Comparative analysis of strong motion (SM) records from the July 2017 Ohrid seismic sequence

Abstract

During the months of June–July 2017, the Lake Ohrid area in the southwestern Macedonia experienced a series of small to intermediate earthquakes. More than a thousand earthquakes occurred in that period, all in the epicentral area about 10 km east-northeast from Ohrid city center. The earthquakes showed characteristics of a swarm with 50 of them having magnitudes of 3 or greater and the strongest reaching magnitude M5.0. The earthquakes caused concern among the people in Ohrid and neighboring cities and villages and prompted the installation of two networks of temporary stations. One network was deployed in the epicentral area to determine in more detail the earthquakes’ depth and source parameters. The other urban network of instruments was installed to monitor the influence of the ground on the amplification of shaking and the dynamics of structures in the city of Ohrid. In this study, a selection of the urban network strong motion (SM) records was analyzed for the first time. Accelerograph records from a magnitude M3.1 earthquake recorded at eight places in the city and from the two earthquakes with magnitudes M4.2 and M5.0 recorded at the permanent seismological station Ohrid (code OHR) were used. The results of the behavior of the instrumented building were also compared with the findings of previous experiments. The differences in the spectral values on the SM records from the stations were investigated in respect to the ground conditions and location where the instruments were installed, i.e., bedrock and sediments at the sites. The power spectra and the H/V ratio of the earthquake records from the 2017 seismic sequence in the Ohrid area were also viewed in respect to the Eurocode-8. In addition to being used as part of standard engineering practice, this information about the local soil conditions can be of interest to geophysicists in seismic and geotechnical investigations, as well as in seismic risk-assessment applications.