Understanding the Generation, Propagation, Near- and Far-Field Impacts of TSUNAMIS
and Planning Strategies to Prepare for Future Events
Chapter 3
CHAPTER 3: TSUNAMI GENERATION MECHANISM
3.1. INITIATION OF TSUNAMIS
3.1.1. Tsunami Initiation by Earthquakes
Earthquakes produce most tsunamis. The parameters of earthquake determine many
of the characteristics of the tsunami. Earthquakes result from slip on faults and many
parameters describe the process; three features however, are most important -moment, mechanism and depth.
Moment measures earthquake strength. Moment Mo is the product of rigidity
the source region’s rocks, fault area A, and average fault slip Δu. Even without a
detailed understanding of tsunami generation, it is safe to suppose that the larger the
earthquake moment, the larger the tsunami, all else fixed.
Mechanism specifies the orientation of the earthquake fault and the direction of slip
on it. Usually, faults are idealized as plane rectangles with normal nˆ . Three angles
summarize earthquake mechanisms -- the strike and dip of the fault and the angle of
slip vector aˆ measured from the horizontal in the plane of the fault. The role of fault
mechanism on tsunami production is not as obvious as the influence of moment.
However earthquakes that affect large vertical displacements of the seafloor would be
more effective than faults that make large horizontal displacements.
Earthquake depth needs no explanation. Because seafloor shifts cause tsunamis, the
distance of the fault from the seafloor should be important. Presumably, deep
earthquakes would produce less potent tsunamis than similar shallow earthquakes.
A tsunami earthquake is defined as an earthquake which excites much larger
tsunamis than expected from its seismic waves. The discrepancy can be quantified in
terms of the surface wave magnitude (Ms) and tsunami magnitude (Mt) scales.
Several proposals have been made for the generation mechanism. (1) Slow rupture
process which is inefficient to excite high frequency seismic waves, (2) subsidiary
high angle faulting in the accretionary prism, (3) source in shallow sedimentary layer
where rigidity is low, and (4) submarine landslides caused by earthquakes.
Although the characteristic earthquake model provides a gross picture of the tsunami
generation process, recent seismological and tsunami studies of great subductionzone earthquakes indicate that the tsunami generation process is more complicated.
Studies show that the fault parameters and locations of tsunami earthquakes share
common features: a narrow and shallow faulting beneath accretionary wedge is
responsible to the large tsunami generation. Furthermore, the location of tsunami
earthquakes seems to be related to submarine geological structure. Also the studies
show that the surface roughness of the ocean bottom near the trench is well
correlated with the large earthquake occurrence.
Fig. 3.1 shows five hours of tsunami waveforms calculated from (8) at distances of
r=200, 500,
45
© A.C Yalciner, H. Karakus, C. Ozer, G. Ozyurt
Understanding the Generation, Propagation, Near- and Far-Field Impacts of TSUNAMIS
and Planning Strategies to Prepare for Future Events
Chapter 3
1000, 2000km from dip slip (Myz=Mzy=1) and strike slip (Mxy=Myx=1) point sources
of magnitude Mw=7.5 buried at 10 km depth. Sea waves from these sources have
radiation patterns of sinθ
θ
Frequency dispersion, with the long periods arriving first, is the most conspicuous
feature of the waveforms. Tsunamis start rapidly. They reach maximum height in the
first few cycles and
then decay slowly over an hour or more. Even for this large earthquake, tsunamis
beyond
500km distance reach just a few cm Note that if the observation direction was west
versus east (Fig. 3.1 top), or northwest versus northeast (Fig. 3.1 bottom), the
tsunami waveforms would be inverted. Whether tsunamis onset in a withdrawal or an
inundation at a particular shoreline strictly depends on the style of faulting and the
relative positions of the shore and the fault. Fig. 3.1 demonstrates that for point
sources, dip slip earthquakes produce 3 or 4 times larger tsunami than strike slip
earthquakes of equal moment.
Figure 3.1 Synthetic record sections of vertical tsunami motions at distances of 200,
500, 1000 and 2000km from point dip slip (top) and strike slip (bottom) earthquakes
of magnitude Mw=7.5 and depth 10 km. Time runs for five hours and the peak
amplitude of each trace is given in cm at the right. The lower half of the focal sphere
are shown toward the right (Stewe Ward, Encyclopedia
of Physical Science and Technology)
46
© A.C Yalciner, H. Karakus, C. Ozer, G. Ozyurt
Understanding the Generation, Propagation, Near- and Far-Field Impacts of TSUNAMIS
and Planning Strategies to Prepare for Future Events
Chapter 3
3.1.2. Tsunami Excitation From Submarine Landslides
Earthquakes parent most tsunamis, but other things do too. For instance, earthquake
shaking often triggers landslides. If the slide happens under the sea, then tsunamis
may form. The total volume of the slide material is expected to be the main parameter
which affects the amplitude of a tsunami generated by a submarine landslide even
though several other parameters also play important roles. Some of these factors are
the following:
a) Depth at which the slide occurs, or rather, depth of water above slide.
b) Angle of the slide from the horizontal (or vertical) direction.
c) Total distance moved by the slide.
d) Duration of the slide.
e) Density of the slide material.
f) Coherent nature of the slide.
g) Grain size and spectrum.
h) Characteristic speed with which the slide moves.
3.1.3. Tsunami Excitation from Volcanic Eruptions
Volcanic eruptions accompanied by mass edifice flank failures and explosion-collapse
processes in forming submarine calderas, are very efficient tsunami generators.
However, volcanic edifice failure events involve relatively small volumes of mass and
the tsunami or tsunami-like waves they generate are of short periods and
wavelengths. Although these waves may be catastrophic locally, their heights
attenuate rapidly as they propagate away from the source.
Tsunami and tsunami-like waves from volcanic sources have complex generation
mechanisms. Destructive water waves may be generated by the explosion and
collapse of a volcanic cone in forming an underwater caldera, by the coupling of
volcanically-generated atmospheric shock waves with the sea surface, by massive
flank failures, by debris avalanches, and by massive aerial and submarine landslides.
To understand the volcanic tsunami generation mechanism, we must examine
submarine caldera formation processes and all other related concurrent geotectonic
activity that takes place before, during and after a volcano's paroxysmal eruption or
mass edifice collapse.
Processes leading to tsunami generation by pyroclastic flows are:
1. Deposition at the shoreline causing a lateral displacement as the zone of
deposition moves offshore.
2. Upward and lateral displacement of water caused by the propagation of a water
supported mass-flow.
3. Downward and lateral displacement of water caused by the sinking of debris from a
segregated flow travelling over the water surface.
4. Upward displacement of a large volume of water due to the deposition of a calderainfill ignimbrite or pyroclastic flow deposit.
47
© A.C Yalciner, H. Karakus, C. Ozer, G. Ozyurt
Understanding the Generation, Propagation, Near- and Far-Field Impacts of TSUNAMIS
and Planning Strategies to Prepare for Future Events
Chapter 3
3.1.4. Tsunami Excitation from Impacts
The sections above discussed tsunamis from earthquakes and landslides. One other
class of tsunamis hold interest – those generated from impacts of comets and
asteroids. Although, for a given location on the Earth's surface, the risk of a "direct"
hit from an asteroid is slight, researchers realized that an ocean impact had the
potential to be much more destructive due to the effects of tsunami.
When an asteroid hits the ocean at 70 000km/h there is a gigantic explosion. The
asteroid and water vaporize and leave a huge crater - typically 20 times the diameter
of the asteroid (that is, a 100m asteroid will create a 2 kilometer diameter crater). The
water rushes back in, overshoots to create a mountain of water at the middle and this
spreads out as a massive wave - a tsunami. The centre of the "crater" oscillates up
and down several times and a series of waves radiate out. An idea of the mechanism
can be obtained by bursting a balloon in a bathtub.
The main items of contention appears to be the initial size of the wave - based on
analysis of the size and shape of the "crater" and the manner in which it collapses,
and the rate at which a tsunami from an asteroid impact dissipates as it travels.
3.2. TSUNAMI PROPAGATION, SHOALING, RUNUP AND INUNDATION
In uniform depth oceans, tsunamis propagate out from their source in circular rings with ray
paths look like spokes on a wheel. In real oceans, tsunami speeds vary place to place (even
at a fixed frequency). Tsunami ray paths refract and become bent according to bottom
topography. Consequently, in real oceans, both tsunami travel time and amplitude have to be
adjusted relative to their values in respective depth distribution..
Toward shore, real oceans shallow and the waves carried on them amplify. Often, the
processes of wave amplification are lumped and labeled “run-up”. Run-up has linear and
non-linear elements.
Shoaling amplification depends on the ratio of group velocity at the nucleation-site and the
coast-site (ocean depth h and hs respectively). Fig. 3.2 shows the effect of shoaling on a
tsunami wave of 150s period. Initially, a unit height wave begins to come ashore from 4000m
of water at the left. As the water shallows, the velocity of the wave decreases and the waves
grows in amplitude. By the time it reaches 125m depth it has slowed from 137m/s group
velocity to 35m/s and grown in vertical height by a factor of two. As the wave comes closer to
the beach it will continue to grow about a factor of two more -- then it will break.
Tsunami run-up is the highest level at land where sea water reached by tsunami. The vertical
distance between the end point of the sea water reached and the still water level of the sea
(without tide) is called run-up height.
Tsunami inundation is the flow of sea water in the coastal area. The horizontal distance
between the end point of the see water reached and the shoreline is called inundation
distance.
48
© A.C Yalciner, H. Karakus, C. Ozer, G. Ozyurt
Understanding the Generation, Propagation, Near- and Far-Field Impacts of TSUNAMIS
and Planning Strategies to Prepare for Future Events
Chapter 3
Figure 3.2. Effect of shoaling on tsunami. The shallowing ocean near shore concentrates
wave energy into smaller and smaller volumes. Tsunami amplitudes grow in response.
(Stewe Ward, Encyclopedia of Physical Science and Technology)
Reference:
Geist, E.L., 1998. Local Tsunamis and Earthquake Source Parameters, Advances in
Geophysics, 39, 117-209.
Okal, E. A., 1988. Seismic Parameters Controlling Farfield tsunami amplitudes: A review,
Natural Hazards,
1, 67-96.
Ward, S. N., 1980. Relationships of tsunami generation and an earthquake source, J. Phys.
Earth, 28, 441-474.
Ritsema, J., S. N. Ward and F. Gonzalez, 1995. Inversion of Deep-Ocean Tsunami Records
for 1987-1988 Gulf
of Alaska Earthquake Parameters, Bull. Seism. Soc.Am., 85, 747-754.
Ward, S. N. and E. Asphaug, 1999. Asteroid Impact Tsunami: A probabilistic hazard
assessment, Icarus, submitted.
http://omzg.sscc.ru/tsulab/paper4.html
T. S. MURTY Tsunami Wave Height Dependence on Landslide Volume
http://www.drgeorgepc.com/TsunamiVolcanicCaribbean.html
De Lange W.P.1; Prasetya G.S.1; Healy T.R.1Modelling of Tsunamis Generated by
Pyroclastic Flows (Ignimbrites)
Crawford & Mader 1998.
49
© A.C Yalciner, H. Karakus, C. Ozer, G. Ozyurt