Reanalysis of historical earthquakes to improve seismic hazard assessment: Case study of the 1880 Zagreb (Croatia) earthquake

Abstract

The earthquake of 9 November 1880 was one of the most important moments in the seismic history of Zagreb (Croatia). It is the strongest earthquake to have occurred in the greater Zagreb area, and as such it defines the seismic hazard in northwestern Croatia, the most populated part of the country. The main objective of this study was to reanalyze the location and magnitude of the earthquake, the input parameters which are crucial for a better assessment of seismic hazard, as there were macroseismic indications that the previous assessments should be revised. In addition, the strongest aftershock occurred two days after the main event, so it can be assumed that the observed intensities were caused by the cumulative effect of these two events. Therefore, a new isoseismal map was created, synthetic macroseismic modelling was performed and additional geophysical and microtremor measurements were taken. Based on all the information collected, the attempt to separate the effects of the strongest aftershock from the effects of the mainshock (to avoid a cumulative effect), a new assessment of the location of the epicentre of the main 1880 earthquake in Zagreb and its magnitude was made. When it comes to historical earthquakes, from a seismological point of view, even small improvements in the definition of the main seismological parameters of an earthquake significant for a given area are very important - for a better understanding of the geodynamics of the area, the earthquake mechanism and the spatial distribution of damage after the earthquake, as well as for the consistent assessment of seismic hazard and thus risk.