Strong Motion and Tsunami Related to the AD 365 Crete Earthquake

Paper: Strong Motion and Tsunami Related to the AD 365 Crete Earthquake

Tsuneo Ohsumi∗,†,YujiDohi∗, and Hemanta Hazarika∗∗

∗National Research Institute for Earth Science and Disaster Resilience (NIED) 3-1 Tennodai, Tsukuba, Ibaraki 305-0006, Japan †Corresponding author, E-mail: t [email protected] ∗∗Department of Civil Engineering, Kyushu University, Fukuoka, Japan [Received April 2, 2018; accepted July 20, 2018]

The West Asian region is a tectonically active area due to crustal deformation; the associated earthquakes occur on a large scale and have been recorded from the historical period to the present. Investigating the most suitable solution for this crustal movement will contribute to this region’s earthquake and tsunami disaster mitigation. The most reliable parameters were de- ﬁned by researchers and applied with a non-uniform distribution in the fault plane based on Papadimitriou et al [1]. The calculated AD 365 earthquake waveform provides an indication of maximum acceleration using the stochastic Green’s function method with the selected parameters. Using this estimation, damage to masonry structures can be calculated. The ancient Crete cities of Aptra and Chania were both hit by the Fig. 1. Tectonic overview of the Mediterranean Sea and AD 365 earthquake. Aptera, built on out-cropping west to India. Also noted are the relevant subduction zones rock, would have been 80% destroyed. In compar- in the region (Be: Betic-Rif, Cb: Calabria, HI: Hellenic, Mk: ison, Chania, in northwest Crete, would have been Makran) and the Anatolian Fault in Turkey (taken from Hori completely destroyed because it was built on thick and Kaneda [2]). sedimentary layers. The subsurface composition at Chania would have made it a high seismic intensity area. This earthquake was followed by a tsunami that are similar to the collision zone in the Himalayas– that devastated the southern and eastern coasts of the Tibet mountain belt. However, this region is characterized Mediterranean. Based on these results, risk mitigation by a subduction zone (solid line in Fig. 1) and a predom- from seismic and tsunami events should focus on high inance of strike-slip faults that partly exist in the north densely populated areas with thick sedimentary layers Anatolia dislocation. These subduction zones cause earth- in the Mediterranean. quakes and tsunamis. In West Asia, the African plate is subducting beneath the Anatolian plate at a rate of 1 to 3.5 cm/yr, which re- Keywords: AD 365 Crete earthquake, stochastic Green’s sults in frequent large magnitude earthquakes along this function method, strong motion, tsunami subduction zone. In AD 365 (Fig. 2), a large magnitude (M8.5) earthquake occurred near Crete (e.g., Fis- cher [3], Shaw et al. [4], Stiros [5], Papadimitriou and 1. Introduction Karakostas [1]). The AD 365 earthquake is one of the best known ancient earthquakes in the eastern Mediterranean. West Asia is an area of active crustal deformation with It caused a tsunami that resulted in great damage to Syria, a history large magnitude earthquakes. Because crustal northern Egypt, and the Greek coast. According to Pi- movement is ongoing, investigating the seismicity in this razzoli [6], who investigated coastline upheaval along the region may contribute to understanding and protecting eastern Mediterranean, the period between 350 and 550 against earthquake and tsunami disasters. was the one of the most seismically active periods in the According to Hori and Kaneda [2], the relative plate past 2,000 years. motion of tectonics from the area around the Mediter- Crete, located 160 km south of the Greek mainland, ranean Sea and west to India is 2–4 cm/yr. This move- is the largest among approximately 3,000 Islands in the ment is smaller than other convergent plate boundaries Aegean, with an area of 8,336 km2. Ancient earthquakes

Journal of Disaster Research Vol.13 No.5, 2018 943 Ohsumi, T., Dohi, Y., and Hazarika, H.

Fig. 3. Left: upheaval history of the oldest (lowermost) layer, assuming a ﬁxed sea-level based on data from Thom- meret et al. [9] and Pirazzoli et al. [10]. Right: a scenario ex- Fig. 2. Epicenter of the AD 365 Crete earthquake. plaining the change in relative sea level in west Crete assuming constant sea level rise and intermittent land uplift [11].

in Crete have been reported in various books by Am- brasseys (e.g., 1994) [7]. In the 4th century, Ammiaus, a historian and military service member, wrote a historical document consisting of 31 volumes. Due to the Christian propagation, which began in the age of the Roman Em- pire, historical documentation was common in the 4th–5th centuries. Secondary tsunami damage caused by the AD 365 earthquake was greater than that from the primary earthquake in the Peloponnese peninsula. Because the magnitude was higher than M8, the tremor propagated across a large area surrounding the Mediterranean. More re- cent smaller magnitude earthquakes in the Greek Is- lands have also affected a wide area around the Mediter- ranean. John Cassian, a theologian in the 4th–5th century, and Sozomenes, a Byzantine historian in the 5th cen- Fig. 4. Contours of upheaval in Crete (up to 9 m on tury, described evidence for widespread flooding from the the southwest part of the island) modified from Pirazzoli tsunami by tracing damage on the roofs of buildings and et al. [12]. subsequent retreat of the coastline. A Byzantine historian, George the Monk, mentioned the tsunami in a 9th century chronicle. The tsunami caused by the AD 365 earthquake was also chronicled by Theopanos in the 8th–9th the sea surface in western Crete due to continuous sea- centuries, Cedrenus in the 11th century, and Glycas in level rise and intermittent land uplift is shown in Fig. 3. the 12th century. According to the literature, the tsunami Pirazzoli et al. [6] suggested that there are traces of destroyed 50,000 houses and caused 5,000 casualties in upheaval along the Greek coast from an earthquake that Alexandria, Egypt. occurred between the mid-4th and mid-6th centuries in Because large magnitude earthquakes and associated the Early Byzantine tectonic paroxysm (EBTP) turbu- disasters will likely occur in the future, this study inves- lent period. Pirazzoli et al. revised his previous inter- tigates and describes the characteristics of the AD 365 pretation [10,12] (Fig. 4) after detailed survey and radio- Crete earthquake. carbon dating of the samples obtained from Antikyhira Island. Significant co-seismic uplift that took place during a short period was demonstrated by over 30 radiocar- 2. Crustal Movement bon dates from 12 regions in Greece and precise sea-level indicators in the eastern Mediterranean. Therefore, it is Flemming [8] evaluated land subsidence and upheaval assumed that the scale of uplift in Crete was 0.5 to 1.0 m and estimated the relative rate of annual sea level rise in general but gradually increased toward the south-west (1.05 mm/yr.) based on research on the south-west coast and reached approximately 9 m. Radiocarbon dates show of Turkey and at about 175 points in Cyprus. Changes in that the largest change occurred between 261 and 425.

944 Journal of Disaster Research Vol.13 No.5, 2018 Strong Motion and Tsunami Related to the AD 365 Crete Earthquake

Fig. 5. Evidence for uplift of the western part of Crete [13]. Fig. 6. ‘Sea marks’ observable on the coast of Sougia (photo by T. Ohsumi).

Fig. 7. Sketch by Spratt [13] showing the position of the ancient port facilities relative to sea level at Phalasarna.

3. Evidence of Uplift in Western Crete

In Travels and Researches in Crete [13], the coastal uplift in western Crete due to the AD 365 earthquake is rec- ognized as a dark band of ‘sea marks’ located at the bottom of Grambousa peninsula in the southwest corner of the island (Fig. 5). These ‘sea marks’ are also observable on the coast of Sougia (Fig. 6). A sketch by Spratt [13], Fig. 7, shows the position of the ancient port facility relative to sea level at Phalasarna, located 48 km from Cha- nia on the northwestern coast. The raised ancient military facilities are about 8.5 m above sea level in ca. BC 333 (Fig. 8). Because the port level is located about 3 m below, the location is in agreement with uplift of about Fig. 8. Uplifted ancient military facilities constructed from 5.5 m. ca. BC 333 at Phalasarna (photo by T. Ohsumi).

4. Trace of Upheaval the subduction plate, and set the average slip to 42 m. Figure 9 compares the crustal displacements and According to Murotani et al. [15], the average slip was upheaval generating areas from the Fischer [3], around 10 m during the Chile earthquake in 1960 and To- Shaw et al. [4], and Stiros [5] models. Pirazzoli [12] hoku earthquake in 2011. A value of 42 m is four times suggested that there is a trace of upheaval along the that of the Chili or Tohoku earthquakes. Moreover, suf- Crete coast from radiocarbon dating and provided a ﬁcient values for upheaval distribution are not obtained detailed survey with evidence for Holocene coseismicity using the equation from Okada [16] (Fig. 9(a)). (Fig. 9). This data was used to compare differing uplift Shaw et al. [4] provided the seismicity and topogra- distributions; Table 1 shows the parameters from each phy in the area of Crete with regional seismicity cor- research group used in the fault model. responding to the AD 365 earthquake. According to Fischer [3] used a dip angle of 13◦, low angle along the authors, the upheaval distribution suggested that the

Journal of Disaster Research Vol.13 No.5, 2018 945 Ohsumi, T., Dohi, Y., and Hazarika, H.

(a) (b) (c)

Fig. 9. Crustal displacements and upheaval generating areas from the Fischer [3], Shaw et al. [4], and Stiros [5] models (Coulomb 3.3 was used, see Toda et al. [14]).

sumed 5.7 × 1028 dyne · cm from the area of dislocation Table 1. Parameters used in the fault model. and an elastic coefficient. According to Papadimitriou and Karakostas [1], seis- Fischer Shaw Stiros Papadimitriou mic coupling has been correlated with the maximum (2007) (2008) (2001) (2008) earthquake sizes that occur at a subduction zone. Sub- ◦ Strike 297 315 292.5 315 duction zones that are strongly seismically coupled ◦ Dip 13 30 ± 5 40 35 periodically produce great earthquakes (Mw > 8.0) Depth km 45 70 5to50 (Kanamori [19]). In contrast, those that are seismically Length km 145 100 105 160 uncoupled produce only moderate to large earthquakes Width km 130 100 80 (Mw < 8.0) (Ruff and Kanamori [20]). Reverse fault- Slip m 42 20 16 8.9 ing is observed on planes with a NW or NE strike, and × 28 × 28 M0 dyne cm 5.04 10 5.7 10 with approximately E-W P axis; the larger of them oc- M 8.5 8.3–8.5 8.5 8.3 w curring in 1982 beneath the Mediterranean ridge. The slip distribution by Papadimitriou and Karakostas [1] had non-uniform distribution in the fault plane (Fig. 10). This AD 365 Crete earthquake occurred not on the subduc- study was in the upheaval by using each of the 128- tion interface beneath Crete, but on a fault dipping at mesh division to obtain the average slip. In addition, about 30◦ within the overriding plate. The shallow branch the geometry of the fault was almost same as in Shaw of the subduction zone dips at low angle to and couples et al. [4]’s setting, except the fault length was set to with the Aegean lithosphere, while the deep branch dips 160 km. Using Shaw et al. [4]’s fault length, 100 km, freely (without coupling) at a high angle beneath the south the Mw scale becomes an 8.0 class using scaling low for- Aegean trough. In this study, the crustal upheaval model- mulae for earthquake size and fault area (e.g., R. Sato: ing from Shaw et al. [4] was combined with the equations log S = M − 4.07 [21]). The M8 level is the difference from Okada [16] (Fig. 9(b)). between the 8.3 to 8.5 classes, which each prior study de- Shaw et al. [4] set the dip angle to 30◦ with a high angle fined. Thus, ground motion can be estimated using the within the overriding plate, the strike to 315◦, average slip stochastic Green’s function method and parameters from to 20 m, and fault depth to 45 km. The strike was the same Papadimitriou and Karakostas [1]. angle, 315◦, used by Papadimitriou and Karakostas [1] in their fault plane solution. Stiros [5] set the dip angle to 40◦ with a high angle within overriding plate, strike angle 5. Estimating Earthquake Ground Motions to 292.5◦, and average slip to 16 m. Stiros [5] analyzed elastic dislocation in the coastal up- 5.1. Methodology heaval data and found that this earthquake was associated with a reverse fault offshore of southwestern Crete, with a In this study, we estimate earthquake ground motion us- minimum magnitude of 8.5. This model is consistent with ing the stochastic Green’s function method. As discussed the approximate seabed trace of the fault; using observed in detail by Irikura [22] and Kamae and Irikura [23], the and calculated displacements from the modelled fault, the Green’s function method simulates ground motions from fault depth was as deep as 70 km. This study applies the an extended fault based on the representation theorem of crustal upheaval from the Stiros [5] modeling study and elastodynamics. equation from Okada [16] (Fig. 9(c)). An extended fault surface with length L and width W Papadimitriou and Karakostas [1] set the direction to is divided into small faults with length ΔL and width ΔW . 315◦ based on the seismology and topography from Pa- Using the representation theorem of elastodynamics, the pazachos [17, 18] (Fig. 10). The earthquake moment as- far-field displacement u(x,t) in a homogeneous, isotropic,

946 Journal of Disaster Research Vol.13 No.5, 2018 Strong Motion and Tsunami Related to the AD 365 Crete Earthquake

Fig. 10. Slip distribution (left) and crustal displacements and upheaval generating areas (right) from Papadimitriou and Karakosta [1].

and layered medium can be expressed in the following in- Table 2. Parameters used in the stochastic Green’s function tegral, from Kanamori and Anderson [24] and Somerville method. et al. [25]: L[m] 100 NL NW ξm+ΔL ηn+ΔW u(x,t)= ∑ ∑ D˙ (ξm,ηn,t − τmn) W[m] 86 ξ η m=1 n=1 m n Strike [deg.] 315 ∗ Dip [deg.] 35.0 G(x,ξm,ηn,t −tmn)dξdη Depth [km] 5 ...... (1) M[N/m] 3.44×1021 =(x,y,z)T D(ξ,η,t) Rise time [s] where, x is the observation point, is 3.0/10.0 the velocity of the source time function at position (ξ,η) Asperity/Background N32.54 on the fault, G(x,ξ,η,t − tξη) is the Green’s function Hypocenter (the impulse response of the medium), and * represents E24.08 Rapture-propagating a convolution. τmn is the rupture propagation time from Radial (m,n) direction the hypocenter of the extended fault to the -th small Cut off frequency [Hz] 13.5 fault and tmn is the propagation time for shear waves to ρ 3] (m,n) [g/cm 2.6 travel from the -th small fault to the observation Vs [km/s] 3.5 point. These two variables are defined by: VR [km/s] 2.7 ζmn Rmn − R τmn = ,tmn = ...... (2) VR CS timeisassumedtobe10s. VR was set to ∼80% of the where, ζmn is the distance from the hypocenter of the ex- S wave velocity from Kataoka et al. [27] and the cutoff tended fault to the (m,n)-th small fault, Rmn is the distance from the (m,n)-th fault to the observation point, R is the high frequency was set to 13.5 Hz from Sato et al. [21]. hypocentral distance of the extended fault, VR is the rupture velocity of the fault, and Vs is the shear wave velocity 5.3. Small Events of the medium. The stochastic Green’s function uses random or realistic phases. This study used realistic phases identified 5.2. Setting Parameters from earthquake records. Earthquake records from Octo- Table 2 shows the parameters used in the stochastic ber 12, 2013 on Crete Island, Mw6.4, provide the phases Green’s function method. As shown in Fig. 10, the ele- for the stochastic Green’s function. The dislocation model ment fault area S (10 km × 10 km = 100 km2) and elas- is based on Papadimitriou and Karakosta [1]. Some ve- tic coefficient μ (from Stiros [5]: 3.0 × 1010,N/m2)are locity records are saturated (over scale) at Antikythera, used in Mo = μ DS to obtain the seismic moment for each Chana, and Iraklio (Fig. 11, upper graph). Therefore, the element. Based on the sum of the seismic moments of horizontal waveforms were converted to the orthogonal each element, the seismic moment of the entire fault is direction from the strike angle of the fault. 3.44 × 1028 dyne · cm (Mw8.3). Considering the scaling relationship, Mw6.4 is a better According to Somerville et al. [25] and Ishii et al. [26], selection for the larger event and smaller records should slip distributions of more than 17.8 m (twice the average be used for smaller magnitude earthquakes. Thus, we slip distribution of 8.9 m) act as the asperity area and the selected data for the aftershock record from the Mw4.0 element less than 17.8 m acts as the background area. In earthquake on October 12, 2013 (Fig. 11, lower graph) at the former, the stress drop is 22.1 MPa and rise time is 3 s, Antikythera, Chana, Aptera and Iraklio. At Phalasarna, while in the latter, the stress drop is 2.6 MPa and the rise there was no recording collected. Thus, the Aptra record

Journal of Disaster Research Vol.13 No.5, 2018 947 Ohsumi, T., Dohi, Y., and Hazarika, H.

Fig. 11. Earthquake records for Crete Island.

was used for the estimation at Phalasarna. At Athens, the surement has become a powerful tool for engineers in es- aftershock record included noise, therefore we used the timating ground motion characteristics, including ampli- main shock phase. fication of soil deposits. The most efficient way to de- termine the dynamic behavior of structures is to use the H/V-ratio technique. Spectral analyses were performed 5.4. Site-Specific Data for measured data at different down-hole locations. The The strong motion simulation technique using deter- horizontal motions of the surface waves were found to be ministic site-specific amplification factors is frequently very similar to the bottom waves. The results in Fig. 12 applied in stochastic Green’s function analyses. In this indicate H/V spectra peaks at 0.4 Hz and 1.2 Hz at the study, we used the existing report for outcropping sites. Chania site where the configuration layer was set at a However, in the thick sedimentary layer sites, we used depth of 100 m for seismic response analyses of horizon- H/V (horizontal/vertical) spectra calculations. Vertical tally layered one-dimensional soil deposits corresponding shear velocity, Vs, in the northwest area was taken from to the H/V spectra. The H/V spectra has two peaks, at Karagianni [28]. Typical Vs values were used for the 3 Hz and 0.7 Hz, at the Iraklio site, where the response Athens, Antikythera, and Aptera sites: 3.5 km/s for 5– from the 100 m horizontally layered one-dimensional soil 2.5 km depth and 1.5 km/s for depths shallower than deposits corresponds to the peak in the primary and sec- 2.5 km. Deep Vsvalues were applied to subsurface struc- ondary waves. Table 3 shows soil profiles for the domi- tures, as provided in Karagianni [28]. Site amplifica- nant frequencies. tion characteristics of the surface layer ground configuration were chosen to fit the transfer function of the one- 5.5. Estimating the AD 365 Earthquake Ground dimensional wave theory. In the surface 200 m at the Motion Waveforms Athens, Antikythera, and Aptera sites, Vsvalues were defined from Karagianni [28] as GL-100 m to 200 m, which 5.5.1. Velocity Synthetic Waveforms and Response is Vs = 700 m/s. Spectrum At the Chania site, with thick sedimentary layers, and Figures 13 and 14 show the estimated AD 365 earth- Iraklio, GL-0 m to 100 m was used to calculate the H/V quake velocity synthetic waveforms and response spec- spectra from the portion of the seismic observation Coda trum. At Phalasarna, 10 km from the epicenter, the esti- waves. The result indicates a predominant configuration mated velocity is 102 cm/s. At Antikythera Island, 50 km frequency of 100 m from GL-0 m. The H/V-ratio mea- from the epicenter, the estimated velocity is 57 cm/s. At

948 Journal of Disaster Research Vol.13 No.5, 2018 Strong Motion and Tsunami Related to the AD 365 Crete Earthquake

Fig. 12. (a) H/V spectra for the Chania and Iraklio sites. Due to the thick sedimentary layers, GL-0 m to 100 m were used to calculate the H/V spectra from the portion of seismic Coda waves shown in (b) for Chania, 140.00 to 221.91 s, and (c) for Iraklio, 230.00 to 311.91 s.

and velocity waveform for different hypocenter positions Table 3. Soil profiles to adapt for the dominant frequencies. at Aptra are shown as Case 1 to 3 in Fig. 17.Thereis Chania not significant change in the velocity response spectrum Layer 1 Layer 2 Layer 3 due to varying the positions (Fig. 18). Case 3 resulted ρ 1.8 1.8 1.8 in a maximum value the 78 cm/s, which corresponds to Vsi [km/s] 60 150 350 328 gal (Fig. 19). Hi [m] 5 15 80 The velocity response spectra and velocity waveform Iraklio for different hypocenter positions at Chania are shown as Layer 1 Layer 2 Layer 3 Case 1 to 3 (Figs. 20 and 21). Case 2 resulted in the max- ρ 1.8 1.8 1.8 imum value of 126 cm/s, which corresponds to 913 gal Vsi [km/s] 60 80 350 (Fig. 22). Hi [m] 5 15 80 By evaluating these hypocenter positions, it is con- cluded that a directivity effect is generated in the velocity waveform. Chania, 50 km from the epicenter, the estimated velocity is 108 cm/s. At Aptra, 75 km from the epicenter, 5.5.3. Seismic Map of Crete Island the estimated velocity is 55 cm/s. At Iraklio, 150 km from the epicenter, the estimated velocity is 41 cm/s. At Sieberg [29] created a seismic map of Levant. Fig. 23 Athene, 270 km from the epicenter, the estimated veloc- shows the distribution of seismic intensity for ancient ity is 2 cm/s. These velocity values decline with distance earthquakes (1886, 1903, 1926, 1956) around Crete. Ac- from the epicenter, although the duration time expands cording to Wyss and Baer [30], the characteristics of these to 200 s. The velocity response spectra for the synthetic earthquakes are summarized as follows. waveforms at the Chania and Irakio sites show a dominant 1) Rupture influenced the whole Hellenic arc. natural period at 0.5 s (2 Hz), which causes non-linear behavior in the sedimentary layers. 2) Earthquake motions were felt over a vast area.

5.5.2. Ancient Earthquake Damage 3) Isoseismal contours were asymmetric. Fairly high Aptra is an ancient city located 30 km east of Chania intensities were observed in areas distant from the and 120 km west of Iraklio. It was built in the 15th to 14th Hellenic arc, while intensities at the other side of the centuries BC and was severely damaged by huge earth- arc decreased sharply. quakes in the 4th and 7th centuries. Roman occupation 4) Epicenters were located in southern Crete Island of Aptra began in 69 BC, as evidenced in ancient temple and the assumed intensities of all these earthquakes ruins, a castle gate, and walls. Catastrophic earthquake reached degree XI. damage occurred in AD 365, concurrent with the fall of the Roman Empire (Fig. 15). Figure 24 shows the Crete region, with extensive The position shown in these historical earthquakes may damage to buildings shown in red. In the 1926 earth- provide an unreliable hypocenter. Thus, at the Aptra and quake (M7.5), the area stretching from Iraklio to Knossos Chania sites, three locations on the lower fault were exam- recorded modiﬁed Mercalli seismic intensities of 9 to 10. ined, as shown in Fig. 16. The velocity response spectra Chania, in northwest Crete, would be an expected high

Journal of Disaster Research Vol.13 No.5, 2018 949 Ohsumi, T., Dohi, Y., and Hazarika, H.

Fig. 13. Estimated synthetic velocity waveforms from the AD 365 earthquake.

Fig. 14. Velocity response spectra comparison for the synthetic waveforms.

seismic intensity area. This seismic map and the exam- ined acceleration at Chania are in agreement. Therefore, seismic risk in high densely area with thick sedimentary Fig. 15. Up: hypothetical representation of the fortiﬁca- layers should be the focus of risk mitigation research and tion tower and entrance (from Information plate of National strategies. Strategic Reference Framework (NSRF) 25th Department of Antiquities). Down: entrance of the castle gate of Aptra (photo by T. Ohsumi). 6. Estimation of Tsunami Propagation

Tsunami propagation was simulated using equations 6.1. Setting Parameters based on non-linear long-wave theory, taking into account friction and advection on the seabed. This simulation used Topographic data were created from Shuttle Radar To- a ﬁnite-difference method (FDM) with a leapfrog scheme pography Mission (SRTM) 30 Plus data [31]. Grid reso- on a staggered grid. The computational time step for lution for land data is 30 arc seconds. Surface altitudes each grid size in the FDM was set according to Courant- of the Earth were derived from data measured with a Friedrickson-Lewy (CFL) conditions to ensure stability of Synthetic Aperture Radar (SAR) onboard a space shut- the calculation. tle. Grid resolution for the ocean data is 1 arc minute.

950 Journal of Disaster Research Vol.13 No.5, 2018 Strong Motion and Tsunami Related to the AD 365 Crete Earthquake 㻌 ] h=0.05 [

) cm/s ( onse Vel. p

Fig. 16. Evaluations of hypocenter positions. Rel. Res

Natural Period (s)

Fig. 18. Comparison of velocity response spectra for differing hypocenter positions at the Aptera site.

Fig. 19. Acceleration waveform for Case 3 at the Aptera site.

than 1,000 km. Therefore, the two-dimensional coordi- nate system and Universal Transverse Mercator UTM 34 was used [34]. These data were interpolated using inverse distance weighting and topographic data with mesh sizes of 1,350 m and 450 m were prepared (Fig. 25). In such analyses, horizontal displacement is normally neglected. However, Tanioka and Satake [35] showed the effect of horizontal deformation. When the tsunami source is on a steep slope, the horizontal displacement is Fig. 17. Comparison of velocity waveforms for differing large relative to the vertical displacement and the effect hypocenter positions at the Aptera site. becomes signiﬁcant. Thus, the initial water level of the tsunami is set as the vertical component obtained from the vertical direction and horizontal deformation effects Offshore bathymetry data were derived from multiple are usually neglected. When the wave source is on a steep sources, including the Coastal Relief Model of the Na- slope and the horizontal displacement is large relative to tional Geophysical Data Center (NGDC). the vertical displacement, the effect becomes signiﬁcant. Thus, we calculated the seabed variation considering the horizontal level. The tsunami propagation was simulated 6.2. Simulation Details using fault parameters to estimate earthquake ground motion (Table 2 and Fig. 10). Table 4 summarizes the details In this study, the computational domain was bounded of the simulation. by latitudes 34◦ and 49◦N, and longitudes 20◦ and 26◦E. The domain covers the Peloponnesus Peninsula, which is located near Crete Island and the fault (Fig. 25). It also 6.3. Results covers Alexandria, where historical observational of the Figures. 26 and 27 show snapshots of the simulation. tsunami have been documented [32, 33]. Table 5 summarizes the maximum tsunami heights at Maximum tsunami heights were calculated at several representative evaluation points around Crete, Pelopon- evaluation points. The distance between the fault and nesus, and Alexandria (see Fig. 25). At Crete, which is furthest evaluation point, located at Alexandria, is less close to the fault, the maximum tsunami height is 14.5 m.

Journal of Disaster Research Vol.13 No.5, 2018 951 Ohsumi, T., Dohi, Y., and Hazarika, H.

㻌 ]

h=0.05 [

)

cm/s (

onse Vel. p

Rel. Res

Natural Period (s)

Fig. 21. Comparison of velocity response spectra for differing hypocenter positions at the Chania site.

Fig. 22. Acceleration waveform for Case 2 at the Chania site. Fig. 20. Comparison of velocity waveforms for differing hypocenter positions at the Chania site. in Alexandria. Additional historical records (e.g., Guidoboni and Ebel [32]) indicate that the AD 365 6 min after the earthquake, the tsunami height increases tsunami destroyed cities and drowned thousands of peo- by 50 cm. At Peloponnesus Peninsula, maximum simu- ple in coastal areas in Africa, Greece, Sicily, and along lated tsunami height is 8.8 m. 11 min after the earthquake, the Adriatic. Flooding in Alexandria in AD 365 following tsunami height increases by 50 cm. At Alexandria, max- the earthquake in Crete was recorded as a major disaster imum simulated tsunami height is 2.4 m. 1 h and 35 min because Alexandria was the most prosperous and densely after the earthquake, the water level rises by 50 cm. populated city in the area. Historical disaster records for this tsunami were also collected from other Ionian coast settlements. 6.4. Comparison with Results from Previous Stud- In our simulation results, the computed inundation ies area in Alexandria (Fig. 27) was not simulated in detail because of changes in bathymetry and land surface Maximum tsunami heights determined from the simu- since AD 365. However, we can assume that Alexandria lation were compared with results from previous studies. was widely inundated because there were (1) few coastal Guidoboni and Ebel [32] and Yamazawa et al. [33] com- structures, (2) low areas, and (3) clear sea level changes pared the tsunami simulations from Lorito et al. [36] and (2.4 m calculated maximum tsunami height). Therefore, Shaw et al. [4]. Lorito et al. [36] did not report quantita- our simulation results are generally consistent with histor- tive estimates but showed that large energy was probably ical records of tsunami damage in Alexandria. captured and carried along the Egyptian coast due to the edge waves. Shaw et al. [4] showed that waves offshore of Alexandria reached approximately 0.6 m in height. How- 6.5. Summary and Conclusions ever, they reported difﬁculty in estimating the run-up in Using the AD 365 earthquake parameters ob- ancient Alexandria because of the many non-linear effects tained from Shaw et al. [4] and Papadimitriou and from near-shore bathymetry and changes in bathymetry Karakostas [1], we calculated motion from this large and land surface since AD 365. earthquake. Observed phase records at seismometer sites According to John Cassian (4th–5th century), boats located on Crete Island were estimated using the Green’s were washed out of the water, and onto rooftops function method for the AD 365 earthquake. Very good

952 Journal of Disaster Research Vol.13 No.5, 2018 Strong Motion and Tsunami Related to the AD 365 Crete Earthquake

August 27, 1886, M7.5 August 11, 1903, M7.9

June 26, 1926, M7.5 Octobert 12, 1956, M7.5

Fig. 23. The distribution of the seismic intensities during ancient earthquakes (1886, 1903, 1926, and 1956) around Crete (modiﬁed from Sieberg [29]).

Fig. 24. Earthquake map of Crete Island (modiﬁed from Sieberg [29]).

results were obtained by applying three Green’s functions and Karakostas [1]’s parameters. Their study used each to the fault geometry over a range of 160 km, where the of the 128 mesh divisions to indicate the dislocation and shallow branch of the subduction zone dips at low angle upheaval. These slip distributions were then used in the to, and couples with, the Aegean lithosphere. Therefore, Green’s function calculations in each of the 128 divisions. in this stochastic simulation, we applied Papadimitriou The velocity response spectra for the synthetic wave-

Journal of Disaster Research Vol.13 No.5, 2018 953 Ohsumi, T., Dohi, Y., and Hazarika, H.

[3] K. D. Fischer, “Modelling the 365 AD Crete Earthquake and its Tsunami,” Geophysical Research Abstracts, Vol.9, 09458, 2007. [4] B. Shaw, N. Ambraseys, P. C. England, M. A. Floyd, G. J. Gorman, T. F. G. Higham, J. A. Jackson, J.-M. Nocquet, C. C. Pain, and M. D. Piggott, “Eastern Mediterranean tectonics and tsunami hazard inferred from the AD 365 earthquake,” Nature Geoscience, Vol.1, pp. 268-276, 2008. [5] S. C. Stiros, “The AD 365 Crete earthquake and possible seismic clustering during the fourth to sixth centuries AD in the Eastern Mediterranean: a review of historical and archaeological data,” J. of Structural Geology, Vol.23, pp. 545-562, 2001. [6] P. A. Pirazzoli, “The Early Byzantine Tectonic Paroxysm,” Zeitschrift fur Geomorphologie, Supplementband, Vol.62, pp. 31- 49, 1986. [7] N. Ambraseys, C. Melville, and R. Adams, “The Seismicity of Egypt, Arabia and the Red Sea,” Cambridge University Press, 1994. [8] N. C. Flemming, “Holocene eustatic changes and coastal tectonics in the northeast Mediterranean: implications for models of crustal consumption,” Philosophical Trans. of the Royal Society A, Vol.289, No.1362, pp. 405-458, 1978. [9] Y. Thommeret, J. Thommeret, P. A. Pirazzoli, L. F. Montaggioni, Fig. 25. Area of the tsunami propagation simulation. and J. Laborel, “Nouvelles donnees sur les rivages souleves de I’Holocene dans l’ouest de la Crete,” Oceanis, Vol.7, No.4, pp. 473- 480, 1981. [10] P. A. Pirazzoli, J. Thommeret, Y. Thommeret, J. Laborel, and L. F. Montaggioni, “Crustal block movements from Holocene shorelines: forms at the Chania and Irakio sites show dominant nat- Crete and Antikythira (Greece),” Tectonophysics, Vol.86, pp. 27-43, 1982. ural periods at 0.5 s (2 Hz) and 1.0 s (1 Hz); these pe- [11] S. C. Stiros, “Late Holocene relative sea level changes in SW Crete: riods will cause non-linear effects in sedimentary layers. evidence of an unusual earthquake cycle,” Annali di Geoﬁsica, Chania and Iraklio sites represent appropriate use of the Vol.39, No.3, pp. 677-687, 1996. [12] P. A. Pirazzoli, J. Laborel, and S. C. Stiros, “Earthquake clustering H/V ratio, which is an effective method for sites with in the Eastern Mediterranean during historical times,” J. of Geo- poor ground information. The synthetic velocity of the physical Research, Vol.101, No.B3, pp. 6083-6097, 1996. [13] T. A. B. Spratt, “Travels and Researches in Crete,” J. van Voorst, Chania site, which is 50 km from the epicenter, is larger London, Vol.2, 1865. than the Phalasarna site velocity, which is 10 km from the [14] S. Toda, R. S. Stein, V. Sevilgen, and J. Lin, “Coulomb 3.3 Graphic- epicenter, due to the sedimentary layers. The velocity re- Rich Deformation and Stress-Change Software for Earthquake, Tectonic, and Volcano Research and Teaching – User Guide,” Revi- sponse spectra for the synthetic waveforms at the Chania sion History for USGS Open-File Report, pp. 2011-1060, 2011. and Irakio sites show the dominant natural period at 2 s, [15] S. Murotani, K. Satake, and Y. Fujii, “Scaling relations of seismic moment, rupture area, average slip, and as-perity size for indicating these periods caused a non-linear effects in the M˜9 subduction-zone earthquakes,” Geo-physical Reserarch Let- sedimentary layers. ters, Vol.40, Issue 19, pp. 5070-5074, 2013. In this study, we estimated maximum tsunami height as [16] Y. Okada, “Internal deformation due to shear and tensile faults in a half-space,” Bull. Seismol. Soc. Am, Vol.82, pp. 1018-1040, 1992. 14.5 m at Crete, which is close to the fault, 8.8 m at Pelo- [17] B. C. Papazachos, B. G. Karakostas, C. B. Papazachos, and E. M. ponnesus Peninsula, and 2.4 m at Alexandria. Accord- Scordilis, “The geometry of the Benioff zone and lithospheric kine- matics in the Hellenic Arc,” Tectonophysics, Vol.319, pp. 275-300, ing to historical records, the AD 365 tsunami destroyed 2000. cities and drowned thousands of people in coastal areas in [18] B. C. Papazachos, D. M. Mountrakis, C. B. Papazachos, M. D. Tra- nos, G. F. Karakaisis, and A. S. Savvaidis, “The faults that caused Africa, Greece, Sicily, and along the Adriatic. These his- the known strong earthquakes in Greece and surrounding areas dur- torical records were are in agreement with our estimated ing 5th century B.C. up to pre-sent,” 2nd Conf. Earthq. Enging. and maximum tsunami heights. Engin. Seism., Thessaloniki, Vol.1, pp. 17-26, 2001. [19] H. Kanamori, “The energy release in great earthquakes,” J. Geo- phys. Res., Vol.82, pp. 2981-2987, 1977. [20] L. Ruff and H. Kanamori, “Seismicity and the subduction process,” Acknowledgements Phys. Earth Planet. Inter., Vol.23, pp. 240-252, 1980. [21] R. Sato, “Japanese seismic dislocation parameter handbook,” Ka- The authors gratefully acknowledge the kind help provided during jima Institute Publishing Co., Ltd., 1989 (in Japanese). their reconnaissance by Professor Gerassimos A. Papadopoulos, [22] K. Irikura, “Prediction of Strong Acceleration Motion using Em- Institute of Geodynamics National Observatory of Athens, who pirical Green’s Function,” Proc. 7th Japan Earthquake Engineering provided the authors with important shear velocity structures in Symp., pp. 151-156, 1986. [23] K. Kamae and K. Irikura, “Simulation of Seismic Intensity Distri- the Aegean area. The authors are indebted to Professor Eleftheria bution During the 1946 Nankai Earthquake Using a Stochastically Papadimitriou and Associate Professor Vassilis Karakostas, Aris- Simulated Green’s Function,” Proc. of 9th Japan Earthquake Engi- totele University of Thessaloniki, who provided the authors with neering Symp., Vol.1, pp. 559-564, 1994. [24] H. Kanamori and D. L. Anderson, “Theoretical Basis of Some Em- important waveform data obtained from Crete Island. The authors pirical Relations in Seismology,” Bull. Seism. Soc. Am., Vol.65, were provided with important information and constructive com- pp. 1073-1095, 1972. ments for the stochastic Green’s function method by Mr. Yasuhiro [25] P. G. Somerville, K. Irikura, R. Graves, S. Sawada, D. Wald, N. Abrahamson, Y. Iwasaki, T. Kagawa, N. Smith, and A. Kowada, Fukushima, Eight-Japan Engineering Consultants Inc. “Characterizing crustal earthquake slip models for the prediction of strongground motion,” Seismological Research Letters, Vol.70, pp. 59-80, 1999. [26] T. Ishii, T. Sato, and P. G. Somerville, “Identiﬁcation of Main Rup- References: ture Areas of Heterogeneous Fault Models for Strong-Motion Esti- [1] E. Papadimitriou and V. Karakostas, “Rupture model of the great mation,” J. Struct. Constr. Eng., AIJ, No.527, pp. 61-70, 2000. AD 365 Crete earthquake in the south-western part of the Hellenic Arc,” Acta Geophysica, Vol.56, No.2, pp. 293-312, 2008. [27] S. Kataoka, T. Kusakabe, J. Murakoshi, and K. Tamura, “Study on a Procedure for Formulating Level 2 Earthquake Motion Based on [2] T. Hori and Y. Kaneda, “Giant earthquakes and tsunamis in the Scenario Earthquakes,” RESEARCH REPORT of National Institute world: Mediterranean Sea,” Report of CCEP, Vol.89, 2013. for Land and Infrastructure Management, No.15, 2003.

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Strong Motion and Tsunami Related to the AD 365 Crete Earthquake

Table 4. Soil profiles to adapt for the dominant frequencies. Governing equation Non-linear long wave theory Numerical solution Finite-difference method (FDM) with a leapfrog scheme on a staggered grid Calculation area Latitudes 34–49◦N, and longitudes 20–26◦E (This area covers the fault, the Peloponnesus Peninsula, the island of Crete, and Alexandria.) Mesh resolutions 1350 m, 450 m Boundary condition Considering tsunami run up in the land area. Transmission border nonreflective in the sea side Structures Not consider Calculation time 3 hours Initial water level Sea bed movement calculated by Okada [16]. Considering the effect of horizontal deformation by Tanioka and Satake [35]. Censored water depth 10−2 m Roughness coefficient 0.025

(a) 0 min (b) 30 min (c) 60 min (d) 90 min

(e) 120 min (f) 150 min (g) 180 min

Fig. 26. Snapshots of tsunami propagation simulations for the whole study area, 1350 m mesh.

(a) 0 min (b) 30 min (c) 60 min (d) 90 min

(e) 120 min (f) 150 min (g) 180 min

Fig. 27. Snapshots of tsunami propagation simulations for Alexandria, 450 m mesh.

Table 5. Maximum tsunami heights at representative evaluation points (see Fig. 25) and duration between the earthquake and arrival of different wave heights at each point.

Maximum Time Evaluation tsunami Maximum Tsunami Tsunami Tsunami Tsunami points heights tsunami heights heights at 50 cm heights at 1 m heights at 3 m heights at 10 m Crete 14.5 m 38 min 6min 8min 30 min 37 min Peloponnesus 8.8 m 46 min 11 min 14 min 31 min – Alexandria 2.4 m 104 min 95 min 102 min – –

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[28] E. E. Karagianni, “Shear velocity structure in the Aegean area obtained by inversion of Rayleigh waves,” Geophysical J. Int., Name: Vol.160, No.1, pp. 127-143, 2005. Yuji Dohi [29] A. Sieberg, “Isoseismal contours Crete ad 365 Crete earthquake intensity Isoseismal contours crete,” Untersuchungen uber Erdbeben und Bruchschollenbau im Oestlichen Mittelmeergebiet, Jena: [s.n.], Afﬁliation: BA49737430, 1932. Associated Research Fellow, Integrated Re- [30] M. Wyss and M. Baer, “Earthquake Hazard in the Hellenic Arc,” search on Disaster Risk Reduction Division, Na- Maurice Ewing Series, Vol.4, pp. 153-172, 1981. tional Research Institute for Earth Science and [31] Scripps Institution of Oceanography: Global Topography Disaster Resilience (NIED) SRTM30 PLUS, http://topex.ucsd.edu/www html/srtm30 plus. html [accessed March 14, 2018] [32] E. Guidoboni and J. E. Ebel, “Earthquakes and Tsunamis in the Past: A Guide to Techniques in Historical Seismology,” Cambridge Address: University Press, 2009. 3-1 Tennodai, Tsukuba, Ibaraki 305-0006, Japan [33] T. Yamasawa, “The Eastern Mediterra-nean Tsunami of 21 July Brief Career: A.D.365: A Review of the Literary,” Departmental Bulletin Paper, 2016- Research Fellow, Japan Society for the Promotion of Science Nara Prefectural University Research Report, Vol.24, No.4, pp. 27- 2017 Dr. of Engineering, Kyoto University 52, 2014. 2017- Associated Research Fellow, NIED [34] C. Goto, F. Imamura, and N. Syuto, “Study on Numerical Simula- tion of the Transoceanic Propagation of Tsunami,” Zisin (J. of the Selected Publications: Seismological Society of Japan. 2nd ser.), Vol.41, No.4, p. 7, 1988. • “Evacuee Generation Model of the 2011 Tohoku Tsunami in [35] Y. Tanioka and K. Satake, “Tsunami generation by horizontal dis- Ishinomaki,” J. of Earthquake and Tsunami, Vol.10, No.2, placement of ocean bottom,” Geophysical Research Letters, Vol.23, pp. 1640010 1-1640010 17, 2016. No.8, pp. 861-864, 1996. Academic Societies & Scientiﬁc Organizations: [36] S. Lorio, M. M. Tiberti, R. Basili, A. Piatanesi, and G. Valiensise, • Japan Society of Civil Engineers (JSCE) “Earthquake-generated tsunamisin the Mediterranean Sea: Scenar- • Japan Society for Natural Disaster Science ios of potential threats to Southern Italy,” J. of Geophysical Re- • Japan Geoscience Union (JGU) search, Vol.113, B01310, 2008.

Name: Hemanta Hazarika

Afﬁliation: Professor, Department of Civil Engineering, Kyushu University

Address: Room No.1124, West Building No.2, 744, Motooka, Nishi-ku, Fukuoka Name: 819-0396, Japan Tsuneo Ohsumi Brief Career: 1996- Geotechnical Engineer Japan Foundation Engineering Co., Ltd. Affiliation: 1998- Assistant Professor, Department of Civil Engineering, Maizuru Principal Research Fellow, Integrated Research National College of Technology on Disaster Risk Reduction Division, National 2001- Assistant Professor, Department of Civil Engineering, Kyushu Research Institute for Earth Science and Disaster Sangyo University Resilience (NIED) 2004- Researcher/Senior Research Engineer, Structural Dynamics Division, Port and Airport Research Institute 2007- Associate Professor, Department of Architecture and Environment System, Akita Prefectural University Address: 2010- Professor, Department of Civil and Structural Engineering, Kyushu 3-1 Tennodai, Tsukuba, Ibaraki 305-0006, Japan University Brief Career: Selected Publications: 1982- Tokyo Electric Power Service Co., Ltd. • “Sustainable Solution for Seawall Protection against Tsunami-induced 1989- Nippon Koei Co., Ltd. Damage,” Int. J. of Geomechanics, DOI: 1999 Dr. of Engineering, Miyazaki University 10.1061/(ASCE)GM.1943-5622.0000687, 2016. 2007 Dr. of Agriculture, Kagoshima University Academic Societies & Scientific Organizations: 2010- Professor, Tokushima University • Japan Society of Civil Engineers (JSCE) 2014- Principal Research Fellow, NIED • American Society of Civil Engineers (ASCE) 2015 Dr. of Science, Tsukuba University • Japanese Geotechnical Society (JGS) 2016 Dr. of Philosophy, Kobe University • International Society for Soil Mechanics and Geotechnical Engineering Selected Publications: (ISSMGE) • “Detection of damaged urban area by using interferometric SAR • Society of Materials Science, Japan (JSMS) coherence change with PALSAR-2,” Earth, Planets and Space, Vol.68, • International Association of Computer Methods and Advances in No.131, 2016. Geomechanics (IACMAG) Academic Societies & Scientific Organizations: • International Society of Offshore and Polar Engineers (ISOPE) • Japan Society of Civil Engineers (JSCE) • Fellow, Indian Geotechnical Society (IGS) • Japanese Geotechical Society (JGS) • Japan Geoscience Union (JGU)

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