Coastal Engineering 64 (2012) Contents lists available at SciVerse ScienceDirect. Coastal Engineering

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1 Coastal Engineering 64 (212) 47 6 Contents lists availale at SciVerse ScienceDirect Coastal Engineering journal homepage: Nearshore tsunami amplitudes off Sri Lanka due to proale worst-case seismic scenarios in the Indian Ocean J.J. Wijetunge Department of Civil Engineering, University of Peradeniya, Peradeniya 24, Sri Lanka article info astract Article history: Received 19 July 211 Received in revised form 7 Feruary 212 Accepted 8 Feruary 212 Availale online 7 March 212 Keywords: Hazard assessment Numerical simulations Suduction earthquakes Coastal zone Early warning This paper descries a deterministic assessment of the tsunami hazard to Sri Lanka from all active suduction zones in the Indian Ocean Basin. High resolution numerical simulations of tsunami propagation have een carried out for eight plausile maximum-credile seismic scenarios in Northern Sumatra Andaman, Southern Sumatra, Arakan and Makran suduction segments. The numerical results have een analyzed to otain wave arrival times and maximum tsunami amplitudes just off the coastline of Sri Lanka. A sensitivity analysis carried out at the outset indicates that the computed tsunami amplitudes off Sri Lanka are only marginally sensitive to the perturations in the source parameters. Moreover, the computed values of peak tsunami amplitudes and arrival times corresponding to an event similar to the tsunami in 24 are in good agreement with the respective field oservations. The numerical simulations also suggest that the maximum tsunami amplitudes around the coastline of Sri Lanka corresponding to worst-case scenarios in Southern Sumatra, Arakan and Makran seismic zones are only less than aout 2 3% of those due to the same in Northern Sumatra Andaman segment. This means that an event similar to the 24 tsunami in the Northern Sumatra Andaman suduction zone could e considered as the worst-case seismogenic tsunami scenario for any part of the coastline of Sri Lanka. The information presented in this paper would help authorities responsile for evacuation to make a etter judgement as to the level of proale maximum tsunami heights in different areas along the coastline of Sri Lanka, and act accordingly, if a large earthquake were to occur in any of the suduction zones in the Indian Ocean Basin. 212 Elsevier B.V. All rights reserved. 1. Introduction The tsunami triggered y the mega-thrust earthquake of M w = (Lay et al., 2; Stein and Okal, 27) in the Northern Sumatra Andaman suduction zone on 26 Decemer 24 caused unprecedented loss of life and damage to property in several Indian Ocean countries including Sri Lanka. The massive tsunami in 24 as well as the two susequent comparatively smaller, yet oceanwide, tsunami due to major earthquakes of M w =8.6 on 28 March 2 (Banerjee et al., 27) and M w =8.4 on 12 Septemer 27 (Lorito et al., 28) off Southern Sumatra, oth of which led to tsunami warnings or alerts in several countries in the Indian Ocean, clearly underscore the need for reliale and timely tsunami early warnings, and preparedness plans for quick evacuation of vulnerale coastal communities to safer areas. The latter two events also highlighted the need for detailed assessments of tsunami threat to each country from seismic activity in all active suduction zones around the Indian Ocean Basin. Lack of such a threat assessment led to tsunami warnings for evacuation issued to the entire coast of Sri Lanka in 2 Tel.: ; fax: address: janakaw@pdn.ac.lk. and in 27. Had such information een availale to the authorities, evacuation orders could e limited to areas at potential risk, thus ensuring optimum utilization of resources and minimizing economic losses and potential injuries during evacuation. Given the paucity of past tsunami oservations for Sri Lanka, we employ a deterministic approach together with numerical simulations of tsunami to assess the level of threat posed to Sri Lanka y mega-thrust earthquakes in the Indian Ocean. In a recent article, Okal and Synolakis (28) have identified four segments of suduction zones in the Indian Ocean that could generate destructive transoceanic tsunami. These seismic zones are located off Southern Sumatra, in the Arakan trench off Myanmar and in the Makran coast of Pakistan and Iran, esides the Northern Sumatra Andaman fault which triggered the 24 tsunami. For Sri Lanka, the Okal and Synolakis account is the most up-to-date source of tsunamigenic fault zones in the Indian Ocean. However, they only present maximum deep-water tsunami amplitudes for the entire Indian Ocean Basin as their simulations do not include the effects of shoaling and refraction in the nearshore regions, for which high-resolution, site-specific models of athymetry are required. Accordingly, this paper descries the numerical simulations carried out with the availale highest resolution nearshore athymetric data to estimate the maximum tsunami amplitudes along the coastline of Sri /$ see front matter 212 Elsevier B.V. All rights reserved. doi:1.116/j.coastaleng

2 48 J.J. Wijetunge / Coastal Engineering 64 (212) 47 6 Lanka corresponding to eight plausile earthquake scenarios in the aforementioned suduction zones. Unfortunately, lack of high-resolution topographic data for the entire coast of Sri Lanka prevents us from carrying out onshore inundation simulations which could lead to a more accurate determination of vulnerale communities. This is ecause, although high-resolution LIDAR data are availale for the south and west coasts of Sri Lanka, only 1:, scale topographic data at 1 m contour intervals are availale for the north and east coasts. 2. Methodology 2.1. Seismic scenarios Besides the 24 tsunamigenic earthquake of M w = in the Northern Sumatra Andaman suduction zone, Okal and Synolakis (28) identified 1 other seismic scenarios capale of causing destructive teletsunami in the Indian Ocean Basin. The fault parameters of these seismic events are given in Tale 1 and the respective segments of the seismic zones are identified in Fig. 1. These events mostly represent the worst-case scenarios of seismic rupture for each segment of the suduction zones under consideration. A detailed description of the tsunamigenic seismic potential of these suduction zones is given in Okal and Synolakis (28). Scenarios 1, 2, 1a and 2a refer to the Southern Sumatra suduction zone: Scenario 1 is a repeat of the great earthquake of 1833 whilst Scenario 2 represents an event with the same focal geometry as Scenario 1, ut extending 3 km farther to the southeast, for a total fault length L=9 km. Both Scenarios 1a and 2a are adaptations of Scenarios 1 and 2, respectively, in order to take into account the partial strain release in its northern segment during the Septemer 27 Bengkulu earthquake off Southern Sumatra. Two Scenarios, 3 and 4, have een chosen to represent the worstcase seismic potential of the Arakan suduction zone: Scenario 3 is a fault model inspired y a repeat of the 1762 earthquake whilst Scenario 4 may e categorized as an extremely low proaility yet plausile event to occur in a 47 km segment immediately north of the termination of the 24 rupture. The Scenarios and 6 are in connection with the Makran suduction zone. The first one, Scenario, considers the simultaneous rupture of the three eastern segments (Byrne et al., 1992) of the fault zone. The second and more speculative one, Scenario 6, adds another 4 km of rupture to include the proale 1483 fault zone on the western Makran as well. This second geometry is clearly an extreme worst-case scenario Fig. 1. Active suduction zones in the Indian Ocean Basin. In each seismic zone, the thick lines represent the fault segments rupturing under each scenario, numered as in Tale 1; in those zones with overlapping scenarios, the shorter fault has een offset oceanwards to enhance clarity. See Tale 1 for fault parameters and text for details. (Modified after Okal and Synolakis, 28). that cannot e totally eliminated given the availale evidence (Okal and Synolakis, 28). The Scenarios 7 and 8 refer to the Java suduction zone extending from the Sunda Straits at 16 E to the island of Suma at 12 E. Historical accounts since 1 AD do not indicate an earthquake in this zone capale of causing a transoceanic tsunami. However, given the possiility of long seismic cycles, the occurrence of an ocean-wide tsunami generated y a mega-thrust earthquake off the coast of Java cannot e ruled out (Okal and Synolakis, 28). Accordingly, Scenario 7 involves the rupture of a 2 km long segment of the fault whilst Scenario 8 envisions the rupture of the entire length of the km Java suduction zone. Finally, Scenario 9 represents the M w = earthquake of 24 as the worst-case event for the Northern Sumatra Andaman segment of the Sunda suduction zone. Note that fault parameters for all scenarios except Scenario 9 are those employed in Okal and Synolakis (28); the fault parameters for Scenario 9 are adopted from the five-segment source model of Grilli et al. (27) Numerical simulations Numerical modelling of tsunami hydrodynamics were carried out y employing COMCOT (COrnell Multi-grid COupled Tsunami model) Tale 1 Fault parameters of seismic scenarios. In Tale 1, h is the depth of the fault plane; L is the length of the fault; M is the seismic moment; W is the width of the fault plane; Φ is the strike angle; δ is the dip angle; λ is the rake angle; Δu is the slip. Seismic Zone Scenario M (dyn cm) Φ (deg) δ (deg) λ (deg) L (km) W (km) h (km) Δu (m) Source Centre Southern Sumatra a a Arakan Makran f Java Northern Sumatra Andaman f E (deg.) N (deg.)

3 J.J. Wijetunge / Coastal Engineering 64 (212) which uses a modified leap-frog finite difference scheme to solve shallow water equations in a staggered finite-difference nested grid system. The model has een validated y experimental data (Liu et al., 199) and has een successfully used to investigate several historical tsunami events, such as the 196 Chilean tsunami, the 1992 Flores Islands (Indonesia) tsunami (Liu et al., 1994, 199), and more recently, the 24 Indian Ocean tsunami (Wang and Liu, 26; Wang et al., 27; Wijetunge et al., 28). The COMCOT model was employed to carry out the numerical simulations of tsunami propagation for all scenarios given in Tale 1, except for Scenarios 7 and 8 for the Java seismic zone and Scenario 6 for the Makran zone. The Scenarios 7 and 8 were not included in the present study ecause the numerical simulations of Okal and Synolakis (28) indicate that the geographical location and orientation of the Java fault plane results in ulk of the tsunami energy directed away from Sri Lanka. On the other hand, in the Makran Zone, maximum offshore tsunami amplitudes in the far-field computed y Okal and Synolakis (28) for Scenario 6 are nearly the same as those for Scenario, so only Scenario is chosen for the present study. A dynamically coupled system of two nested grids was employed to simulate the tsunami propagation from each of the seismic zones towards the shoreline of Sri Lanka. Whilst Grid-2 was the same for all simulations, four different versions of Grid-1 were used to accommodate the different locations of the suduction segments. The athymetry data for the largest grid employed in the simulations, i.e., Grid-1 shown in Fig. 2, was otained y interpolating GEBCO (21) data with a resolution of 1 arc-min to a grid of 1.36 arc-min (~2 m) spacing. Grid-2, which is emedded in Grid-1 for the simulation of tsunami propagation over the shallow continental shelf off Sri Lanka at a finer resolution of.2712 arc-min (~ m), is also shown in Fig. 2. The athymetry for Grid-2 shown in Fig. 3 was at first interpolated from GEBCO (21) grid and was then updated with data from navigation charts. These navigation charts typically covered depths down to aout 3 4 m at scales of 1:1, or 1:3,. The nearshore athymetry at some localities was further updated with data from higher resolution navigation charts at scales of 1:1, and 1:1,. As discussed y Wijetunge et al. (28), the amplitude of the 24 Indian Ocean tsunami during its propagation in the ocean asin and continental shelf was of the order of magnitude of 1 2 m, whilst the typical water depth is around 3 4 km in ocean asin and is of the order of magnitude of 1 m on the shelf. Therefore, the nonlinearity is relatively small and can e ignored. In addition, the wavelength of the leading wave was of the order of magnitude of 1 km in ocean asin and 1 km nearshore, aout two orders of magnitude larger than water depth, indicating that the dispersive effect is not of importance. Thus, linear shallow water equations are adequate to solve tsunami propagation in Grids-1 and 2. Further details of COMCOT model including governing equations and numerical formulation can e found in Wijetunge et al. (28). 3. Results and discussion At the outset, the sensitivity of numerical results to the perturations in source parameters and to the grid size was examined, and the results are given in Sections 3.1 and 3.2, respectively. The spatial variations of the maximum values of tsunami amplitude as well as tsunami arrival time contours were computed for all eight seismic scenarios simulated in the present study. However, owing to space constraints, we present the numerical results in detail for only four tsunami events corresponding to each of the four suduction zones, in Sections Accordingly, although more than one tsunamigenic earthquake scenario was simulated for Southern Sumatra and Arakan suduction zones, the results pertaining to only Scenarios 2 and 3, which are the worst-cases for the respective segments, are selected for detailed discussion, esides the Scenarios and 9, respectively, for Makran and Northern Sumatra Andaman suduction zones. Nonetheless, a summary of computed maximum tsunami amplitudes as well as arrival times for Sri Lanka is provided in Section 3. for all scenarios considered. The numerical results of tsunami time series, amplitudes and arrival times corresponding to Scenario 9, i.e., an event similar to the tsunami in 24, are also compared with field oservations to verify the reliaility of our modelling procedure Sensitivity to perturations in source parameters Okal and Synolakis (28) concluded that the characteristics of the tsunami in the far-field appear to e roust with respect to perturations in the properties of the parent earthquake, as long as the seismic moment M (or in the case of a change of dip angle δ, the product (M sin δ) remains constant. However, Sri Lanka qualifies only marginally as far-field with respect to tsunami originating in some of the seismic zones considered in the present analysis. This is ecause the far-field is defined as those regions more than 1 km from a tsunami source (Furumoto, 1993) whilst, for instance, the Northern Sumatra Andaman segment is located only aout 11 km to the east of Sri Lanka. Accordingly, in order to examine the sensitivity of the source parameters to the wave field computed around Sri Lanka, a series of numerical simulations were carried out y varying, one at a time, several parameters of a simplified source model employed y Okal and Synolakis (28) for the Northern Sumatra Andaman earthquake in 24. This model of total moment M = dyn cm corresponds to a seismic slip of Δu=1 m along a fault plane stretching north south over L=11 km with a fault width W=1 km, strike= 39, dip=8 and rake=11. The computed maximum tsunami amplitudes across the entire computational domain are compared in Fig. 4 with the original reference frame shown in frame (e). In Fig. 4a d, we vary the location of the source y moving its centroid 1 (~11 km) in all four directions. We see that shifting the source north and south has little effect on the tsunami wave field approaching Sri Lanka although the tsunami amplitudes directed towards Maldives are slightly lower and higher in frames (a) and(), respectively, compared to the reference frame (e). However, when the source is moved eastward (frame c) and westward (frame d) y 1, respectively, we see slightly lesser and higher tsunami amplitudes towards Sri Lanka. The tsunami amplitudes appear to e slightly higher when the depth of the source is increased from 1 km (frame e) to 2 km (frame f). Frame (g) indicates that an increase of the value of the dip to δ=12, whilst keeping the product M sin δ constant, does not result in any significant change in the wave field directed towards Sri Lanka. However, we see in frame (h) that an increase in slip to 2 m (whilst reducing the fault length to 82 km) results in slightly higher tsunami amplitudes towards Sri Lanka. Finally, when we suppress the strike slip component of the source mechanism (i.e., λ=9 ) in frame (j), we see no significant change in the tsunami amplitudes approaching Sri Lanka. So, it appears that, on the whole, the computed tsunami amplitudes offshore of Sri Lanka are only marginally sensitive to the perturations in the source parameters corresponding to an earthquake in the Northern Sumatra Andaman seismic zone. Seismic zone 4 (Fig. 1) lying at a minimum distance of aout 13 km may also e categorized as marginally far-field. However, the remaining tsunamigenic seismic zones, 1, 2, 3 and lying at a minimum distance of 2 km, 18 km, 16 km and 24 km, respectively, from Sri Lanka could e considered far-field Sensitivity to grid size In order to examine the sensitivity of the model results to the grid size, model simulations for one scenario, i.e., Scenario 3, were carried out with four different grid sizes, i.e., grid spacing, Δx=2 m, m, 12 m, and 2 m. The maximum values of the amplitude of the simulated water levels were first otained from Δx=2 m grid for the

4 J.J. Wijetunge / Coastal Engineering 64 (212) 47 6 o N a o N Grid-2 Grid-2 o E o N Elevation (m) c o N o E Elevation (m) d Grid-2 Grid-2 o E o E Elevation (m) Elevation (m) Fig. 2. Grid-1 of the computational domains employed for simulation of: (a) Scenario 9;() Scenarios 1,1a,2,and 2a;(c) Scenarios 3 and 4; and (d)scenario.location of Grid-2 is also shown. o N water points nearest the shore together with their geographical coordinates (longitude and latitude). The peak amplitudes corresponding to the grid points nearest the aove locations were then extracted from Δx=12 m, m, and 2 m grids. A comparison of the maximum amplitudes from Δx=2 m with those from Δx=2 m gave a correlation (r 2 value) of only.24. Similar comparisons of Δx=12 m and m grids with Δx=2 mgaver 2 values of.4 and.89. Therefore, it appears that Δx=2 m and 12 m grids cannot sufficiently resolve the variaility of nearshore athymetry whilst the results from Δx= m are significantly more reliale giving etter agreement with those from Δx=2 m. Accordingly, given the consideraly more computational time required with a grid of Δx=2 m, it was decided to adopt Δx= m for the inner grids Source directivity o E Elevation (m) Fig. 3. Grid-2 of the computational domain. Fig. gives the distriution of the maximum offshore tsunami amplitudes across the entire computational domain of respective outer grids for each seismic scenario. These figures also show the directivity of tsunami energy thus providing an indication of areas of potential high risk in the far-field. We see in Fig. a, for the Northern Sumatra Andaman segment, that the southern part of the rupture with larger slips show strong source directivity towards west to southwest, slightly away from Sri Lanka yet with large enough amplitudes to significantly affect the southeastern and southern coasts directly. On the other hand, much

5 J.J. Wijetunge / Coastal Engineering 64 (212) North 2 1 deg. South 1 deg. East 4 1 deg. Max. Amp (m) a c West 1 deg. Reference 1 Depth=2 1 km d e f Dip=12 11 deg 12 Slip=2 m Rake=9 13 deg Latitude (in deg.) g h j Longitude (in deg.) Fig. 4. Sensitivity of numerical simulations of the 24 Northern Sumatra Andaman tsunami to perturations in source parameters. The computed maximum tsunami amplitudes for the unpertured source is given in frame (e) whilst the same in other frames correspond to: (a) source displaced y 1 to the north; () source displaced y 1 to the south; (c)source displaced y 1 to the east; (d) source displaced y 1 to the west; (f) depth of the source increased from 1 km to 2 km; (g) dip angle increased from 8 to 12 ; (h)slipincreasedfrom 1 m to 2 m; (j) rake angle changed from 11 to 9. energy from the northern part of the rupture radiates towards the east coasts of Sri Lanka and India. The amplification of the tsunami waves caused y the nearshore athymetry off southern and eastern shores of Sri Lanka also can e seen on this figure, which we will e examining further in Section 3.. The maximum tsunami amplitudes shown in Fig. for Scenario 2 in Southern Sumatra suggest strong source directivity towards southwest, away from Sri Lanka. Consequently, offshore tsunami amplitudes approaching Sri Lanka appear to e mostly lower than. m. We also see a sliver of comparatively higher energy directed towards the southeastern coast from the northern part of the rupture. Fig. c shows that, the location and orientation of the Arakan fault line for Scenario 3 is such that, in the far field, the maximum energy is directed towards the southern part of the east coast of India etween 1 N 1 N, and to a lesser extent, towards the north coast of Sri Lanka, particularly, the Jaffna peninsula (~8 E, 1 N). It also appears that, although located lateral to the maximum energy zone, certain stretches of the east coast of India north of 1 N also could e hit y larger wave heights owing to the effects of shoaling and energy focusing caused y the athymetry. Finally, we see in Fig. d for the Makran suduction zone that, in the far-field, maximum energy is radiated in a southeasterly direction towards Maldives. Fig. d also indicates that the west and southwest coasts of India effectively shield Sri Lanka from direct tsunami attack, and therefore, it appears that only little energy from a Makran tsunami would reach the western and southern coasts of the island Tsunami arrival times Prior information relating to the time it takes for the first wave of a tsunami to arrive a given coastline is essential for emergency planning and in early warning. Accordingly, the computed arrival time contours corresponding to potential tsunamigenic earthquakes identified in Scenarios 9, 2, 3 and (Tale 1) are shown in Fig. 6. Note that the arrival times given are in minutes after the occurrence of the respective earthquakes and are ased on the first 1 cm rise of the mean water level. As the tsunami propagation speed depends only on the water depth, these calculated arrival times ought to e applicale to a tsunamigenic earthquake of any magnitude in the corresponding fault geometries. Let us first consider the contours of computed tsunami arrival times corresponding to Scenario 9 in Fig. 6a. We see that the tsunami waves first reach the southern part of the east coast of Sri Lanka in aout 9 min after the earthquake. We also compare in Fig. 7 the computed arrival times for the aove event with those reported at 33 locations along the coastline of Sri Lanka y Inoue et al. (27) ased on eyewitness accounts. Note that oserved arrival times of the first wave are not availale at Location Nos. 4 and 12. We see that, despite the scatter, the computed arrival times on the whole show reasonaly good agreement with those reported. Moreover, the only tide gauge record availale in Sri Lanka for the aove event (Pattiaratchi and Wijeratne, 29), shown in Fig. 8 with the tide filtered out, indicates that the tsunami arrived in the vicinity of Colomo Port (79.8 E, 6.96 N) as a leading elevation wave 2 h 2 min after the earthquake; note that, a portion of the record is missing due to the floats getting stuck at the ottom of the stilling well during the tsunami. The model simulated time series of sea surface elevation extracted from Grid-2 at a location nearest to the tide gauge is also shown in Fig. 8. We see that, there is a discrepancy in the arrival time of aout 1 min with the oserved tidal record lagging the computed time series. The propagation simulations of Grilli et al. (27) and Arcas and Titov (26), respectively, also indicated tsunami arrival times aout 7 and 2 min earlier than that recorded with respect to the same tide gauge.

6 2 J.J. Wijetunge / Coastal Engineering 64 (212) 47 6 Fig.. Spatial distriution of maximum values of the amplitude of tsunami generated y: (a) Scenario 9 in Northern Sumatra Andaman Suduction Zone, () Scenario 2 in Southern Sumatra Suduction Zone, (c) Scenario 3 in Arakan Suduction Zone, and (d) Scenario in Makran Suduction Zone. We also see in Fig. 8 that the numerical simulations underestimate the amplitude of the first wave y aout 2%; however, this is not entirely surprising ecause, in the asence of onshore inundation simulations, the computed time series corresponds to an offshore location at a water depth of aout 8 m whereas the tide gauge is located in aout 1 m deep water. It must e added that there are several factors that should also e taken into consideration when comparing the arrival times from the numerical simulations with those from tide gauge records or eyewitness accounts (Wijetunge et al., 28). One is that the numerical simulations assume an instantaneous rupture, although in reality, the rupture process lasted aout 8 1 min. Therefore, it is likely that, considering the inclination of the longitudinal axis of the rupture zone, the primary forcing for the tsunami that arrived in Sri Lanka was from Nicoar Andaman area and the rupture in that region may have occurred aout 1 min later. Therefore, aout 1 min of time lag in oth tide records and eyewitness accounts with respect to model simulations may e attriuted to the aove approximation of instantaneous rupture. Secondly, in the case of eyewitness oservations, most people may have noted the crest of the wave whilst the computed arrival times correspond to the first 1 cm rise of water level, which would have occurred aout 1 min earlier depending on the period of the wave. Thirdly, as mentioned y Inoue et al. (27), some of the eyewitnesses may not have een wearing watches during the tsunami, so they depended only on their sense of time; furthermore, some eyewitnesses may not have seen the first wave ecause they were not near the ocean at that time, so the first wave of an eyewitness does not necessarily correspond to the actual first wave. The arrival time contours shown in Fig. 6 for the Southern Sumatra segment indicate that the tsunami waves first reach the southeastern coast of Sri Lanka in aout 16 min after the earthquake. Similarly, Fig. 6c suggests that tsunami waves will hit parts of the north and east coasts of Sri Lanka in aout 16 min in the event of a teletsunami generated y an earthquake in the Arakan segment defined y Scenario 3. On the other hand, a teletsunami originating in the Makran suduction zone (Scenario ) will take as long as ~28 min to reach parts of the west and southwestern coasts of the island. 3.. Maximum nearshore tsunami amplitudes The computed variation of the maximum values of the amplitude of the tsunami extracted from the computational domain of Grid-2 at offshore grid points adjacent to the shoreline of Sri Lanka (water depths 7 1 m) are shown in Figs for Scenarios 9, 2, 3 and in Northern Sumatra Andaman, Southern Sumatra, Arakan and Makran suduction zones, respectively. In each figure, the four plots give the computed maximum amplitudes along the coastlines of: (a) Western Province and part of North-Western Province, () Southern Province, (c) Eastern Province, and (d) Northern Province, of Sri Lanka. Moreover, some asic statistics relating to the computed tsunami amplitudes for each province and the earliest tsunami arrival time for any part of the island for all eight scenarios simulated are summarized in Tale 2. Let us first consider the maximum tsunami amplitudes shown in Fig. 9 for Scenario 9, i.e., the tsunami in 24. Fig. 9 also shows the maximum water levels computed y Ioualalen et al. (21) using a fully non-linear, dispersive Boussinesq-type model utilizing the same

7 J.J. Wijetunge / Coastal Engineering 64 (212) Fig. 6. Contours of arrival time in minutes after earthquake for tsunami generated y: (a) Scenario 9 in Northern Sumatra Andaman Suduction Zone, () Scenario 2 in Southern Sumatra Suduction Zone, (c) Scenario 3 in Arakan Suduction Zone, and (d) Scenario in Makran Suduction Zone; SL Sri Lanka. Tsunami arrival time (min. after earthquake) Sri Lanka a Computed Inoue et al. (27) Location No. Fig. 7. (a) Comparison of computed tsunami arrival times with eyewitness accounts of the arrival time of the first wave reported in Inoue et al. (27) for Scenario 9, () Locations of oservations along the coastline of Sri Lanka, reproduced from Inoue et al., 27. source parameters as in Scenario 9 and the field measurements of maximum water levels reported in Liu et al. (2), Synolakis et al. (2), Choi et al. (2), Sato et al. (2), Goff et al. (26), Shiayama et al. (26), and Tomita et al. (26) in the immediate aftermath of the 24 tsunami. It must e added that the field measurements of water levels shown in Fig. 9 have een made at locations typically within 1 m onshore, although, in the asence of any inundation simulations overland, the computed values from the present study correspond to grid points immediately seaward of the shoreline; however, the numerical study of Ioualalen et al. (21) on a grid of spatial resolution.278 arcmin (~2 m) included onshore inundation simulations as well. We see in Fig. 9 that the field measurements on the whole show satisfactory agreement with the simulated variation at most locations thus further verifying the reliaility of the numerical modelling Sea-surface elevation (m) Computed Tide Gauge Time after earthquake (hrs) Fig. 8. Comparison of the tide gauge record at Colomo (Pattiaratchi and Wijeratne, 29) with the tide filtered out and the model simulated sea-surface elevation extracted at the nearest availale offshore point in Grid-2. Note that, a portion of the tide gauge record is missing due to the floats getting stuck at the ottom of the stilling well during the tsunami. 6

8 4 J.J. Wijetunge / Coastal Engineering 64 (212) 47 6 Max. Tsunami Amplitude (m) 1 d o E Jaffna Max. Tsunami Amplitude (m) d o E Jaffna a o N Chilaw 1 Colomo Galle Trincomalee Sri Lanka Batticaloa Yala o E procedure employed in the present study. Moreover, the computed water levels from the present study also agree reasonaly well with those of Ioualalen et al. (21), where data points from the latter study are availale. However, at several locations particularly on the south and west coasts, the computed values from Ioualalen et al. (21) appear to fall slightly aove those from the present study despite the fact that oth studies have utilized nearshore athymetries generated from navigation charts of similar resolution, approximately the same grid spacing as well as the same source model, i.e., Grilli et al. (27). However, one primary difference etween the two sets of simulations is that the present simulation is ased on linear shallow water equations whilst that of Ioualalen et al. (21) on non-linear and dispersive Boussinesq-type equations. Moreover, the present results have een extracted at grid points immediately seaward of the shoreline at water depths of aout 7 1 m whereas those of Ioualalen et al. (21) presumaly at locations further landward or onshore since they modeled overland inundation as well. It is, however, not entirely clear whether this slight discrepancy is due to non-inclusion of frequency dispersion and non-linear effects in the present numerical formulation or due to extraction of wave heights at water points further offshore. The envelopes of the spatial variation of the computed tsunami amplitude, for instance, that shown in Fig. 9c for the Eastern Province, exhiit several prominent troughs and peaks. As discussed in Wijetunge (29), the troughs in the computed tsunami amplitudes at around 7.3 N, 7. N, 7.8 N and 8.6 N are all as a result of energy defocusing o N c 1 Fig. 9. Computed maximum tsunami amplitudes extracted from Grid-2 of the present study at water points adjacent to the shoreline of Sri Lanka corresponding to Scenario 9 (grey); field measurements of maximum water levels (lack lines) from Liu et al. (2), Synolakis et al. (2), Choi et al. (2), Sato et al. (2), Goff et al. (26), Shiayama et al. (26), and Tomita et al. (26); and maximum water levels computed y Ioualalen et al. (21) (open circles) using a fully non-linear, dispersive Boussinesq-type model utilizing the same source parameters as in Scenario 9: (a) Western Province and part of North-Western Province, () Southern Province,(c) Eastern Province, and (d) NorthernProvince. a o N Chilaw Colomo Galle Trincomalee Batticaloa Ya la o E along the valleys of sumarine canyons at these locations. On the other hand, the ridges on either side of these canyons help focus tsunami energy thus resulting in increased tsunami amplitudes seen at around 7.4 N, N and 7.9 N. We see several notale peaks in the variation of tsunami amplitude shown in Fig. 9d for the Northern Province also, for example, those at ~8.37 E and at ~8.8 E are due to the focusing of wave energy y the athymetry. Similarly, several peaks in the computed tsunami amplitude distriution along the southern province (Fig. 9), for instance, those at ~81.2 E and at ~81. E, could also e attriuted to strong focusing y the shelf athymetry (Wijetunge, 29a). The aforementioned oservations of focusing and defocusing of tsunami energy due to large-scale features of the shelf athymetry off the eastern, southern and northern coastlines also agree well with the spatial distriution of computed peak wave heights reported in Ioualalen et al. (21) for the entire seaoard of Sri Lanka. However, good qualitative agreement etween the two studies here is not entirely surprising ecause dispersion and non-linear effects are perhaps not significant as the wave length is several orders of magnitude larger than oth the water depth and the wave heights associated with tsunami propagation over the large-scale athymetric features that are responsile for the aove hydrodynamic processes. The largest peaks in tsunami amplitude, 2.8 m and 2.3 m, due to Scenario 2 for Southern Sumatra segment appear in the eastern part of the southern province at ~81.2 N and 81. N, respectively (Fig. 1). We see that, the maximum tsunami amplitudes are smaller than aout 2 m and 1 m, respectively, along the east (Fig. 1c) and the west (Fig. 1a) coasts for this scenario. Apart from the two peaks in tsunami amplitude etween ~8.1 E and ~8.4 E, the amplitudes are lower than ~1 m in the remaining stretches of the Northern Province o N c Fig. 1. Computed maximum tsunami amplitudes at water points adjacent to the shoreline of Sri Lanka corresponding to Scenario 2: (a) Western Province and part of North- Western Province, () SouthernProvince,(c) Eastern Province, and (d) NorthernProvince.

9 J.J. Wijetunge / Coastal Engineering 64 (212) 47 6 a Max. Tsunami Amplitude (m) o N d o N a Max. Tsunami Amplitude (m) o E Jaffna Chilaw Colomo Galle Trincomalee o E Jaffna Chilaw Colomo Galle Batticaloa Yala Trincomalee Batticaloa Yala o E Fig. 12. Computed maximum tsunami amplitudes at water points adjacent to the shoreline of Sri Lanka corresponding to Scenario : (a) Western Province and part of North-Western Province, and () Southern Province o N Fig. 11. Computed maximum tsunami amplitudes at water points adjacent to the shoreline of Sri Lanka corresponding to Scenario 3: (a) Western Province and part of North-Western Province, () Southern Province, (c) Eastern Province, and (d) Northern Province. c The computed tsunami amplitudes for Sri Lanka corresponding to Scenario 3 in Arakan suduction zone suggest that the Northern Province (Fig. 11d) will e exposed to a comparatively greater level of threat with amplitudes ranging from ~ m with a mean value of 1.7 m. The Eastern Province (Fig. 11c) is likely to experience wave amplitudes ranging from ~.4 2. m with a mean value of 1.1 m. However, the computed maximum tsunami amplitudes are lower than ~1 m off the coast of the southern province (Fig. 11), and negligile for the western province (Fig. 11a). Finally, the computed tsunami amplitudes corresponding to Scenario for the Makran Suduction Zone (Fig. 12) indicate that the maximum amplitudes are likely to e lower than ~1 m off the coasts of western (Fig. 12a) and southern (Fig. 12) provinces whilst negligile for the eastern and northern provinces, and therefore, not shown. 4. Concluding remarks The numerical results suggest that the peak tsunami amplitudes around the coastline of Sri Lanka corresponding to maximumcredile seismic scenarios in Southern Sumatra (Scenario 2), Arakan (Scenario 3) and Makran (Scenario ) zones are less than aout 2 3% of those due to the same in Northern Sumatra Andaman segment (Scenario 9) which triggered the M w = earthquake in 24. This means that an event similar to the 24 tsunami that originated in the Northern Sumatra Andaman suduction zone could e considered as the worst-case seismogenic tsunami scenario for any part of the coastline of Sri Lanka. The estimated tsunami amplitudes given in Tale 2 indicate that it may not e necessary to evacuate the entire coastline of Sri Lanka if a large earthquake capale of generating a transoceanic tsunami were to occur in some of the seismic zones concerned. In such cases, a warning requiring immediate evacuation could e issued to areas that appear to e more vulnerale ased on a threshold wave height as determined y the local authorities whilst a tsunami watch would e sufficient for the rest of the coastline. We emphasize that the numerical results such as those presented in Figs only show which parts of the coastline of Sri Lanka are likely to e exposed to greater risk in terms of the amplitude of tsunami waves approaching the shoreline. However, factors such as the onshore topography, the population density and the construction material and standards could further enhance or reduce the vulneraility of a given coastal area. For example, the topography of onshore lands significantly influences the inundation pattern as well as the flow depth and the eventual run-up height whilst ordinary rick and mortar housing are more susceptile to tsunami loading than reinforced-concrete uildings. Moreover, the next tsunamigenic earthquake in any of the suduction zones concerned need not necessarily e as large as those adopted in the present simulations. If the seismic moment of the next earthquake in a given suduction zone is smaller compared to that used in the present simulations, the tsunami amplitudes reaching the shoreline of Sri Lanka are likely to e smaller than those computed; nevertheless the spatial variation of the tsunami amplitudes is expected to e qualitatively similar. The information presented in this paper relating to the spatial distriution of likely maximum tsunami amplitudes and arrival times for Sri Lanka would help authorities responsile for evacuation to make a etter judgement as to the proale maximum heights of oncoming tsunami waves in different areas along the coastline, and act accordingly, if a large earthquake were to occur in any of the suduction zones in the Indian Ocean Basin. Acknowledgement The author wishes to thank Prof. Emile A. Okal for kindly providing him with source parameters of most of the seismic scenarios considered in the present study. He gratefully acknowledges Prof. Costas E. Synolakis

10 6 J.J. Wijetunge / Coastal Engineering 64 (212) 47 6 Tale 2 Summary of numerical results for all scenarios simulated. Seismic Zone Scenario M (dyn cm) Range and mean value (in rackets) of computed maximum tsunami amplitudes adjacent to shoreline (m) Northern Province Eastern Province Southern Province Western Province Southern Sumatra (.9) (1.) (.9) (.) 1a (.) (.) (.4) (.3) (1.1) (1.1) (1.3) (.6) 2a (.6) (.8) (.7) (.4) Arakan (1.7) (1.1) (.4) (.13) (.9) (.6) (.3) (.13) Makran (.3) (.4) Northern Sumatra Andaman (6.7) (.) (6.3) (2.4) Both Western Province and part of North-Western Province up to ~8.4 N. Arrival Time (min) and Dr A. Bareropoulou of the University of Southern California Tsunami Research Centre for useful discussions that he had with them whilst working on an earlier draft of this paper during his stay there on saatical leave. 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