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NEAR
AND FAR-FIELD EFFECTS OF TSUNAMIS GENERATED BY THE PAROXYSMAL
ERUPTIONS, EXPLOSIONS, CALDERA COLLAPSES AND MASSIVE SLOPE FAILURES
OF THE KRAKATAU VOLCANO IN INDONESIA ON AUGUST 26-27, 1883
George
Pararas-Carayannis
Presentation
for the International Seminar/Workshop on Tsunami
"In Memoriam 120 Years Of Krakatau Eruption
Tsunami And Lesson Learned From Large Tsunami"
August 26th 29th 2003, Jakarta
and Anyer, Indonesia
Also published in the Journal
of Tsunami Hazards, Volume 21, Number 4. 2003 http://www.STHJOURNAL.ORG
Copyright © 2003. All Rights
Reserved
ABSTRACT
The paroxysmal phases
of Krakatau's volcanic activity on August 26-27, 1883, included
numerous submarine Surtsean (phreatomagmatic) eruptions, three
sub aerial Plinian eruptions from the three main craters of Krakatau
on Rakata island, followed by a fourth gigantic, sub aerial,
Ultra-Plinian explosion. Landslides, flank failures, subsidences
and a multiphase massive caldera collapse of the volcano - beginning
near the Perbowetan crater on the northern portion of Rakata
and followed by a collapse of the Danan crater - occurred over
a period of at least 10 hours. The first of the three violent
explosions occurred at 17: 07 Greenwich time (GMT) on August
26. The second and third eruptions occurred at 05:30 GMT and
at 06:44 GMT on August 27. Each of these events, as well as expanding
gases from the submarine phreatomagmatic eruptions, lifted the
water surrounding the island into domes or truncated cones that
must have been about 100 meters or more in height. The height
of the resulting waves attenuated rapidly away from the source
because of their short periods and wavelengths. It was the fourth
colossal explosion (VEI=6) and the subsequent massive flank failure
and caldera collapse of two thirds of Rakata Island, at 10:02
a.m., on August 27 that generated the most formidable of the
destructive tsunami waves. A smaller fifth explosion that occurred
at 10:52 a.m. must have generated another large water cone and
sizable waves. The final collapse of a still standing wall of
Krakatau - which occurred several hours later, at 16:38, generated
additional waves.
 The near field effects of the main tsunami along
the Sunda Strait in Western Java and Southern Sumatra were devastating.
Within an hour after the fourth explosion/caldera collapse, waves
reaching heights of up to 37 m (120 feet) destroyed 295 towns
and villages and drowned a total of 36,417 people. However, because
of their short period and wavelength, the wave heights attenuated
rapidly with distance away from the source region. It took approximately
2.5 hours for the tsunami waves to refract around Java and reach
Batavia (Jakarta) where the only operating tide gauge existed.
Waves of 2.4 meters in height, were recorded - but with an unusually
long period of 122 minutes. The long period is attributed to
modification due to resonance effects and did not reflect source
characteristics. The tsunami travel time to Surabaya at the eastern
part of Java was 11.9 hours, and the reported wave was only 0.2
meters.
The far field effects of the tsunami were noticeable around the
world, but insignificant. Small sea level oscillations were recorded
by tide gauges at Port Blair in the Andaman Sea, at Port Elizabeth
in South Africa, and as far away as Australia, New Zealand, Japan,
Hawaii, Alaska, the North-American West Coast, South America,
and even as far away as the English Channel, in France and England.
It took 12 hours for the tsunami to reach Aden on the southern
tip of the Arabian Peninsula, about 3800 nautical miles away.
The wave reported at Aden, at Port Blair and at Port Elizabeth,
represents the one generated in the Sunda Strait. There were
no land boundaries on the Indian Ocean side of Krakatau to prevent
the tsunami energy from spreading in that direction. The tsunami
travel time of a little over 300 nautical miles per hour to Aden
appears reasonable. However, it is doubtful that the waves that
were reported at distant locations in the Pacific or in the Atlantic
Ocean represented the actual tsunami generated in the Sunda Strait.
Very little, if any at all, of the tsunami energy could have
escaped the surrounding inland seas to the east of the Sunda
Strait. Most probably, the small waves that were observed in
the Pacific as well as in the Atlantic were generated by the
atmospheric pressure wave of the major Krakatoa explosion, and
not from the actual tsunami generated in the Sunda Strait. The
unusual flooding that occurred at the Bay of Cardiff, in the
UK. was caused by atmospheric coupling of the pressure wave from
the major Krakatau eruption.
INTRODUCTION
 The effects of the volcanic explosions and collapses
of the volcano of Krakatau in Indonesia on August 26-27, 1883,
including the effects of its destructive tsunami, were extensively
reported in early scientific reports based on geological surveys
and data collected in the affected area (Verbeek, 1884; Fuchs,
1884; Warton & Evans, 1888). Most of the original publications
include descriptive documentation of the tsunami near field effects
in the Sunda Strait and elsewhere in Indonesia, with speculations
as to the tsunami source mechanism. Analysis of the geological
data collected by early and subsequent surveys resulted in publications
that further documented the caldera collapse and provided additional
evaluation as to what occurred in 1883 and about tsunamis generated
from subsequent eruptions of Anak Krakatau (Montessus de Ballore,
1907; Escher, B.G., 1919, 1928; Stehn, 1929; Wilson et al 1973).
Additional descriptions of the tsunami effects - based on original
and subsequent references were included in historical catalogs
of earthquakes and tsunamis (Milne, 1912; Svyatlowski, 1957,
Kawasumi, 1963; Heck, 1947; Imamura, 1949; Iida et al., 1967).
Similarly, water level disturbances at tide gauges from the great
explosion were extensively reviewed and evaluated (Ewing &
Press, 1955). Subsequent computations of the tsunami travel time
and the modeling of the tsunami were undertaken, using a point
tsunami source and a simplified tsunami generation mechanism
(Nomanbhoy & Satake 1997). Subsequent publications drew comparisons
and analogies between Krakatau's explosion/collapse tsunami mechanism
with that of the multi-phase explosions and collapses of the
volcano of Santorin in the Bronze Age (Pararas-Carayannis, 1973,
1974, 1983, 1992).
Based on original surveys and a very extensive review of the
geologic data in the scientific literature, a comprehensive publication
further described Krakatau's eruption and its effects (Simkin
& Fiske, 1983). The accuracy of its historical tsunami data
was further verified by a review of original documents
some translated from Dutch, German and French publications
prior to the above publication and for corrections of earlier
historical catalogs of tsunamis (Pararas-Carayannis, 1983)
The atmospheric pressure effects of the Krakatau's explosion
were also researched and documented (Hirota, 1983). Additional
tsunami descriptions were provided in numerous other publications
(Anon., 1883; Whitney, 1992; Simkin & Siebert, 1994; Decker
& Hadikusumo, 1961; Furneaux, 1964; Decker & Decker,
1989; Sea Frontiers, 1971). Over the years, numerous articles
in magazines and newspapers about the1883 Krakatau disaster and
its destructive tsunami have fascinated and continue to fascinate
readers. Similarly, museums and Internet web sites have provided
extensive descriptions and bibliographies.
More recent papers have evaluated the tsunami generation mechanisms
for different types of volcanoes with varying eruption intensities,
have assessed the risk of postulated mega tsunami generation
from massive edifice failures of island stratovolcanoes such
as Krakatau, Santorin, Piton de la Fournaise Cumbre Vieja, and
Kilauea (Pararas-Carayannis, 2002a,b).
The present study is part of an ongoing investigation of tsunami
source mechanisms from large-scale sub aerial and submarine volcanic
and non-volcanic, mass waste processes. It examines briefly only
the kinematic processes of flank failure and caldera collapses
during the paroxysmal phases of Krakatau's eruption on August
26-27, 1883, comments on specific source characteristics and
mechanisms of tsunami generation resulting from such mass waste
of volcanic edifice structure, and summarizes the near and far-field
tsunami effects including the generation of atmospherically-induced
sea level disturbances at great distances from the source region.
GEOLOGIC
SETTING
Krakatau (Krakatoa)
is one of the volcanoes of the Sunda volcanic arc in Indonesia,
located in the Sunda Strait, at 16.7 S. Latitude and 105.4 E.
Longitude, 40 km off the west coast of Java. The stratovolcano
was formed by the subduction of the Indian-Australia Plate under
the Eurasian Plate. Great volcanic eruptions have occurred in
this region in the distant geologic past. A mega-colossal volcanic
explosion/collapse during the Quaternary period of the Ice Age,
approximately 75,000 years ago, devastated the center of the
island of Sumatra and created a 100 km long caldera, now the
site of Lake Toba. The volume of tephra discharge from this massive
volcanic eruption is estimated at 2,000 cubic km. The massive
1815 Ultra-Plinian eruption/explosion of the Mount Tambora volcano
ejected between 100 and 200 cubic kilometers of tephra.
BRIEF HISTORY
OF KRAKATA'S VOLCANIC ACTIVITY AND TSUNAMI GENERATION
At its peak, the
island of Rakata, which the volcano of Krakatau had formed, had
reached a height of 790 m (2,600 ft.) above sea level. According
to ancient Japanese scriptures, the first known super-colossal
eruption of Krakatau occurred in the year 416 A. D. Some
have reported it to occur in 535 A.D. The energy of this eruption
is estimated to have been about 400 megatons of TNT, or the equivalent
of 20,000 Hiroshima bombs. This violent early eruption destroyed
the volcano, which collapsed and created a 7 km (4-mile) wide
submarine caldera. The remnants of this earlier violent volcanic
explosion were the three islands of Krakatau, Verlaten and Lang
(Rakata, Panjang, and Sertung). Undoubtedly the 416 A.D. eruption/explosion/collapse
generated a series of catastrophic tsunamis, which must have
been much greater than those generated in 1883. However, there
are no records to document the size of these early tsunamis or
the destruction they caused.
Subsequent to the 416 A.D. eruption and prior to 1883, three
volcanic cones of Krakatau and at least one older caldera had
combined again to form the island of Rakata. The volcanic cones
on the island were aligned in a north-south direction. The northernmost
was called Poeboewetan and the southernmost was called Rakata.
Overall approximate dimensions of the island were 5 by 9 Kilometers.
A long period of relative inactivity of Krakatau was interrupted
by a moderate eruption that occurred between May 1680 and November
1681. This activity destroyed all the lush vegetation that had
grown on the island. Large quantities of rock, pumice, and ash
fell into the sea. Undoubtedly, volcanic edifice mass waste,
subsidence and partial flank failure events associated with this
activity, generated large local tsunamis. However, the geomorphology
of the island was not altered significantly. Thus, prior to the
great 1883 eruption, Krakatau was the remnant of the older volcano
that had not erupted for 200 years.
The great 1883 eruptions, explosions, mass waste and collapse
events of Krakatau generated the catastrophic tsunamis along
the Sunda Strait. Subsequent local tsunamis in the Sunda Strait
were generated by the 1927 and 1928 eruptions of the new volcano
of Anak Krakatau (Child of Krakatau) that formed in the area.
Although large tsunamis were generated from these recent events,
the heights of the waves attenuated rapidly away from the source
region, because their periods and wavelengths were very short.
There was no report of damage from these more recent tsunamis
in the Sunda Strait.
 CHRONOLOGY OF EVENTS
PRIOR TO KAKATAU'S PAROXYSMAL PHASE
As mentioned, following
about 200 years period of inactivity, Krakatau became active
again in early 1883 when a large earthquake struck the area.
There was subsequent increase in seismic activity. On May 20,
1883 Krakatau begun to erupt again. The initial explosive eruptions
could be heard 160 km away. Steam and ash could be seen rising
11km above the summit of the volcano. Most of the activity was
from the 3 main vents Perboewatan being the most active.
The eruptions continued with varying degrees of severity to August
producing only gas, steam and ashes. During that period
the Danan vent got progressively wider because of collapses.
By August 11, 1883. Three major vents of Krakatau were actively
erupting. Eleven other vents were ejecting smaller quantities
of steam, ash and dust.
Figure 1. Schematic of Rakata Island
prior to Krakatau's 1883 paroxysmal eruptions (After H. J. G.
Ferzenaar, included in Verbeek)
CHRONOLOGY
OF EVENTS DURING KRAKATAU'S PAROXYSMAL PHASE
The paroxysmal phase
of Krakatau's eruption took place in less than two days
on August 26 and 27, 1883 with remarkably few reported
earthquakes. This final phase included numerous submarine Surtsean
(phreatomagmatic) eruptions, three sub aerial Plinian eruptions
from the three main craters, followed by a fourth gigantic, sub
aerial, ultra-Plinian explosion, mass waste, flank failures and
massive caldera collapse.
The first of the four violent explosions begun with extraordinary
intensity at 17: 07 Greenwich time (GMT) on August 26, 1883 (27
August local date). Subsequently, eruptions of lesser magnitude
became more frequent, occurring on the average every 10 minutes.
The first and subsequent smaller explosions sent huge volumes
of airborne tephra that completely blocked out the sun and brought
darkness to the Sunda Straits. A black cloud of smoke rose 27
kilometers (17 miles) above the volcano and was seen and reported
by sailors on a ship, 120 km away.
At the time, a plug
of solid lava apparently blocked Krakatau's central vent. Underneath
it pressure was rapidly building up. Subsequently, there was
a partial caldera collapse which increased Krakatau's Perboewatan
crater to approximately 1,000 meters in diameter and its average
depth to about 50 meters. Also, several submarine Surtsean (phreatomagmatic)
eruptions and partial flank collapses of the island must have
occurred during this initial period. Probably, these continued
intermittently during the following 10- hour period.
The second and third
of Krakatau's violent explosions occurred at 05:30 GMT and at
06:44 GMT on August 27, 1883, respectively. Finally, at 10:02
(GMT) on August 27, the fourth paroxysmal eruption/explosion
blew away the northern two-thirds of Rakata Island. Almost instantaneously
this explosion was followed by a very substantial collapse of
the unsupported volcanic chambers of Krakatau, thus forming a
huge underwater caldera.
A smaller
fifth explosion occurred at 10:52 a.m. on August 27. The final
collapse of a still standing wall of Krakatau occurred
several hours later, at 16:38. The entire northern part of Rakata
Island had disintegrated completely. The combined explosions
and collapses of the volcano destroyed a good part of the island.
Its remnant is now known as the island of Krakatau.
Figure 2. Schematic of Krakatau seven
weeks after the paroxysmal phase (After R. D. M. Verbeek, 1884)
MAGNITUDE
OF THE 1883 ERUPTIONS/EXPLOSIONS OF THE KRAKATAU VOLCANO
The fourth explosion
of Krakatau at 10:02 (GMT) on August 27 resulted in the ejection
of 15-20 cubic km of material. At least 2 cubic Km of the finer
material was blown to a height of 27 Km. The event was assigned
a Volcanic Explosivity Index of VEI=6 which rates as "colossal".
To be assigned a VEI rating of 6, a volcanic eruption must have
a plume height over 25 km and a displacement volume ranging between
10 and 100 km3. Eruptions of this size occur only once every
few hundred years on earth.
The total thermal energy released by the four main events of
the 1883 eruptions is estimated to be equivalent to 200 megatons
of TNT. Most of this energy was released by the fourth paroxysmal
explosion, which is estimated to be the thermal energy release
of about 150 175 megatons of TNT, or the equivalent of about
7,500 8,750 Hiroshima atomic bombs (the Hiroshima bomb
released about 20 kilotons of thermal energy).
Krakatau's tremendous explosions were heard throughout the area
and beyond, over 1/3rd of the earth's surface. They were heard
as far away as 3,540 kilometers (2,200 miles) away in Australia,
and even as far away as Rodrigues Island which is 4,653 km (2,908
miles) away to the west-southwest, in the Indian Ocean, about
1,000 miles (1,600 km) east of Madagascar. People on Rodrigues
Island described the sounds to be like the distant roar of firing
canons. The sounds continued at intervals of three to four hours
during the night of August 26th.
SOURCE
DIMENSIONS AND MECHANISMS OF TSUNAMI GENERATION
The 1883 eruption
of Krakatau provides the best understanding of the tsunamigenic
potential from mass failures and collapses of island volcanoes.
It is believed that not one but several tsunamis were generated
as a result of several events that occurred during Krakatau's
paroxysmal phase. The following source dimensions and mechanisms
of tsunami generation are inferred from examination of historical
records, chronology of events, underwater topography of Krakatau's
post 1883 caldera and geological evidence. The 1927 birth of
Anak Krakatau volcano (Stern, 1929a) in the caldera of Krakatau
altered significantly the underwater topography. However, observations
from Strombolian intensity eruptions of Anak Krakatau in 1928
shed additional light on the mechanisms of tsunami generation
from volcanic explosions.
Mechanisms: Violent
Plinian and Ultra-Plinian eruptions/explosions, submarine phreatomagmatic
activity and other associated processes such as atmospheric shock
waves, magmatically-induced earthquakes, gravitational settling,
sudden coastal subsidence, rock falls, landslides, flank failures
and massive caldera collapses, are some of the kinematic mechanisms
by which several destructive local tsunamis were generated during
Krakatau's paroxysmal phase on August 26 and 27, 1983. Geologic
evidence and the chronology of events support the following sequential
tsunami generation mechanisms.
During the first
10 hours of Krakatau's paroxysmal eruptive phase, local tsunamis
were generated in the immediate area primarily from numerous
landslides, rock falls, flank failures, subsidences, falling
ejecta, submarine phreatomagmatic eruptions and the atmospheric
shock waves associated with the first three explosions. Also,
during this period, sub aerial, multiphase collapses of the volcanic
vents occurred on Rakata Island. However, none of these nor the
major caldera collapse which begun near the Perbowetan crater
on the northern portion of the island or the subsequent partial
collapse of the Danan crater, contributed to tsunami generation.
These collapses occurred above sea level.
Tsunami Generation
During the Early Paroxysmal Phase:
The first three
explosions ejected material primarily upwards and did not displace
much water to generate a large tsunami. Other concurrent mass
waste phenomena and falling ejecta must have contributed to the
generation of large waves. These waves had relatively short periods
and wavelength and their heights attenuated rapidly as they propagated
away from the source region. However, the atmospheric shock waves
from these explosions and concurrent phreatomagmatic eruptions
must have generated larger waves. The expanding gases from submarine
phreatomagmatic activity must have lifted the water surrounding
the island into domes or truncated cones that, at times, could
have been as much as a 30-50 meters in height. Also, the height
of the waves that were thus generated attenuated very rapidly
away from the source area because of the limited source dimensions
and the resulting short periods and wavelengths. This conclusion
is supported by observations of the tsunami waves observed during
the 1928 eruptions of Anak Krakatau the Son of Krakatau
that subsequently formed in the area.
Tsunami Generation
During Final Paroxysmal Phase: The fourth colossal paroxysmal eruption/explosion
which occurred at 10:02 (GMT) on August 27, blew away the northern
two-thirds of Rakata island, resulting in the ejection of about
15-20 cubic km of material, and generating the major destructive
tsunami in the Sunda Strait. In spite of limited observations
and data, it is very likely that the destructive tsunami occurred
in accordance to the following sequential scenario.
Most of the force of the fourth explosion was also directed upwards,
so there was no significant contribution to tsunami generation.
However, the atmospheric shock wave was much more powerful than
those of the earlier explosions. Also greater was the concurrent
submarine phreatomagmatic activity. Expanding gases from such
activity must have lifted the water surrounding the island into
a dome or a truncated cone, rounded at the top and forming an
acute angle with the surface of the water. The water cone thus
raised must have been about100 meters or more in height. Substantial
waves of short period were generated but attenuated rapidly away
from the source. The waves thus generated were more significant
than those generated during the earlier phase of the paroxysmal
activity. Supporting this scenario are the previously mentioned
observations of wave generation during eruptions of the Anak
Krakatau volcano.
Between January 12-20, 1928 eruptions of this new volcano generated
water cones with heights of up to 26 meters which were photographed
from Lang island. A subsequent eruption on January 24, 1928 generated
a water mass of cylindrical shape. The waves were generated from
Strombolian eruptions that lacked the intensity of the 1883 Plinian
and super Plinian eruptions of Krakatau. The 1928 waves were
sizeable but attenuated rapidly because of short periods and
wavelengths.
Following the fourth eruption/explosion of the 1883 paroxysmal
phase of Krakatau, additional waves were generated in the immediate
area from the falling pyroclastics and the large blocks of pumice.
These, too, were of extremely short periods and wavelengths and
did not contribute significantly to tsunami energy propagation
away from the source. Most probably the waves from falling pyroclastics
created standing waves and a chaotic sea surface in the immediate
area. The larger waves traveling in the same direction away from
the source region quickly sorted out according to their periods
and added to their heights.
Immediately following the fourth paroxysmal explosion of Krakatau,
there must have been several successive massive flank failures
on all sides of the volcano. The waves thus generated were sizeable
but also of short period and wavelength. However within minutes
after the fourth explosion, what was left of Krakatau's basaltic
peak begun to collapse into the volcano's unsupported magmatic
chambers resulting in the large depression of the sea floor which
created the eastern branch of the 1883 submarine caldera. It
was this massive caldera collapse and the associated flank failures
that generated the major catastrophic tsunami in the Sunda Strait.
A possible second explosion and caldera collapse contributed
to the generation of an additional tsunami.
 Tsunami Generation After the Major Paroxysmal
Phase: What
is concluded from the above is that the total engulfment and
collapse of Krakatau did not occur as a single event. According
to Verbeek (1894), fifty minutes after the major explosion, at
10:52 a.m., there was another severe detonation that was heard
at great distance. This fifth explosion appears to have been
primarily hydromagmatic and is not known to be associated with
additional volcanic collapse. However, this event must have also
generated another large, truncated cone of water and sizable
waves. When waves form this subsequent event reached the nearest
shores in the Sunda Strait about fifty minutes later, they were
indistinguishable from the waves of the earlier destructive tsunami.
Figure 3. Map showing
the remnant of Rakata Island (now called Krakatau Island) after
the 1983 explosion and collapse and the new volcano of Anak Krakatau
that subsequently emerged from Krakatau's sunken caldera (Modified
after Simkin and Fiske, 1983)
Also, the final collapse
of a still standing wall of Krakatau which occurred several hours
later at 16:38, must have generated additional tsunami waves
of short wavelegth and period also indistinguishable from
the earlier major destructive tsunami. Given the catastrophic
magnitude of the earlier tsunami waves that followed the 10:02
explosion, waves generated by both earlier and subsequent events
were not noticed. The only tide gauge that operated in Batavia
was too far to record these smaller tsunamis as separate discreet
events. The small sea level disturbances that are evident on
the earlier Batavia tide gauge record may have been caused by
the atmospheric shock waves of the volcanic explosions, rather
by the short period tsunami waves which were probably filtered
out.
The submarine eruptions
of Krakatau continued in the evening and night of the 27th of
August but all activity stopped in the morning of the 28th. However,
on October l7, 1883 there was again a small submarine eruption,
when mud was ejected and a small wave was generated (Verbeek,
1884).
Tsunami Source
Dimensions: The dimensions
of the tsunami generating area, as well as the volume of water
displaced by the mass waste of the volcanic structure edifice
due to the 1883 explosions and collapses of Krakatau, can be
estimated from post disaster data collection and the bathymetry
of the sea floor.
Prior to Krakatau's paroxysmal phase, the island of Rakata had
an average elevation of about 212 m (700 feet). However, following
the fourth major explosion, 40 sq. Km of the island were reduced
to an extensive depression with the depth of more than 275 m
(900 feet) below sea level. The huge underwater caldera that
was formed by the massive collapse was about 7 km in diameter.
The three remaining islands of Verlaten, Lang and Krakatau
the latter being the remnant of Rakata, surround the caldera
after the final collapse. The total horizontal extent of the
area affected by the collapse and other kinematic processes which
contributed to tsunami generation, is estimated at about 40 sq.
Kms.
Although the bathymetry of Krakatau's underwater caldera changed
considerably after the birth of the Anak Krakatau volcano, review
of original and subsequent bathymetry shows a complex relief
of the sea floor between the three remaining islands. Following
the 1883 events, the water depth between the islands of Lang
and Verlaten was about 70 m. However, within this area there
were two small basins with depths of about 120 m. The depth of
the main caldera depression was about 270 m. The relief within
this caldera depression appeared to have an eccentric uplift
on one side which was about 60 m higher that the bottom of the
basin. Further examination of the bathymetry showed that a central
ridge of about 25m in height separated the two deeper basins
on the sea floor. The existence of this ridge indicates that
two events of major explosion/collapse took place, rather that
a single one. The significance of two separate explosion/collapse
events is that there were two discreet tsunamigenic sources and
that two major tsunamis were generated on August 27, 1883
although there is no data to support how far apart in time these
tsunamis were generated. Perhaps the second depression of the
underwater caldera was caused by a collapse that followed the
fifth explosion at 10:52, but this cannot be concluded with certainty.
Finally, the total volume of seawater displaced by the massive
kinematic processes described above, is estimated to have been
at least 20 cubic Kilometers. Therefore, given these approximate
source dimensions, the tsunami's greatest initial wavelegth is
estimated to have been as much as about 7 km the size of
the caldera and the greatest period of the resulting waves,
no more than 4-5 minutes. Unless the two explosions/collapses
described above occurred at a very close time interval,
the wavelengths and periods of the resulting waves from each
tsunami event would be expected to have had even shorter wavelengths
and periods.
 TSUNAMI TRAVEL TIMES
The origin time of
the major tsunami has been assumed to be 10:02 (GMT) on August
27 which was the time of the fourth colossal paroxysmal
eruption/explosion/collapse. However this origin time may be
somewhat wrong as it took several minutes for the volcano to
collapse and for the tsunami to be generated.
Figure 4. Travel Time
of Major Tsunami from Krakatau's Fourth Explosion and Collapse
in five minute intervals(Modified after Yokohama, 1981)
Furthermore, as the
bathymetric data indicates, there were two separate tsunamis
generated with origin times that may have been different. Using
the 10:02 (GMT) as the major tsunami origin time, it took about
25 minutes for the first wave to reach the nearest land point
on Sumatra. It took considerably longer - in some cases as much
as an hour - for the waves to reach major cities and villages
on the Sunda Strait.
 NEAR FIELD TSUNAMI
EFFECTS
The combination of
the fourth colossal explosion and subsequent massive flank failures
and caldera collapses of Krakatau generated catastrophic tsunami
waves.
Figure 5. The extent
of maximum inundation from the tsunami(s) generated by the August
27, 1883 explosions and collapse of the volcano of Krakatau (Modified
after Symons, 1888)
Because of the relative
shallow bathymetry of the Sunda Strait, it took almost an hour
for the destructive waves to reach the nearest coastal settlements
of western Java and southern Sumatra. In certain areas, the waves
swept inland for several kilometers, destroying virtually everything
in their path. A total of 295 towns and villages were washed
away. A total of 36,417 people were drowned. Maximum runup was
as high as 37 m. (120 ft.) along certain areas.
 The huge tsunami was well documented in terms
of visual observations of heights reached along the coasts of
Java and Sumatra as well from a recording at a tide gauge at
Batavia (Jakarta). Since there is good descriptive documentation
of the tsunami effects, the following is only a very brief description
of the near field effects of the 1883 tsunami in Java and Sumatra.
Many of the names of towns and villages of this region provided
here were taken from older accounts. Some of these names have
since changed. For example Batavia is now Jakarta.
Figure 6. Maximum
runup heights (in meters) of the tsunami(s) of August 27, 1883
at coastal towns of Southern Sumatra and Western Java (Modified
after Symons, 1888)
Island of Sumatra
(Telok Batong, Vlakke Hook) - The tsunami travel time to the
closest villages of Sumatra was about 1 hour after the explosion
of Krakatoa. At Telok Batong, tsunami waves up to 22 meters (72
feet) completely submerged the village. At Vlakke Hook the maximum
tsunami wave height was 15 meters.
Island of Java {Sirik, Anjer, Tyringen,
Merak, Batavia (Jakarta) and Surabaya} - Destructive tsunami waves reached the Western
coast of Java within an hour after the fourth explosion and ensuing
collapse of Krakatau. The village of Sirik was almost entirely
swept away. It took also about one hour for the destructive waves
to reach Anjer where a 10-meter wave completely overwhelmed the
lower part of town. At Tyringen, waves ranged from 15 - 20 meters
in height (up to 6O ft) while at Merak, the waves reached a maximum
of 35 meters.
 Figure 7. The tsunami(s) from the August 27,
1883 explosions and collapse of the volcano of Krakatau as recorded
by the tide gauge at Batavia (Jakarta). Superimposed on the tide
gauge record is a barograph record, which shows the early arrival
of the atmospheric pressure waves and the sea level oscillations,
recorded by the tide gauge prior and after the arrival of the
tsunami. (Modified after Verbeek, 1884)
It took approximately
2.5 hours for the tsunami waves to refract around the western
end of the island of Java and to reach Batavia (Jakarta). Maximum
waves of 2.4 meters were reported there with a very long period
of 122 minutes. By the time the tsunami reached Surabaya, at
the eastern part of Java, the reported wave height was only 0.2
meters. The tsunami travel time to Surabaya was 11.9 hours.
FAR
FIELD TSUNAMI EFFECTS
Because of the short
periods and wavelengths, the height of the tsunami waves attenuated
considerably with distance away from the source. The far field
effects were negligible. However, small sea level oscillations
from Krakatau's major explosion and collapse were observed or
recorded by tide gauges around the world, as far away as Hawaii,
the American West Coast, South America, and even as far away
as the English Channel in France and England. Some of the tide
gauge records were of the actual tsunami waves, while other recorded
or observed sea level disturbances appear to have been caused
by the atmospheric shock pressure of the powerful explosions
of the volcano.
 Figure 8. The tsunami(s) from the August 27,
1883 explosions and collapse of the volcano of Krakatau as recorded
by tide gauges at Port Blair in the Andaman Islands and at Port
Elizabeth, South Africa (Modified after Wharton & Evans,
1888).
Using the time of
Krakatau's fourth explosion as the tsunami origin time, it is
estimated that it took 12 hours for the tsunami waves to reach
Aden on the southern tip of the Arabian Peninsula, some 3800
nautical miles away from the Sunda Strait. Unfortunately there
was no operating tide gauge. The wave reported at Aden probably
represents the one generated in the Sunda Strait. The travel
time of a little over 300 nautical miles per hour to Aden appears
reasonable. There were no land boundaries on the Indian Ocean
side of the Sunda Strait to prevent the tsunami waves from Krakatau
from spreading and traveling in a westward direction. The tide
gauges operating at Port Blair in the Andaman Islands and at
Port Elizabeth in South Africa recorded these direct tsunami
waves.
There were many recordings or observations of small sea level
oscillations around the world. The oscillations were detected
by tide gauges in South Africa (4,690 miles away), at Cape Horn
(7,820 miles away), and Panama (11,470 away). However, some of
the observed or recorded disturbances on the coasts of America
and Europe, originally attributed to the tsunami from Krakatau,
did not have arrival times that corresponded to actual tsunami
travel times.
 Figure 9. The tsunami(s) from
the August 27, 1883 explosions and collapse of the volcano of
Krakatau as recorded by tide gauges at San Francisco, Honolulu.
and at Moltke Harbor, South Georgia (Modified after Press and
Harkrider 1962).
It is doubtful that
the sea level oscillations reported or recorded at distant locations
in the Pacific or in the Atlantic Ocean represent the actual
tsunami generated in the Sunda Strait. Very little, if any at
all, of the tsunami energy could have escaped the surrounding
inland seas to the east of the Sunda Strait. Most probably, the
small waves that were observed or recorded in the Pacific as
well as in the Atlantic were generated by the atmospheric pressure
waves from the major Krakatau explosion, and not from the actual
tsunami generated in the Sunda Strait.
The Honolulu tide gauge in the Hawaiian Islands recorded a small
oscillation of 0.24 meters, 17 hours after the explosion of Krakatau.
In Alaska's Kodiak Island a small oscillation of 0.1 meter was
recorded. In San Francisco, California, a 0.1-meter sea level
oscillation was recorded 20 hours after Krakatau's explosion.
In Japan, a small sea level oscillation was recorded at Honshu-Sagami
and at Shikoku-Satsuma. In Australia, a trace of the tsunami
was recorded. It was less than 0.1 meter. In New Zealand a 0.3-meter
change in water level was reported.
ATMOSPHERICALLY
GENERATED TSUNAMIS
A rapidly moving
atmospheric pressure front moving over a shallow sea can couple
with the sea surface and generate tsunami-like waves. It has
been clearly established that the atmospheric pressure shock
waves from the explosions of Krakatau were significant. They
circled the earth and were recorded by barographs throughout
the world. In fact, some as many as seven times as the wave bounced
back and forth between the eruption site and its antipode (located
near Bogota, Colombia) for 5 days after the explosion.
Such atmospheric pressure wave coupling from the Krakatau main
explosion, traveling around the earth, gave rise to unusual sea
level oscillations in bays and estuaries far distantly from the
source region. Also, the atmospheric pressure wave appears to
have been responsible for sea level oscillations recorded by
many tide gauges.
In all probability, the small sea level oscillations that were
observed in the Pacific as well as in the Atlantic were generated
by the atmospheric pressure waves that resulted from the major
Krakatau explosion, and not from the actual tsunami generated
in the Sunda Strait. The waves recorded by gauges in Honolulu,
San Francisco and elsewhere were caused by the atmospheric pressure
waves, because their timing is not consistent with tsunami travel
times. For example, if it took 11.9 hours for the actual tsunami
to reach Surabaya on the eastern end of Java, how could it take
only 17 hours to reach Honolulu and 20 hours to reach San Francisco,
particularly since little or no tsunami energy could escape the
inland seas to the east of the Sunda Strait? Similarly, the unusual
sea level disturbances observed in such distant locations as
the Bay of Cardiff can only be explained by coupling of the sea
surface with the atmospheric pressure wave from the major explosion
of Krakatau.
Since the shock wave travel at the speed of sound (approx. 340
meters/sec - about 1225 Km/hour) the travel time of the atmospheric
pressure wave to Cardiff could be estimated. At that speed, and
the shorter distance in a westward rather than eastward direction,
an 18-hour travel time of the atmospheric wave from Krakatau's
fourth paroxysmal explosion to Cardiff seems possible. There
is no way that a direct wave could travel on the surface of the
ocean to reach Cardiff in such a short time particularly
considering that it took 12 hours for the actual waves to just
reach Aden on the Arabian Peninsula. Additionally, it appears
the configuration and geometry of the Bay at Cardiff, the offshore
bathymetry, and the direction of approach of the shock wave were
optimum for atmospheric coupling that caused the observed sea
level disturbances. Additional studies of microbarograph recordings
of the atmospheric wave caused by the Krakatau explosions would
be very helpful to further confirm the atmospheric origin of
the sea level disturbances that were observed or recorded.
SUMMARY
AND CONCLUSIONS
Geologic evidence
and observations reported in the scientific literature indicate
that several tsunamis were generated in the Sunda Strait during
the paroxysmal phases of Krakatau's volcanic activity on August
26-27, 1883. During a ten hour period, Rakata Island was significantly
altered by several subsidences, explosions and large waves following
four Plinian and several submarine Surtsean (phreatomagmatic)
eruptions and explosions.
A sub aerial collapse of Krakatau's caldera begun in the northern
part of Rakata in the vicinity of the Perboewatan crater. Subsequent
tsunami waves - generated by flank collapse activity - triggered
extensive landslides of previously deposited pumice on Verlaten
and Lang islands and generated additional large local tsunami-like
waves. One but more likely two - major explosions followed
by extensive flank failures and massive caldera collapses of
Krakatau - on what used to be Rakata Island - generated the more
destructive of the tsunamis observed in the Sunda Strait. The
existence of a ridge separating the resulting submarine caldera
depression is indicative of two distinct large explosion/collapse
events. The composition of deposits on neighbor islands also
supports such mechanism of tsunami generation during Krakatau's
paroxysmal phase.
The largest of the destructive tsunami waves were generated a
little after 10:02 on August 27th (GMT), from the combination
of explosion, subsidences, caldera collapses, landslides and
massive underwater flank failures. Another substantial tsunami
was generated at 16:38 by an additional phase of Krakatau's caldera
collapse.
The small waves and other sea level disturbances that were observed
at great distances were generated by atmospheric, air-to-sea
coupling of shock waves from the major Krakatoa explosion, and
not from the actual tsunami generated in the Sunda Strait.
See Also: The
Great Tsunami of August 26, 1883 from the Explosion of the Krakatau
Volcano ("Krakatoa") in Indonesia
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