An underestimated risk factor
The natural hazard of earthquakes is one of the most well-documented dangers worldwide. Through highly sensitive seismic stations, even the slightest earthquakes can be detected, and the position of the epicenter can be determined. Nevertheless, it is impossible to make a prediction of when and where the next earthquake will occur, or even to provide an adequate advance warning.
When the media reports on earthquakes, the magnitude of the earthquake is generally given as a measure of its strength. Magnitude is, in the broadest sense, a numerical value for the energy released at the hypocenter of an earthquake (earthquake magnitude). However, if one wants to make statements about the effects of earthquakes at a specific location, possibly far from the earthquake’s center, it is necessary to consider the earthquake’s intensity. Intensity does not describe the strength of an earthquake but rather its effects felt or observed at the Earth’s surface on a 12-point scale (Modified Mercalli Intensity (MMI)) represented by Roman numerals. Alternatively, it could be expressed as the measurable ground acceleration at the observation site.
Figure 1: Intensity of an earthquake – depending on the distance to the hypocentre.
With increasing distance from the earthquake’s epicenter, the intensity decreases significantly. It is also highly dependent on the nature of the local geological subsoil. Soft soil consisting of fine-grained sediments, especially when saturated with water, can significantly increase the local intensity of an earthquake. This is particularly true for artificially filled subsoil.
A good analogy for this relationship is a 100-watt light bulb – or, in the spirit of energy conservation, let’s consider an LED of equivalent brightness! The 100 watts represent the measure of the light source’s released energy, corresponding to magnitude. When standing directly in front of this light source, one experiences the maximum intensity and may be dazzled. If one moves far enough away from it, the light source will hardly be recognizable—the intensity decreases with increasing distance.
Figure 2: Light bulb (symbolic image).
IIn Germany, the earthquake risk is relatively low in global comparison, but not negligible. There are zones in the Rhine region and the Swabian Jura in particular where damage-causing earthquakes are possible. However, these earthquakes are rare events in Germany. To illustrate, one can examine the intensities of an event that has a 10% probability of occurring in 50 years – also referred to as a 475-year event – at two different locations:
In Cologne, such an event would be expected to have an intensity of “VI” (strong) on the MMI scale. The definition is: “Felt by all, and many are frightened. Some heavy furniture is moved; a few instances of fallen plaster occur. Damage is slight.” (Modified Mercalli Scale).
It is assumed that the Roman Praetorium in Cologne – the seat of the city administration at the time – was destroyed by a somewhat stronger earthquake during the transition from antiquity to the Middle Ages. However, this theory is not confirmed. The remains of the Praetorium can still be seen, hidden under today’s town hall (“Spanischer Bau”).
In Kobe, Japan, for the same return period, an intensity of “VIII” (destructive) would be expected. The description includes: “Damage slight in specially designed structures; considerable damage in ordinary substantial buildings with partial collapse. Damage great in poorly built structures. Fall of chimneys, factory stacks, columns, monuments, walls. Heavy furniture overturned. Sand and mud ejected in small amounts. Changes in well water. Motorists are disturbed.” (Modified Mercalli Scale).
Figure 3: Various earthquake waves in a seismogram.
Given such potential for damage, the question arises about pre-warning or even prediction of earthquakes. Since 2007, Japan has been operating an early warning system that takes advantage of the fact that the so-called primary waves propagate faster than the secondary and surface waves responsible for the majority of damage. However, depending on the distance from the earthquake’s epicenter, this system provides only pre-warning times of a few seconds to a few tens of seconds. In favorable cases, this may be sufficient to initiate measures aimed at reducing the risk of damage: trains can be slowed down, buses and cars can come to a halt, and gas pipelines can be shut off.
But is it also possible to predict or warn with a lead time longer than just seconds? Time and again, noticeable behavior of animals, the appearance of so called earthquake lightning, and similar phenomena have been described in individual cases just before an earthquake.
Figure 4: Animals on the run (symbolic image).
In April 2009, an English zoologist observed toads in Abruzzo and although the animals normally mate in spring when the moon is full, they disappeared from their breeding grounds at precisely this time. Shortly afterward, an earthquake occurred. On the same evening, the toads returned. However, the success rate was 50 % and is therefore not statistically significant. One could have just as easily flipped a coin.
For several years, a project called Icarus (International Cooperation for Animal Research Using Space) at the Max Planck Institute for Behavioral Biology in Konstanz has been attempting to increase the success rate or at least answer the question of whether strange animal behavior could ever be used as a means of short-term earthquake protection (Source: https://www.icarus.mpg.de/28810/animals-warning-sensors).
In this project, researchers equip animals in earthquake-prone regions with acceleration sensors attached to collars and record their movements over months using satellite technology. The results indicate that the closer the animals were to the epicenter of an impending earthquake, the earlier they exhibited peculiar behavior. The movement profiles of different animal species in various regions could potentially provide clues about the location and timing of an impending earthquake.
Before this data can be used for an early warning system, the data quality must be improved: more catastrophic events and more species need to be analyzed to determine which animals react to which events and how reliably these reactions can be associated with the respective natural event. Even then, it remains a challenge to have a group of “measurement animals” equipped with sensors and under constant observation at all times in every earthquake-prone zone on Earth.
Until this is achieved, the realization stands: based on the current state of science and technology, there is no reliable method for earthquake prediction.
Finally, there is the concept of the seismic gap. These are areas at the edges of tectonic plates with recurring strong earthquakes, but where no further earthquakes of comparable magnitude have occurred over a long period of time since the last event. If one averages the displacement between two tectonic plates over very long periods, it remains nearly constant. Therefore, it is assumed that seismic gaps indicate the locations of future strong earthquakes.
Figure 5: Illustration of the concept of the “seismic gap”.
With this assumption, a “target value” is established, indicating the expected release of seismic energy with the uniform displacement of plates—somewhat like an electricity meter. The actual value then refers to the measured release of energy through earthquakes of various magnitudes, summing up the seismic energy released over time.
Taking the energy release along the Japanese coast near Fukushima in 2011 as an example, it is observed that there was a gap between the target and actual values of seismic energy release during the earthquakes in 2011. This gap would have been sufficient for an earthquake with a magnitude well over 8.5. On March 11, 2011, the energy balance was famously more than offset. Japan is still dealing with the consequences of the magnitude 9.1 earthquake and the resulting tsunami.
This concept provides a possibility for long-term earthquake prediction. However, it is a “prediction” in the sense that the energy balance indicates that a strong earthquake is imminent in the medium term (years to decades in the future). The exact date will remain a mystery.
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