Citation: Tatapu, C., B. Shibazaki, Y. Fujii (2021), Tsunami simulation and hazard assessment formegathrust earthquakes along the coasts of the Solomon Islands, Synopsis of IISEE-GRIPS MasterʼsThesis.

TSUNAMI SIMULATION AND HAZARDASSESSMENT FOR MEGATHRUST EARTHQUAKES
ALONG THE COASTS OF THE SOLOMON ISLANDS
Carlos Tatapu1,2Supervisor: Bunichiro SHIBAZAKI3, Yushiro FUJII3
ABSTRACTThe Solomon Island has experienced earthquakes that were highly tsunamic genic in nature. Recentlarge earthquakes occurred near the plate convergence zone between the Australian and the PacificPlates. Still, seismic gaps exist in areas along the subduction zone. Therefore, this study looks intoscenario earthquakes along with these seismic gaps in the western and eastern parts of the Solomon
Islands. TUNAMI numerical simulation code was adopted to investigate tsunami hazards in the study
area. We assumed the two scenario rectangular fault models with different amounts of slips (i.e., 2 m,5 m, and 10 m) and calculated seafloor deformation and tsunami propagation. We used GEBCO Datafor bathymetry, SRTM data for topography in inundation modeling. We assumed output points to gettsunami waveforms for different islands and to get maximum wave heights and their arrival time on thetarget locations. Here, we discuss the results for the assumed fault model with 10m slip for two scenario
earthquakes. For the western Solomon study areas, the maximum wave height is 4.8 m on Rennell Islandand is 3.6 m at the coastline of Guadalcanal Island. For the eastern Solomon study areas, the maximum
wave heights of 4.9 m and 1.98 m are observed at Nama and Avita on Santa Cruz Islands, with arrival
times of 20 and 10 minutes, respectively. The tsunami inundation modeling shows run-up distance ofmore than 100 m for Managakiki coastlines and 84m for the coastline of Nama settlement in Santa CruzIslands. The study demonstrated an overview of possible seismic events that will help tsunami disastermanagement and planning for the Solomon Islands. Accurate data and finer bathymetric data are crucial
in improving simulations within other seismic-prone areas within the Solomon Islands.
Keywords: Tsunami propagation, Tsunami simulation, Tsunami Inundation, Tsunami Height, GEBCO.
1. INTRODUCTION
The Solomon Islands archipelago is located along the ring of fire and has been prone to earthquake,volcano, and tsunami hazards. Records of earthquakes, tsunami, and volcanic events have shown thedamages and casualties in the Solomon Islands in the past. The Solomon Island's chain of islands islocated close to the active subduction zone and a trench system as described by Peterson et al. (1999).
The 2007 western Solomon Islands earthquake with an Mw 8.1 generated a tsunami and caused
damages and casualties to villages and provincial centers close to its epicenter (e.g., Wei et al., 2015).
Following the western Solomon event, 2010 and 2013 events occurred in the western and eastern partof the Solomon Islands. Still, seismic gaps exist in areas along the Solomon subduction zone (e.g., Chenet al., 2011). Thus, this study develops a tsunami propagation model and estimates tsunami arrival timesand maximum wave height to assess tsunami hazards along the coast of the Solomon Islands and1

Senior Information Officer, Geological Survey Division (MME&RE), Solomon Islands.
IISEE-GRIPS Master’s course student.
3International Institute of Seismology and Earthquake Engineering, Building Research Institute.
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develop a tsunami inundation model for scenario earthquakes that can be useful for rapid assessment
and disaster planning.
2. DATAThe General Bathymetric Chart of the Oceans (GEBCO) is downloaded for use in this study. TheGEBCO
data
isavailable
throughthewebsite(https://www.gebco.net/data_and_products/gridded_bathymetrry_data/gebco_2020/),
andwedownload the 15-arc-second grid for use in this study. The 15-arc-second grid, resampled to 1 min-arc
grid, forms the basis of data that are used in tsunami modeling along the subduction zones (Figure 1,Table 1). The Shuttle Radar Topography Mission (SRTM) is one arc minute data available from theUSGS site. We used the SRTM data for populated islands near the subduction zones and used it tosimulate inundation. The SRTM data is merged with GEBCO data to get finer grid data to simulate
inundation for a selected region within the study area (Figure 2).

Figure 1 GEBCO Data for bathymetry of Western Solomon (left) and Eastern Solomon (right).

Figure 2 Merged SRTM Data with GEBCO data for Guadalcanal (Western Solomon) (left) and
Vanikoro island (Eastern Solomon) (right).
To simulate wave propagation and to investigate tsunami heights and arrival times for thestudy areas, the computation parameters for the broader regions are given in Table 1. Output points fortsunami wave calculation to estimate tsunami height and arrival times are shown in Figure 3.
Table 1 Computational settings for wave propagation
RegionData Source

Grid dimension(nx /ny)

Spatial grid size(∆x /∆y) (m)Temporal gridsize (sec)WesternSolomon

GEBCO 1-arcminute360 /360

1832 / 1843

32

EasternSolomon

GEBCO 1-arc
minute480 / 480

1821 / 1843

3Figure 3 Output points used for tsunami wave
height and arrival time calculations for western(left) and eastern (right) Solomon study area

For tsunami inundation modelling, we use a nesting grid system. Tables 2 and 3 give four
regions and resolutions for the nesting grid system for the western and eastern Solomon Islands.

Table 2 Nesting grid parameters for western region tsunami simulation
RegionLongitude

Latitude

Number ofGrid nx * ny
480x360

Data Source-12/-6Grid size(arcsec)60

1156/1622
3159/161
159.2/160.30-10/-8.8-9.214/-9.075

206.6667

300x210225 x 252

GEBCO_2020
GEBCO_2020

4159.45/159.75-9.46/-9.23

2.2222405 x 540

GEBCO_2014/2020+SRTM

GEBCO_2020

Table 3 Nesting grid parameters for eastern region tsunami simulation
RegionLongitude

Latitude1

160/168-14/-82
3

165/167.8
166.085/167.433

4

166.75/167-12.5/-10.5-12.214/11.075-11.8333/11.5

Grid size(arcsec)
60

Number of
Grid nx * ny
480x360

Data Source

206.6667

510x360
728 x 615

GEBCO_2020
GEBCO_2020

2.2222405 x 540

GEBCO_2014/2020+SRTM

GEBCO_2020

3. METHODOLOGY
Tsunami wave propagation can be explained by the shallow water theory. Here, we introduce equations
used in tsunami numerical simulation (Koshimura, 2021). The continuity equation and the momentum
equation derived from the Euler's equation set (Koshimura, 2021) are adopted to describe the watermotion. This study adopts the Tohoku University's Numerical Analysis Model to investigate Near-fieldtsunamis (TUNAMI) code. More specifically, we use the modified version of TUNAMI code by Prof.
3Koshimura (Fuji, 2021), to calculate tsunami propagation and simulation along the coastlines of thestudy area. We use the inundation simulation model (Yanagisawa, 2021), which adopts the sphericalcoordinate system for both study areas.
4. RESULTS AND DISCUSSION
We simulate tsunami wave propagation for two different regions using the GEBCO bathymetry data.
We use different fault slips (2, 5, 10 m) for two earthquake scenarios. Figure 4 shows snapshots oftsunami wave propagation for case I-3 (10 m slip) in western Solomon and for case II-3 (10 m slip) ineastern Solomon, respectively, at the elapsed time of zero minutes (initial time), 15 minutes, and 30minutes.
Figures 5 and 6 show the calculated tsunami waveforms for the study regions for the caseof 10 m slip in the western Solomon and for the case of 10 m slip in the eastern Solomon, respectively.
For the western Solomon
study region, there are 11 output
points (the results of four points areshown here), and for the eastern
Solomon study region, there are fouroutpoints. From the calculatedwaveforms, the tsunami arrival times
and the maximum tsunami height canbe determined. Travel time and themaximum tsunami height areimportant in this simulation study. We
investigate three models (2, 5, 10 m)
for western and eastern study regions
to examine tsunami wave heights andthe travel time for coastlines close to
the hypocenter.

Figure 4 Snapshots of tsunami wave propagation (0, 15, 30 min.)for western (top) and eastern (bottom) Solomon study area

Figure 5 Waveforms at output points (Figure 3 left) for the western Solomon study area.

Figure 6 Waveforms at output points (Figure 3 right) for the eastern Solomon study area.
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Figure 7 shows the calculated maximum
tsunami wave heights for different output
points for the western and eastern Solomon
study areas. Hereafter, we discuss theresults of 10m slip models.
For the western Solomon study
areas, the maximum wave height is 4.8 m on
Rennell Island and is 3.6 m at the coastlineof Guadalcanal Island. The arrival time of
the maximum tsunami wave is 11 minutes
at Rennell Island (Figure 8a).
The maximum tsunami waveheights are observed on Nama (4.9 m)
coastline and Avita (1.95 m). The arrival
time for maximum tsunami heights on thecoastline of Avita is within 10 minutes, andfor Nama, the arrival time is within 20minutes (Figure 8b) of earthquake event
Figure 7 Maximum tsunami heights graphs for both western
4.1 Tsunami inundation(top) and eastern (bottom) study area(a)100Time (min)

80

8182

8260
40

23 22
20203330

11103090LataNama

Avita

Nantinoga

Outpoint(b)2M_slip

5M_Slip

10M_slip

Figure 8 Maximum tsunami arrival time for bothwestern (a) and eastern (b) study areas.

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Inundation modeling is performed for
Mangakiki in the western Solomon andone of the islands in the Santa Cruz regionsin the eastern Solomon. The run-up heightand inundation area are important fortsunami hazard assessment and disasterplanning. Figure 9 below shows
inundation depths and run up distance for
Mangakiki and Nama. Figure 9 shows
tsunami height for Mangakiki and nearby
villages.
Themaximum
tsunamiinundation depth for the coastline of thetarget area shows significant wave heights.
More than 100m of inundation distance ismeasured. The inundation for thecoastlines of Nama and nearby villages, asseen in Figure 9, shows significant run-up
distance and inundation depths. The run-up
distance is between 84 m and 300 m onthese coastlines, with inundation depths
that vary between 0 to 6 m.

(a)Inundation
depth (m)(b)

Inundation
depth (m)Figure 9 Inundation depths for Mangakiki (a) and coastline of Nama (b).
5. CONCLUSIONSThe study used numerical simulation to investigate tsunami hazards caused by scenario earthquakes
occurring along the subduction zone between the Australian Plate and the Pacific Plate in the vicinityof the Solomon Islands Archipelago. We adopted the TUNAMI numerical code to simulate wave
propagation caused by scenario earthquakes for the western Solomon and eastern Solomon regions. We
then estimated important parameters such as the maximum tsunami heights and the arrival times thatwill be very useful for disaster planning and future mitigation. For the western Solomon study area, thecoastline of Rennell Island and Guadalcanal (Mangakiki) shows a maximum tsunami wave height of
4.8 m and 3.6 m. The inundation depth along the coastlines of Mangakiki was 4 m with a run-up distanceof more than 100 m. For the eastern Solomon region, maximum tsunami wave heights of 4.8 m and1.95 were observed on Nama and Avita coastlines. The run-up distance for parts of the coastline ofNama was 84 m with a maximum inundation depth of 6 m. The arrival times for maximum tsunami
wave heights for the study areas (western and eastern Solomon study regions) is within 20 minutes ofan earthquake as simulated, which is crucial for tsunami disasterACKNOWLEDGEMENTS
This research was conducted as the individual study of the training course “Seismology, Earthquake
Engineering and Tsunami Disaster Mitigation” by IISEE/BRI, JICA, and GRIPS. I would like to express
my sincere gratitude to Dr. Shibazaki for supervising me and giving me very useful advice andsuggestions during my individual study and all the other staff members at IISEE/BRI for their supportand encouragement during this training program. Furthermore, I am extremely thankful to Dr. Fujji(IISEE, BRI) for his constructive comments regarding this work
REFERENCES
Chen, M. C., Frohlich, C., Taylor, F. W., Burr, G., and Van Ufford, A. Q., 2011, Arc segmentation andseismicity in the Solomon Islands arc, SW Pacific. Tectonophysics, 507, 47-69.
Fujii, Y., 2021, Lecture notes on tsunami simulation, IISEE/BRI.
Koshimura, S., 2021. Lecture notes on tsunami hazard assessment: theory of tsunami propagation andinundation simulation, IISEE/BRI.
Petterson, M. G., et al., 1999, Geological-tectonic framework of Solomon Islands, SW Pacific: Crustalaccretion and growth within an intra-oceanic setting, Tectonophysics, 301, 35–60.
Yanagisawa, H., 2021, IISEE Lecture note on numerical simulation of tsunami, IISEE/BRI.
Wei, Y., Fritz, H. M., Titov, V. V., Uslu, B., Chamberlin, C., and Kalligeris, N., 2015, Source models
and near-field impact of the 1 April 2007 Solomon Islands tsunami. Pure and Applied Geophysics,
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