Breaking News:Hrvoje Tkalčić on When Worlds Quake– What Just Happened

When Worlds Quake is a fascinating account written by Hrvoje Tkalčić of how scientists around the globe seek to use quakes to answer tantalizing questions about the structure and inner dynamics of our planet and to discover the deepest secrets of our nearest neighbors in the solar system.

Your book opens with a cow named Matilda waiting to be milked on the morning of the 1906 San Francisco earthquake, and with a giant catfish beneath Japan causing tremors. Why start with these characters rather than diving straight into the science?

Hrvoje Tkalčić: I’ve always believed that earthquakes are, at their core, deeply human stories. Yes, they’re geological phenomena governed by physics, but they’ve shaped civilizations, sparked myths, and changed the course of history. Matilda the cow was real—she was grazing on Point Reyes when the San Andreas fault ruptured that April morning, and the ground literally shifted several meters beneath her hooves. That image has stayed with me: this patient creature, oblivious to the tectonic forces building beneath her for centuries, suddenly caught in one of history’s most significant earthquakes.

And Namazu, the giant catfish from Japanese mythology—I love this myth because it reveals something profound about how humans have always tried to make sense of the terrifying unpredictability of earthquakes. Before we had seismology, we had stories. The Japanese didn’t just fear earthquakes; they created an entire iconography around them, especially after the devastating 1855 Edo earthquake. These namazu-e woodblock prints that appeared afterward weren’t just art—they were social commentary, expressions of anger, fear, and even dark humor in the face of catastrophe.

So yes, I could have started with plate tectonics and seismic waves, but I wanted readers to understand first that earthquakes have been part of the human experience since the beginning. The science came later, and it’s all the more remarkable for it.

You’re a seismologist, but much of this book reads like an adventure memoir—stormy seas in the Southern Ocean, the Australian Outback, seasickness on research vessels. How did fieldwork shape your understanding of earthquakes?

HT: There’s this romantic notion of scientists sitting in comfortable offices, analyzing data on computer screens. And yes, that’s part of it—I spend plenty of time staring at seismograms! But the truth is, if you want to understand what’s happening deep inside the Earth, you have to go to some of the most inhospitable places on the planet to collect that data.

I’ll never forget my first days aboard the RV Investigator in the Southern Ocean. I was supposed to be co-leading this expedition to deploy ocean-bottom seismographs, and there I was, debilitated by seasickness, unable to function normally, questioning every decision that led me there. The ship was rolling 25 degrees in each direction, the wind was howling at 50 knots, and we had these incredibly sensitive, expensive instruments we needed to lower precisely to the ocean floor—each one worth about 200,000 Australian dollars.

But here’s what I learned: those moments of physical struggle, of fighting the elements to recover an instrument in the middle of the night with waves crashing over the deck—those experiences connect you to the science in a way that’s impossible to replicate in a laboratory. When you’ve spent weeks at sea, when you’ve watched albatrosses circle the ship and seen a remote island covered in fresh snow, when you’ve felt the ship heave beneath you forcefully, you develop a different relationship with the Earth. You realize you’re not just studying an abstract object; you’re on it, you’re part of it, and it’s alive in ways that equations alone can never fully capture. And then there are moments of unexpected grace—watching the aurora australis dance above Antarctic waters, or returning to port in Hobart and seeing crew members’ children waving from the dock, and realizing how much this work costs us, but also how much it gives back.

The fieldwork also taught me humility. We, humans, are quite small compared to the forces we’re trying to understand. But we’re also remarkably clever—we’ve figured out how to listen to the Earth’s interior by placing sensors on the ocean floor, 5 kilometers beneath the surface, in regions so remote that we’re among the first humans to ever study them. That’s simultaneously humbling and exhilarating.

The book pays tribute to many scientists, but among them is Andrija Mohorovičić, the Croatian scientist who discovered the boundary between Earth’s crust and mantle. What drew you to his story?

HT: Mohorovičić is my scientific hero, and as a Croatian myself, I feel a deep personal connection to his legacy. But beyond national pride, his story exemplifies everything I love about science: meticulous observation, creative thinking, and the courage to propose something entirely new.

Picture this: It’s October 1909, and a magnitude 6.1 earthquake strikes near Pokupsko, about 35 kilometers south of Zagreb. Mohorovičić is already middle-aged, working at the Zagreb Meteorological Observatory with limited resources. When he examines the seismograms from this earthquake, collected from various European cities, he notices something unexpected: there are extra signals present that shouldn’t be there according to the existing understanding of how seismic waves travel through the Earth.

A lesser scientist might have dismissed these as noise or errors. But Mohorovičić was extraordinarily thorough—obsessively so. He recognized that these signals were telling him something fundamental about the Earth’s structure. After months of painstaking analysis, comparing seismograms, doing calculations by hand, he realized that these extra arrivals indicated a sharp boundary about 54 kilometers below the surface, where seismic waves suddenly speed up. This was the transition from the Earth’s crust to its mantle.

What strikes me most is the inverse problem he solved—working backward from observations on the surface to infer something about the Earth’s invisible interior. This is the bread and butter of geophysics, but he was pioneering it with 1909 technology. No computers, no digital data, just careful observation, physical understanding, and brilliant reasoning.

The boundary he discovered—now called the Mohorovičić discontinuity, or “Moho” for short—fundamentally changed how we understand our planet’s structure. Every geology textbook shows the crust-mantle boundary; we drill toward it, we map its depth variations, and we use it to understand plate tectonics. All from those unexpected signals on century-old seismograms that Mohorovičić refused to ignore.

One of the most compelling chapters in your book examines earthquake prediction efforts in China during the 1970s—both a remarkable success and a devastating failure. Can we predict earthquakes, and should we even try?

HT: This is the question everyone asks me, and there’s a story from Tangshan that still haunts me—one I describe in the book that I won’t spoil here, except to say it involves a mother’s impossible choice in the rubble. It’s a reminder that behind every statistic about earthquake casualties, there are stories of unimaginable human suffering.

In February 1975, Chinese authorities evacuated the city of Haicheng in response to foreshocks and other warning signs. Hours later, a magnitude 7.3 earthquake struck. Thousands of lives were saved. It seemed like we’d cracked the code.

Then came Tangshan. July 28, 1976, magnitude 7.6, striking during the night with no warning. Around 240,000 people died. Each one has a story.

The painful truth is that Haicheng was partly lucky. Most large earthquakes occur without clear precursors—no foreshocks, no groundwater changes, nothing. The warning signs we sometimes observe aren’t reliable enough to justify evacuations. Imagine ordering a city to evacuate based on ambiguous signals, the enormous disruption and cost, and then… nothing happens. Do it twice, three times, and people stop listening. Then the earthquake comes without warning anyway.

Here’s what we can do: earthquake forecasting —assessing long-term probabilities for regions based on their geology and history. This helps with building codes and preparedness. And early warning systems, like Japan’s, which detect an earthquake the moment it starts and send alerts seconds before the worst shaking arrives. Those few seconds save lives.

But prediction? Saying exactly when and where? We’re not there, and anyone who claims otherwise is selling false hope to vulnerable people. The best we can offer is honest science: better buildings, better preparedness, better warnings when an earthquake begins. It’s not magic, but it’s real, and it works.

You write about deploying seismographs on Mars as part of the InSight mission. How does studying marsquakes help us understand Earth—and what does it tell us about the possibility of life elsewhere?

HT:  This is one of the most exciting frontiers in seismology—we’re now truly planetary seismologists, not just Earth scientists. The NASA’s InSight mission, which landed on Mars in 2018 with a seismograph, is doing for Mars what thousands of seismographs have done for Earth: revealing its hidden interior.

But here’s the fascinating thing: studying Mars actually helps us understand Earth better because Mars is, in some ways, a simpler planet. It’s smaller, it doesn’t have plate tectonics as we know them, and its geological history has been less turbulent than Earth’s. The early Solar System history—those first few tens of millions of years when planets were differentiating into cores, mantles, and crusts—is still relatively preserved inside Mars. On Earth, four billion years of plate tectonics, erosion, and recycling have erased most of those early traces.

When InSight detected its first marsquake, it was a profound moment. We’re listening to another world’s interior! The seismic waves traveling through Mars tell us about its core size, whether that core is liquid or solid, the thickness of its crust, and the structure of its mantle. From gravity measurements alone, we could only estimate Mars’s core depth to within 600 kilometers of uncertainty. With seismology, we can pinpoint it to within a few or at least a few tens of kilometers.

Now, about life: Here’s where it gets really interesting. On Earth, plate tectonics plays a crucial role in making our planet habitable. It recycles carbon dioxide—pulling it down into the mantle at subduction zones and releasing it back through volcanoes. This carbon cycle regulates Earth’s climate and has prevented us from becoming a runaway greenhouse like Venus. Plate tectonics also creates diverse habitats, releases nutrients into the ocean, and may have been essential for the origin of life.

Mars doesn’t have plate tectonics today. Its interior has cooled more than Earth’s because it’s smaller. But did it ever have plate tectonics? Could there have been a window of time when Mars was warm enough, dynamic enough, to support life? These are questions that seismology can help answer by revealing the thermal and compositional structure of Mars’s interior.

The bigger philosophical question is: Does life require plate tectonics? Or could life adapt to different planetary dynamics? We’ve evolved on a tectonically active world, so earthquakes and volcanoes are part of our planet’s life-giving system. Understanding whether other planets had or have similar dynamics helps us think about where else in the universe life might exist.

The book’s title is When Worlds Quake —plural. You’re clearly thinking beyond Earth. What’s next for planetary seismology?

HT:  We’re living in a golden age for planetary seismology, and it’s only going to get more exciting. When I was a student, planetary seismology meant the Apollo lunar seismometers from the 1970s, and that was it. Now we have records from seismographs on Mars, plans for new lunar missions (Australia is going to the Moon, too), and serious proposals for seismology on Venus, Europa, Titan—even the Sun!

The Moon is particularly interesting because we’re planning to return, and this time we want to stay. NASA’s Artemis program envisions a sustainable human presence on the lunar surface, and if you’re going to build habitats there, you need to understand moonquakes. The Apollo seismometers revealed that the Moon experiences deep moonquakes triggered by tidal forces from Earth, plus impacts from meteoroids, plus shallow moonquakes that can be quite strong. We need better seismic networks to map lunar structure and assess seismic hazards.

Venus is the real frontier. It’s Earth’s twin in size, but it’s a hellish world with surface temperatures hot enough to melt lead and atmospheric pressure 90 times greater than Earth’s. No spacecraft has survived more than a couple of hours on its surface. But if we could get seismographs there—and engineers are working on electronics that could withstand those conditions—we could answer profound questions: Does Venus have an inner core like Earth? Is its mantle still convecting? Did Venus once have plate tectonics and water oceans before the runaway greenhouse effect?

Ocean worlds like Europa and Enceladus are another target. These moons have subsurface oceans beneath icy shells, and understanding the thickness of that ice and the depth of those oceans requires seismology. Imagine dropping a seismograph on the ice of Europa and listening to “icequakes” and the sloshing of a subsurface ocean!

What excites me most is that each world teaches us something new. The Moon taught us that small bodies can still be seismically active. Mars is teaching us about planetary evolution without plate tectonics. Future missions will reveal how common or rare Earth’s particular style of planetary dynamics really is. And every time we listen to a new world quake, we’re asking the same fundamental question: How do planets work?

After reading this book, what do you hope readers will understand differently about earthquakes—and about Earth itself?

HT:  I hope readers come away with a sense of earthquakes not as random disasters, but as windows into the profound processes that make Earth the dynamic, living planet it is. Every earthquake—even a devastating one—is also a gift to science. Those seismic waves racing through the Earth’s interior illuminate structures we could never see otherwise. We’ve essentially turned the entire planet into a CAT scan machine, using earthquakes as our X-rays.

But more than that, I want readers to appreciate the deep connections between Earth’s interior dynamics and life on the surface. The heat from planetary accretion 4.5 billion years ago—the collisions of countless planetesimals that built our planet—that heat is still driving convection in the mantle, moving tectonic plates, and generating our magnetic field. Without those early violent collisions, we wouldn’t have the internal heat engine that powers plate tectonics. Without plate tectonics, we wouldn’t have carbon recycling. Without carbon recycling, we’d be a dead world like Venus.

So when you feel an earthquake, yes, it’s frightening. Yes, it can be destructive. But it’s also a reminder that you’re standing on a living planet, one that’s still cooling from its violent birth, still convecting, still evolving. The same forces that occasionally cause the ground to shake are the forces that gave us breathable air, liquid water oceans, and the conditions for life to flourish.

I also hope readers gain an appreciation for the scientists—past and present—who’ve dedicated their lives to understanding our planet. From Mohorovičić painstakingly analyzing seismograms by candlelight in 1909 Zagreb, to Inge Lehmann discovering the solid inner core in 1936, to today’s researchers deploying seismographs at the bottom of the Southern Ocean or on the surface of Mars—we’re all part of this grand human project of trying to understand our place in the universe.

And finally, I hope readers understand that this work isn’t finished. There are still profound mysteries: Why does the inner core rotate differently from the rest of the planet? What’s happening at the base of the mantle where ancient subducted slabs accumulate? How common is plate tectonics in the universe? These are questions that future generations of geophysicists and planetary scientists will answer, perhaps inspired by books like this one.

The Earth is still quaking, still revealing its secrets, and we’re still listening. That sense of ongoing discovery, of new worlds to explore both beneath our feet and across the solar system—that’s what drives me, and I hope it comes through in every page of this book.

Hrvoje Tkalčić is a professor and head of Geophysics at the Australian National University, where he is director of the Warramunga Seismic and Infrasound Monitoring Facility. Recipient of the Price Medal of the Royal Astronomical Society, he is a fellow of the Australian Academy of Science, the American Geophysical Union, and the Australian Laureate Fellow, recognized for his fundamental contributions to the study of Earth’s interior. His books include The Earth’s Inner Core . When Worlds Quake: The Quest to Understand the Interior of Earth and Beyond will be published by Princeton University Press on January 13, 2026.