V. Kapetanidis

Seismic site characterization at the western Cephalonia Island in the aftermath of the 2014 earthquake series

BackgroundThe site response during a strong earthquake event may be proven crucial for earthquake hazard assessment and risk mitigation. Two moderate magnitude earthquakes that occurred in early 2014 in Cephalonia produced the largest ground motion values ever recorded in Greece, highly exceeding the provisions of the effective seismic code implying for local effects. This motivated the investigation of site response in the epicentral area presented herein.MethodsWe applied the HVSR method on free-field ambient noise measurements obtained during an in situ survey. 68 measurements were adopted for site characterization after their validation using earthquakes and geotechnical data. The site response was approximated by the peak frequency and the amplification ratio of the HVSR curves.ResultsThe majority of measurements exhibit smooth lateral variations in the frequency range 0.7–17 Hz, at a factor up to 7 and they are clearly classified in two bands, a low (0.7–4 Hz) and a high one (5–17 Hz). Some discrepancies that are observed between microtremor measurements and earthquake recordings for peak frequencies <2 Hz and overall underestimated ambient noise HVSR amplification are likely explained by near-source, radiation pattern and/or nonlinear soil effects.ConclusionsHigh frequencies combined with low amplification correlate with damage in the hardest hit areas. Low frequencies are aligned in a NNE-SSW direction in the epicentral area, similar to the strike of the activated fault, indicating that the properties of rocks along the fault zone have possibly been affected by slippage and/or dynamic effects.

Resolving the Tectonic Stress by the Inversion of Earthquake Focal Mechanisms. Application in the Region of Greece. A Tutorial

This work describes the derivation of the tectonic stress from the inversion of focal mechanisms of double-couple earthquakes. The presented material is based, in large part, on several review papers, lecture notes and practices on the matter, developed by the authors during the last years.

Seismicity and Tomographic Imaging of the Broader Nisyros Region (Greece)

Nisyros Island is a Quaternary composite volcano located close to the eastern termination of the South Aegean Volcanic Arc. Large destructive earthquakes have been reported in the study area. Nevertheless, seismic activity during the last decades is moderate to low, consisting of both shallow and intermediate depth earthquakes. The main regions of the broader area with observed spatiotemporally clustered seismicity are between Nisyros and Karpathos, east of Kos and in the gulf of Symi. Major events of intermediate depth have occurred near Karpathos and Rhodes Islands while the most significant zone of deep earthquakes is identified in the western Nisyros basin. Evidence for a non-systematic temporal co-incidence of deep events at different regions as well as increase in shallow seismicity after the occurrence of a strong deep event have been observed. Moment tensor inversion , using recordings in local and regional distances, was applied to determine the focal mechanisms of recent moderate events. The solutions, obtained by minimizing the difference between observed and synthetic waveforms, revealed that shallow events are mainly related to normal faulting, whereas intermediate depth events to reverse faulting with important strike-slip component. A tomography study was performed, using manually located events, and identified two areas of high V p/V s ratio and low velocity perturbations. The first is located SW of Nisyros and can be attributed to magma intrusion of deeper composition containing fluids and melts. The second, reaching 15 km depth, is possibly related to the magmatic chambers that feed the Yali and Strongyli volcanic centers.