A Comparative Study of Building Damage in Ston, Croatia, Caused by the Earthquakes of 1850 and 1996

The small medieval city of Ston in southern Dalmatia (Croatia) has been hit by several strong earthquakes since mid-nineteenth century. The two most important are the ones from the years of 1850 and 1996. Although various aspects of the 1996 event have been well studied so far, the earthquake of 1850 has only recently been macroseismically analyzed. It turned out that the macroseismic epicenters, focal depths, and the epicentral intensities of the two events are virtually the same. As the categories of damage to buildings in Ston caused by the 1996 event were available from previous studies, we here combine details on damage and property ownership from reports in 1850 with the cadastral records from 1837. This allowed us to geolocate most of the damaged houses and thus directly compare the spatial distribution of damage from the two earthquakes. Although the building identification was not straight-forward and unambiguous due to unknown history of each building during the 13 yr between the cadastral survey and the earthquake, the overall damage distributions of both events are found to be similar. They both show the largest damage confined to the plain terrain below the Bartolomija hill characterized by a sedimentary cover 10-30 m thick and expected ground-motion amplification by factors of 3-5. Minimal damage for both events is observed on the hillslopes of Bartolomija in the northern part of the city, where the bedrock is shallow or outcropping. To our knowledge, this observation of the shaking effects for two strong similar earthquakes in a city that has changed little in the 146 yr between them is the only one of its kind in Croatia. It confirms consistency of spatial distribution of earthquake ground-motion amplification for comparable input earthquake motion.