Ancient Tsunamis and their Modern Significance

Ōtsuchi, Iwate, Japan (March 18, 2011). An aerial view of damage to Ōtsuchi, Japan, a week after a 9.0 magnitude earthquake and subsequent tsunami devastated the area. (U.S. Navy photo by Mass Communication Specialist 3rd Class Dylan McCord/Released)

An aerial view of damage to Ōtsuchi, Japan, a week after a 9.0 magnitude earthquake and subsequent tsunami devastated the area.

By: Beverly Goodman

On March 11, 2011, the word “tsunami” went from being an esoteric term to a household word. The world’s television screens were filled with images of destruction and carnage when massive waves generated by an offshore earthquake devastated large portions of northeastern Japan. Waves reaching as high as 40 meters resulted in more than 19,000 people either killed or missing, almost one million damaged or destroyed buildings, and $230 billion in damages. To make matters worse, the Fukushima Nuclear Power Plant was severely damaged, causing a meltdown and explosions that released radioactive contamination into the air and water. According to Forbes, more than 315,000 people remain displaced today.

Just two years after this catastrophe, we are still asking whether any of the devastation could have been prevented. Should houses have been built differently? Should nuclear plants have been sited differently? How safe is it to live near any coast? In the aftermath of Hurricane Sandy in October 2012, such questions are even more pressing for Americans living on the east coast. While archaeology cannot answer all of these questions, it can contribute to our understanding of tsunamis. In turn, the geological study of tsunamis helps us understand important archaeological phenomena in the eastern Mediterranean.

The perception that tsunamis—waves generated by geological events such as earthquakes and landslides—are occurring more regularly is not incorrect but it has been magnified through the smart-phone lens. Never before has information been passed so quickly, so widely, and so personally by witnesses to these infrequent events. The news media and video documentation allows us to experience these events like never before. But just as earthquakes existed long before the era of human perception, so too have tsunamis. Our problem is that we only have instrument-derived data on tsunamis for the last 100 years, though we know that tsunamis occurred in antiquity even when modern observations have yet to document evidence of them. In particular, geoarchaeological evidence also suggests that the Eastern Mediterranean region is overdue for this type of natural catastrophe.

Mediterranean coastal archaeological sites provide an invaluable opportunity to supplement the last century of instrument data and allow us to identify regions of tsunami risk. These archaeological data allow us to postulate the timing, magnitude, and impact of tsunami events in the past, and they provide vital information for future predictions. However, locating the remains of a tsunami event in coastal archaeological contexts is difficult and involves major methodological challenges. Since it is not a simple endeavor, archaeological evidence of tsunami events is grossly under-researched.

The same limitations with measured data are found in earthquake studies, and archaeological data have provided important details to define earthquake magnitudes in the past, using both textual and material remains. All geologists are familiar with historical catalogues that summarize all of the known earthquake events in a given geographical area and their textual sources. David Amiran’s famous catalogue of earthquakes in Palestine, published in the Israel Exploration Journal in 1951, and Nicholas Ambraseys’ 2009 book Earthquakes in the Mediterranean and Middle East are two important examples. Great effort and attention has been paid to historical texts and passing remarks that included any mention of seismic-like events. Archaeological evidence has been helpful for corroborating these records or determining exactly how ‘completely destroyed’ a site or region really was. Visitors to Beth Shean are quickly impressed by the evidence of the 749 CE earthquake. In turn, these catalogues have been critical for civil engineers and authorities who design modern building codes and disaster plans.

Tsunami records ride the coattails of earthquake catalogues, and until recently have been treated as curious but less-significant events. Recent articles and books summarizing tsunami events in regions including the Mediterranean have helped focus attention on the power of these events to alter human settlement as dramatically as earthquakes or other natural disasters. And as with earthquake catalogues, historical tsunami catalogues must be continually reassessed to determine the nature and extent of any given event. Geoarchaeological data have only begun to make a contribution.

Utilizing historical information about tsunamis, however, depends first on the ability to use geological tools to look at and recognize tsunami deposits for what they are. Sedimentological studyies of tsunamis were significantly expanded in the decade following the major tsunamis that devastated Japan in 2011 and the Indian Ocean in 2004. Following these events, scientists flocked to the impact sites to measure and record the deposits left by the waves and to determine the extent of damage. In the process, expansive sets of sedimentological descriptions became available, far overshadowing all the literature available previously, and this research created a richer database for interpreting tsunami deposits. However, it also complicated matters by exposing the wide range of sometimes contradictory geological signatures left behind. For example, in the case of the 2004 Indian Ocean tsunami, most post-tsunami surveys consisted of trenches oriented perpendicular to a shore. These recorded both sediment sequences that were fining upward (that is, where grain size increases going down in a horizon) and fining downward (where grain size decreases going down in a horizon). In some areas the wave also produced more erosional than depositional remains.

Until recently, sedimentology suggested a relatively limited set of requirements to consider a deposit tsunami-derived. The primary test consisted of a marine deposit, for example sand and shells, located in a non-marine context far beyond the limits of a storm’s influence. The challenge is differentiating such a deposit from an average storm deposit.

Underwater coring in progress. Photo courtesy of Dr. Beverly Goodman.

Underwater coring in progress. Photo courtesy of Dr. Beverly Goodman.

It is at this point in the investigative process that archaeological sites are especially useful. First, archaeological remains can provide more precise dates than the absolute methods used by geosciences. Second, the condition and location of the marine deposit helps model and characterize the tsunami event. For example, deposits without immediate resettlement allow us to posit a more significant event than those deposits within an area of continuous occupation. When contemporaneous written data are available, archaeological deposits provide complementary or comparative data. In non-literate contexts or where a tsunami is not mentioned within the written record, geoarchaeological data can fill important gaps.

To understand how archaeology can contribute to the issue at hand, it is helpful to look at the evidence from Caesarea. At this site, underwater cores reveal evidence of four tsunami events at ca. 1500 BCE, 100-200 CE, 500-600 CE, and 1100-1200 CE. The first event (ca. 1500 BCE) was produced by the eruption of the Aegean island of Santorini and produced immense deposits as much as 40 centimeters thick. The next event can be dated precisely to December 13, 115 CE, thanks to the Roman historian Cassius Dio and to observations in the Talmud. Cassius Dio noted a tsunami and earthquake destroyed Antioch in December 115, while the Talmud, in Baba Metzia, noted that a tsunami resulted in the destruction of Caesarea’s Herodian harbor and reached as far south as Yavne. Although there was a tsunami in 502 CE, multiple texts speak to the devastation wrought across the Eastern Mediterranean by the event of July 9, 551 CE. Other texts attest to seismic activity in Greece and Anatolia during that year. John of Ephesus wrote of the sea receding at Beirut and the crowds who rushed to gather fish and treasure only to be engulfed by the wave itself. Modern crowds familiar with video of tsunamis are unlikely to be fooled the same way and would hopefully run to higher ground.

To the modern tsunami specialist, ancient deposits, for all their usefulness, are still rare. But it may be that tsunami deposits in the archaeological record have actually been grossly underestimated. A wide range of factors work against the preservation of such deposits, even in the short term, and we still have limited abilities to recognize the deposits.

It has been nearly a decade since the Indian Ocean tsunami of 2004, and researchers are returning to re-measure and record many of the same tsunami related features examined soon after the event. Just seven or eight years later, these deposits are now only rarely visible to the naked eye. One study found that over 70% of the deposits recorded in 2005 were virtually non-existent in 2011. Presumably, older tsunami deposits, which have also been subject to human modification such as rebuilding, are even less likely to be preserved and identified. The deposits that do remain and are recognized are thus especially important.

In coastal archaeological deposits the issue of preservation is compounded in cases of continued rebuilding. Just as the coast of Thailand has not become a vacant ghost town filled with memorials to the deceased, ancient coastal sites were not necessarily abandoned and left for paleo-tsunami scientists and archaeologists to investigate. People return, rebuild, rearrange, and remove the remains of the tsunami events and carry on with their lives. All the Mediterranean must be described this way. And even in areas where tsunami deposits are likely to be preserved, differentiating a typical coastal fill horizon made up of shell, ceramic, and sand from one containing the same materials but caused by a tsunami is well beyond typical excavation parameters. New methods are necessary.

Many proxies for recognizing tsunami deposits are proving useful. These include marine micropaleontological indicators, unique distributions of fine grain size fractions, mineralogical signatures, and the distribution of optically stimulated luminescence signals. In addition, work in underwater archaeological excavations as well as offshore coring have revealed a nearly untapped area for discovery of less-disturbed, better preserved tsunami horizons.

Underwater core being examined. Photo courtesy of Dr. Beverly Goodman.

Underwater core being examined. Photo courtesy of Dr. Beverly Goodman.

All of these finds provide a rare window into understanding the minimum impacts of past events and producing better tsunami records as a means to estimate future risk. Three hundred thousand people were killed by the tsunami that struck Southeast Asia in 2004. Tens of millions of people live around the Mediterranean and the number of people at risk from tsunamis there is vast. Tsunami studies offer important opportunities for pioneering new geoarchaeological methods and for using scientific and textual data in creative ways. Putting these insights to use to mitigate the risks of the inevitable tsunami, through early warning systems, guidelines for construction and infrastructure, and disaster response, is a challenge facing governments and scientists regardless of nationality.

 Beverly Goodman is Assistant Professor in the Leon Charney School of Marine Sciences at the University of Haifa. Her specialty is Marine Geoarchaeology and coastal environments. She has participated and directed fieldwork in Israel, Turkey, Yemen, and Mexico.

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