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.

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.

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.

Amir Sadeghi-Bagherabadi, university researcher

The LUOVA system is a service intended for authorities. It warns about natural disasters that take place in Finland and abroad. LUOVA was established after the earthquake and tsunami disaster that occurred on Boxing Day 2004 in Asia. The system’s aim was to improve and clarify communication related to natural disasters. Institute of Seismology provides information and assessments of hazardous impacts of earthquakes to the LUOVA system.

Structure and functioning of LUOVA

The LUOVA system gathers forecasts, assessments, and warnings from various sources, and creates a real-time situational picture to support the decisions of the authorities. The basis of LUOVA is an information system that enables research institutes to monitor different natural phenomena worldwide and analyze threats, risks, and actual natural disasters. LUOVA offers a uniform reporting method for all types of warnings – one, fast channel in which authorities find the necessary information about natural disasters.

The LUOVA operation center is located at the Finnish Meteorological Institute (FMI) together with the on-call weather services. Experts deliver real-time reviews on natural disasters to the LUOVA web portal. Information providers of LUOVA are Finnish Meteorological Institute (dangerous weather phenomena, sea floods, tsunamis), SYKE (water floods) and the Institute of Seismology (earthquakes, tsunamis).

Earthquake observations in LUOVA

Institute of Seismology provides information and assessments of hazardous impacts of earthquakes to the LUOVA system. The focus is on hazardously big earthquakes abroad mainly at the tourist but also at high-risk destinations, but authorities are also informed about notable earthquakes in Finland. The earthquake alerts and estimates of the LUOVA are based on the Institute’s automatic observation system and on the on-call duty related to it. The on-call seismologist gets information on earthquakes primarily from the SeisComP-based automated system. More information on the events can be found on the websites of international organizations and centers, and on their warning messages.

The on-call seismologist checks, evaluates, and corrects automatic alerts. The risk of the event is assessed, and the event is classified into risk categories 1-4 (1 = extremely dangerous, 2 = dangerous, 3 = potentially dangerous, 4 = not dangerous). The on-call seismologist provides an impact assessment with a more detailed description of the possible consequences for the risk categories 1 to 3 events via the LUOVA system to the authorities. In addition, on-call seismologists will provide an expert opinion and assistance in issuing warnings and assessing the effects of damage, if necessary.

In 2023, 296 events came to the LUOVA system. Of these, 237 were evaluated as risk category 4 events, 51 risk category 3 events, 6 risk category 2 events and 2 risk category 1 events.

Assessment of effects caused by earthquakes

The magnitude of an earthquake and the earthquake effects and consequences are not directly interdependent. When assessing the destructive effects, without direct observations from the event area, priority is given to the magnitude of the earthquake (released energy, destructive power) as well as to the depth of the earthquake (surface waves, tsunami) and the location of the earthquake (centers of population, age and condition of buildings, critical infrastructure). That is, even a small earthquake (magnitude 4-5) occurring at shallow depth below a large population center can cause great damage. On the other hand, a large earthquake (magnitude 6 – 7) in the middle of the wilderness or ocean may not attract any attention (other than that it is detected by the global seismic station network).

Tsunamis relate to large earthquakes (typically above magnitude 7) that occur on the seabed due to vertical movement. Therefore, tsunamis are most commonly generated in subduction zones. Tsunami is different from wind-generated waves in such a way that it puts in motion the sea water in motion at its entire depth, while the effect of wind-generated waves is limited to the surface layer. Tsunami wavelengths and speeds are also significantly higher. The LUOVA on-call seismologist will also issue a warning in connection with the impact assessment if a tsunami can arise from an earthquake.

Recent earthquakes globally can be found on the Institute’s website (https://www.helsinki.fi/en/institute-seismology/earthquakes/earthquakes-globally).

Niina Junno, seismologist