Introduction

Abstract

I have first shown that most intermediate and deep earthquakes occur in " subduction zones ", tectonic plate convergence zones where one of the plates (usually oceanic) plunges beneath the other. The hypocentres of these earthquakes are remarkably aligned along inclined planar surfaces that follow the dip of the plunging plate. The frequency of these earthquakes is not uniformly distributed as a function of depth, with a minimum around 300 km, which makes it possible to distinguish two classes of earthquakes: intermediate and deep. The strongest earthquakes occur at depths greater than 600 km and can reach magnitudes greater than Mw 8 (magnitude of " moment ", defined in the lecture). The strongest instrumentally recorded earthquake (Mw 8.3) occurred recently, on 24 May 2013, beneath the Sea of Okhotsk west of the Kamchatka peninsula (Russia). Although generally harmless, some intermediate-depth earthquakes cause significant damage : this is the case in Romania, for example, in the Vrancea area, where earthquakes of magnitude > 7 destroyed part of the city of Bucharest in 1940, and again in 1977.

In this lecture, we took a world tour of subduction zones and associated deep earthquakes, all around the Pacific Ocean, but also in Europe : Sicily-Calabria, along the Hellenic arc, or southern Spain, including " a nest " of deep earthquakes around 650 km depth, and in Central Asia, particularly in the Hindu-Kush region. Intermediate and deep earthquakes not apparently associated with subduction zones, however, occur in zones of " convergence " and are associated with pieces of the Earth's mantle where the temperature is lower than in surrounding regions, as indicated by seismic imaging.

After a few reminders of the two types of seismic waves, their characteristics and the modern way in which they are used to locate and determine the depth of an earthquake, we retraced the history of the discovery of deep earthquakes in the years 1920 and the important role played by Kiyoo Wadati (Japan). He devised an original geometric method for determining the epicenter and depth of earthquakes, and demonstrated the existence of three classes of earthquakes under Japan : superficial, intermediate and deep, a classification retained to this day. He then demonstrated the presence of inclined planes along which non-superficial earthquakes are aligned. These planes are now known as " Wadati-Benioff zones " - the latter, working in the USA, was instrumental in proposing a physical explanation for the existence of these planes - and were observed at the time near the boundary between the ocean and the continent.