Natural Hazards Editors' Vox

Plate Boundaries and Natural Hazards

The editors of a new book on tectonics discuss the origins of the science and its importance in a new millennium.

By and

On the morning of All Saints Day of 1755, a Giant Earthquake struck Lisbon. Most of its population was inside churches celebrating the holy day. Many died due to the collapse of roofs. Candles set fires that would last for days. The ones that escaped fled to the margin of the Tagus River, but a few minutes after the main shock three giant waves raised the river and flooded the city’s downtown area. At the time, Lisbon was the capital of a multi-continental empire, an empire that would start to collapse that day. The socio-economical and physiological implications were enormous. In 1756, the Prussian philosopher Emmanuel Kant wrote three essays about the origin of earthquakes. These essays were largely inspired by the new physics of Isaac Newton and are today considered pioneer works in what we now know as seismology. Kant understood that the earthquake, and the consequent tsunami, was caused by the sudden movement of the seafloor. He also made many other prescient observations. For example, he realized that earthquakes were displayed along linear features that run parallel to major mountain chains and that some of these alignments correspond to the margin of continents while others do not. He went as far as proposing that there was a connection between the source of the 1755 quake and Iceland! Kant was certainly exploring what we know today as plate boundaries.

The fragmentation, rigidity, and mobility of the Earth’s outer shell was already implicit in the ideas of continental drift of Alfred Wegener and Arthur Holmes, as well as in the pioneer works on seafloor spreading by Harry Hess. However, it was only after the confirmation of the Vine–Matthews–Morley hypothesis that the symmetric pattern of seafloor magnetization near the ridges was a result of seafloor spreading that the idea of Earth’s surface mobility was finally accepted. The term “plate” was introduced for the first time in Wilson’s seminal paper of 1965 and subsequently developed by Le Pichon, Morgan, McKenzie, and others. The theory of plate tectonics was born!

Plate Tectonics is the grand unifying theory of the solid earth sciences. It allows describing consistently and within one logical framework many geological processes that were until then perceived as being unrelated. According to the modern conception of plate tectonics, the surface of the Earth is composed of rigid lithospheric plates that incorporate the crust and the upper (strong) portion of the mantle and move coherently relative to one another over the asthenosphere through geological time, such that deformation, seismicity, and volcanism occur at their boundaries. Some of the most destructive natural hazards that occur on Earth—earthquakes, tsunamis and volcanic eruptions—are associated with tectonic plate boundaries. It is, therefore, no coincidence that two of the greatest earthquakes ever recorded, the 1960 MW 9.5 Chile earthquake and the 1964 MW 9.2 Alaska earthquake, occurred at the start of the decade in which the theory of plate tectonics was developed. Today, approximately 40% of the world’s population lives near plate boundaries.

The present millennium has been particularly devastating in terms of plate boundary natural hazards. The MW ~9.1-9.3 Sumatra-Andaman earthquake in 2004, the MW 8.8 Chilean earthquake in 2010, and the MW 9 Tōhoku earthquake in 2011, all with subsequent deadly tsunamis, and the 2010 Haiti earthquake and the 2015 Nepal earthquake all had devastating effects and increased our awareness of the destructive power of natural hazards. In total, half a million people were killed. In 2010, the eruption of the Eyjafjallajkull volcano in Iceland closed the airspace of several European countries to commercial jet traffic for almost 10 days, affecting about 10 million travelers. It was therefore timely to put together a volume dealing with these subjects.

The recent book, Plate Boundaries and Natural Hazards, reviews some of the main concepts associated with tectonic plate boundaries and presents new studies on associated natural hazards. The volume was designed to contain different levels of information and complexity so that it can be used not only by scientists but also by students, policy makers, journalists, and the informed public. Our intention was not to cover all the subjects in the field (that would be impossible) but, instead, provide the reader with insight into what is currently being done.

Although our knowledge of Earth dynamics and plate tectonics has improved enormously, there are still fundamental uncertainties in our understanding of these hazards. Increased understanding is crucial to improve our capacity to predict such natural hazards. Nevertheless, we may have to rely on prevention strategies for the time being. However, we are convinced that more studies and ever-increasing periods of continuous recording and monitoring of the Earth system will allow improvements to be made in natural hazard prediction and mitigation. Progress has been made in predicting volcanic eruptions by monitoring volcano inflation, allowing timely evacuations of the surrounding regions before major eruptions.

Finally, there are still major unsolved problems in the field of plate tectonics waiting to be unraveled. How does mantle convection operate? How do new subduction zones initiate within pristine oceans? What is the rheology of tectonic faults and how does it control the earthquake’s cycle? What are the forces driving and resisting plate tectonics and what are their magnitudes? The theory of plate tectonics has probably just reached its adulthood and the next few decades will surely be exciting times.

João C. Duarte, University of Lisbon, Portugal; email: [email protected]; and Wouter P. Schellart, Monash University, Australia; email: [email protected]