Strong Motion and Tsunami Related to the AD 365 Crete Earthquake

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 bot- tom 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.

Load more