Category Archives: seismic observatory

Daily analysis of seismic events investigates the sources of seismic signals

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The Institute of Seismology frequently receives inquiries from the media and citizens about various shaking observations.

The Institute has several channels to report or send inquiries on these observations. In addition to calling the info-phone of the Institute of Seismology, you can send an email or fill out an observation form on our website. Links and other contact information can be found on the Institute’s website. Every observation will be examined, and if necessary, we will carry out an analysis of the event. An explanation cannot always be found simply by looking seismograms, as some of the event sources are very weak and local. In these cases, only a very close seismic station can detect the signal. On the other hand, the source may be elsewhere than in bedrock, for example, an air-borne pressure wave can also cause windows to vibrate.

Based on the observation sent to us, we check the data from the nearest seismic stations for the time indicated. If the seismic event is strong enough, it may already be visible on our website “Seismic bulletins” where you can find “Local explosions and earthquakes today“. This page contains a map and a list of seismic events automatically detected and located by our software system. Because it is an automated list, i.e. a person has not checked it, the locations can be inaccurate at times, and there may even be incorrect events when the software combines signal peaks caused by frost, for example.

An analyst, i.e. a seismic data analyzer, searches for data from seismic stations at the requested observation time. Data is searched for a possible seismic event, the source of which is usually an explosion or an earthquake, but sometimes cave-in, landslide or frost quake can cause a signal. The first signal is marked as the longitudinal wave or P wave, and a slightly slower wave is marked as transverse wave or S wave. When these waves have been determined for several seismic stations, the difference in the velocity between P and S waves can be used to calculate the distance between the source and the station, and the source of the signal is likely to be found at or near the intersection of these distances. The more seismic stations are available around the source, the more precise location can be determined for the event. For example, seismic events in Finland tend to be located closer to the real source than seismic events outside Finland.

The analyst will also try to determine the seismic event mechanism as best as possible. Most seismic events observed in Finland (about 18,000 per year) are explosions at mines, quarries or construction sites. Explosions in mines and quarries are typically made as a series of multiple blasts, where bets are placed at certain distances from each other and are detonated with a slight delay. For explosions such as these, the signal is characterized by a steady distribution of energy in all directions and the typical waveforms generated by different blasting techniques (Figure 1). The explosions aim to dampen some energy frequencies and typically form a striped pattern in energy spectra (Figure 2).

Figure 1. Typical signal from an explosion observed in seismograms.

Figure 2. Typical striped energy distribution caused by an explosion. The left vertical axis denotes frequency, and the color scale describes the signal strength (i.e. the amount of energy). The image shows the OUF station’s observation of the explosion at Kaustinen.

However, with earthquakes, energy is distributed in different directions depending on the type of movement in the bedrock that caused the signal (Figure 3). In the energy spectra of the earthquake, energy has been distributed evenly across all frequencies, especially at closer seismic stations (Figure 4). A small seismic event, overlying locations, or duplication of another seismic event can make it more difficult to determine the source.

Figure 3. Earthquake happened in Kuusamo on 2 May 2024. Figure shows registrations from different seismic stations in Finland.

Figure 4. The energy distribution produced by the Kuusamo quake (from MSF station) is characteristic of the earthquake. The left vertical axis denotes frequency, and the color scale describes the signal strength (i.e. the amount of energy).

Finally, the analyst determines the magnitude of the event by measuring the amplitude of the strongest signal at different stations, from which the local magnitude is calculated to the seismic event. Once all parameters have been obtained, the result will be published on our website. Earthquakes are immediately visible through an earthquake search. In addition, earthquakes will appear within a few working days after the remaining seismic events of that day have been analysed, “Seismic bulletins” from the tab “Analyst-reviewed bulletins” of the page “daily bulletins of seismic events in Northern Europe” with explosions and other seismic events.

Kati Oinonen, seismologist

About timescales in seismology

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Finland’s first seismograph began operating in Helsinki a hundred years ago. It is a milestone worth noticing and a long time for humans.

However, one hundred years is a brief time to collect evidence of earthquakes. In many regions, no strong earthquake has occurred in over a century, and even in regions with recent large earthquakes, a century is too brief to properly assess the long-term seismic hazard of the region. Although seismologists generally do not work on geological timescales spanning up to millions to billions of years, their work does include extending the series of earthquake observations as far back in the past as possible and forward into the future.

Earthquakes in the distant past can be investigated by using non-instrumental seismological methods. The likelihood of a strong earthquake occurring goes up the longer the time of observation is. The price of looking very far back, however, is an increase in uncertainty about the earthquake size – but there are ways of using available evidence to model sizes of earthquakes in the past.

Before measuring devices became common, the impacts of earthquakes could be observed directly by the naked eye. Especially after earthquakes caused damage, people have written documents about the event for authorities and other contemporaries, and sometimes also for scientific purposes. The most reliable of these reports are from eyewitness accounts. In the best cases, detailed written records have survived to modern times and been found by researchers. Through modern research methods, the seismic history of a target location showing how often strong earthquakes have occurred and how those earthquakes have impacted society over the centuries, or a couple of millennia in some parts of the world, can be extracted.

The geological traces left by strong earthquakes in the natural environment can go back to at least ten thousand years ago. The suddenness and violence of the earthquake remains as displacement movements, subsidence or rise of the ground compared to the environment, old landslides, and ancient water paths in the soil. The location and timing of these traces adds information about the frequency of strong earthquakes. Searching the landscape for evidence of large earthquakes is called paleoseismology.

The Fennoscandian Shield in Northern Europe has ancient bedrock and is seismically relatively quiet. There are written records of earthquake effects as far north as Lapland for almost three centuries and in the south for a slightly longer time. At the end of the last Ice Age, the retreat of the ice mass changed the stress conditions of the earth’s crust, causing strong earthquakes associated with several southwest-northeast trending fault escarpments in northern Fennoscandia. They have been dated to about 9–11 thousand years ago.

In the 2000s, high-resolution LiDAR (Light Detection and Ranging)-images, trenching, and seismic reflection campaigns have advanced paleoseismology in Fennoscandia. Traces of paleoearthquakes have been identified in lake sediments in the south. It has been proposed that paleoseismicity had three maxima in the Finnish territory around 10-12 thousand years ago, 5-7 thousand years ago, and 1.5-3 thousand years ago. The highest magnitude earthquakes, likely around Mw7 and above, were associated with the oldest earthquakes, and lower magnitudes (around Mw6) with the younger events (https://doi.org/10.1016/j.tecto.2019.228227). Traces of strong earthquakes 700–4000 years ago have been reported from the Stuoragurra postglacial fault in northern Norway(https://doi.org/10.1017/9781108779906.015). What is special is their size range – up to magnitude 7 – which challenges the old notion that the strongest earthquakes occurred relatively shortly after the Ice Age.

Knowing the seismicity of the past is critical in assessing the seismic risk in the near future. Because we cannot predict exactly when and where an earthquake will occur, probabilites must be used. The usual way of talking about seismic risk is that an earthquake of a given size or a given ground motion occurs in a target area with a given probability in 30 or 50 years. This 30-to-50-year period is related to construction and engineering building codes, and the hazard maps are aimed at ensuring that appropriate seismic building codes are used so that strong ground movements do not damage buildings and society’s infrastructure.

Seismologists therefore have at their disposal a catalog of instrumental measurements from the last 100 years or so, and usually a longer period of non-instrumental observations. Combined, these give seismologists the best data available for working with engineers to plan for the strong earthquakes that will inevitably occur in the future.

Päivi Mäntyniemi, University Researcher