The immediate aftermath of a great urban earthquake is a dramatic and terrible event, comparable to a massive terrorist attack. Yet the shocking impact soon fades from the public mind and receives surprisingly little attention from historians, unlike wars and human atrocities. In 1923, the Great Kanto earthquake and its subsequent fires demolished most of Tokyo and Yokohama and killed around 140,000 Japanese: a level of devastation and fatalities comparable with the atomic bombing of Hiroshima and Nagasaki in 1945. But the second event has infinitely more resonance in public consciousness and historical studies than the first. Indeed, most people would be challenged to name a single earthquake with an indisputable historical impact, including even the most famous of all earthquakes: the San Francisco earthquake and fire of 1906. In truth, however, great earthquakes, from ancient times—as recorded by Greek and biblical writers—to the present day, have had major cultural, economic, and political consequences—often a combination of all three—some of which were beneficial. Thus, the current prime minister of India owes his election in 2014 to an earthquake that devastated part of his home state of Gujarat in 2001, which led to its striking economic growth. The martial law imposed on Tokyo and Yokohama after the 1923 earthquake gave new authority to the Japanese army, which eventually took over the Japanese government and led Japan to war with China and the world. The destruction of San Francisco in 1906 produced a boom in rebuilding and financial and technological development of the surrounding area on the San Andreas Fault, including what became Silicon Valley. A great earthquake in Venezuela in 1812 was the principal cause of the temporary defeat of its leader Simon Bolivar by the Spanish colonial regime, but his subsequent exile led to his permanent freeing of Bolivia, Colombia, Ecuador, Peru, and Venezuela from Spanish rule. The catastrophic Lisbon earthquake of 1755—as well known in the early 19th century as the 1945 atomic bombings are today—was a pivotal factor in the freeing of Enlightenment science from Catholic religious orthodoxy, as epitomized by Voltaire’s satirical novel Candide, written in response to the earthquake. Even the minor earthquakes in Britain in 1750, the so-called Year of Earthquakes, produced the earliest scientific understanding of earthquakes, published by the Royal Society: the beginning of seismology. The long-term impact of a great earthquake depends on its epicenter, magnitude, and timing—and also on human factors: the political, social, intellectual, religious, and cultural resources specific to a region’s history. Each earthquake-struck society offers its own particular lesson, and yet, taken together, such earth-shattering events have important shared consequences for the history of the world.
Earthquakes involve sudden shear sliding motion between large rock masses across internal contact surfaces called faults. The slip on the fault releases strain energy previously stored in the surrounding rock that accumulated due to frictional resistance to sliding. Most earthquakes are directly caused by plate tectonics, and locate in the cool, brittle rock near Earth’s surface. Events with seismic magnitude measured 8.0 or greater are called great earthquakes and involve slip of from several to tens of meters across faults with lengths from 100 to more than 1,000 kilometers. These huge ruptures tend to occur on or near plate boundaries; the largest are on shallow-dipping plate boundary faults (megathrusts) found in compressional regions called subduction zones, where one tectonic plate is thrusting under another. Some great earthquakes occur within bending or detaching plates as they deform seaward of or below a subduction zone. Yet others occur on plate boundary strike-slip faults where two plates are shearing horizontally past one another, or within deforming plate interiors. Elastic wave energy released during the fault sliding is recorded and studied by seismologists to determine the fault location, orientation and sense of sliding motion, amount of radiated elastic wave energy, and distribution of slip on the fault during the event (co-seismic slip). Geodetic methods measure elastic strain accumulation prior to an earthquake, co-seismic slip, and afterslip on the fault that occurs without earthquakes, along with viscous deformation of the mantle as it responds to the fault offset. Great earthquakes commonly locate under the ocean, and the sudden motion of the seafloor generates tsunami—gravitational water waves that can be recorded with ocean floor pressure sensors (these waves are also used to determine co-seismic slip). As seismic, geodetic. and tsunami modeling methods have progressed over the past 50 years, our understanding of great earthquake rupture processes and earthquake interactions has advanced steadily in the context of plate tectonics and improved understanding of rock friction. All faults have heterogeneous frictional properties inferred from non-uniform sliding during each event, with areas of large slip instabilities called asperities having slip-velocity weakening friction and other areas having slip-velocity strengthening friction that results in stable sliding. The seismic wave shaking and tsunami waves can cause great devastation for humanity, so efforts are made to anticipate future earthquake hazards. As plate tectonics steadily move Earth’s plates, elastic strain around plate boundary faults accumulates and releases in a repeated stick-slip sliding process that causes a limited degree of regularity of faulting. Given the history of prior earthquakes on a given fault, we can identify seismic gaps where future slip events are likely to occur. With geodesy we can also now measure locations of accumulating slip deficit relative to plate motions, as well as variation in seismic coupling, which characterizes the fraction of plate motion accounted for by earthquake failure.