Concepts, Terminology and Sources of Earthquakes

During an earthquake an individual could be thrown out of bed at night, be unable to stand upright and be forced to kneel on the ground, fall down stairs, or even be tossed out of the swimming pool by the violent sloshing of the water. Buildings and infrastructure may be severely damaged or even destroyed altogether.

Figure 1: Parking structure that collapsed during the 1994 Northridge earthquake,
California State University, Northridge Campus.

Figure 2: Inner structure of Earth

There are ways of making structures safer than the current ones. Researchers and the engineering community have mobilized to achieve that goal, working on removing shortcomings in the design of structures that have not performed well in seismic events and coming up with improved versions capable of standing up to a certain level of earthquakes.

1. Specific Gravity

The specific gravity of a substance refers to how much heavier than water a unit volume of the substance is. Some specific gravities related to earthquake engineering are as follows:

Figure 3: Discontinuity of seismic waves, named the Mohorovicic discontinuity.

Earth’s crust 2.7–3.0
Mantle (inner periphery) 5.7
Core (periphery) 9.7
Center 12.3

The earth’s crust floats on the surface of the mantle (Figure 2 & 3), which possesses a viscoelastic character. This equilibrium is called isostasy.

2. Orogenic Movements and Crust Convection (Conveyor Belt)

Orogenic movements and crust convection are mainly responsible for mountain building and valley forming—in other words, the constant changes affecting the surface of the earth. In the first half of the twentieth century Alfred Wegener asserted that at one time continents such as Africa and South America were connected and then drifted away from each other. Wegener, who was ridiculed at the time for his continental drift concept, perished on an expedition to the North Pole.

Since then, fossil and geological evidence has substantiated the fact that these continents were once one massive piece. High-technology developments of the 1960s and deep-diving submarines have produced interesting findings about ocean floor fissures and left-and-right movements that, like a giant conveyor belt, have the power of moving continents that float on the viscous mantle. A similar movement at Lake Victoria in Africa is slowly splitting the African continent.

3. Subduction Zones

Figure 4: Subduction process

As the ocean floor exerts pressure on the coastline of the continent, the leading edge of the ocean floor is pushed under the continent, carrying down sea deposits, including the remains of organisms (Figure 4). The matter reaches intensive heat under the continent and produces geothermal irregularities— gases and molten matter that tend to rise to the surface. This subduction process can be seen in the series of active volcanoes along the Pacific shoreline of the American continent from Alaska to Chile, and is responsible for the earthquakes that affected Chile, Colombia, California, and Washington State.

4. Volcanoes

Around 900 years ago the Sunset Crater volcano eruption coupled with strong ground motion caused panic among the native population in what is today Flagstaff and the surrounding areas of Arizona. We can still see the geologically fresh lava flow. In northern California, Mount Shasta bombarded the neighboring region with boulders that scattered for miles, some weighing about 5 tons. The upheaval was accompanied by severe ground motion.

5. Land Erosion

The 1812 New Madrid earthquake in Missouri is considered the largest earthquake in what is regarded a low-seismicity area. What could have caused such an event was the mighty Mississippi constantly eroding the land mass and making the earth’s crust lighter. Since the earth’s crust cannot adjust immediately to the river’s action, from time to time it springs up.

Summary of main sources of Earthquakes:

1. Orogenic movements such as mountain building
2. Subduction and plate convection followed by geothermal and mechanical disturbances
3. Volcanic activity
4. Land erosion

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