We investigate controls on tsunami generation and
propagation in the near-field of great megathrust earthquakes
using a series of numerical simulations of subduction and
tsunamigenesis on the Sumatran forearc. The Sunda
megathrust here is advanced in its seismic cycle and may be
ready for another great earthquake. We calculate the seafloor
displacements and tsunami wave heights for about 100
complex earthquake ruptures whose synthesis was informed
by reference to geodetic and stress accumulation studies.
Remarkably, results show that, for any near-field location:
(1) the timing of tsunami inundation is independent of slipdistribution
on the earthquake or even of its magnitude, and
(2) the maximum wave height is directly proportional to the
vertical coseismic displacement experienced at that location.
Both observations are explained by the dominance of long
wavelength crustal flexure in near-field tsunamigenesis. The
results show, for the first time, that a single estimate of vertical
coseismic displacement might provide a reliable short-term
forecast of the maximum height of tsunami waves

Antonioli, A.

Cocco, M.

Dunlop, P.

Giunchi, C.

Huang, J. D.

McCloskey, J.

Nalbant, S.

Piatanesi, A.

Sieh, K.

Steacy, S.

Earth-prints Repository

 
 
 
 
 5 
 
 
Near-field Propagation of Tsunamis from Megathrust 
Earthquakes 
 10 
John McCloskey1*, Andrea Antonioli1, Alessio Piatanesi2, Kerry Sieh3, Sandy Steacy1, 
Suleyman S. Nalbant1, Massimo Cocco2, Carlo Giunchi2, JianDong Huang1 and Paul 
Dunlop1 
                                                 
1 Geophysics Research Group, School of Environmental Sciences, University of Ulster, 
Coleraine, Co Derry,BT52 1SA, N.  Ireland.  
2 Seismology and Tectonophysics Department, Istituto Nazionale di Geofisica e 
Vulcanologia, Via di Vigna Murata 605, 00143 - Rome, Italy. 
3 Tectonics Observatory, California Institute of Technology, Pasadena 
* Corresponding Author, j.mccloskey@ulster.ac.uk, +442870324769 
 
McCloskey et al (2007) Propagation of Megathrust Tsunamis 
 2
 
Abstract 15 
We investigate controls on tsunami generation and propagation in the near-field of 
great megathrust earthquakes using a series of numerical simulations of 
subduction and tsunamigenesis on the Sumatran forearc. The Sunda megathrust 
here is advanced in its seismic cycle and may be ready for another great 
earthquake. We calculate the seafloor displacements and tsunami wave heights for 20 
about 100 complex earthquake ruptures whose synthesis was informed by 
reference to geodetic, and stress accumulation studies. Remarkably, results show 
that, for any near-field location: 1) the timing of tsunami inundation is 
independent of slip-distribution on the earthquake or even of its magnitude and 2) 
the maximum wave height is directly proportional to the vertical coseismic 25 
displacement experienced at that location. Both observations are explained by the 
dominance of long wavelength crustal flexure in near-field tsunamigenesis. The 
results show, for the first time, that a single estimate of vertical coseismic 
displacement might provide a reliable short-term forecast of the maximum height 
of tsunami waves.  30 
 
Introduction 
The great magnitude 9.2 Sumatra-Andaman earthquake of 26 December 2004 produced 
vertical seafloor displacements approaching 5m above the Sunda trench southwest of 
the Nicobar Islands and offshore Aceh (Subarya, et al. 2006; Vigny, et al. 2005; 35 
Piatanesi & Lorito S. 2007; Chlieh,  et al. 2006) creating a large tsunami that 
propagated throughout the Indian Ocean, killing more than 250,000 people. Waves 
incident on western Aceh reached 30m in height. On March 28 2005 the megathrust 
ruptured again in the magnitude 8.7 Simeulue-Nias earthquake but in this case the 
McCloskey et al (2007) Propagation of Megathrust Tsunamis 
 3
waves nowhere exceeded 4m and few people were killed by them. The Simeulue-Nias 40 
earthquake nucleated in an area whose stress had been increased by the Sumatra-
Andaman earthquake (McCloskey et al. 2005) Follow-up studies (Nalbant, et al. 2005; 
Pollitz, et al. 2006) show that it has additionally perturbed the surrounding stress field 
and has, in particular, brought the megathrust under the Batu and Mentawai Islands 
closer to failure. Recent aseismic slip (Briggs et al. 2006) has further increased the 45 
stress (Fig. 1). Paleogeodetic studies show that the megathrust under the Batu Islands is 
slipping at about the rate of plate convergence (Natawidjaja et al. 2004) while under 
Siberut Island it has been locked since the great 1797 earthquake and has recovered 
nearly all the strain released then (Natawidjaja et al. 2006) 
The contrasting 2004 and 2005 events highlight the difficulties attendant on preparing 50 
coastal communities for the impact of tsunamis from earthquakes whose slip-
distributions and even magnitudes are essentially unknowable even where, as is the case 
on the Sunda megathrust to the west of Sumatra, there is clear evidence of an impending 
great earthquake. Cities on the west coast of Sumatra, notably Padang and Bengkulu 
with combined populations in excess of 1 million, lie on low coastal plains and are 55 
particularly threatened by tsunamis generated by Mentawai segment earthquakes. Here 
we attempt to understand these threats by simulating tsunamis which would result from 
a wide range of plausible earthquakes sources. 
 
Modelling Scheme 60 
Our simulations, which will be described in detail elsewhere, combine sophisticated 
numerical modelling with the best current geologically-constrained understanding of the 
state of the Sunda megathrust to forecast the range of possible tsunamis which might be 
experienced following the next great Mentawai Island earthquake. We define four likely 
fault segments which are suggested by the structural geology of the megathrust, by 65 
McCloskey et al (2007) Propagation of Megathrust Tsunamis 
 4
historical earthquakes and by long-term and recent stress accumulation. All simulated 
earthquakes are on the same 3D structure. The Sunda trench in the area of interest is 
approximately linear, strikes at about 140° and extends from the equator to about 6.5°S. 
The plate interface dips at about 15° resulting in a down-dip seismogenic width of about 
180km. We simulate about 100 or so complex slip distributions, around 25 for each 70 
fault segment length, which have been judged, by reference to paleoseismic and 
paleogeodetic data, to be likely candidates for the future event (see for example, Briggs 
et al. 2006; Prawirodirdjo, L. et al., 1997). We make no assumptions about the location 
of maximum slip on the fault, whether shallow near the trench or deep under the 
volcanic arc, but the slip models conform to the observed fractal distribution (Mai & 75 
Beroza, 2002) though our main results are not sensitive to a wide range of plausible slip 
distributions. We note that these slip distributions conform to constraints on the gradient 
of slip which are set by material and constitutive properties of the lithosphere and have 
been used elsewhere to model slip heterogeneity in tsunamigenesis (Geist 2002). Using 
a finite-element model of the elastic structure of the lithosphere customised for the 80 
western Sumatran forearc and including the effects of topography, we calculate the 
seafloor displacements which would result from each selected slip distribution. These 
displacements define boundary conditions for the tsunami simulation. The non-linear 
shallow water equations are solved numerically using a finite difference scheme on a 
staggered grid (Mader 2004). The initial sea-surface elevation is assumed to be equal to 85 
the coseismic vertical displacement of the seafloor calculated using the elastic model, 
and the initial velocity field is assumed to be zero everywhere (Satake, 2002). We apply 
a pure reflection boundary condition along the true coastline at which the depth has 
everywhere been set to 10m to avoid numerical instabilities. This boundary condition 
ensures that all the tsunami kinetic energy is converted into potential energy at the coast 90 
and thus, while we do not simulate the complex processes of inundation which are 
McCloskey et al (2007) Propagation of Megathrust Tsunamis 
 5
controlled by fine scale details of the near-shore topography, our predicted coastal wave 
heights include both the effect of shoaling to 10m depth and the interaction with the 
solid boundary.  
 95 
Results  
We report on the systematic control of tsunami waveforms in the near-field, Formally 
defined here as that region which experiences vertical co-seismic displacement which is 
measurable with current GPS technology. We find that the shape of the tsunami wave 
train recorded at any tide gauge is, to first order, independent of the slip-distribution or 100 
even of the magnitude of the earthquake that caused it. Figure 2 illustrates this 
independence with respect of two very different simulated Mentawai earthquakes. Event 
I is a 330km long re-rupture of the 1797 segment and with magnitude 8.3 while Event II 
is a 630km rupture of both the 1797 and 1833 segments with magnitude 9.0. Despite the 
great difference in both magnitude and location of high slip regions in the rupture with 105 
respect to the tide gauge, the shapes of the wave-height time-series are different only in 
detail; the timing of the main tsunami phases is constant. Conversely, the maximum 
height of the waves differs by an order of magnitude. This similarity, which is observed 
for all 100 simulations at all simulated tide gauges, allows the accurate prediction of the 
arrival time of flooding phases. The first wave crest, for example, arrives at Padang 110 
33.5±2.5 (2σ) minutes after the event origin. Similar predictions can be made for the 
other five near-field tide gauges in this study. 
Another feature of these curves is the visual similarity of the z-component of coseismic 
deformation experienced at the tide gauges, as indicated by the intercept on the height 
axes, despite the axes being scaled for the maximum height of the wave and not for the 115 
intercept; the ratio of coseismic displacement to maximum wave height is constant for 
McCloskey et al (2007) Propagation of Megathrust Tsunamis 
 6
these two events. Surprisingly, this observation is robust for all simulations and for all 
simulated tide gauges. Figure 3 shows the relationship between near-field vertical 
coseismic displacement and maximum observed tsunami height for three stations. This 
relationship holds for the other three tide gauges in the study though the scatter on the 120 
data is significantly higher for stations to seaward of the Islands. The coseismic 
displacement also predicts the depth of the deepest tsunami trough. Note that these 
results are not related to Plafker’s rule of thumb (Okal and Synolakis, 2004), which is, 
incidently, reproduced in this study, relating the maximum slip on the fault to the 
maximum observed wave height. These results show that the local tsunami energy is 125 
controlled by the local coseismic deformation, rather than the maximum deformation 
which may occur at many hundreds of kilometres distance and which generally do not 
predict the local tsunami at any specific point. 
 
Discussion and Conclusions 130 
The explanation for these relationships is straightforward. The entire near-field region 
experiences a well defined pattern of vertical coseismic deformation, upward under the 
forearc high and downward under the forearc basin and the Sumatran coast, which is 
controlled by the geometry of the subduction interface, and which is extended laterally 
along the length of the rupture (Fig. 4a). Whereas the amplitude of this wave varies 135 
strongly with the earthquake, to first order, the wavelength is always about 300km and 
its ends, where vertical coseismic deformation is zero, are fixed at the trench and just 
landward of the coast (Fig. 4b). These features are largely independent of the slip-
distribution or magnitude of the event. Since the initial tsunami waves are driven by the 
coseismic seafloor displacement their initial locations are controled by this 140 
instantaneous long wavelength crustal flexing, no matter what its amplitude, and 
propagate perpendicularly to the strike of the megathrust in the near field. Wave phase 
McCloskey et al (2007) Propagation of Megathrust Tsunamis 
 7
velocities are controlled by bathymetry and the observed waveforms at every site are, 
therefore, also largely independent of the details of the causal event.  
The strong correlation between maximum wave height (and minimum trough depth) and 145 
the vertical coseismic displacement at any point can also be understood by reference to 
this long wavelength crustal flexure. Since local tsunamis propagate normal to the axis 
of deformation, tsunami energy at any point is controlled, again to first order, by the 
potential energy of the coseismic tsunami wave along a line perpendicular to this axis 
through the point of interest. The potential energy is therefore proportional to the 150 
integral of the coseismic seafloor movement . Now we have seen that the amplitude of 
this profile is strongly earthquake dependent, thus the height of the resulting tsunami 
depends strongly on the event. However, since the general shape of the deformation 
wave is fixed both in wavelength and phase, an estimate of its amplitude at any point, 
ideally some distance from a node of the flexure, is a good first order predictor of the 155 
entire potential energy line integral and thus the amplitude of the resulting waves. Given 
the generality of this explanation we expect that the relationships reported in this paper 
will be applicable to any subduction zone though their details will be modified by local 
crustal geometry. 
These results may assist planning of preparedness strategies throughout the western 160 
Sumatran forearc complex. They show that the travel times of damaging tsunami phases 
in the near-field are subject to strong lower bounds, of about 30 minutes for the 
Sumatran coast and somewhat less for the off-shore islands, which are independent of 
the nature of the seismic source. Validation of these results using recent earthquakes is 
not straightforward. The accurate measurement of phase arrival times requires the 165 
operation of tide gauges with high-frequency sampling and are not available in western 
Sumatra for the recent earthquakes. Travel times simulated here are, however, 
consistent with field observations made after the 2004 tsunami (eg. 
McCloskey et al (2007) Propagation of Megathrust Tsunamis 
 8
http://ioc.unesco.org/iosurveys) and by the low-frequency tide gauge in Sibolga 
following the 2005 event (P. Manurung personal communication). These short travel 170 
times preclude the possibility of using ocean wide tsunami warning systems in 
preparedness planning for western Sumatra. On the other hand, the strong correlations 
between coseismic displacement and the height of the tsunami wave, which have been 
demonstrated here for failure of the Sunda megathrust under the Mentawai Islands, offer 
real hope of producing accurate short-term forecasts of tsunami height on the basis of a 175 
single GPS vertical coseismic displacement estimate which could be made in a few 
minutes following the earthquake origin (see also Blewitt et al. 2006). These 
correlations, of course, are valid only for tsunamigenesis by dip-slip failure on the 
megathrust without significant contributions from other processes such as submarine 
landslide or normal fault rupture in the hanging wall block which have been invoked to 180 
explain anomalous tsunami energy following other earthquakes (Pelayo & Wiens, 1992; 
Heinrich et al. 2000). They also assume that slip on the earthquake is rapid, unlike the 
slow 2006 Java earthquake which efficiently generated a large tsunami in the absence of 
strong shaking on shore. Recent and historical earthquakes in western Sumatra would 
appear to satisfy these conditions.  185 
 
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Acknowledgements We thank Rory Quinn for assistance in the bathymetric modelling, 
Spina Cianetti for assistance in construction of the finite element model of subduction 
and Chris Bean for constructive criticism of the manuscript. The Landsat ETM+ data is 235 
used courtesy of the Global Land Cover Facility, (http://www.landcover.org). We 
acknowledge financial support from the Natural Environmental Research Council . 
 
 






Near-field propagation of tsunamis from megathrust earthquakes

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Near-field propagation of tsunamis from megathrust earthquakes

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