Andrija Mohorovičić, Croatian meteorologist and seismologist (d. 1936)

Andrija Mohorovičić (23 January 1857 – 18 December 1936) was an eminent Croatian geophysicist whose groundbreaking work fundamentally reshaped our understanding of Earth's internal structure. Revered as one of the principal founders of modern seismology, his name is indelibly linked to the discovery of a crucial geological boundary: the eponymous Mohorovičić discontinuity, often simply referred to as the "Moho."

Who was Andrija Mohorovičić?

Born in Volosko, Opatija, then part of the Austro-Hungarian Empire, Mohorovičić embarked on an impressive academic journey. He pursued studies in mathematics and physics at the prestigious University of Prague, laying a robust theoretical foundation for his future endeavors in Earth sciences. His early career included teaching at a nautical school in Bakar, where he developed a keen interest in meteorology, designing instruments and establishing meteorological stations. This early multidisciplinary engagement honed his observational skills, which would prove invaluable in his later seismological research.

In 1892, he joined the University of Zagreb, eventually ascending to the position of professor and director of the Geodynamic Institute. It was here, at the heart of seismological research in Croatia, that Mohorovičić focused his profound intellect on deciphering the mysteries of seismic waves – the vibrations that travel through Earth's interior following an earthquake.

The Discovery of the Mohorovičić Discontinuity (The Moho)

Mohorovičić's most celebrated achievement came in 1909. Following a powerful earthquake in the Kulpa Valley (Pokuplje region) in Croatia on October 8, 1909, he meticulously analyzed seismograph records from various stations. His careful observation revealed a puzzling phenomenon: some seismic waves (specifically P-waves, or primary waves, which are compressional, and S-waves, or secondary waves, which are shear waves) arrived earlier than expected at stations located further from the epicenter, while others arrived later, as if taking a longer, more direct path. This deviation from expected travel times baffled many of his contemporaries.

This anomaly led him to a revolutionary conclusion. He hypothesized that these waves were not traveling through a uniform medium but were instead refracted and reflected at a distinct boundary within Earth's interior. He deduced that there must be a sharp increase in the velocity of seismic waves at a certain depth. This boundary, where the Earth's less dense crust abruptly transitions into the denser underlying mantle, became known globally as the Mohorovičić discontinuity. It effectively marks the base of the crust and the top of the mantle, forming a fundamental division in Earth's structure.

• Location: The Moho's depth varies significantly depending on the geological setting. It lies approximately 5 to 10 kilometers beneath the ocean floor, where the crust is relatively thin. Under continental landmasses, it typically ranges between 30 to 50 kilometers in depth, though it can extend up to 70 kilometers beneath major mountain ranges like the Himalayas, reflecting the thickness of the continental crust.
• Significance: The discovery of the Moho provided the first concrete empirical evidence of a layered Earth, moving beyond mere speculation. It offered a fundamental framework for subsequent studies of planetary structure and was a pivotal moment that transformed theoretical geological models into verifiable scientific facts through quantitative seismological analysis.

Andrija Mohorovičić's Enduring Legacy in Seismology

Beyond the groundbreaking identification of the Moho, Mohorovičić's broader contributions cemented his status as a true pioneer in seismology. He was among the first to correctly interpret the complex travel times of seismic waves to infer subsurface structures, advocating tirelessly for the establishment of a robust global network of seismological stations for more accurate earthquake monitoring and research. His methodologies and insights significantly advanced the nascent field of seismology, moving it from largely descriptive observations to a quantitative, analytical science capable of revealing Earth's hidden complexities.

His meticulous approach to data analysis, his ability to deduce complex internal structures from subtle seismic wave patterns, and his relentless pursuit of empirical evidence set an extraordinarily high standard for future generations of geophysicists. Mohorovičić's work remains a cornerstone of Earth science, continually referenced in studies of plate tectonics, seismic tomography (imaging Earth's interior), and the deep Earth, underscoring his lasting impact on our understanding of the planet.

Frequently Asked Questions About Andrija Mohorovičić and the Moho

What is the Mohorovičić discontinuity?

The Mohorovičić discontinuity, commonly known as the Moho, is the boundary within Earth's interior that separates the crust from the underlying mantle. It is primarily identified by a distinct and rapid increase in the velocity of seismic waves, indicating a significant change in rock density and composition.

How was the Moho discovered?

Andrija Mohorovičić discovered the Moho in 1909 by analyzing seismogram records from the Kulpa Valley earthquake in Croatia. He observed that certain seismic waves arrived at varying times, leading him to hypothesize the existence of an internal boundary where wave velocities abruptly changed, a phenomenon consistent with reflection and refraction.

Why is the Mohorovičić discontinuity important?

The Moho is crucially important because it provided the first direct seismological evidence that Earth has distinct layers, specifically separating the crust from the mantle. This discovery revolutionized our understanding of Earth's internal structure and laid a foundational basis for modern geology and geophysics, allowing scientists to model the Earth's interior with greater accuracy.

Where is the Moho located?

The depth of the Moho varies significantly across the globe. Under the oceans, it is relatively shallow, typically found at depths of 5 to 10 kilometers. Beneath continents, it averages 30 to 50 kilometers in depth, but it can extend much deeper, up to 70 kilometers, under major mountain ranges due to the thicker crust in these areas.