The
first known written record of an earthquake felt in the NMSZ was from a
French missionary traveling up the Mississippi with a party of
explorers. At 1 pm on Christmas Day 1699, at a site near the present-day location of Memphis, the party was startled by a short period of ground shaking.
The Great Earthquake at New Madrid, a 19th-century woodcut from Devens' Our First Century (1877)
December 16, 1811, 0815 UTC (2:15 am); (M about 7.5) epicenter in northeast Arkansas, probably on the Cottonwood Grove fault;
it caused only slight damage to man-made structures, mainly due to the
sparse population in the epicentral area. The future location of
Memphis, Tennessee, was shaken at Mercalli level IX intensity. A seismic
seiche propagated upriver, and Little Prairie was destroyed by liquefaction.
Local uplifts of the ground and the sight of water waves moving
upstream gave observers the impression that the Mississippi River was
flowing backwards.
At New Madrid, trees were knocked down and riverbanks collapsed.
This event shook windows and furniture in Washington, DC, rang bells in
Richmond, Virginia, sloshed well water and shook houses in Charleston,
South Carolina, and knocked plaster off of houses in Columbia, South
Carolina. In Jefferson, Indiana, furniture moved, and in Lebanon, Ohio,
residents fled their homes. Observers in Herculaneum, Missouri, called
it "severe" and said it had a duration of 10–12 minutes.
Aftershocks
were felt every 6-10 minutes, a total of 27, in New Madrid until what
was called the Daylight Shock, which was of the same intensity as the
first. Many of these were also felt throughout the eastern US, though
with less intensity than the initial earthquake.
December 16, 1811, sometimes termed the "Dawn Shock" or
"Daylight Shock", occurred at 1315 UTC (7:15 am); (M about 7) with the
epicenter in northeast Arkansas.
January 23, 1812, 1515 UTC (9:15 am); (M about 7.3) epicenter around New Madrid, although this is disputed.
This was probably the smallest of the three main shocks but resulted in
widespread ground deformation, landslides, fissuring, and stream-bank
caving in the meizoseismal area. Johnston and Schweig attribute this earthquake to a rupture on the New Madrid North Fault.
A minority viewpoint holds that this earthquake's epicenter was in
southern Illinois. A 2011 expert panel urged further research to clarify
this point, stating that the Illinois hypothesis would mean that an
extended section of fault exists, perhaps still loaded and capable of
hosting a great earthquake in the future.
February 7, 1812, 0945 UTC (3:45 am); (M about 7.5) epicenter near New Madrid, Missouri. This was the largest event in the series, and it destroyed the town of New Madrid. At St. Louis, Missouri, many houses were severely damaged, and their chimneys were toppled. It appears to have occurred on Reelfoot fault, a reverse fault segment that crosses under the Mississippi River just south of Kentucky Bend and continues to the east as the Lake County Uplift.
In this event, uplift along the fault created temporary waterfalls on
the Mississippi River, created a wave that propagated upstream, and
caused the formation of Reelfoot Lake by damming streams.
A map of more than 4,000 earthquakes in the area since 1974
1812–1900
Hundreds
of aftershocks of the 1811–12 series followed over a period of several
years. Aftershocks strong enough to be felt occurred until 1817. The
largest earthquakes to have occurred since then were on January 4, 1843,
and October 31, 1895, with magnitude estimates of 6.0 and 6.6, respectively. The 1895 event had its epicenter near Charleston, Missouri. The quake damaged virtually all the buildings in Charleston, created sand volcanoes by the city, cracked a pier on the Cairo Rail Bridge, and toppled chimneys in St. Louis, Missouri; Memphis, Tennessee; Gadsden, Alabama; and Evansville, Indiana.
Modern activity
The largest NMSZ earthquake of the 20th century was a 5.4-magnitude quake on November 9, 1968, near Dale, Illinois. The quake damaged the civic building at Henderson, Kentucky, and was felt in 23 states. People in Boston
said their buildings swayed. At the time of the quake, it was the
biggest recorded quake with an epicenter in Illinois in that state's
recorded history. In 2008 in the nearby Wabash Valley seismic zone, a similar magnitude 5.4 earthquake occurred with its epicenter in Illinois near West Salem and Mount Carmel.
Instruments were installed in and around the area in 1974 to
closely monitor seismic activity. Since then, more than 4,000
earthquakes have been recorded, most of which were too small to be felt.
On average, one earthquake per year is large enough to be felt in the
area.
Potential for future earthquakes
In a report filed in November 2008, the U.S. Federal Emergency Management Agency
warns that a serious earthquake in the NMSZ could result in "the
highest economic losses due to a natural disaster in the United States,"
further predicting "widespread and catastrophic" damage across Alabama,
Arkansas, Illinois, Indiana, Kansas, Kentucky, Mississippi, Missouri,
Oklahoma, Texas, and particularly Tennessee, where a 7.7 magnitude quake
would cause damage to tens of thousands of structures affecting water distribution, transportation systems, and other vital infrastructure.
The earthquake is expected to result in many thousands of fatalities,
with more than 4,000 of the fatalities expected in Memphis alone.
The potential for the recurrence of large earthquakes and their
effects today on densely populated cities in and around the seismic zone
has generated much research devoted to understanding the NMSZ. By
studying evidence of past quakes and closely monitoring ground motion and current earthquake activity, scientists attempt to understand their causes and recurrence intervals.
In October 2009, a team composed of University of Illinois and Virginia Tech researchers headed by Amr S. Elnashai, funded by the Federal Emergency Management Agency,
considered a scenario where all three segments of the New Madrid fault
ruptured simultaneously with a total earthquake magnitude of 7.7. The
report found that there would be significant damage in the eight states
studied – Alabama, Arkansas, Illinois, Indiana, Kentucky, Mississippi,
Missouri, and Tennessee – with the probability of additional damage in
states farther from the NMSZ. Tennessee, Arkansas, and Missouri would be
most severely impacted, and Memphis and St. Louis would be severely
damaged. The report estimated 86,000 casualties, including 3,500
fatalities, 715,000 damaged buildings, and 7.2 million people displaced,
with two million of those seeking shelter, primarily due to the lack of
utility services. Direct economic losses, according to the report,
would be at least $300 billion.
Iben Browning's 1990 prediction
Beginning in February 1989, self-proclaimed climatologist Iben Browning, who claimed to have predicted the 1980 eruption of Mount St. Helens and the 1989 Loma Prieta earthquake
– predicted a 50% probability of a magnitude 6.5 to 7.5 earthquake in
the New Madrid area sometime between December 1 and December 5, 1990.Browning appears to have based this prediction on particularly strong
tidal forces being expected during that time, and his opinion that a New
Madrid earthquake was "overdue;" however, seismologists generally agree
that no correlation exists between tides and earthquakes.
The United States Geological Survey
(USGS) requested an evaluation of the prediction by an advisory board
of earth scientists, who concluded, "the prediction does not have
scientific validity."
Despite the lack of scientific support, Browning's prediction was
widely reported in international media, causing public alarm. The period
passed with no major earthquake activity in New Madrid or along the
120-mile (190 km) fault line.
Uncertainty over recurrence potential
The
lack of apparent land movement along the New Madrid fault system has
long puzzled scientists. In 2009, two studies based on eight years of
GPS measurements indicated that the faults were moving at no more than
0.2 mm (0.008 in.) per year. This contrasts to the rate of slip on the San Andreas Fault, which averages up to 37 mm (1.5 in) per year across California.
On March 13, 2009, a research group based out of Northwestern University and Purdue University, funded by the USGS, reported in Science
and other journals that the New Madrid system may be "shutting down"
and that tectonic stress may now be accumulating elsewhere. Seth Stein, the leader of the research group, published these views in a book, Disaster Deferred,
in 2008. Although some of these ideas have gained some acceptance among
researchers, they have not been accepted by the National Earthquake
Prediction Evaluation Council, which advises the USGS. In the November 5, 2009, issue of Nature, researchers from Northwestern University and the University of Missouri said that due to the lack of fault movement, the quakes along the faults may only be aftershocks of the 1811–12 earthquakes.
According to the USGS, a broad consensus exists that the
possibility of major earthquakes in the NMSZ remains a concern, and that
the GPS data do not provide a compelling case for lessening perceived
earthquake hazards in the region. One concern is that the small
earthquakes that still happen are not diminishing over time, as would be
if they were aftershocks of the 1811–12 events; another is that the
4,500-year archaeological record of large earthquakes in the region is
more significant than 10 years of direct strain measurement. The USGS
issued a fact sheet in 2009 stating the estimate of a 7–10% chance of a
New Madrid earthquake of magnitude comparable to one of the 1811–12
quakes within the next 50 years, and a 25–40% chance of a magnitude 6
earthquake in the same time frame. In July 2014, the USGS increased the risk assessment for the New Madrid area.
Tsunami waves do not resemble normal undersea currents or sea waves because their wavelength is far longer. Rather than appearing as a breaking wave, a tsunami may instead initially resemble a rapidly rising tide. For this reason, it is often referred to as a tidal wave,
although this usage is not favoured by the scientific community because
it might give the false impression of a causal relationship between
tides and tsunamis. Tsunamis generally consist of a series of waves, with periods ranging from minutes to hours, arriving in a so-called "wave train".
Wave heights of tens of metres can be generated by large events.
Although the impact of tsunamis is limited to coastal areas, their
destructive power can be enormous, and they can affect entire ocean
basins. The 2004 Indian Ocean tsunami
was among the deadliest natural disasters in human history, with at
least 230,000 people killed or missing in 14 countries bordering the Indian Ocean.
The Ancient Greek historian Thucydides suggested in his 5th century BC History of the Peloponnesian War that tsunamis were related to submarine earthquakes,
but the understanding of tsunamis remained slim until the 20th century,
and much remains unknown. Major areas of current research include
determining why some large earthquakes do not generate tsunamis while
other smaller ones do. This ongoing research is designed to help
accurately forecast the passage of tsunamis across oceans as well as how
tsunami waves interact with shorelines.
Terminology
Tsunami
The term "tsunami" is a borrowing from the Japanese tsunami津波, meaning "harbour wave." For the plural, one can either follow ordinary English practice and add an s, or use an invariable plural as in the Japanese. Some English speakers alter the word's initial /ts/ to an /s/ by dropping the "t," since English does not natively permit /ts/ at the beginning of words, though the original Japanese pronunciation is /ts/.
The term has become commonly accepted in English, although its literal
Japanese meaning is not necessarily descriptive of the waves, which do
not occur only in harbours.
Tsunamis are sometimes referred to as tidal waves. This once-popular term derives from the most common appearance of a tsunami, which is that of an extraordinarily high tidal bore.
Tsunamis and tides both produce waves of water that move inland, but in
the case of a tsunami, the inland movement of water may be much
greater, giving the impression of an incredibly high and forceful tide.
In recent years, the term "tidal wave" has fallen out of favour,
especially in the scientific community, because the causes of tsunamis
have nothing to do with those of tides,
which are produced by the gravitational pull of the moon and sun rather
than the displacement of water. Although the meanings of "tidal"
include "resembling" or "having the form or character of" tides, use of the term tidal wave is discouraged by geologists and oceanographers.
A 1969 episode of the TV crime show Hawaii Five-O entitled "Forty Feet High and It Kills!" used the terms "tsunami" and "tidal wave" interchangeably.
Seismic sea wave
The term seismic sea wave is also used to refer to the phenomenon because the waves most often are generated by seismic activity such as earthquakes. Prior to the rise of the use of the term tsunami in English, scientists generally encouraged the use of the term seismic sea wave rather than tidal wave. However, like tidal wave, seismic sea wave is not a completely accurate term, as forces other than earthquakes—including underwater landslides, volcanic eruptions, underwater explosions, land or ice slumping into the ocean, meteorite impacts, and the weather when the atmospheric pressure changes very rapidly—can generate such waves by displacing water.
Other terms
The use of the term tsunami for waves created by landslides
entering bodies of water has become internationally widespread in both
scientific and popular literature, although such waves are distinct in
origin from large waves generated by earthquakes. This distinction
sometimes leads to the use of other terms for landslide-generated waves,
including landslide-triggered tsunami, displacement wave, non-seismic wave, impact wave, and, simply, giant wave.
While Japan may have the longest recorded history of tsunamis, the sheer destruction caused by the 2004 Indian Ocean earthquake and tsunami event mark it as the most devastating of its kind in modern times, killing around 230,000 people. The Sumatran region is also accustomed to tsunamis, with earthquakes of varying magnitudes regularly occurring off the coast of the island.
As early as 426 BC the Greek historian Thucydides inquired in his book History of the Peloponnesian War about the causes of tsunami, and was the first to argue that ocean earthquakes must be the cause. The oldest human record of a tsunami dates back to 479 BC, in the Greek colony of Potidaea, thought to be triggered by an earthquake. The tsunami may have saved the colony from an invasion by the Achaemenid Empire.
The cause, in my opinion, of this phenomenon must be
sought in the earthquake. At the point where its shock has been the most
violent the sea is driven back, and suddenly recoiling with redoubled
force, causes the inundation. Without an earthquake I do not see how
such an accident could happen.
The Roman historian Ammianus Marcellinus (Res Gestae
26.10.15–19) described the typical sequence of a tsunami, including an
incipient earthquake, the sudden retreat of the sea and a following
gigantic wave, after the 365 AD tsunami devastated Alexandria.
Causes
The principal generation mechanism of a tsunami is the displacement of a substantial volume of water or perturbation of the sea. This displacement of water is usually caused by earthquakes, but can also be attributed to landslides, volcanic eruptions, glacier calvings or more rarely by meteorites and nuclear tests.However, the possibility of a meteorite causing a tsunami is debated.
Seismicity
Tsunamis can be generated when the sea floor abruptly deforms and
vertically displaces the overlying water. Tectonic earthquakes are a
particular kind of earthquake that are associated with the Earth's
crustal deformation; when these earthquakes occur beneath the sea, the
water above the deformed area is displaced from its equilibrium
position. More specifically, a tsunami can be generated when thrust faults associated with convergent or destructive plate boundaries move abruptly, resulting in water displacement, owing to the vertical component of movement involved. Movement on normal (extensional) faults can also cause displacement of the seabed, but only the largest of such events (typically related to flexure in the outer trench swell) cause enough displacement to give rise to a significant tsunami, such as the 1977 Sumba and 1933 Sanriku events.
Over-riding plate bulges under strain, causing tectonic uplift.
Plate slips, causing subsidence and releasing energy into water.
The energy released produces tsunami waves.
Tsunamis have a small wave height offshore, and a very long wavelength (often hundreds of kilometres long, whereas normal ocean waves have a wavelength of only 30 or 40 metres),
which is why they generally pass unnoticed at sea, forming only a
slight swell usually about 300 millimetres (12 in) above the normal sea
surface. They grow in height when they reach shallower water, in a wave shoaling process described below. A tsunami can occur in any tidal state and even at low tide can still inundate coastal areas.
On April 1, 1946, the 8.6 MwAleutian Islands earthquake occurred with a maximum Mercalli intensity of VI (Strong). It generated a tsunami which inundated Hilo
on the island of Hawaii with a 14-metre high (46 ft) surge. Between 165
and 173 were killed. The area where the earthquake occurred is where
the Pacific Ocean floor is subducting (or being pushed downwards) under Alaska.
Examples of tsunamis originating at locations away from convergent boundaries include Storegga about 8,000 years ago, Grand Banks in 1929, and Papua New Guinea
in 1998 (Tappin, 2001). The Grand Banks and Papua New Guinea tsunamis
came from earthquakes which destabilised sediments, causing them to flow
into the ocean and generate a tsunami. They dissipated before
travelling transoceanic distances.
The cause of the Storegga sediment failure is unknown.
Possibilities include an overloading of the sediments, an earthquake or a
release of gas hydrates (methane etc.).
The Tauredunum event was a large tsunami on Lake Geneva in 563 CE, caused by sedimentary deposits destabilised by a landslide.
In the 1950s, it was discovered that tsunamis larger than had previously been believed possible can be caused by giant submarine landslides.
These large volumes of rapidly displaced water transfer energy at a
faster rate than the water can absorb. Their existence was confirmed in
1958, when a giant landslide in Lituya Bay, Alaska, caused the highest wave ever recorded, which had a height of 524 metres (1,719 ft).[40]
The wave did not travel far as it struck land almost immediately. The
wave struck three boats—each with two people aboard—anchored in the bay.
One boat rode out the wave, but the wave sank the other two, killing
both people aboard one of them.
Another landslide-tsunami event occurred in 1963 when a massive landslide from Monte Toc entered the reservoir behind the Vajont Dam
in Italy. The resulting wave surged over the 262-metre (860 ft)-high
dam by 250 metres (820 ft) and destroyed several towns. Around 2,000
people died.Scientists named these waves megatsunamis.
Some geologists claim that large landslides from volcanic islands, e.g. Cumbre Vieja on La Palma (Cumbre Vieja tsunami hazard) in the Canary Islands, may be able to generate megatsunamis that can cross oceans, but this is disputed by many others.
In general, landslides generate displacements mainly in the
shallower parts of the coastline, and there is conjecture about the
nature of large landslides that enter the water. This has been shown to
subsequently affect water in enclosed bays and lakes, but a landslide
large enough to cause a transoceanic tsunami has not occurred within
recorded history. Susceptible locations are believed to be the Big Island of Hawaii, Fogo in the Cape Verde Islands, La Reunion in the Indian Ocean, and Cumbre Vieja on the island of La Palma in the Canary Islands;
along with other volcanic ocean islands. This is because large masses
of relatively unconsolidated volcanic material occurs on the flanks and
in some cases detachment planes are believed to be developing. However,
there is growing controversy about how dangerous these slopes actually
are.
Other than by landslides or sector collapse, volcanoes may be able to generate waves by pyroclastic flow submergence, caldera collapse, or underwater explosions. Tsunamis have been triggered by a number of volcanic eruptions, including the 1883 eruption of Krakatoa, and the 2022 Hunga Tonga–Hunga Ha'apai eruption. Over 20% of all fatalities caused by volcanism during the past 250 years are estimated to have been caused by volcanogenic tsunamis.
Debate has persisted over the origins and source mechanisms of
these types of tsunamis, such as those generated by Krakatoa in 1883,
and they remain lesser understood than their seismic relatives. This
poses a large problem of awareness and preparedness, as exemplified by
the eruption and collapse of Anak Krakatoa in 2018, which killed 426 and injured thousands when no warning was available.
It is still regarded that lateral landslides and ocean-entering
pyroclastic currents are most likely to generate the largest and most
hazardous waves from volcanism; however, field investigation of the Tongan event,
as well as developments in numerical modelling methods, currently aim
to expand the understanding of the other source mechanisms.
Some meteorological
conditions, especially rapid changes in barometric pressure, as seen
with the passing of a front, can displace bodies of water enough to
cause trains of waves with wavelengths. These are comparable to seismic
tsunamis, but usually with lower energies. Essentially, they are
dynamically equivalent to seismic tsunamis, the only differences being
1) that meteotsunamis lack the transoceanic reach of significant seismic
tsunamis, and 2) that the force that displaces the water is sustained
over some length of time such that meteotsunamis cannot be modelled as
having been caused instantaneously. In spite of their lower energies, on
shorelines where they can be amplified by resonance, they are sometimes
powerful enough to cause localised damage and potential for loss of
life. They have been documented in many places, including the Great
Lakes, the Aegean Sea, the English Channel, and the Balearic Islands,
where they are common enough to have a local name, rissaga. In Sicily they are called marubbio and in Nagasaki Bay, they are called abiki.
Some examples of destructive meteotsunamis include 31 March 1979 at
Nagasaki and 15 June 2006 at Menorca, the latter causing damage in the
tens of millions of euros.
Meteotsunamis should not be confused with storm surges,
which are local increases in sea level associated with the low
barometric pressure of passing tropical cyclones, nor should they be
confused with setup, the temporary local raising of sea level caused by
strong on-shore winds. Storm surges and setup are also dangerous causes
of coastal flooding in severe weather but their dynamics are completely unrelated to tsunami waves. They are unable to propagate beyond their sources, as waves do.
There has been considerable speculation about the possibility of using nuclear weapons to cause tsunamis near an enemy coastline. Nuclear testing in the Pacific Proving Ground by the United States generated poor results. In Operation Crossroads
in July 1946, two 20-kilotonne-of-TNT (84 TJ) bombs were detonated, one
in the air over and one underwater within the shallow waters of the
50-metre (164 ft) deep lagoon at Bikini Atoll.
The bombs detonated about 6 km (3.7 mi; 3.2 nmi) from the nearest
island, where the waves were no higher than 3 to 4 m (9.8 to 13.1 ft)
when they reached the shoreline. Other underwater tests, mainly Operation Hardtack I/Wahoo in deep water and Operation Hardtack I/Umbrella in shallow water, confirmed the results. Analysis of the effects of shallow and deep
underwater explosions indicate that the energy of the explosions does
not easily generate the kind of deep, all-ocean waveforms typical of
tsunamis because most of the energy creates steam, causes vertical fountains above the water, and creates compressional waveforms.
Tsunamis are hallmarked by permanent large vertical displacements of
very large volumes of water which do not occur in explosions.
Characteristics
When the wave enters shallow water, it slows down and its amplitude (height) increases.The wave further slows and amplifies as it hits land. Only the largest waves crest.
Tsunamis are caused by earthquakes, landslides, volcanic explosions, glacier calvings, and bolides.
They cause damage by two mechanisms: the smashing force of a wall of
water travelling at high speed, and the destructive power of a large
volume of water draining off the land and carrying a large amount of
debris with it, even with waves that do not appear to be large.
While everyday wind waves have a wavelength
(from crest to crest) of about 100 metres (330 ft) and a height of
roughly 2 metres (6.6 ft), a tsunami in the deep ocean has a much larger
wavelength of up to 200 kilometres (120 mi). Such a wave travels at
well over 800 kilometres per hour (500 mph), but owing to the enormous
wavelength the wave oscillation at any given point takes 20 or 30
minutes to complete a cycle and has an amplitude of only about 1 metre
(3.3 ft). This makes tsunamis difficult to detect over deep water, where ships are unable to feel their passage.
The velocity of a tsunami can be calculated by obtaining the
square root of the depth of the water in metres multiplied by the
acceleration due to gravity (approximated to 10 m/s2). For example, if the Pacific Ocean is considered to have a depth of 5000 metres, the velocity of a tsunami would be √5000 × 10 = √50000
≈ 224 metres per second (730 ft/s), which equates to a speed of about
806 kilometres per hour (501 mph). This is the formula used for
calculating the velocity of shallow-water
waves. Even the deep ocean is shallow in this sense because a tsunami
wave is so long (horizontally from crest to crest) by comparison.
The reason for the Japanese name "harbour wave" is that sometimes a village's fishermen
would sail out, and encounter no unusual waves while out at sea
fishing, and come back to land to find their village devastated by a
huge wave.
As the tsunami approaches the coast and the waters become shallow, wave shoaling
compresses the wave and its speed decreases below 80 kilometres per
hour (50 mph). Its wavelength diminishes to less than 20 kilometres
(12 mi) and its amplitude grows enormously—in accord with Green's law. Since the wave still has the same very long period, the tsunami may take minutes to reach full height. Except for the very largest tsunamis, the approaching wave does not break, but rather appears like a fast-moving tidal bore.
Open bays and coastlines adjacent to very deep water may shape the
tsunami further into a step-like wave with a steep-breaking front.
When the tsunami's wave peak reaches the shore, the resulting temporary rise in sea level is termed run up. Run up is measured in metres above a reference sea level.
A large tsunami may feature multiple waves arriving over a period of
hours, with significant time between the wave crests. The first wave to
reach the shore may not have the highest run-up.
About 80% of tsunamis occur in the Pacific Ocean, but they are
possible wherever there are large bodies of water, including lakes.
However, tsunami interactions with shorelines and the seafloor
topography are extremely complex, which leaves some countries more
vulnerable than others. For example, the Pacific coasts of the United
States and Mexico lie adjacent to each other, but the United States has
recorded ten tsunamis in the region since 1788, while Mexico has
recorded twenty-five since 1732.
Similarly, Japan has had more than a hundred tsunamis in recorded
history, while the neighbouring island of Taiwan has registered only
two, in 1781 and 1867.
Drawback
An
illustration of the rhythmic "drawback" of surface water associated
with a wave. It follows that a very large drawback may herald the
arrival of a very large wave.
All waves
have a positive and negative peak; that is, a ridge and a trough. In
the case of a propagating wave like a tsunami, either may be the first
to arrive. If the first part to arrive at the shore is the ridge, a
massive breaking wave or sudden flooding will be the first effect
noticed on land. However, if the first part to arrive is a trough, a
drawback will occur as the shoreline recedes dramatically, exposing
normally submerged areas. The drawback can exceed hundreds of metres,
and people unaware of the danger sometimes remain near the shore to
satisfy their curiosity or to collect fish from the exposed seabed.
A typical wave period for a damaging tsunami is about twelve
minutes. Thus, the sea recedes in the drawback phase, with areas well
below sea level exposed after three minutes. For the next six minutes,
the wave trough builds into a ridge which may flood the coast, and
destruction ensues. During the next six minutes, the wave changes from a
ridge to a trough, and the flood waters recede in a second drawback.
Victims and debris may be swept into the ocean. The process repeats
with succeeding waves.
Scales of intensity and magnitude
As with earthquakes, several attempts have been made to set up scales
of tsunami intensity or magnitude to allow comparison between different
events.
Intensity scales
The first scales used routinely to measure the intensity of tsunamis were the Sieberg-Ambraseys scale (1962), used in the Mediterranean Sea and the Imamura-Iida intensity scale (1963), used in the Pacific Ocean. The latter scale was modified by Soloviev (1972), who calculated the tsunami intensity "I" according to the formula:
where
is the "tsunami height" in metres, averaged along the nearest
coastline, with the tsunami height defined as the rise of the water
level above the normal tidal level at the time of occurrence of the
tsunami. This scale, known as the Soloviev-Imamura tsunami intensity scale, is used in the global tsunami catalogues compiled by the NGDC/NOAA and the Novosibirsk Tsunami Laboratory as the main parameter for the size of the tsunami.
This formula yields:
I = 2 for = 2.8 metres
I = 3 for = 5.5 metres
I = 4 for = 11 metres
I = 5 for = 22.5 metres
etc.
In 2013, following the intensively studied tsunamis in 2004 and 2011,
a new 12-point scale was proposed, the Integrated Tsunami Intensity
Scale (ITIS-2012), intended to match as closely as possible to the
modified ESI2007 and EMS earthquake intensity scales.
Magnitude scales
The first scale that genuinely calculated a magnitude for a tsunami,
rather than an intensity at a particular location was the ML scale
proposed by Murty & Loomis based on the potential energy. Difficulties in calculating the potential energy of the tsunami mean that this scale is rarely used. Abe introduced the tsunami magnitude scale , calculated from,
where h is the maximum tsunami-wave amplitude (in m) measured by a tide gauge at a distance R from the epicentre, a, b and D are constants used to make the Mt scale match as closely as possible with the moment magnitude scale.
Tsunami heights
Diagram showing several measures to describe a tsunami size, including height, inundation and run-up
Several terms are used to describe the different characteristics of tsunami in terms of their height:
Amplitude, Wave Height, or Tsunami Height: Refers to the height
of a tsunami relative to the normal sea level at the time of the
tsunami, which may be tidal High Water, or Low Water. It is different
from the crest-to-trough height which is commonly used to measure other
type of wave height.
Run-up Height, or Inundation Height: The height reached by a tsunami
on the ground above sea level, Maximum run-up height refers to the
maximum height reached by water above sea level, which is sometimes
reported as the maximum height reached by a tsunami.
Flow Depth: Refers to the height of tsunami above ground, regardless of the height of the location or sea level.
(Maximum) Water Level: Maximum height above sea level as seen from
trace or water mark. Different from maximum run-up height in the sense
that they are not necessarily water marks at inundation line/limit.
Drawbacks can serve as a brief warning. People who observe drawback
(many survivors report an accompanying sucking sound) can survive only
if they immediately run for high ground or seek the upper floors of
nearby buildings.
In 2004, ten-year-old Tilly Smith of Surrey, England, was on Maikhao beach in Phuket,
Thailand with her parents and sister, and having learned about tsunamis
recently in school, told her family that a tsunami might be imminent.
Her parents warned others minutes before the wave arrived, saving dozens
of lives. She credited her geography teacher, Andrew Kearney.
In the 2004 Indian Ocean tsunami
drawback was not reported on the African coast or any other east-facing
coasts that it reached. This was because the initial wave moved
downwards on the eastern side of the megathrust and upwards on the western side. The western pulse hit coastal Africa and other western areas.
A tsunami cannot be precisely predicted, even if the magnitude and location of an earthquake is known. Geologists, oceanographers, and seismologists
analyse each earthquake and based on many factors may or may not issue a
tsunami warning. However, there are some warning signs of an impending
tsunami, and automated systems can provide warnings immediately after an
earthquake in time to save lives. One of the most successful systems
uses bottom pressure sensors, attached to buoys, which constantly
monitor the pressure of the overlying water column.
Regions with a high tsunami risk typically use tsunami warning systems
to warn the population before the wave reaches land. On the west coast
of the United States, which is prone to tsunamis from the Pacific Ocean,
warning signs indicate evacuation routes. In Japan, the populace is
well-educated about earthquakes and tsunamis, and along Japanese
shorelines, tsunami warning signs remind people of the natural hazards
along with a network of warning sirens, typically at the top of the
cliffs of surrounding hills.
The Pacific Tsunami Warning System is based in Honolulu, Hawaiʻi.
It monitors Pacific Ocean seismic activity. A sufficiently large
earthquake magnitude and other information triggers a tsunami warning.
While the subduction zones around the Pacific are seismically active,
not all earthquakes generate a tsunami. Computers assist in analysing
the tsunami risk of every earthquake that occurs in the Pacific Ocean
and the adjoining land masses.
As a direct result of the Indian Ocean tsunami, a re-appraisal of the
tsunami threat for all coastal areas is being undertaken by national
governments and the United Nations Disaster Mitigation Committee. A
tsunami warning system is being installed in the Indian Ocean.
One of the deep water buoys used in the DART tsunami warning system
Computer models can predict tsunami arrival, usually within minutes of the arrival time. Bottom pressure sensors can relay information in real time. Based on these pressure readings and other seismic information and the seafloor's shape (bathymetry) and coastal topography, the models estimate the amplitude and surge height of the approaching tsunami. All Pacific Rim
countries collaborate in the Tsunami Warning System and most regularly
practise evacuation and other procedures. In Japan, such preparation is
mandatory for government, local authorities, emergency services and the
population.
Some zoologists hypothesise that some animal species have an ability to sense subsonic Rayleigh waves
from an earthquake or a tsunami. If correct, monitoring their behaviour
could provide advance warning of earthquakes and tsunamis. However, the
evidence is controversial and is not widely accepted. There are
unsubstantiated claims about the Lisbon quake
that some animals escaped to higher ground, while many other animals in
the same areas drowned. The phenomenon was also noted by media sources
in Sri Lanka in the 2004 Indian Ocean earthquake. It is possible that certain animals (e.g., elephants)
may have heard the sounds of the tsunami as it approached the coast.
The elephants' reaction was to move away from the approaching noise. By
contrast, some humans went to the shore to investigate and many drowned
as a result.
In some tsunami-prone countries, earthquake engineering measures have been taken to reduce the damage caused onshore.
Japan, where tsunami science and response measures first began following a disaster in 1896, has produced ever-more elaborate countermeasures and response plans.
The country has built many tsunami walls of up to 12 metres (39 ft)
high to protect populated coastal areas. Other localities have built floodgates
of up to 15.5 metres (51 ft) high and channels to redirect the water
from an incoming tsunami. However, their effectiveness has been
questioned, as tsunamis often overtop the barriers.
The Fukushima Daiichi nuclear disaster was directly triggered by the 2011 Tōhoku earthquake and tsunami, when waves exceeded the height of the plant's sea wall and flooded the emergency generators. Iwate Prefecture, which is an area at high risk from tsunami, had tsunami barriers walls (Taro sea wall)
totalling 25 kilometres (16 mi) long at coastal towns. The 2011
tsunami toppled more than 50% of the walls and caused catastrophic
damage.
The Okushiri, Hokkaidō tsunami, which struck within two to five minutes of the earthquake on July 12, 1993, created waves 30 metres (100 ft) tall—as high as a 10-storey building. The port town of Aonae
was completely surrounded by a tsunami wall, but the waves washed right
over the wall and destroyed all the wood-framed structures in the area.
The wall may have succeeded in slowing down and moderating the height
of the tsunami, but it did not prevent major destruction and loss of
life.
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