August 6, 1870 Tsunami: Adriatic Sea, Croatia

1870 Aug 06. Adriatic Sea, Dalmatia. [...] The tide gauge in Lesina started to record secondary oscillations of the water level at 18h on August 6; they reached the maximum amplitude by 18h and ceased by midnight (Perrey, 1875a). (reference #2130)

1870 Aug 06, Hvar, Vis. [...] Grgur Bucic from Hvar, an amateur scientist who installed a tide shaft and made the first simultaneous meteorological and sea-level observations on the eastern Adriatic coast, published an elaborate description of these earthquakes in Hvar (Bucic 1870): ‘The strongest earthquake was on 6 August around 9:22 in the evening, especially significant because it lasted 10 s…. On barogram there was no sign of any disturbance, and wind speeds were: at 9:15–17.2 km/h, at 9:29–8.8 km/h, at 8:50–23.0 km/h. On the graph of autograph which measures the tide (Fluthmesser) one can observe secondary oscillations of 4 inches, while the height between the highest and the lowest sea level on that day was 11 inches. That secondary oscillation started around 6 o’clock in the afternoon, reached its maximum around 9 o’clock and finished around midnight. Such anomalies are not rare, and they depend on air-pressure change, e.g., by gusts of bora wind or at collision of two large wave motions in the Adriatic, when one wind of considerable intensity ceases and gives way to the one from opposite direction. However, on this occasion these circumstances did not exist, at least not at Hvar’. This report, cited later by Perrey (1875), Kispatic (1891–1895), Soloviev et al. (2000) and Paulatto et al. (2007) (Fig. 22), strongly suggests that the high frequency oscillations observed at Hvar could have originated from a tsunami. However, we also have detailed weather reports from Vis (Geologische Reichsanstalt 1870): ‘On the 6 around 9 am there was significant swelter, then light south easterly breeze which later changed to north westerly wind of strength 3. Land towards south east was covered by thick, grey clouds, the sea was slightly disturbed. Thermometer and barometer were as on the 5. At 4 h 50 min pm weak earthquake in vertical direction with detonations lasting for 2 s. After the earthquake the land was still covered by big obscure clouds; the sea remained as before, that is slightly disturbed. At around 9 h 20 min pm followed a very strong shock with thunder, at first in the vertical direction, afterwards undulatory form west to east lasting 3–4 s … On water one could observe nothing but slight movement and increasing sea level. During the night there was light rain, heavy lightning and thundering from time to time’. Hence, the high-frequency oscillations at Hvar started at 6 o’clock, 3 h prior to the earthquake (no data in Herak et al. 1996, or in Italian catalogues and newspapers). They reached a maximum at the time of the earthquake. However, weather conditions before and after the earthquake were very unstable. Thus, remote meteorological forcing could also be considered to have been responsible for the sea-level oscillations at Hvar. (reference #9531)