Understanding Earthquakes

To understand earthquakes and volcanic eruptions the fundamental role of tectonic plates needs to be examined. At their points of collision with a neighbor, or when tension or pressure builds up through encounters with obstructions, earthquakes occur and the area affected by the interference is shaken. Waves of different types and strengths are radiated from the shaken location, referred to as the epicenter. The fastest and most widely distributed of these waves are known as “P” waves. They transmit shock waves of alternating compression and dilation that can pass through gases, liquids, and solids and so reach most of the earth at all levels of depth depending on their strengths. There are other, slower types of waves that emanate from the epicenter. One of them, known as an “L” wave, moves rapidly in and out of the outermost layer of the earth and is responsible for much of the physical damage caused by earthquakes.

The Alaskan earthquake of 1964 was one of the strongest to be recorded since seismograph records began. Its “L” or long waves lasted for five minutes and, since their strength was very high, they were felt in places all over the world. Their strength was so high that the instruments at that time were unable to measure the maximum shocks. Later, as evidences of its destructive power were examined in detail, its strength according to the Richter Scale was known to be 9.2. See Appendix 4 for explanations of the measures in use for different types of disaster. Because of the strength of its long waves, the Alaskan quake made the surface of the earth shake up and down like the ringing of a bell. Three thousand miles away, in the Great Lakes on the U.S./Canada border, the same long waves set up several series of surface waves known as seiches and, on the other side of the globe in South Africa, there were seiches in lakes and on the ocean. The same long waves lifted up the surface of the surrounding earth surface by more than forty feet and pulled it down ten feet below its normal level. This is still regarded as the greatest deformation ever known from an earthquake.

A powerful tsunami emanated from the Alaskan earthquake. This is a phenomenon commonly associated with earthquakes and will be described in more detail in the next section of this introduction. The tsunami resulting from the 1964 earthquake carried wave heights greater than thirty feet and they swept down the west coasts of Canada and the United States all the way to San Diego, causing damage wherever they went. In Crescent City, California, a location that always experiences some destruction whenever a tsunami reaches northern California, over thirty city blocks were flooded. In Canada, at the head of an ocean inlet leading to the city of Port Alberni, eighty miles from the sea, a small subdivision was destroyed. All along the coastal area of southern Alaska, landslides and land subsidence occurred in places where the underlying soil happened to be clay. Additionally, liquefaction happened in areas close to the epicenter. All three of these surface disruptions—landslides, land subsidence, and liquefaction—are common concomitants of earthquakes.

The first two of these three disruptions, landslides and land subsidence, are familiar because they occur in small ways wherever there are steep slopes and the danger of their recurrence can be anticipated by putting up protective barriers. Liquefaction is not so easy to anticipate because it occurs on flat ground and most frequently on shorelines that are presently above the elevation of high water. During an earthquake, the long waves vibrate the surface, create spaces among the particles of hardened soil, and allow water to seep in and change the surface to a muddy surface that cannot hold up buildings. Again and again, as will be found in accounts of earthquakes, developers of land for residential or industrial uses discovered that these shorelines are low cost flat land that needs no clearing and so they have built on them, often in violation of building regulations. Such developments are low-cost because they are built on dangerous locations. Sometimes, as in the case of the San Francisco earthquake of 1906, in which liquefaction destroyed areas that were close to the water, those same areas were rebuilt in later years, only to be destroyed again by liquefaction in a subsequent earthquake.

In certain earthquakes, landslides have proved to be extraordinarily destructive forces. In the case of the greatest earthquake of all time, as far as loss of human life is concerned, the Shaanxi Province earthquake of 1556 in China, the long waves of the earthquake encountered one of the lightest types of soil found anywhere on earth, those of the loess regions of northwest China. They were deep deposits, often hundreds of feet thick, easy to demolish and, in modern times, their destruction would never have led to the death toll of 1556—more than 800,000. But, in that time, loess soil, because it was dry and easy to excavate, provided low cost homes for the people who lived and farmed there. People dug out loess caves and they proved to be warm in the cold winters and cool in the hot summers. When the long waves created high, massive landslides of loess, hundreds of thousands of Chinese were smothered or crushed before they could escape. A similar story of landslide power related to an underwater earthquake in Unimak, Alaska, in 1946, from which a massive landslide was triggered. The amount of seawater displaced by the landslide gave rise to one of the biggest tsunamis ever experienced in the Pacific Ocean. It destroyed local areas of Alaska and swept across the Pacific to do enormous damage in Hawaii.

The vast majority of the half-million earthquakes that occur every year are located at the margins of tectonic plates. However, as will be seen from two events, one in the interior of the United States and the other in Siberia, some very powerful earthquakes occur in the middle of continents, far from the margins of any tectonic plate. The causes of these interior earthquakes relate to the fault lines that develop at weak places underground as continents and ocean floors together continue to move across the surface of the earth. Tensions build up over time and, since the rate of movement in the interior of a continent is slower than on the ocean floor, it takes a long time for stresses to build up to a crisis. Something finally gives way at the place of tension and there is an earthquake. The quake is strong because it has built up for a long time and the local population is unprepared for it because they have lived through a long quiescent period with no evidence to indicate an impending quake.

Every year, on average, earthquakes kill thousands of people and cause damage that adds up to hundreds of millions of dollars. In one fifteen-year period toward the end of the twentieth century, half a billion people lost their lives from earthquakes. Most of these deaths occurred in countries that are less developed than North America or Northwest Europe. Why is this? One explanation may be the type of surface layers of rock beneath the cities or rural areas hit. If there are numerous faults as was the case in the Guatemala City earthquake of 1976 the long waves will and they did shatter the surface rocks, with the result that a very large number of people died and damage was enormous and widespread. Additionally, in Guatemala City, and this was also true in many other places in Latin American earthquakes, the typical adobe-type homes collapse easily and people are killed by falling buildings. Engineers who work in earthquake zones repeatedly point out that buildings, not earthquakes, kill people, and they urge authorities to push for safer building codes.

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