Compilation of Seismotectonic hazard map of Hungary based on geomorphology, structural analyses and seismology

Our team has compiled Seismotectonic hazard map of Hungary. One of the main contents of the map are Eurocode 8 categories whose production steps are described here in detail. In engineering, site response to earthquakes has been classified to national and international standards. Eurocode 8 standard is partly based on Vs₃₀ that is the time averaged shear-wave velocity in the uppermost 30 m sediment. We have compiled 67 Vs30 measurements and collected 103 Vs₃₀ values from PhD theses and industrial reports. The values could be divided to soil class A–D of Eurocode 8 which are defined by Vs₃₀ thresholds. The special soil class E (hard rock beneath 5–20 m thick loose sediment) needed a deeper investigation. The Vs trend was plotted and plots with obvious knickpoint has been analysed further. In case of one knickpoint in Vs trend two-layered model was used. We were defined the thickness and the theoretical Vs₃₀ of the upper and the lower strata. In case if the site fit to class E, original Eurocode 8 class have been overwritten. Other advantage of the extrapolation of Vs trend of the uppermost strata is to derive the theoretical Vs₃₀ of the given geomorphological feature if its sediment would fill up the whole 30 m.

In Hungary only the youngest and lowest level of alluvial and lacustrine features fall into the most critical class D. Therefore that features have been mapped. In case of the youngest sediment’s thickness was not exceeded 20 m in each places, that site would classified as „shallow D” which is not a Eurocode 8 soil class. This process could be done using the borehole database of Geomega Ltd. Classification of soil class E have derived using the same method: thousands of borehole data have been checked to delineate the margin of the categories around the rock outcrops. For soil classes A–D topographical slope – Vs₃₀ relation has established. For Hungary, we recommend to use 0.3%, 3% and 11% as topographical slope barriers between soil classes D-C-B-A (in advance).

Secondly, active faults were mapped using the methodology described by the European Facilities for Earthquake Hazard and Risk. Third, earthquake database was use to present area affected by frequent ground motions. We have divided the database to historical and to instrumental detections due to their differences in the accuracy and reliability of magnitude and epicentre location.

Historically Komárom-Oroszlány-Balatonfő line was most affected by earthquakes. Our map revealed that in the Middle Hungarian Shear Zone consists of still active fault lines. Some spots are affected by densely located small earthquakes such as the neighbourhood of Zalaszengrót, Répcelak, Nagyigmánd, the DIósjenő fault, Heves, Csepel, Jászberény, Nagykanizsa, Nagyatád, Pincehely, Szabadszállás, Kecskemét, and Miskolc. In almost all cases the most critical soil class D can be found in the neighbourhood of mentioned sites, while class E appears only in some locations.

The research project was supported by the National Research, Development and Innovation Office of Hungary (2018-1.2.1-NKP-2018-00007). Map can be downloaded among others and vector data can be requested at Geomega website (www.geomega.hu).