Frost quakes in wetlands in northern Finland during extreme winter weather conditions and related hazard to urban infrastructure

Abstract. The paper reports the first results of experiment in northern Finland during winter, 2022–2023, that aimed to study seismic events caused by seasonal freezing in so-called Critical Zone (CZ) of the Earth. These events have attracted public attention recently, as multiple reports about them from local inhabitants in Arctic and sub-Arctic areas appeared recently in social networks. To make an instrumental study of such events, to reveal relationship between their occurrence and winter weather conditions and to evaluate the possible hazard, we installed two high-resolution seismic arrays with co-located soil temperature stations at two sites in Finland, one of them being in the city of Oulu in the sub-Arctic area (65.04° N, 25.61° E) and the other one, above the Polar Circle in municipality of Sodankylä (67.36° N, 26.63° E). The equipment recorded continuous seismic and soil temperature data during November 2022–April 2023. Based on reports from the inhabitants of Talvikangas (Oulu) about ground shaking and unusual noises on 6.1.2023 and their observations of new fractures on the roads there, we selected the time interval for identification of frost quakes originated during that day from continuous seismic records in Talvikangas and in Sodankylä. During the selected time interval, the extremely rapid air temperature drop of about -1.4 °C/hour in Talvikangas and -0.88 °C/hour in Sodankylä were observed. We identified and located two types of seismic events, namely, frost quakes with frequencies of about 10–20 Hz, with waveforms like those of tectonic events, and irregular-shape frost tremors with frequencies of about 120–180 Hz. The sources of frost quakes in Talvikangas are mainly located on irrigated wetland while in Sodankylä about 50 % of registered frost quakes were caused by ice fracturing on the Kitinen river. However, several relatively strong events with origin in wetlands were also recorded. A significant number of sources of frost tremors are confined to wetland areas cut by irrigation channels and to roads cleaned from snow during winter, both in Talvikangas and in Sodankylä. We calculated ground accelerations and ground velocities for strongest events from both groups and compared them to equivalent properties of other seismic signals, like distant local earthquakes in the area, mining production blasts, cargo trains vibration. Our study shows that high-frequency frost tremors corresponding to surface fracture opening in the uppermost frozen surface layer of the thickness up to 5 cm can directly damage infrastructure objects like roads and basements of buildings. Surface waves, produced by frost quakes and propagating inside the shallow soil layer, have large enough ground accelerations at epicentral distances of hundreds of meters. Therefore, frost quakes should be considered as phenomenon, that potentially can damage infrastructures and they have to be taken into account in seismic hazard assessment. Our research is the first instrumental study of seismic events originated from wetland areas. These events occur as a result of interaction between the uppermost layer of the solid Earth’s CZ and atmosphere processes that deserves further study.