2nd Croatian Conference on Earthquake Engineering 2CroCEE

In micro tidal settings, processes of bioerosion and bioconstruction can lead to the development of distinct morphological features that define the modern shoreline. When such features are discovered above or below the present-day mean sea level, they reveal relative sea-level (RSL) change. Among the best sea-level indicators on rocky coasts are fixed biological and geomorphological markers – algal rims, Lithophaga lithophaga borehole upper limits, and tidal notches. For instance, the coralline alga Lithophyllum byssoides can build algal rims that are considered one of the most precise RSL markers along rocky coasts. Consequently, studying RSL markers on Lopud, Koločep and Grebeni two elevated palaeoshorelines have been distinguished. Their detailed survey allowed the distinction of seismotectonically uplifted sectors of the coast. The established high-resolution geochronology enables separation of coseismic uplift events from the periods of interseismic subsidence. Studying the local tectonic contribution to RSL change we approach aspects of neotectonics and palaeoearthquakes offering new insights in a timetable (500 BC up to 1800 AD) where earthquake related data are either misssing or incorporate high uncertainties, both regarding their epicentral localities and estimated magnitudes.
Our reconstruction provides evidence on two successive major earthquakes which affected approximately 5 km of coasts in the Pelješac-Dubrovnik fault zone, with an uplift amplitude between 40 and 80 ± 15 cm per event. The earlier, older events, caused larger displacements (60-80 cm), while the later, younger events, revealed on average lower displacements (40-55 cm) corresponding to the 1520 AD quake and the 1667 Dubrovnik earthquake. This new approach refers to algal rims as a possible tool for constraining palaeoseismic events, allowing to supplement the database of instrumental records and historical observations through field-based evidence.
This research was supported by Croatian Science Foundation project HRZZ-IP-2019-04-9445 – Relative sea-level change and climate change along the eastern Adriatic coast – SEALeveL.