TSUNAMI HAZARD ASSESSMENT ALONG THE
COAST OF LINGAYEN GULF, PANGASINAN,PHILIPPINES
Julius Galdiano
MEE12621

Supervisor:

Yushiro FUJII

ABSTRACT
We performed tsunami simulations for the Lingayen Gulf using the earthquake parameters of the twoearthquake scenarios along the Manila Trench. The Case 1 earthquake scenario (M7.6) with a slip
amount of 1.2 m produced smaller maximum wave height of 0.3 m and it arrived in about 1 hour in theinner bay of the Lingayen Gulf. The simulation results showed that Case 2 earthquake scenario (M8.4)
with a slip amount of 3.7 m produced the maximum wave height of 1.6 m that arrived in about 1 hour inthe inner bay of the Lingayen Gulf. The inundation computations showed that Case 1 produced amaximum height of 0.7 m and inundated approximately 0.13 km2 in a target area in Dagupan City while
Case 2 produced a maximum height of 3 m and inundated approximately 0.70 km2 in the same area. The
existing tsunami sensor (ASTI tide gauge station) is effective in the inner bay as it alarms the communityof an impending tsunami in 30 min and 35 min in Cases 1 and 2, respectively, while it is ineffective inBolinao coast because there will be a sudden tsunami arrival. Combining ASTI and JICA (proposedtsunami sensor project) tide gauge stations would advance to a better and robust tsunami early warning
system in the Lingayen Gulf.
Keywords: Scenario earthquakes, Manila Trench, Tsunami numerical simulation, Tsunami sensor

1. INTRODUCTION
The geographical location of the Philippines makes the wholearchipelago seismically active and prone to earthquakes, andoffshore earthquakes can cause tsunamis.
PHIVOLCS has taken research on tsunami hazard andrisk assessment in various areas to mitigate the future tsunami
effects. One of the projects that were accomplished is thedevelopment of the first ever Community Tsunami Detection andWarning System in the Philippines in Pangasinan (Figure 1). The
detection system is composed of three types of sensors: wet, dry andultrasonic sea level monitoring (PHIVOLCS Website). As soon as
the system detects a tsunami, the alarm systems installed in various
areas will be triggered and alerts the communities to an impendingtsunami threat.
Lingayen Gulf (see Figure 1) was selected as the Figure 1. Location of the study area withstudy area because of its vulnerability to tsunami hazards and is the existing tsunami early warning systemclose to Manila Trench which was the source of tsunamis that (yellow star: tsunami sensor, red stars:
sirensaffected the nearby coastal areas in the past.
The purpose of the study is to assess the tsunami
Philippine Institute of Volcanology and Seismology (PHIVOLCS)- Department of Science and Technology(DOST), Philippines
International Institute of Earthquake Engineering (IISEE), Building Research Institute (BRI), Japan1

hazard along the coasts of the Pangasinan Province where earthquakes and tsunamis have occurred as
early as about 6,000 years ago (Ramos and Tsutsumi, 2010). We also discuss the effectiveness of theexisting near real-time tsunami detector and warning systems in the Lingayen Gulf.

2. THEORETICAL CONCEPTS
2.1.

Governing Equations in Cartesian and Spherical Coordinate Systems

In Cartesian coordinates, the shallow water wave equations (Imamura et al., 2006) can be written as
(1)
()

()

√(2)

()

( )

√(3)

where,
andis the total water depth, is the wave amplitude, t is the time, is the roughness coefficient,
are the discharge fluxes in x and y directions, is the gravitational acceleration.
Employing the spherical coordinate system in the numerical simulation is imperative toaccommodate the spheroidal shape of the earth in the computation (Yanagisawa, 2013). The continuity
and momentum equations are expressed as([(())(())]

(4))
()

√√

(5)
(6)

where,
is the discharge flux in the (along a parallel of latitude) direction,is the discharge fluxin the (along a parallel of longitude) direction, and is the radius of the earth at 6378.137 km.
2.2.

Ocean Floor Deformation

Ocean floor deformation is the initial condition to generate a tsunami. Vertical deformation of the ocean
floor is assumed to be the same as the water surface displacement. To calculate this deformation, theelastic fault model by Okada (1985) using the fault plane model is used.
In this study, the effect of the horizontal displacement (Tanioka and Satake, 1996) is also
considered. The vertical displacement of water due to the horizontal movement of the slope,
iscalculated as(7)where,

is the water depth andandare the horizontal seafloor displacements due to a faulting.

23. DATA AND METHODOLOGY
3.1.

Bathymetry and Topography Data

In this study, we used the bathymetry data from theGeneral Bathymetry Chart of the Ocean (GEBCO) withresolutions of 1 arc-min (hereafter referred to as G1m) and
30 arc-sec (hereafter referred to as G30c) that were
downloaded from the GEBCO website. To generate a finer
resolution, the Philippine topography map withbathymetry data produced by the National Mapping andFigure 2. Comparison of the bathymetry data of
Resource Information Authority (NAMRIA, 1987) wereG1m (right) and G30cD (left).
digitized and merged to the GEBCO 30 arc-sec
bathymetry data (hereafter G30cD) (Figure 2). The Shuttle
Radar Topography Mission version 2 (SRTM_v2) datasets retrieved from the United States GeologicalSurvey (USGS) website were overlaid with the G30cD and used in performing the inundation modeling.
3.2.

Tide Gauge Stations

We set eight tide gauges as output points (Figure 3) to obtain thetsunami height and arrival time. Among these stations, only
Advanced Science and Technology Institute (ASTI) is an actual tidegauge station. Japan International Cooperation Agency (JICA) is a
proposed tide gauge station to be installed in early part of 2014.
NAMRIA tide gauge is a pre-existing tide level monitoring station
maintained by NAMRIA. The other 5 tide gauge stations are assumed
in this study.
3.3.

Tsunami Simulation

To model the tsunami Figure 3. Location of the tide gaugestations set as output points.
propagation, we used thetsunami simulation codes;
Tohoku University Numerical Analysis Model for Investigationof Near-field Tsunamis, No. 2 (TUNAMI-N2) by Imamura et al.
(2006). In this study, we used the tsunami simulation codes
provided by Koshimura (2013), which were modified by Fujii(2013) for the NetCDF input and output. We also used thesimulation codes for tsunami inundation by Yanagisawa (2013).
The tsunami height calculations using the uniform grid systemof 1’ in Cartesian coordinates were set in region 1 (Figure 4).
The tsunami inundation calculations using the Sphericalcoordinates were set in four regions as seen in Figure 4. The
spatial grid sizes of regions 1 to 4 are 1’, 20”, 6.66667”, and2.22222”, respectively.
3.4.
Figure 4. Computation regions for tsunami
height and arrival time (region 1) andtsunami inundation (regions 1 to 4).

Earthquake Source

In this study, we used the fault parameters of the segment 2 ofManila Trench (MT2) as shown in Table 1. We simulated twoearthquake case scenarios of the historical (Case 1) and3

maximum credible (Case 2) tsunamis. For the case scenarios, we considered the biggest historical event(M7.6) that ever occurred within this segment and the maximum credible event (M8.4) that can happen
along this fault segment (Salcedo, 2010).
Table 1. Fault parameters for each case scenario earthquake.
Scenario

M

Historical
Max Credible

7.6
8.4

3.5.

Location(lon., lat. (deg))
119.10 16.06
119.10 16.06

Length(km)
97.72
254.0

Width(km)
53.21
91.16

strike(deg)
11

dip(deg)3636rake(deg)95
95

Slip(m)1.21
3.69

Depth(m)00

Numerical Stability

The Courant Friedrics Lewy (CFL) condition is given by the formula√(Imamura et al.,2006), whereand
are the temporal and spatial length of grids respectively, is the gravity
acceleration (9.8 m/ ) andis the maximum still water depth in a computation region. In thisstudy, we used temporal grid size ( ) of 3.0 s in simulations of tsunami height and travel time and 1.0 s
in inundation calculation to stabilize the numerical computations.

4. RESULTS AND DISCUSSIONS
4.1.

Ocean Floor Deformation and Tsunami Propagation

Figure 5 shows the seafloor deformation ofthe two earthquake scenarios. The Case 1
earthquake scenario had a computedmaximum uplift of 0.64 m while themaximum subsidence was 0.26 m. Case 2 had
a computed maximum uplift of 1.87 m and themaximum subsidence was 0.31 m.
4.2.

Tsunami Waveforms

Figure 6(a) shows the comparison of thetsunami waveforms produced using the G1mand G30c in Case 2. The waveforms
calculated at TGS01, TGS02, TGS03, TGS04,
and TGS05 have remarkable differences. The

Figure 5. Seafloor deformations for Cases 1 (left) and 2 (right).
The contour intervals of uplift (red lines) and subsidence (bluebroken lines) are 0.1 m and 0.05 m, respectively.

Figure 6. Comparison of tsunami waveforms in Case 2 produced (a) using
G1m (red) and G30c (blue) and (b) using G30c (blue) and G30cD (red).

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tide gauge stations were located in the
inner bay of the Lingayen Gulf, while
the tide gauge stations that calculated
almost the same waveform amplitudes
were located outside the Lingayen
Gulf.
The comparison of thewaveforms calculated using the G30cand G30cD in Case 2 are shown in
Figure 6(b). The waveforms calculated
at all tide gauge stations have almostthe same amplitudes. This shows that the depths of the two bathymetry data are almost identical. Thisfurther demonstrates that the use of the two bathymetry data is more realistic than the use of the G1m.
4.3 Tsunami Travel Times (TTT) and Tsunami Heights
Figure 7 shows the estimated TTTs at each tide station for Cases 1 and 2 using the three bathymetry data.
The TTTs at tide gauges located in the open sea (ASTI, JICA, and NAMRIA) or near the open sea(TGS01) are almost the same for both the earthquake scenario cases, while the TTTs in the inner bay(TGS02, TGS03, TGS04 and TGS05) using both G30c and G30cD are almost the same. On the other
hand, the TTTs at the same tide gauges using G1m are longer. The first tsunami arrives at the Lingayen
Gulf (referred also as inner bay) coast in 38 min and 35 min as estimated at TGS02 for the Cases 1 and 2,respectively. Figure 8 shows the tsunami heights at each tide gauge for Cases 1 and 2.The maximum
wave height of Case 1 was 0.3 m at TGS03. In Case 2, the maximum tsunami height was 1.6 m at
TGS04.

Figure 7. Estimated TTTs from the source point to thetide gauge stations in Cases 1 (left) and 2 (right) using
the three different bathymetry data.

Figure 8. Maximum heights computed at each tide gauge
station in Cases 1 (left) and 2 (right) using the threedifferent bathymetry data.

4.4. Tsunami Inundation
We simulated tsunami inundations for Cases 1 and 2
taking their vertical and horizontal effects into
consideration. We also simulated Case 2 neglecting
the horizontal effect to compare the results with a
combination of vertical and horizontal effects. To
measure the inundation height, we set three watergauges (WG) as output points in the target area in
Figure 9. Location of the inundation area analysis (orangebox) and water gauges set as output points and thecorresponding waveforms at each water gauge (blue;considering the vertical and horizontal effects, red; withvertical effects only).

Dagupan City (Figure 9). The results show thatwhen vertical and horizontal effects are bothconsidered, the highest calculated tsunami heightis higher by about 17% as compared to the one
calculated
neglecting the horizontal effects.
Figure 10. Magnified view of the target area in Dagupan CityThe maximum height calculated at
for inundation analysis of Cases 1 (left) and 2 (right) in WG 2.
WG 2 was 0.7 m in Case 1. The maximum height
among the WGs set was 3.0 m at WG 2 in Case 2. The calculated inundation areas in the target area asshown in Figure 10 were approximately 0.13
and 0.70
for Cases 1 and 2, respectively.

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4.5.

Tsunami Sensor

Tsunami sensors can be effective as an early warning system in the inner bay along the Lingayen Gulf
coast because the first tsunamis arrive in 38 min and 35 min both recorded at TGS02 for Cases 1 and 2,
respectively. Bolinao (see Figure 3) will be hit by tsunamis with a maximum height of 0.8 m (asrecorded at ASTI) that will be arrived in 8 min after the earthquake in Case 1. In this case, a quick
evacuation is needed to escape the dangers of tsunami. In Case 2, the initial tsunami will attack Bolinao
coast in less than 1 min and the wave will reach the positive peak of 2.0 m in 10 min and a maximum
height of 2.4 m in about 30 min (as recorded at ASTI). In this case, the aim of the tsunami sensors will beineffective in the coastal areas of Bolinao. By combining ASTI and JICA tide gauge stations as tsunami
sensors, a better and robust early warning system would be achieved as they complemented with eachother.

5. CONCLUSIONS
Tsunami simulations were performed for Lingayen Gulf, setting Dagupan City as the target area using
the two earthquake scenarios of M8.4 and M7.6 from a segment of the Manila Trench. The use ofGEBCO 30 arc-sec data and digitized depth points from a bathymetry map produces a more realistic
output as compared to the use of GEBCO 1 arc-min. Also, considering the horizontal displacement
effect of tsunami in the simulation is needed to accommodate the influence of the slope of the ocean
floor near the trench axis.
ASTI tide gauge station is effective as tsunami warning system in the inner bay ofLingayen Gulf coast while it is ineffective in the Bolinao coast. By combining ASTI and JICA tidegauge stations as tsunami sensors, a more accurate early warning system will be in placed.
The results of the tsunami simulation will give better understanding of the community’stsunami risk that can be used in designing the evacuation strategies and maps.
As part of my future plan, I would like to do tsunami hazard assessments in the populated
coastal areas in the Philippines that are prone to tsunamis. The results are essential inputs in thedevelopment of tsunami hazard maps. Likewise, I would like to formulate a database of earthquake
scenarios that can be useful in establishing a tsunami early warning system in the Philippines.

REFERENCES
Fujii, Y., 2012-2013, Lecture notes on Tsunami simulation, IISEE/BRI.
Imamura F., Yalciner A.C., Ozyurt G., 2006, DCRC, Tohoku University, Japan.
Koshimura S., 2013, TUNAMI-N2 CODE, Tohoku University, 2012-2013 IISEE/BRI Lecture notes.
NAMRIA, 1987, Philippine National Topographic Map Series.
Okada Y., 1985, Surface deformation due to shear and tensile faults in a half space: Bulletin,
Seismological Society of America, 75, 1135-1154.
Ramos, N. T., Tsutsumi, H., 2010, Tectonophysics (2010), doi: 10.10 16/j.tecto.2010.08.007.
Salcedo, J., 2010, Individual studies by participant at the IISEE.
Tanioka, Y. and Satake, K., 1996, Tsunami generation by horizontal displacement of ocean bottom,Geophysical Research Letters, Vol. 23, No. 8, 861-864.
Website: NOAA, General bathymetric chart of the oceans (GEBCO) http://ngdc.noaa.gov/mgg/gebco/.
Website: PHIVOLCS official website www.phivolcs.dost.gov.ph.
Website: USGS, http://dds.cr.usgs.gov/srtm/version2_1/SRTM3/Eurasia/.
Yanagisawa, H., 2012-2013, Lecture Notes on Numerical simulation of tsunami and its application,
IISEE/BRI.

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