American Scientist

WHEN WORLDS QUAKE: The Quest to Understand the Interior of Earth and Beyond. Hrvoje Tkalčić. 312 pp. Princeton University Press, 2026. $29.95.

Earthquakes are nature’s hidden giants, lying dormant until they awaken to unleash powerful forces that can reshape the land and disrupt human lives. In Japan, to explain the shaking that occurred during earthquakes, people once imagined a giant catfish named Namazu thrashing underground. Namazu was believed to destroy buildings, rupture streets, and spread fear throughout human history. Yet beneath the destructive power of earthquakes lies a quieter, more revealing truth: They also offer a window into the planet’s interior.

In When Worlds Quake: The Quest to Understand the Interior of Earth and Beyond, Australian-Croatian seismologist Hrvoje Tkalčić takes readers on a journey of listening to the whispers and rumbles of the Earth across continents, and of exploring historical events, cultural myths, and stories related to earthquakes. Much like doctors rely on x-rays and CT imaging to see inside the human body, geophysicists have adopted similar methods to image the Earth’s interior: “Sources or transmitters are replaced by earthquakes, x-ray and radio waves by seismic waves, and receivers by seismometers,” Tkalčić writes. These waves reveal what no drill or expedition ever could, tracing hidden structures of the planet’s core and mantle.

Unlike in medical imaging, however, humans cannot control when or where earthquakes occur. Earthquakes follow faults, cracks in the Earth's crust that offset the ground, usually along tectonic plate edges but sometimes far from them. In March 2020, a magnitude-5.5 earthquake that shook the Croatian capital of Zagreb, where Tkalčić’s sister lives, was far from the edges of major tectonic plates. Confused and alarmed, residents sought explanations, so Tkalčić engaged in online conversations with people across Croatia. “I felt a duty to clarify the phenomenon of earthquakes and explain the sensations caused by passing seismic waves in a deeper and more expansive way,” Tkalčić writes. These short social media posts became the early foundation for this accessible, story-driven book.

Tkalčić moves easily between stories, science, and the small wonders that catch his eye. Andrija Mohorovičić, the Croatian geophysicist who discovered the boundary between Earth’s crust and mantle—the Mohorovičić discontinuity, commonly shortened to the Moho—once said, “The task of seismology is to study the interior of Earth and to continue where geology stops . . . ” Tkalčić uses earthquakes as both scientific tools and narrative guides, blending personal anecdotes, history, and physics to reveal the Earth’s hidden interior in a way that is accessible and engaging.

Tkalčić illustrates Mohorovičić’s point when he guides readers through the seismograms of the 2020 earthquake at Zagreb, as recorded at different locations. A seismometer acts like a recorder to capture an earthquake's vibrations, and the seismogram it produces is the resulting graph of the ground’s vibrations. When an earthquake occurs, two waves propagate at different speeds, and the arrival time difference between them tells scientists how far the quake is from each seismic station. By doing this at three or more stations and drawing circles around the stations with those distances on a map, seismologists can use the point where the circles overlap to triangulate the earthquake’s location and depth.

Tkalčić then takes us further, showing the many ways seismologists can use earthquake data to uncover the hidden movements and structures of the Earth. His explanations feel less like lessons and more like walking beside someone who has spent a lifetime listening to quakes and wants you to hear them too. Tkalčić reflects on the 1906 San Francisco earthquake through the anecdotal tale of a dairy cow that supposedly vanished into a fault crack: “The cow Matilda could not have imagined that deep in the rocks under her feet, a drama was unfolding that would permanently change our understanding of earthquakes and shape several scientific disciplines just emerging at the start of the 20th century.” 

Tkalčić also shares how he became an accomplished seismologist. After several postdoctoral positions in California, he moved to Australia, where he established himself as a leader in geophysics. Western Australia can feel almost like another planet—harsh, extreme, and otherworldly. There, he set up seismometers in deserts to record real-time seismic waves, surrounded by seismically active places in all directions, from Indonesia in the northwest to New Zealand in the southeast, an astonishing natural laboratory for geophysics. 

But placing seismographs on land is not enough; to image Earth’s inner core, seismic waves must be recorded nearly on the opposite side of the planet from large earthquakes; for example, an earthquake in Chile needs to be recorded in Southwest Asia. Because approximately 70 percent of the planet is covered by ocean, many of these critical recording locations lie beneath the sea. So Tkalčić and his colleagues set out to close these gaps by venturing into the Southern Ocean aboard research vessels, deploying seismometers on the ocean floor in October 2020. (Reviewer’s note: After a year, the 27 instruments were retrieved, and the data were analyzed and used, mainly for modeling the Macquarie Ridge Complex, a seismically active tectonic plate boundary in the southwest Pacific Ocean. However, the recovery part is not included in the book, since the original Croatian version of this book was written in 2022 and has not been updated. To date, I have not seen studies about the inner core.)

Tkalčić ends the book by leaving Earth behind, examining quakes on other worlds and what they tell us about their interior. When Neil Armstrong first set foot on the Moon, his team installed seismometers that went on to record the first of more than 12,000 moonquakes during the Apollo missions. On Mars, the InSight (Interior Exploration using Seismic Investigations, Geodesy, and Heat Transport) lander carried the first successful seismometer to the Red Planet in 2018, capturing faint signals never recorded before. Just as doctors study the internal organs of many different people to understand how bodies work, seismologists look inside multiple planets. By comparing their interiors, Tkalčić reveals patterns and surprises that put Earth in context and help answer the big questions—ultimately including, “Are we alone in the universe?”

When Worlds Quake offers an engaging and accessible introduction to the science of earthquakes and seismology. The book is more than a text about how seismologists study the Earth’s inner workings, it is also the recounting of the human stories and cultures that are intertwined with these natural events. Tkalčić takes readers on a journey that is as thrilling as it is enlightening.  Perhaps one day, with the tools and insights he describes, scientists may be able to predict when and where the Namazu will thrash once more.