Of Cwacians and Eorthequakynges
From the Old English, Cwacian means “to shake or vibrate usually from shock or instability.” In the twelfth century it evolved into the word Quake. Eorthequakynge entered the Old English lexicon to mean “a shaking or trembling of the Earth that is volcanic or tectonic in origin.” In the fourteenth century it evolved into the now familiar Earthquake.
The Earth, however, has been experiencing quakes long before words were devised to describe them in any language — probably soon after it began hurtling around the Sun at 19 miles per second. The earliest record of an earthquake dates from 1831 BC in the Shandong province of China. A more complete record keeping began in 780 BC, during the Zhou dynasty under Emperor You, when a massive quake jammed the source of the Jing, Wei, and Luo rivers. This event was interpreted as a case of the Yin and Yang of the Earth being out of balance, and a sign of the impending collapse of the Zhou dynasty.
The ancients would likely have made the same assessment of climate change today and be right on, although they weren’t about earthquakes. Other beliefs surrounding earthquakes have included the myth of “earthquake weather” when the weather is unusually hot and calm; an invasion of toads as a predictor of an earthquake; the predictability of earthquakes; and invisible monsters that cause houses to creak and shake.
Earthquakes are one of the deadliest natural disasters both to life and property. The deadliest quake killed around 830,000 people in China in 1556. And there are approximately half a million quakes each year all over the planet of which nearly 100,000 can be felt while approximately 100 cause damage. The ever-present danger of earthquakes drove attempts at making sense of the Earth’s innards, which eventually led to our current understanding of earthquakes.
In 1760, John Michell, a British philosopher and clergyman, stated in a groundbreaking essay that earthquakes travel in waves and are caused by shifting masses of rock miles below the Earth’s surface.
In 1833, Charles Lyell, a Scottish Geologist published “Principles of Geology” in which he stated that geologic changes result from slow, continuous processes that have been developing over long periods. This served as an inspiration for Jules Verne’s 1864 classic, “Journey to the center of the Earth,” in which the main characters descend into a volcano in Iceland, travel to the center of the Earth and resurface in southern Italy, encountering strange animals and other adventures along the way. Over a hundred years later scientists would attempt to drill deep holes into the Earth to make sense of its structure.
While interest in the study of earthquakes — seismology — goes far back in time, modern interest dates back to the mid-eighteenth century. It is intertwined with attempts to understand the structure of the Earth.
Seismic waves from large earthquakes pass throughout the Earth. In the nineteenth century, instruments were devised to measure ground motions from seismic waves. Like light refracting through a prism, geologists observed that seismic waves were bent as they passed through the Earth. This advanced our understanding of the structure of the Earth.
Since the density of the material determines the speed at which seismic waves travel, calculations led Emil Weichert, a German geophysicist to suggest in 1898 that the Earth was composed of a mantle surrounding a core of iron. Early in the twentieth century, Richard Oldham, an English seismologist, found that the speed of seismic waves increases deeper into the Earth but only up to a certain depth. Oldham stated that the Earth consists of a solid mantle that increases in stiffness and surrounds a liquid core. In 1936, Inge Lehmann, a Danish seismologist posited that the liquid core had a solid inner core.
The Earth’s structure is thought to consist of an outer crust that ranges in thickness from up to 6 miles under oceans to 40 miles under continents. Under the crust lies the increasingly dense mantle with a thickness of 1800 miles. The boundary between the crust and the mantle is called the Mohorovicic discontinuity in honor of the Croatian seismologist who first postulated it. The liquid inner core extends for nearly 1400 miles under the mantle with a solid core that is nearly 800 miles thick.
The Earth’s Lithosphere refers to the rigid part of the crust and the upper solid mantle. It is also the coolest of Earth’s layers. The Asthenosphere is the viscous part of the mantle below the Lithosphere. Imagine the Earth’s layers to be a hot multi-layered chocolate cake that is cooling. The outer layer that cools fastest and hardens into a crust is the Lithosphere. The hotter and slow moving viscous layer beneath is the Asthenosphere.
In an attempt to physically verify the various postulations about the Earth’s structure, scientists in the US launched project Mohole in 1957 to penetrate the crust under the Pacific Ocean off Mexico. The project failed due to a lack of funds.
In 1962, scientists in the Soviet Union began to dig a hole that today holds the record for being the deepest artificial point on Earth. The Kola Superdeep Borehole was over 7 miles (12.2 kilometers) deep. The project was eventually discontinued due to the heat and pressure at that depth. Had he been alive, Jules Verne would have loved keeping tabs on the progress of the Kola Superdeep Borehole. And the theory of plate tectonics would have given him plenty of writing material.
In order to explain how continents drift over time, in the 1960’s the theory of plate tectonics was formulated and is now widely accepted. In this theory, the Lithosphere is broken up into 7 major and 10 minor slabs or plates. While geologists still don’t know how continents were formed and whether they can be broken apart, they now know that the continental plate is between 80 to 120 miles deep while the oceanic plates are thinner. Tectonic plates move very slowly relative to each other (a few centimeters each year) with oceanic plates sliding below continental plates. Over time stresses build up at plate boundaries due to this motion and become so great that eventually a rupture occurs releasing this stress. And that’s what causes most Cwacians and Eorthequakynges.
