Geological evidence for tsunami of the past: a contribution to the tsunami hazard assessmentA. Smedile, P.M. De Martini, and D. Pantosti The Augusta bay in Eastern Sicily (Italy) hosts one of the largest industrial and military infrastructures of Italy. Most of the coastal area is thus covered by critical facilities and by the town of Augusta that counts a population of about 35,000. Eastern Sicily, and the Augusta Bay in particular, was repeatedly hit by tsunami waves related to large local historical earthquakes (e.g. 1908, 1693, 1169) (CPTI Working group, 2004) as well as to far-field sources (e.g. AD 365 Crete earthquake) (Jerome, 380). To verify the extent of inundation of the historical events, and to reconstruct a history of tsunami inundation in the Bay for the past millennia, we were looking for the geological signature of past tsunamis. Well-targeted geomorphic analyses and sediment sampling have been performed both inland and offshore. Small ponds, marshes and lagoons were the targets in the coastal area, whereas a single core in the relatively wide continental shelf was performed offshore, in front of the Augusta town. The integrated interpretation of geophysical and geological data has been carried out to recognize, date and correlate key-layers in the sediment column that may be directly or indirectly related to tsunami events. A total number of 22 cores were collected inland at two different sites with a maximum distance of 530 m from the present coastline (De Martini et al., 2010). The dominant fine to very fine stratigraphy is intercalated by at least 6 high-energy depositional layers, repeatedly found in several cores. These are relatively thin (about 10 cm), single, massive, and structureless beds with abrupt erosional lower contacts. These beds are composed of coarse to fine sand and comprise a bioclastic component (sometimes predominant) with of microfauna (benthic and planktonic foraminifera, from both shallow and open marine environment) and shell fragments both indicating a marine origin. Chronological constraints on the age of these deposits are based on 8 AMS radiocarbon datings and on the attribution of a tephra layer to the 122 BC Mt. Etna eruption. For the marine shell samples (details in De Martini et al., 2010), measured C14 ages were dendro-chronologically corrected using a marine calibration curve (Reimer et al., 2009) and an appropriate ΔR offset (Reimer and Reimer, 2001) On the basis of the combination of all the data collected, the inland sequence of paleotsunami deposits spans the last 4100 yrs. In terms of tsunami timing, we could list them as follow (PR= Priolo Reserve site; AU= Augusta Hospital site): younger than 1420-1690 AD (PR-01), 650-770 AD (AU-00), 160-320 AD (PR-02), 600-400 BC (AU-01), 800-600 BC (PR-03), 975-800 BC (AU-02) and 2100-1635 BC (PR-04). Three of the tsunami deposits found at the Priolo Reserve site may be associated with historical or already geologically known tsunamis, these are: PR-01 with the 1693 local event, PR-02 with the 365 AD Crete event and PR-04 with the ca. 3600 BP Santorini event. A 6.7 m-long piston-core was sampled 2 km offshore the Augusta harbor at 72 m water depth, in a low energy area far from human modification. The core study includes X-ray imaging, isotopic dating, tephrachronology, grain-size and foraminiferal analyses (Smedile et al., 2011). The homogeneous sequence of dark grey mud is interrupted at -2.9 m b.s.f. (below sea floor) by the same Mt. Etna tephra deposit found inland. Through the analysis of tephrostratigraphy, radiocarbon dating and radioactive tracers, the entire core sequence has been dated back to the past 4500 yrs. The quantitative micropaleontological (on the benthic foraminifera assemblage) and sedimentological analyses highlighted 12 anomalous layers marked by high concentration of displaced epiphytic foraminifera (species growing in vegetated substrates like the Posidonia oceanica) and subtle grain size increase that are likely to have been caused by high-energy events, with tsunamis (back-wash wave) as best candidates. We estimated an age range for these 12 layers as follows: E1 (AD 1820-1920), E2 (AD 1430-1810), E3 (AD 930-1170), E4 (AD 590-800), E5 (AD 430-660), Ex (AD 90-370), E6 (BC 350-130), E7 (BC 580-320), E8 (BC 660-400), E9 (BC 800-560), E10 (BC 1130-810), E11 (BC 1720-1200). The hypothesis that these layers represent the geologic signature of past tsunamis is also supported by the fact that the ages of 5 of them coincide with that of historical tsunamis [1908 (E1), 1693 (E2), 1169 (E3), AD 365 Crete (Ex) and ca. 3600 BP Santorini (E11)]. The comparison of historical tsunami data with the geological offshore and inland observations show a good correlation among the ages of events. Evidence for the main local historical tsunamis (1908, 1693, 1169) was found in inland and/or offshore, evidence for two already known far generated-tsunamis (AD365 Crete, and BC 1600 Santorini) was found too. Moreover, correlative unknown tsunami deposits are found in both marine and coastal environments. On the basis of these new data we can propose, for the past 4 ka, an inland and offshore geologic average tsunami recurrence interval in the Augusta Bay of about 550-650 and 320 years, respectively. This difference is conceivably due to the better preservation of the stratigraphic record in the offshore with respect to coastal areas, commonly affected by intermittent erosional and sedimentation events as well as by antrophic activities. It is interesting to notice that from a methodological point of view the marine record provides a history of past tsunamis that appears to be as complete as the historical one that for the Augusta Bay furnishes an average tsunami recurrence interval of 250-400 yr. From a tsunami hazard assessment point of view, both geological approaches can contribute critical information on frequency, run-up and extent of inundated area that can be used both for time-dependent probabilistic models and for preparing/testing inundation scenarios. Information on the frequency of tsunamis appears more complete if derived from the marine geological record, however, this approach may not be applicable everywhere and is certainly more costly. Information on run-up and extent of inundated area can be retrieved only form inland geology and historical records. The Augusta Bay represents a unique case study in the Mediterranean because it allows a comparison between geological (both inland and offshore) and historical records. References De Martini, P.M., Barbano, M.S., Smedile, A., Gerardi, F., Pantosti, D., Del Carlo, P., Pirrotta, C, (2010). A 4000 yrs long record of tsunami deposits along the coast of the Augusta Bay (eastern Sicily, Italy): paleoseismological implications. Marine Geology, 276, 42-57, doi: 10.1016/j.margeo.2010.07.005 Smedile A., P.M. De Martini, D. Pantosti, L. Bellucci, P. Del Carlo, L. Gasperini, C. Pirrotta, A. Polonia, E. Boschi (2011). Possible tsunamis signatures from an integrated study in the Augusta Bay offshore (Eastern Sicily-Italy), Marine Geology, 281, 1-13, doi: 10.1016/j.margeo.2011.01.002. Reimer, P.J., Reimer, R.W., 2001. A marine reservoir correction database and on-line interface. Radiocarbon, 43, 461-463 suppl. mat.URL: http://www.calib.org. Reimer, P.J., Baillie, M.G.L., Bard, E., Bayliss, A., Beck, J.W., Blackwell, P.G., Bronk Ramsey, C., Buck, C.E., Burr, G.S., Edwards, R.L., Friedrich,M., Grootes, P.M.,Guilderson, T.P.,Hajdas, I., Heaton, T.J., Hogg, A.G., Hughen, K.A., Kaiser, K.F., Kromer, B., McCormac, F.G., Manning, S.W., Reimer, R.W., Richards, D.A., Southon, J.R., Talamo, S., Turney, C.S.M., van der Plicht, J., Weyhenmeyer, C.E., 2009. IntCal09 and Marine09 radiocarbon age calibration curves, 0-50, 000 years cal BP. Radiocarbon, 51 (4), 1111-1150. |
