The Japan M9.0 earthquake and the following tsunami are well documented by videos, photographs, sea-level measurements, seismograms etc. But how do we recognize such huge events if they happened some thousands of years ago? If there’s no historical report we would use earthquake environmental effects (EEE) for characterizing the earthquake and paleoseismicity. Let’s look what would be left from a 5000 year old earthquake and tsunami.The Environmental Seismic Intensity Scale (ESI2007) was created by the INQUA. The idea is to characterize earthquake intensities not via building damage patterns, but by having a look at what the event did to the environment. How did it change our nature? The scale includes observations on primary effects like
- surface ruptures
and on secondary effects like
- mass movements (rockfalls and landslides)
- ground cracks
- waves and tsunamis
- hydrogeological anomalies
- tree shaking and
- the extend of the affected area.
If the earthquake and the tsunami would have happened in 3000 BC, some of these effects would have been preserved over time, and that’s what we could use. Obviously, it will not be possible to find the fault that caused the earthquake because it’s offshore. Bad luck, we would have loved to trench it and see the enormous offset of up to 17.5 m!
Maybe the first thing we came across was the tsunami sediments. Different kinds of mechanisms were observed:
The Sendai airport was hit by a rather slow wave that increased its height. Here, the amount of debris and coarse grained sediments appeared rather low. There will be, however, sands and silty-clayey deposits for sure.
In case of the impressive wavefront with burning houses on top and thousands of tons of debris, there will be unusual sediments like gravels, corals, trees etc.
All those sediments will likely have in common that they include closed sea shells (they open when they die, so if you find a closed ones in the sediment, this could mean they have been “surprised”) and fauna typical for deeper water. Tsunamis can bring sediments even from the deep sea, since they move the entire water column. Scientists observed that after the 2004 tsunami in Sumatra and at other locations.
Until now I haven’t found reports if there was only one big tsunami wave or if we had a tsunami train – a number of waves coming one after the other. There’s some hint for that by this video from Crescent City, California, where the waves arrived hours after the earthquake happened:
If that was the case, parts of our deposits might have been removed by later waves. On the other handside, we might also find more than one tsunami layer but two or three or even five. Those layers would possibly not only show rip-off clasts and strange fauna, but also a fining upward if they were deposited in a flat area. We learned from the news that parts of Honshu are still flooded – a perfect situation for the preservation of tsunamites. So called “back-wash” sediments would include not only marine deposits, but also material from further inland brought back by huge suction of the tsunami waves.
Given an average sedimentation rate of, let’s say 1 mm/yr, we will have to drill 5 m for reaching those kind of layers after 5000 years. Maybe not all of those sediments will preserved due to erosion – we will have drill a lot of places on Honshu for getting to know about the extend and size of the tsunami! And once we have the idea that we found a bigger event, we would start drilling on Kamchatka, Hawaii, Alaska, Vanuatu, Chile, California and all the other coasts in order to get an idea on the source of the tsunami. Then we would try to find material for 14C dating: wood, trees, plants, shells and so on. We could also try to date by OSL in order to find out when our sample has seen sunlight for the last time.
On some videos we have seen that the wave came in more rapidly, there we would probably encounter block deposits and large debris that could be visible over thousands of years at the surface. Also compare this image – a car “deposited” on top of a three-storey house! Boulders do not swim like cars, but you’ll get an idea on the energy.
Watch this video, from 1:30:
Videos showed that there were rockfalls and landslides in the north-eastern Honshu area.
Some of them can be visible for a long time. If we find out that all those mass movements occured at the same time, more or less, then we will have another hint for our earthquake.
Liquefaction was reported from a lot of places, and again liquefaction features might be preserved in the sediment record.
This is what liquefaction looks like if it’s some thousands of years old:
If we are lucky, we can detect surface cracks like those:
Of course, they will most likely be buried under a thick sediment layer, too.
If we put all our information together, we could get an idea on what happened 5000 years before. We would know about the age of the event (more or less) and get an idea on its magnitude due to the effects observed and the area where we found them. However, most likely we will perform not better than saying: “At least M8, somewhere at the subduction zone offshore, 5000 +/- 150 years ago”…
More on the ESI scale:
Guerrieri, L., Vittori, E. (2007): Intensity Scale ESI 2007. Memoria Descritiva de la Carta
Geologica d’Italia, 74. 41 pp, Tech. rep., Servizio Geologico d’Italia, APAT, Rome, http://
Michetti, A. M., Esposito, E., Guerrieri, L., Porfido, S., Serva, L., Tatevossian, R., Vittori, E.,
Audemard, F., Azuma, T., Clague, J., Comerci, V., Gürpinar, A., McCalpin, J., Mohammadioun, B., Mörner, N.-A., Ota, Y., Roghozin, E. (2007): Intensity Scale ESI 2007, Memorie Descrittive della Carta Geologica D’Italia, 74-41, 1-41.
Papanikolaou, I., Papanikolaou, D., Lekkas, E. L. (2009): Advances and limitations of the Environmental Seismic Intensity scale (ESI 2007) regarding near-field and far-field effects from recent earthquakes in Greece: implications for the seismic hazard assessment, in: Palaeoseismology: Historical and Prehistorical Records of Earthquake Ground Effects for Seismic Hazard Assessment., edited by Reicherter, K., Michetti, A., Silva, P., vol. 316 of Special Publications, pp. 11-30, The Geological Society, London, doi:10.1144/SP316.2.
Silva, P. G., Rodríguez Pascua, M. A., R. Pérez López, R., Giner-Robles, J. L., Lario, J., Bardají, T., Goy, J. L., Zazo, C. (2009): Geological and archaeological record of the 1504 AD Carmona earthquake (Guadalquivir Basin, South Spain): A review after Bonsor (1918) based on the ESI-2007 Scale, in: Archaeoseismology and Palaeoseismology in the Alpine-Himalayan Collisional Zone. 1st INQUA-IGCP 567 International Workshop on Earthquake Archaeology and Palaeoseismology. 7-13 September 2009, Baelo Claudia (Cádiz, Spain), edited by Pérez-López, R., Grützner, C., Lario, J., Reicherter, K., Silva, P. G., pp. 139-142, ISBN: 978-84-7484-217-3.