Built on Moving Ground

Read the german version here.

Hrvoje Tkalčić belongs to a rare category of scientists who can translate deep time into human time. In his book “When Worlds Quake”, the seismologist takes the reader deep beneath the surface, explaining both theory and practice. The book is an introduction to the science of seismology and a personal account of what it means to dedicate one’s life to studying a restless planet. At a time when humanity is concentrating its populations in ever denser cities and building increasingly complex infrastructure, understanding the restless physics beneath our feet is a civilizational necessity. This has taken Tkalčić from seeing the Aurora Borealis in Alaska to sunny California to the red-soil fields of the Australian outback.

Tkalčić describes how we have come to understand better how the interior of our planet works, leaving old superstitions behind. Fascinating facts include his description of seismic waves, which propagate through the planet after each quake and human-made explosions. These waves can be detected with the right tools like rays in a CT scan, a capability that has allowed the detection of clandestine nuclear tests. We have also learned that the Earth’s core is not uniform: the inner core is solid, while the outer core consists of convecting liquid iron. The motion of this electrically conducting fluid generates Earth’s magnetic field through a process known as the geodynamo. Extending far into space, this field forms the magnetosphere, a protective shield that deflects charged particles from the solar wind, making life on Earth possible. The same dynamics that produce earthquakes and take lives also create a shield against unbearable radiation that gives life to this planet. The very forces that make Earth habitable guarantee it will never be fully safe.

Earth’s interior moves because the planet is still hot, retaining vast reserves of primordial heat from its violent formation. That energy leaks only slowly into the vacuum of space, insulated by kilometers of rock, air and vacuum while radioactive decay within the planet continuously replenishes part of what is lost.

Most of the time, the constant motion in our crust is imperceptible to humans. In 1976, in Tangshan, China, that motion became perceptible in the most tragic way. An earthquake that lasted just sixteen seconds caused the death of 250 000 people. This is why it is extremely important to place seismometers that constantly monitor Earth’s movements, to understand how its interior works, predict when and how the crust will shift, and ultimately mitigate deaths. Unfortunately, seismometers are not homogeneously placed, with poorer areas and the sea being particularly underrepresented. Storms, funding constraints, and seasickness all stand in the way of placing new seismographs. This lack of seismometers limits our ability to predict precisely where the next earthquakes will occur. Without them, we remain seismically deaf to vast areas of the globe. The data from the seismographs has to be later processed to create a high-resolution map, running supercomputers for days or weeks at a time, which requires further funding.

In Switzerland, we are perhaps more accustomed to thinking about avalanches or torrential downpours as natural threats. Yet where earthquakes are common, they determine almost every aspect of life: how buildings are constructed, how bridges are reinforced, how insurance premiums are calculated, how and where cities expand, and even how safe people feel in their own homes.

Because earthquake forecasts are inherently probabilistic, modern societies must learn to act under conditions of permanent uncertainty. Seismology is a science of probabilities: more seismometers mean more data, and better models. Bayesian methods allow researchers to update risk estimates as new information becomes available.

Short-term seismic predictions give us early-warning systems. These provide seconds of advance notice which can halt trains, shut down gas lines or allow people to seek – or avoid – cover. Each marginal second can save thousands of lives.

Long-term predictions, also known as seismic maps, inform building codes. This means building infrastructure robust to earthquakes where they are needed and not building it where it is not needed.

When Worlds Quake shows how much trust we deposit civilizationally in our current probabilistic models, which, if correct, can prevent future disasters on the scale of Tangshan from happening. It shows how many lives and billions of dollars can be saved if we get seismic predictions right, as well as the disasters that occur when we get them wrong. Trust in seismology is not abstract, it is embedded in engineering norms, public warning systems, and insurance prices.

Stability is not Earth’s natural state but a human achievement built atop moving ground. The ground will continue to move. The question is how much we can improve our behavior by increasing our understanding of the laws governing how and why it moves.