CHARACTERISTICS OF LANDSLIDE GENERATED TSUNAMIS FROM OBSERVATIONAL DATA G.A. PAPADOPOULOS and S. KORTEKAAS Institute of Geodynamics, National Observatory of Athens, 11810 Athens, Greece Abstract Characteristic features of landslide tsunamis are examined from a set of documented cases. The studied events affected a coastal zone of no more than 10-15 km at both sides of the slump area. The only two tsunamis that were observed over a considerably greater distance are the Izmit 1999 and particularly PNG 1998 tsunamis, both of them being possibly associated with co-seismic fault displacement component. Slump volume seems to control both maximum wave height and maximum length of affected coastline. A rapid quasiexponential attenuation of wave heights with distance from the source is observed because of strong wave dispersion. Keywords: Landslide, tsunami, wave height, attenuation 1. Introduction Tsunamis generated by subaerial or submarine slides or slumps associated with either seismic or fully aseismic mechanisms, have attracted interest since at least the 60’s (e.g. Ambraseys 1960, Miller 1960, Galanopoulos et al. 1964, Müller 1964, Jørstad 1968, Murty 1979, Papadopoulos 1993, Rabinovich et al. 1999, Assier-Rzadkiewicz 2000, Perissoratis and Papadopoulos 2000). However, since the occurrence of the Papua New Guinea (PNG) 17th July 1998 catastrophic tsunami (Kawata et al. 1999) interest in landslide tsunamis has drastically increased. The reason for this is that although the PNG event generation mechanism is still controversial (e.g. Matsuyama 1999, Geist 2000, 2001, Okal and Synolakis 2001), there is evidence based on modern observational and modelling techniques that supports a slump as source over fault displacement (e.g. Tappin et al. 1999, 2001, Heinrich et al. 2000, Okal and Synolakis 2001). Submarine and subaerial landslides generate tsunamis with significant wave heights on coasts close to the source. They have wavelengths of a few kilometres and rapidly attenuating wave heights due to frequency dispersion. On the contrary, fault-generated tsunamis are more widely distributed and their wave heights are magnitude controlled. They possess wavelengths in the order of hundreds of kilometres while their wave height decay is slow because of weak dispersion effects. Due to the concentrated large wave heights, landslide tsunamis result in catastrophic consequences (e.g. Aegion 1963, Vaiont 1963, Nice 1979, Skagway 1994, PNG 1998, Izmit 1999). Therefore, the problem of landslide generated water waves either on open coastlines or in enclosed water bodies is of great practical interest to design engineers (Noda 1970). Apart from studying particular cases (see selected references above and in sections 3.1 and 3.2 below), significant progress has been made in modelling landslide tsunamis (e.g. Wiegel 367 368 Papadopoulos and Kortekaas 1955, Noda 1970, Striem and Miloh 1976, Koutitas 1977, Murty 1979, Iwasaki 1987, Heinrich 1992, Harbitz et al. 1993, Watts 1998, Grilli and Watts 1999, Rabinovich et al. 1999, Tinti and Bortolucci 2000a,b, Ward 2001, Grilli et al. 2002; see also references about PNG above and in sections 3.1 and 3.2 below). However, the comparative study of observational data regarding landslide tsunami wave height has been rather neglected so far. This study is based on several well-documented cases. The results are useful in (1) the recognition of some empirical patterns regarding the wave height of landslide tsunamis, (2) their theoretical explanation, and (3) their implementation in hazard mitigation plans. Table 1: The upper part of the table lists the ten case studies and the lower part records events for which no wave height distribution data are available. nop = number of observation points, d = maximum distance from the source where the tsunami was observed, h = maximum recorded wave height and V = slump volume. (data from: Galanopoulos et al. 1964, Koutitas and Papadopoulos 1999 (Aegion); Assier-Rzadkiewicz et al. 2000 (Nice); Okal et al. 2002, Herbert et al. 2002 (Fatu Hiva); Harbitz et al. 1993, Jørstad 1968 (Tafjord), Tappin et al. 2001, internet (PNG), Altinok et al. 1999, Yalciner pers. com. 2001 (Izmit); Miller 1960 (Lituya); Murty 1979 (Kitimat); Rabinovich et al. 1999 (Skagway); Müller 1964 (Vaiont) and Jørstad 1968 for all Norwegian cases). Event Case studies (nop) d (km) h (m) V (m3) Aegion 1963 (16) Nice 1979 (7) Fatu Hiva 1999 (16) Tafjord 1934 (65) Loenvann 1905 (9) Loenvann 1936 (102) Eikesdalsvann 1966 (6) PNG 1998 (100) 19.61 25 10.2 8.3 8 8 11.3 184.7 6 3 10 62.3 40.5 74.2 3 15.03 1.00E +07 1.00E +07 2.40E +06 2-3.00E +06 3.50E +05 1.00E +06 4-5.00E +06 5-10.00E +09 Izmit 1999 (29) 47.37 2.9 3.78E +05 ? Lituya Bay 1958 (25) 12.95 524.26 3.06E +07 Cause Tsunamis caused by slides in loose deposits Kitimat 1975 Skagway 1994 Trondheim 1888 Gjerstadvann 1929 Orkdalsfjord 1930 Songevann 1935 Selbusjøen 1950 Nordset 1956 Sokkelvik 1959 Hammerås 1963 Follerø 1964 >1 >1 20 4.6 9-11 4-5 3 5 8 >3 4 1 1-2 2.60E +07 0.81E +06 > 2-3.00E +06 1.00E +05 2.50E +07 > 5.00E +04 2.00E +05 1.00E +04 2-3.00E +06 7.00E +03 3.00E +04 ? Tsunamis caused by rock falls Vaiont 1963 Rammerfjell 1731 Tjelle 1756 Ravnefjell 1936 Ravnefjell 1950 Stegane 1948 > 40 >8 >8 6 100 77 38 > 49 12-15 3-5 ? 2.50E +08 > 1.00E +05 1.50E +07 1.00E +06 ? 1.00E +06 ? 3.00E +04 aseismic coastal slide aseismic submarine slide aseismic subaerial rockslide aseismic subaerial rockslide aseismic subaerial rock fall aseismic subaerial rock fall aseismic subaerial rock fall seismic submarine slide + fault displacement? seismic coastal slide + fault displacement? seismic subaerial rock fall (no fault displacement) Characteristics of landslide generated tsunamis 369 2. Methodology Landslide generated tsunamis may have different characteristics from seismic tsunamis. To investigate patterns of landslide tsunami wave height, data were collected for ten events for which sufficient observation points of wave heights were available (Table 1). These events include seven aseismic tsunamis and three tsunamis occurring after an earthquake but that are thought to have been triggered by landslides induced by the earthquake (Izmit 1999, PNG 1998 and Lituya 1958). For each one of these cases the wave heights were plotted against the distance from the slump area (Fig. 1). Moreover, empirical attenuation laws were investigated (Fig. 2). In addition, we utilised another set of eighteen aseismic tsunami events for which no wave height distributions are available (Table 1). They were used along with the ten case studies to investigate possible correlation between slump volume and maximum wave height and maximum length of affected coastline. For this purpose maximum height and maximum attenuation distance were plotted against slump volume (Fig. 3). Table 2: Empirical laws of tsunami wave height h (in m) decay with distance d (in km) from the source. r is correlation coefficient (see Fig. 2). Event Best fit all data Best fit source side data Comments Aegion 1963 PNG 1998 h= 4.756(exp-0.156d) r 2 = 0.77 See text Tafjord 1934 Lituya 1958 h = 4.456(exp-0.123d) r 2 = 0.58 h = -1.161ln(d) + 8.468 r 2 = 0.49 h = 9.021(exp-0.046d) r 2 = 0.44 h = 28.02(exp-0.324d) r 2 = 0.55 h = 160.21(exp-0.220d) r 2 = 0.70 h = 96.75(exp-0.182d) r 2 = 0.88 Loenvann 1905 h = 136.91(exp-0.204d) r 2 = 0.74 h = 11.71(d-0.651) r 2 = 0.66 Loenvann 1936 h = -6.016log(d) +13.80 r 2 = 0.50 h = -3.198log(d) +9.152 r 2 = 0.21 h= 24.34(exp-0.263d) r 2 = 0.61 h = 147.95(exp-0.225d) r 2 = 0.67 Including 9 far-field data points Excluding 9 far-field data points See text One splash point included (h = 524 m) One splash point excluded Only one observation point at source side See text 3. The data Although all tsunamis of this data set are triggered by landslides, the particular mechanisms differ (Table 1). The tsunamis of Aegion 1963 and Nice 1979 were triggered by partly coastal and partly submarine slides. The events of Loenvann 1905 and 1936, Tafjord 1934, Lituya Bay 1958 and Fatu Hiva 1999 were generated by aseismic rock falls, while Eikesdalsvann 1966 was caused by a snow avalanche. Although the rockslide in Lituya Bay was triggered by an earthquake, laboratory simulation of the event indicates that no seismic component, that is co-seismic fault displacement, was involved in the tsunami generation (Fritz et al. 2001). The PNG 1998 and Izmit 1999 waves are also attributed to seismically induced submarine slumps. 370 Papadopoulos and Kortekaas ,] PLW 31* VRXWKFRDVW QRUWKFRDVW P K P K GNP 1LFH P K P K QRUWKFRDVW $HJLRQ VRXWKFRDVW GNP GNP GNP /LWX\D%D\ )DWX+LYD QRUWKFRDVW VRXWKFRDVW P K P K GNP GNP /RHQYDQQ /RHQYDQQ ZHVWFRDVW ZHVWFRDVW HDVWFRDVW HDVWFRDVW P K P K 7DIMRUG HDVWFRDVW K K P GNP ZHVWFRDVW P (LNHVGDOVYDQQ 1(FRDVW GNP GNP 6:FRDVW GNP Figure 1. Wave height against distance from the slump area. In enclosed, the closed diamonds indicate the coast where the slump occured and the open diamonds the opposite coast (e.g. Izmit 1999). Note that the scale are different in each graph. Characteristics of landslide generated tsunamis $HJLRQ 31* G K H G K H U U P K P K GNP GNP 31* /LWX\D G G H U K K H U P P K 371 K GNP GNP /RHQYDQQ /RHQYDQQ K G K ORJG U U P P K K GNP GNP Figure 2: Attenuation curves for six case studies. The curves for Aegion, Tafjord and Lituya represent the data set for the coast on which the slump occurred. The curves for PNG, Loenvann 1905 and 1936 represent the whole data set, excluding some far field points (see text). However, the contribution of the co-seismic fault displacement component in the tsunami generation can not be excluded. The majority of the events occurred in enclosed seas or fjords, in which case wave reflection and refraction may have been more important than on open coasts. Most of the additional events are slides and rock falls that occurred in Norwegian fjords (Jørstad 1968). Exceptions are: Skagway 1994 (Rabinovich et al. 1999), Kitimat 1975 (Murty 1979) and Vaiont 1963 (Müller 1964). 4. Results and discussion In general, the studied landslide tsunamis affected a coastal zone of no more than 10-15 km at both sides of the slump area (Fig.1). The only two tsunamis that were observed over a considerably greater distance are the Izmit and PNG tsunamis, both of them being possibly associated with co-seismic fault displacement component. However, the Nice event includes one data point, the harbour of Cannes, at 25 km from the slide, where damped oscillations of ≤ 1 m were reported. 372 Papadopoulos and Kortekaas Tsunami wave height decay when propagating away from the source location is one of the critical processes to be controlled in landslide tsunami hazard estimation (Ward 2001). By assuming that wave height decay is a measure of wave attenuation we modelled wave height observations, h (in m), as a function of distance, d (in km), from the source location for six out of ten case studies to formulate empirical laws of tsunami wave height decay (Fig. 2). Nice 1979 and Eikesdalsvann 1966 were excluded because of the small number of observation points available. Izmit 1999 was also excluded because of the very complex source nature, which involved not only coastal and possibly submarine sliding failures but also faulting with strike-slip and normal components of co-seismic displacement. Fatu Hiva 1999 was excluded because the event took place on a nearly circular island with the majority of observation points located at the opposite side with respect to the slide location. To avoid possible strong shoaling and topographic effects, data points located in the very narrow Vassenden bay of Loenvann as well as data points from the Tafjord and Norddal bays of Tafjord were not included. D UXQXSKHLJKWVOLGHVRQO\ 31* P K [ D P $HJLRQ ,]PLW 31* P N G [ D 6NDJZD\ E PD[GLVWDQFH P 1LFH ,]PLW ORJYRO Figure 3: Empirical envelops for (a) the maximum wave height (in m) as a function of the slump volume (in m3), (b) the maximum length of coast affected by the tsunami (in km) as a function of the slump volume for the whole data set and (c) the same as b but without the PNG and Izmit cases. Trendline equations are: a: h = 13.131 logV -14.522 r2 = 0.9704 b: h = 233.15 logV -352.78 r2 = 0.9856 c: h = 76.838 logV -111.43 r2 = 0.9886 ORJYRO F PD[GLVWDQFHZLWKRXW31*,]PLW P N G [ D 1LFH P )DWX+LYD /RHQ /RHQ $HJLRQ /LWX\D 7DIMRUG ORJYRO The decay patterns that fit best to the observational data are summarised in table 2. In most cases exponential decays of the form h = a exp (-b d) are dominant, where a and b are constants. This implies a decay faster than the attenuation according to 1/d predicted for an impulsive axial-symmetric wave decay in water of finite depth or according to d – ½ , dictated by long-wave theory for large slides or even tectonic tsunamis (Ward 2001). However, in the case of PNG a decay law of the form h = a ln (d) + b holds, which signifies an even faster decay in the very near-field but slow decay over larger distances. This may be interpreted as the possible effect of the fault component involved in the source mechanism which is consistent with the tsunami waveform analysis made by Matsuyama (1999) from Japanese tide-gauges at distances in the order of d ~ 3,000 km. A similar behaviour is observed in the Loenvann aseismic cases because of the very narrow fjord shape (fjord width ~ 1 km), which results in out of proportional high wave heights near the slide area. This was also described by Belousov et al. (2000) in relation to a volcanic tsunami in Kamchatka. In general, fast decay of landslide tsunamis is well explainable by strong wave dispersion. Characteristics of landslide generated tsunamis 373 Empirical envelope curves were produced for the maximum length of coast affected and the maximum wave height as a function of the slump volume (Fig. 3). Rock falls (e.g. Lituya Bay 1958, Vaiont 1963) cause tsunamis with significantly higher wave height than landslides or submarine slumps. Therefore, rock falls were excluded in the calculation of the empirical envelope curves for the maximum wave height. Each prediction curve was calculated from three limiting points (Fig. 3). According to these curves, slump volume seems to control both maximum wave height, h, and maximum length of affected coastline, d. However, a major problem often is the uncertainty involved in the slump volume, since in many cases this is based on rough estimation. For example, for the Izmit event only the dimensions of the coastal area that slid into the sea are known, although a slide of similar or larger volume was observed offshore in seismic profiles (Altinok et al. 1999). The uncertainties in slump volume along with bathymetry differences and total displacement of slump mass from case to case may explain the large variations of h and d for a given slump volume (Fig. 3). 5. Conclusion The landslide tsunamis studied here show a rapid quasi-exponential attenuation of wave heights with distance from the source because of strong wave dispersion and therefore, only affect a relatively small part of the coastline of about 10-15 km from the source. However, there are two exceptions: PNG 1998 and Izmit 1999. These are the only two tsunami events in the data set with a possible co-seismic fault component involved in the source. Slump volume seems to control both maximum wave height and maximum length of affected coastline. 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