Historical Seismicity of the Rijeka Region (Northwest External Dinarides, Croatia)—Part II: The Klana Earthquakes of 1870

The Klana earthquake sequence with the mainshock on 1 March 1870 (Imax VIII Medvedev–Sponheuer–Karnik [MSK]) is one of the most important events that occurred in the Rijeka (Croatia) epicentral region. It is remarkable not just because of its impact to the local community and its significance for the seismic hazard in the Rijeka area but also because it provided some of important observations on earthquake phenomena during the infancy of seismology. In particular, Stur (1871) seems to have been the first to attempt to explain the observed variation in damage by interaction of different local geological units during shaking; Hoernes (1878) heavily relied on Stur’s data from the Klana epicentral region when he proposed that the vast majority of global earthquakes is related to the mountain-building process and thus named them tectonic earthquakes. Despite this, the Klana earthquake so far has escaped any detailed consideration by seismologists. In this study, we present the historical data sources consulted to extract useful reports on damage and other observed effects, compile macroseismic maps for the five strongest events, and invert the estimated intensity data to compute macroseismic hypocenters and epicentral intensities for the mainshock and the two strongest aftershocks. We also present an earthquake chronicle with data for 92 identified earthquakes in the area for the January– August 1870 period. The mainshock’s macroseismic epicenter is located near the active Raša fault–Klana-Novi Vinodolski fault system, which we thus propose to be the primary source of the sequence. Macroseismic epicenters of the aftershocks lie close to the surface trace of the nearby Skadanšˇcina-Rijeka reverse fault