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INSTABILITY
OF KILAUEA VOLCANO'S SOUTHERN FLANK - EVALUATION OF MASS EDIFICE
FAILURES, FLANK COLLAPSES AND POTENTIAL TSUNAMI GENERATION
George Pararas-Carayannis
Copyright
© 2005. All Rights Reserved
 Introduction
Throughout
Kilauea volcano's geologic history there have been numerous mass
edifice failures, collapses and subsidence along its southern
flank - some associated with earthquakes and local destructive
tsunamis such as those of 2 April 1868 and 29 November 1975.
There is
geologic evidence of a zone of weakness along the volcano's southern
flank. Paralleling the Puna rift zone is an extensive system
of coastal faults (palis), which appear to be gravitational features
associated with ongoing subsidence caused by both seismic and
aseismic events - the latter also documented by recent GPS satellite
measurements. There is also evidence of other parallel submarine
volcanic rift zones formed in an evolutionary sequence. The following
report evaluates:
a) The instability of Kilauea' southern flank;
b) On going subsidence and kinematic processes;
c) Crustal displacements associated with flank failures of the
southern flank
d) Expected future failures and frequency of recurrence
e) Tsunami generation mechanisms of oceanic shield stratovolcanoes
in general;
f) Tsunami generation efficiency of volcanic flank failures;
in particular: and,
g) Tsunami generation from Kilauea's past and future flank failures.
 The Instability of Kilauea's
Southern Flank
Marine geophysical
data, including SEA BEAM bathymetry, HAWAII MR1 sidescan, and
seismic reflection profiles, indicate that the southern slope
of the Island of Hawaii comprises of the three active hot spot
volcanoes, which are Mauna Loa, Kilauea and the submarine volcano
Loihi. This is the locus of the Hawaiian hot spot (Smith et al,
1999).
Review of
the coastal geology along Kilauea's southern flank, indicates
a complex pattern of kinematic processes and of resulting geomorphological
features.
A number
of large coastal fault scarps (palis), some as high as 500 meters,
parallel the Puna rift zone and are the tops of an extensive
fault zone along which substantial movement has occurred in the
past. Large fault blocks are tilted back, by as much as 8 degrees
towards the rift zone, indicating a pattern of gradual subsidence.
This continuous subsidence has created the feature known as the
Hilina Fault System.
Hawaii's
southern slope showing coastal faults parallel to the east rift
zone of the Kilauea volcano, and the Hilina Slump along which
slope failures have been occurring (Modified after Morgan et
al. 2001).
The Hilina
Slump and the Papa`u Seamount - The Hilina Slump and the Papa`u Seamount
are the offshore continuation of the mobile Kilauea volcano's
south flank that has resulted from extensive subsidence and slope
failure along a deeper detachment surface. (Morgan et al 2001).
High-resolution
side scan surveys of Kilauea's southern slope (Clague et al 1998;
Dartnell and Gardner 1999) show a number of cuspate normal faults
near the head of the Slump, as well as grabens and horsts in
the offshore region. These are indicative of past successive,
crustal movements - some associated with major earthquakes.
Presently,
the sub-aerial portion of the Slump creeps seaward at a rate
of approximately 10 cm/year. It is presumed that the underwater
portion is also moving at the same rate.
Also, review of the submarine geology of the Island of Hawaii
shows evidence of debris avalanches on the ocean floor along
the southwestern flank of the Mauna Loa and the southern flank
of Kilauea. The debris avalanches are indicative of large prehistoric
slides (Moore et al. 1989, 1994; Lipman, 1995; Moore and Chadwick,
1995; Clague et al., 1998; Dartnell and Gardner, 1999).
Crustal Displacements
Associated with Flank Failures of Kilauea's Southern Flank
Crustal
movements associated with flank failures of Kilauea volcano's
southern flank appear to involve uplift, subsidence and slope
failure along the Hilina Slump. The most recent and extensively
studied event was the flank failure associated with the Kalapana
tsunamigenic earthquake of 29 November 1975.
 The Flank Failure of
29 November 1975 -
The 1975 flank failure uplifted the sea floor offshore as the
onshore section of the volcano down-dropped and moved outward.
Post-earthquake surveys showed that a large crustal block slid
horizontally towards the ocean but that there was a great deal
of variation in the degree of crustal displacements (Pararas-Carayannis
1975). The maximum horizontal displacement, near Keauhou Landing
several kilometers east of Halape, was about 26 feet. The displacements
decreased to the east and west from this area. The dimensions
of the sea floor affected by the crustal movements of the 29
November 1975 flank failure were approximately 70 km long, and
30 km wide with the long axis of the displaced block being parallel
to the coast. This entire offshore region rose approximately
1.2 meters (3.9 feet). The total volume of displaced material
was roughly estimated to be only 2.52 cubic km (Pararas-Carayannis
1976a,b). These appear to be typical displacements associated
with flank failures of Kilauea's southern flank.
Oblique
sidescan view of the Papa`u Seamount (Modified after Smith et
al, 1999)
Inspection
of tide gauge records of the tsunami on the Island of Hawaii
and the other islands indicated that the initial tsunami wave
motion from this flank failure was upwards at all stations. This
confirmed also that the initial offshore crustal displacement
for this volcanic flank failure was a uplift, as the onshore
section subsided and moved outward. This also supported the conclusion
that the resulting slope failure and earthquake were not entirely
due to gravitational effects of instability, but may have been
partially caused by compressional lateral magma migration from
shallow magmatic chambers of Kilauea, or by lateral magmastatic
forces along an arcuate failure surface,or along a secondary
zone of crustal weakness on the upper slope of the Hilina Slump.
In fact, recent paleomagnetic studies show that differential
rates of movement and rotation occur between sections of the
Hilina Slump (Riley et al., 1999). Also, this had become evident
during the survey of crustal displacements associated with the
29 November 1975 event (Pararas-Carayannis 1975).
Kinematic
Movements Associated With Kilauea's Flank Failures
 Seismic studies indicate
that Kilauea is sliding along the seafloor or along parallel
zones of weakness within the southern rift zone, thus creating
the Hilina Slump. Because of such movement cuspate normal faults
have been formed near the head of the Slump. Also, the paleomagnetic
studies of changes in lava flow directions on the Hilina Fault
scarps have helped determine the pattern and speed of subsidence
and kinematic movement along the southern flank of the volcano.
In addition to subsidence, these studies have determined a pattern
of counterclockwise rotation, indicating slippage between blocks,
occurring along listric normal faults (Riley et al., 1999).
Submarine
Topography of the Loihi Volcano (Alexander Malahoff graphic)
As previously
stated, during the 1990's Kilauea appeared to be sliding easily
along its base in a seaward direction at an average speed of
about 10 cm a year. When this movement is blocked large earthquakes
occur. The growth of the submarine volcano Loihi has formed a
boundary to the southwest, which appears to limit Hilina's failure
and movement in that direction.
The Aseismic
Slip of November 2000 Aseismic kinematic movements and slippage of
the Hilina block were measured by satellites of the Global Positioning
System (GPS) in November 2000. The measurements documented that
a 12- by 6-mile area (amounting to about 72-square-miles) of
the south slope of Hawaii's Kilauea Volcano moved 3.5 inches
toward the sea, over a 36-hour period.
Although
indicative of instability, the motions were imperceptible and
occurred along the upper slope of Kilauea's southern flank (Cervelli
et al, 2000). Such slow aseismic movements have probably occurred
over thousands of years. These imperceptible kinematic changes
are now being detected and measured on a regular basis only because
the new satellite technology and instrumentation have made it
possible.
Triggering
Mechanisms of Kilauea Volcano's Flank Failures
Volcanic
flank failures can be triggered by isostatic load adjustments,
extensive erosion, gaseous pressures, violent phreatomagmatic
eruptions, magmastatic pressures, gravitational collapse of magmatic
chambers, dike and cryptodome intrusions as well as buildup of
hydrothermal and supra hydrostatic pore fluid pressures. For
a more complete discussion of triggering mechanisms of volcanic
flank failures and gravitational flank collapses of island stratovolcanoes
go to http://drgeorgepc.com/ TsunamiMegaEvaluation.
For this
report it will suffice to discuss only the most common triggering
mechanisms of major flank failure for shield volcanoes such as
Kilauea. The primary triggering mechanisms of Kilauea's flank
failure appear to be mechanical magma intrusion, magmastatic
pressures and lava dike intrusions. Isostacy may also play a
significant role but there is no sufficient data to determine
its influence.
The south
flank of the Kilauea volcano is characterized by frequent, low-intensity
seismicity. Most of the sudden crustal movements which have generated
occasional strong shallow earthquakes in the past appear to be
triggered by sudden flank failures caused by magmatic intrusions
and lava movements.
Essentially,
the movement of magma in the volcanic chambers acts as a piston
in creating hydraulic pressure - mainly upwards but also laterally.
Such pressure spreads the volcano's base. Lava moving higher
in the volcanic dikes along a rift zone exerts lateral as well
as vertical pressures. Compression by such magma and lava movements
can push large crustal blocks outward and trigger substantial
coastal crustal displacements and shallow-focus tsunamigenic
earthquakes. The many grabens and horsts on the southern offshore
flank of Kilauea are indicative of such gradual failure events.
These submarine geomorphological features parallel the Kau rift
zone and are as indicative of compression activity.
Also, similar
lava movements or gravitational settling, by activity termed
as "silent earthquakes" may induce gradual coastal
subsidence. As reported above, the November 2000 aseismic slippage
detected by the satellites of the Global Positioning System is
indicative of such gradual movement and settling along the south
slope of Hawaii's Kilauea Volcano.
Either process
is capable of triggering larger underwater slumps or landslides.
The Hilina slump on the southern slope of Kilauea and the Koolau
slumps off the Island of Oahu are some of the larger scale features
of underwater slope failures that appear to have been triggered
by such mechanisms. Whether these are singular, sudden events,
or composite events that have occurred over a period of time,
will be analyzed and discussed in detail in another report.
Future Failures
of Kilauea Volcano's Southern Flank
Kilauea
is being actively monitored by satellite instrumentation which
permits accurate geodetic measurements and of rates of movement
along its south flank. These measurements indicate that stresses
are building up at the present time. Once a threshold limit is
exceeded; there will be another release of accumulated energy
along the volcano's southern flank. Magmatic pressures from within
Kilauea's own chambers or lava dike intrusion will probably trigger
such failures. Pressures from within Mauna Loa's magmatic chambers
may also contribute to instability and future failure of Kilauea's
southern flank.
In spite
of such apparent instability, a massive flank failure of Kilauea
along a detachment fault zone - as postulated (Ward 2001) - is
very unlikely to occur. Neither the 1868 nor the 1975 earthquakes
were associated with massive flank failures of Mauna Loa or Kilauea
or generated an ocean-wide mega tsunami (Pararas-Carayannis 1976a,
1976b, 2002; Pararas-Carayannis and Calebaugh, 1977).
Future mass
edifice failure events and major flank collapses of Kilauea's
south flank can be expected to result in large, shallow focus
earthquakes on a nearly flat-lying fault plane. A repeat of the
1975 flank failure and associated large earthquake, can be expected
on the south flank of Kilauea every 200 years or even more frequently
(Pararas-Carayannis,1976a,b). Smaller events can be expected
to occur every 100 years more or less - depending on volcanic
activity and internal pressure build up. Most failures will occur
as discreet events perhaps over a period of time.
Tsunami Generation
Mechanisms of Oceanic Shield Volcanoes
 Since shield volcanoes
are normally characterized by sporadic and weak explosive activity,
tsunamis cannot be effectively generated by their eruptions.
However, near the coast, secondary eruptions associated with
explosive phreatomagmatic activity known as Surtseyan, can generate
small local tsunamis. For example, the small cones of Diamond
Head, Coco Head and Punchbowl are examples of secondary activity
of the Koolau volcano on the island of Oahu.
These violent
Surtseyan eruptions occurred in the last 10,000-40,000 years.
The small fragments of coral, which can still be seen embedded
within the consolidated, volcanic ash, indicate the strength
of their explosive activity. Each of these violent phreatomagmatic
eruptions lasted only a few hours, with some extreme episodes,
which, undoubtedly, generated local tsunamis.
Larger and
more destructive local tsunamis may be also generated by the
mass edifice failure and collapses of the flanks of active oceanic
shield volcanoes such as Kilauea. As previously discussed, the
evolutionary development of such volcanoes generates lateral
forces at right angles to the major rift zones, forcing volcanic
mass outward and thus triggering faulting subsidence and flank
collapses.
Though most
of the collapse processes of shield volcanoes are closely associated
with regional volcanic activity, slow aseismic tectonic subsidence
and gravitational isostatic settling can occur also along fractures
controlled by regional fault patterns as described previously.
The fault patterns may be localized along ring fractures and
may indeed form circular caldera-like depressions which may continue
underwater - as at the Piton De La Fournaise volcano on Reunion
Island in the Indian Ocean, or as extensive flank rifts as along
the southern part of the volcanoes of Santorin or Kilauea.
An arcuate
coastline on a volcanic island may be indicative of such failure
along a ring dike, or along a rift zone. Such coastal slope failures
may be triggered concurrently or subsequently to an eruption.
Interstitial ground water saturation, gravitational settling,
or tectonic earthquakes along major fracture zones, can also
trigger sizeable flank failures and thus generate tsunamis -
often independently of eruptive processes. Earth tides and oceanic
tidal loading may also play a role in triggering volcanic flank
failures.
Such flank
failures may be massive and be associated with major earthquakes
and large local tsunamis. However, the focal depths of such earthquakes
are normally fairly shallow and most of the destruction is caused
by the tsunami. The extent of the crustal displacements associated
with these failures may range from moderate to large and may
occur in phases over a period of time, rather than as single
discreet events. Thus, one or more tsunamis may be generated.
Other factors may limit the extent of flank failures.
For example,
oceanic shield stratovolcanoes - such as Kilauea - have underwater
slopes composed primarily of fairly stable pillow lavas with
a smaller percentage of unstable pyroclastics. As already described,
collapses and flank failures can be expected to occur periodically.
However, most of these failures may be distinct, rather than
massive events. They may result from step faulting such as that
along the Hilina slide on the southern underwater slope of Kilauea.
The following is an evaluation of flank failures of shield stratovolcanoes
such as Kilauea, and of the mechanisms of tsunami generation
of such processes.
Tsunami Generation
Efficiency of Volcanic Flank Failures
The efficiency
of tsunami generation depends on the volume of crustal material
involved in the slope failure and on its time history, speed
and efficiency of coupling with water displacement. Although
such slope failures can be expected to generate destructive local
tsunamis, the source dimensions would result in waves of relatively
short wavelength and period. The energy and height of tsunami
waves would attenuate rapidly away from the source.
Finally,
earthquakes induced by movement of lava in the magmatic chambers
of a volcano may result in compressional effects and cause both
coastal subsidence and offshore uplift - with the formation of
underwater grabens and horsts along the southern slope of Kilauea.
However, because of the relatively shallow depth of such earthquakes,
small area extent, and the relatively small volume of crustal
displacements, the resulting tsunamis have relatively short periods.
Their destructiveness is usually confined in the immediate area
of generation.
Tsunami Generation
from Flank Failures of the Kilauea Volcano
Slope failures
and subsidence along Kilauea southern flank have occurred with
frequency in the past. The failures appear to occur in phases
over a period of time, rather than as single, large-scale events
involving great volumes of crustal material. Other, large scale
hydromagmatic explosions, like those of Krakatoa, Santorin or
Tambora, also occur in stages but are usually associated with
different mechanisms which are presented elsewhere at this website.
Also to be discussed separately and evaluated in the future,
will be the postulated massive Koolau slide off the eastern coast
of the island of Oahu.
 The rarer and more
massive volcanic flank failures can be expected to result in
voluminous crustal displacements and debris avalanches which
may extend to the ocean floor and form large debris toes. There
is evidence of such a toe and debris along the southeastern end
of the Hilina Slide. Although an isopach has not been constructed,
the volume and dimensions of this prehistoric slide do not appear
to be as great, as that of 1868 along the flank of the Mauna
Loa volcano.
As indicated previously, inspection of the offshore bathymetry
of the southern coastal region of the island of Hawaii reveals
a pattern of "horsts" and 'grabens" - topographical
features suggestive of a long history of large crustal block
movements, subsidence and flank failures. These features parallel
the displacements on land (the palis).
Thus, mass
edifice failures appear to have occurred with regularity along
Kilauea's southern flank. Magmatic movements near the magmatic
chambers of Kilauea's Puna Volcanic rift zone induce most failures
and associated earthquakes. Such failures must have generated
destructive tsunamis in prehistoric times, and as recently as
1868 and 1975. Even smaller failures occurred as recently as
1989. These resulted in smaller but still damaging earthquakes
but did not generate tsunamis. There was no apparent evidence
of slope instability and failure. The ocean slope of the island
consists primarily of pillow lavas - not particularly susceptible
to large-scale slope failures. Slope failures can occur but not
necessarily as single, large scale events. The geomorphology
of the Hilina slump indicates slope failures, which occurred
as a series of discrete events over a period of time.
The movement
of lava in the magmatic chambers below a shield volcano, by acting
as a piston, has the potential of inducing shallow focus earthquakes
with substantial coastal crustal displacement. For example, offshore
submarine geomorphological features, such as the grabens and
horsts observed south of the volcano of Kilauea and paralleling
the Kay rift zone, are indicative of such activity. Such volcanic
earthquakes near the coast have the potential of generating very
destructive local tsunamis. However, because of the relatively
shallow depth of the earthquake and the relatively small volume
of crustal displacements, tsunamis destructiveness is usually
confined in the immediate area of generation.
The destructive
local tsunamis of August 29, 1975 on the southern coast of the
island of Hawaii is an example of such activity. There was no
tsunami of significance outside the source region. Although there
was extensive coastal subsidence along the southern coast, the
first wave motions of the tsunami were up, indicating offshore
uplift.
In contrast,
the first water motions of the destructive, local April 2,1868
tsunami were down, indicating tsunami generation entirely by
subsidence, or slope failure, along the underwater slope of Mauna
Loa rather than Kilauea. Maximum coastal subsidence occurred
somewhat westward of the 1975 event. The main event was preceded
by a series of small earthquakes caused by significant magmatic
movements. Also, there was a flank eruption of Mauna Loa.
Conclusions
In summary,
there is no indication that Kilauea's southern flank is unusually
unstable at this time or that a catastrophic massive failure
will occur soon. The subsidence process on the Hilina Slump appears
to be continuous and gradual. The 1975 flank failure and major
earthquake generated only a local destructive tsunami along the
southern coast. with limited far field effects elsewhere in the
Hawaiian Islands and the Pacific.
However,
a major crustal adjustment can be expected to occur again on
Kilauea's southern flank in the future. Such an event could occur
in the mid-21st Century or even sooner - unless the stresses
that are building up presently are relieved by aseismic slippage.
When a flank failure occurs again a major earthquake with an
upper magnitude limit of about 7.4 can be expected. As with the
1868 and the 1975 events a very destructive local tsunami will
be generated.
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