Monthly Archives: June 2024

When the Earth Spins: Unraveling the Importance of Ground Rotational Motion in Seismology and Engineering

When we think of earthquakes, the first thing that comes to mind is the shaking of the ground. However, there’s more to this movement than just the back-and-forth or up-and-down motion. Ground rotational motion, the twisting and tilting of the Earth’s surface locally, plays a crucial role in understanding and preparing for seismic events.

Daily Life Examples of Rotational Motion

To make the concept of rotational motion more relatable, let’s look at some everyday examples:

  1. Spinning Top: Just like a spinning top that wobbles as it slows down, the ground can twist and turn during an earthquake.
  2. Dizzy Dance Moves: Ever tried spinning around in circles for fun? The dizzy feeling you get is a small-scale version of what happens during rotational ground motion, minus the risk of knocking over a building.

Everyday examples of the rotational motion.

These everyday activities help us visualize how the ground might move during an earthquake. Now, let’s delve deeper into what ground rotational motion is and why it’s important.

Breaking Down Ground Motion: Translation, Rotation, and the Dance of the Earth

Ground rotational motion refers to the local rotation of the ground around about the vertical and horizontal axes (we nerds call them torsion and tilt or rocking motions!). Dream about standing on a giant turntable that starts to spin; you would experience rotational motion. Similarly, during an earthquake, different parts of the ground can move in circular or twisting patterns.

Assume that you’re standing in an open field during an earthquake. Now, let’s break down what happens to the ground beneath you:

  • Translation Motion:

Imagine someone gives you a gentle push from the north. You’ll feel yourself moving southward. This movement, where the ground shifts back and forth in one direction, is translation motion. It’s like someone gently nudging you in a straight line. Similarly, another person can give you a gentle push from the east that makes you feel yourself moving westward.

  • Rotation Motion:

Now, let’s say another person starts spinning you around in a circle while you’re still being pushed from the north and east. You’re not just moving south and west anymore; you’re also spinning around. This spinning motion, where the ground twists and turns, is rotation motion. It’s like being on a spinning merry-go-round while still feeling the push from the north and east.

How do you like it so far? Doesn’t it feel like that you are in an amusement park? You already have a couple of friends pushing and spinning you in different directions. So, it doesn’t hurt to have another pushing friend! Now, let’s add another dimension:

  • Up-and-Down Motion:

While all this is happening, imagine someone else starts lifting you up and down. So not only are you being pushed south, spun around, but you’re also bobbing up and down. This up-and-down motion completes the trio of translational motions, adding depth to the movement.

So, in summary, during an earthquake, the ground can move in six different ways simultaneously: it can shift back and forth, left and right, and rise and fall (up-and-down). These three motions are called translational motion. The other three components of ground motion are rotations about the three perpendicular axes. These six motions together make up the complex movement of the ground, and understanding each component is crucial preparing for seismic events.

Decomposition of the ground motion to three translational and three rotational components

Why Is It Even Important?

  • Seismology

In seismology, understanding rotational motion helps in improving models of earthquake behavior. These models are used to predict how different types of ground movement, including both translational (back-and-forth) and rotational (twisting) motions, affect the Earth’s surface. By incorporating rotational motion into these models, scientists can gain a more complete understanding of ground shaking, leading to better predictions and preparedness for future earthquakes.

  • Engineering

For engineers, especially those designing buildings, bridges, and other infrastructure, knowing how structures respond to rotational motion can lead to better designs that are more resistant to earthquakes. Additionally, assessing the impact of rotational forces helps in evaluating the safety and stability of existing structures (watch this https://www.usgs.gov/media/videos/shaking-atwood-building-anchorage-alaska).

Measuring Ground Rotational Motion

Measuring rotational motion is challenging, but several techniques have been developed. These techniques can be divided into two main categories: rotational sensors and array-derived rotational motion.

  • Rotational Sensors

Rotational sensors directly measure the twisting and turning of the ground. These include gyroscopes, ring laser gyroscopes, and fiber optic gyroscopes, which detect and measure rotational movements during seismic events. These sensors provide critical data that helps in understanding the complexities of ground motion. Think of them as the seismologist’s version of a ballerina’s sense of balance—except way more precise and without the tutu.

  • Array-Derived Rotational Motion

The array technique for estimation of rotational motion involves using an array of seismometers (instruments that measure ground translational motion) to infer rotational motion. By strategically placing multiple seismometers in a pattern, scientists can analyze the data from these instruments to calculate the rotational components of ground motion.

How Array Technique Works

Imagine placing several seismometers in a grid. Each seismometer records the motion of the ground at its specific location. By comparing the differences in the signals recorded by each seismometer, researchers can derive how the ground beneath the array rotates. This method is highly effective because it uses widely accessible existing technology.

Rotational Motion in Helsinki

Here at the Institute of Seismology, University of Helsinki, we estimate the array-derived rotational motions about the vertical and horizontal axes for almost 500 small-magnitude seismic events that were induced by the 2018 stimulation of the Espoo/Helsinki geothermal reservoir. We process data from the six geophone arrays that were installed in the Helsinki metropolitan area. Despite the small magnitude of the earthquakes, we obtained clean rotational motion records showing the applicability of the array technique for estimating rotational motions caused by such small-magnitude earthquakes.

The arrays of geophones used for recording translational motions from the seismicity induced by the 2018 stimulation of the Espoo/Helsinki geothermal reservoir (read more https://doi.org/10.1785/0220190253)

Some sample array-derived rotational motion timeseries obtained from the translational motion records (read more https://doi.org/10.1029/2020GL090403)

Amir Sadeghi-Bagherabadi, university researcher

Return of Seismic Stations to Helsinki

Helsinki Day is celebrated again on 12 June 2024 to celebrate the City of Helsinki’s birthday. Seismological observation in the City of Helsinki started a century earlier when Mainka seismographs ordered from Göttingen in Germany and received from the Finnish Science Society started their operations at the premises of the University of Helsinki Department of Physics at Siltavuorenpenger.

In seismological observations, the trend was for a long time from cities towards more trouble-free environments. It is usually easier to detect natural seismicity in Röykkä, Nurmijärvi, where seismic observation started in 1958 and continues. In Helsinki, observation ended. However, the seismic sources originating from human activities are growing in the urban environment, which interests both the city and the Institute of Seismology. Thus, in early 2019, the city approached the Institute in an effort to re-establish seismic observation stations in its area. The agreement to establish three seismic stations and a special HelsinkiNet observation network was signed in August of the same year.

After the purchase of seismic devices, the Institute of Seismology began the search for suitable station locations as soon as autumn 2019. Based on favorable observation geometry and low seismic noise, the locations were selected as the western coast of Lauttasaari, Kuninkaantammi and Kallahdenniemi. The stations are equipped with Canadian Nanometrics Centaur digitizers and Trillium Compact sensors. It is not only the direct connection of the sensor to the bedrock, but also the continuous data communication link to the Institute of Seismology with the router that is important for the most uninterrupted operation. Stations also had to be able to be placed in places protected from weather fluctuations and vandalism, either in existing buildings or in equipment shelters built for the purpose.

HelsinkiNet’s three stations KUNI, LAUT and VUOS started operating in the first half of 2020. At the same time, in western Helsinki and Espoo, a temporary seismic research network was continued, which had been established specifically for the supervision of the Espoo deep geothermal drillhole project of St1 Inc. From the stations of that network, the Seurasaari station HEL1 became part of HelsinkiNet at the beginning of 2024. In addition, in May 2021, HelsinkiNet grew with the RSUO station, intended for the supervision of the Ruskeasuo geothermal heat plant project of Helen Inc. The network now includes five seismic stations, the data of which is utilized in the Institute’s daily analysis. All the HelsinkiNet stations are part of the Institute’s national seismic station network.

Test measurements in the city's winter garden did not eventually lead to the establishment of a station in the area.

Test measurements in the city’s winter garden did not eventually lead to the establishment of a station in the area.

Most of the observations of HelsinkiNet stations have been explosions of quarries and construction sites. Every year of the network’s operations there have also been natural or human-induced earthquakes. The increasing use of geothermal heat production with the help of shallow and medium-deep heat wells is linked to the carbon neutrality target set by the City of Helsinki in 2035 and emphasizes the importance of the station network due to the seismic risk associated with the heat production. At times, even distant events have been discovered in data, such as the Taiwan earthquake on 3 April 2024.

Under the agreement between the Institute of Seismology and the City of Helsinki, the Institute publishes a public report on the activities and observations of HelsinkiNet annually. The report is mainly in Finnish. All reports can be read in the Helda publication archive and the latest of them can be found here.

Toni Veikkolainen, seismologist