Can low-magnitude earthquakes act as a triggering factor for landslide activity? Examples from the Western Carpathian Mts, Poland

There are numerous examples of the impact of strong earthquakes on landslide activity. There is however very little information about the ability of low-magnitude earthquakes (M < 5.0) to affect the stability of pre-existing landslide slopes. Two landslides were studied (Hołowiec and Kamień, Western Carpathians, Poland). These are located 20-30 km from seismoactive zones where earthquakes of M ≤ 4.4 occur. We have used tree rings to date past landslide movements possibly caused by earthquakes. We have dated events of tree tilting by landslide activity, after which the trees developed eccentric rings and reaction wood, in a sample of 40 Norway spruces (Hołowiec) and 51 European silver firs (Kamień). The results of dendrochronological dating were compared with the occurrence of earthquakes in the study area (magnitudes, epicentral distance, etc.). We have also analysed precipitation data to help to disentangle the impact of rainfall and earthquakes as triggering factors. We were able to distinguish: earthquake-triggered landsliding with no impact of precipitation, precipitation-triggered landsliding with no seismic impact and landsliding triggered by the overlapping impact of precipitation and earthquakes. The results show that the combination of both triggering factors has the strongest influence on the stability of landslides under study. The study demonstrates the ability of low-magnitude (M < 5.0) earthquakes to trigger landslide activity, even at distances larger than appears from limiting curves published for co-seismic landslides. The results also suggest that the activity of the Kamień landslide can possibly be influenced by the long-distance (over 500 km) influence of strong earthquakes (M 6.8-7.4) from outside of the study area. The study demonstrates that the seismic factor, both local, low-magnitude earthquakes and distant, strong earthquakes, can be an important trigger of landslide activity. Their role may be underestimated in the study area and other areas considered as seismically non-active or of low seismic activity.