Earthquake
Earthquakes have been around for as long as the planet and have plagued humans throughout history. With no warning, major earthquakes strike populated areas of the world every year, killing hundreds, injuring thousands, and causing hundreds of millions of dollars in damage. Yet despite millions of dollars and decades of research, seismologists (scientists who study earthquakes) are still unable to predict precisely when and where an earthquake will happen.
An earthquake is a geological event in which rock masses below the surface of the earth suddenly shift, releasing energy and sending out strong vibrations to the surface. Most earthquakes are caused by movement along a fault line, which is a fracture in the earth's crust. Thousands of earthquakes happen each day around the world, but most are too small to be felt.
Earth is covered by a crust of rock that is broken into numerous plates. The plates float on a layer of molten (liquid) rock within the earth called the mantel. This molten rock moves and flows, and this movement is thought to cause the shifting of the plates. When plates move, they either slide past, bump into, overrun, or pull away from each other. The movement of plates is called plate tectonics. Boundaries between plates are called faults.
Earthquakes can occur when there are any of the four types of movement along a fault. Earthquakes along the San Andreas and Hayward faults in California occur because of two plates sliding past one another. Earthquakes also occur if one plate overruns another. When this happens one plate is pushed under the other plate, as on the western coast of South America, the northwest coast of North America, and in Japan. If plates collide but neither is pushed downwards, as they do crossing Europe and Asia from Spain to Vietnam, earthquakes result as the plates are pushed into each other and are forced upwards, creating high mountain ranges. Many faults at the floor of the ocean are between two plates moving apart. Many earthquakes with centers at the floor of the ocean are caused by this kind of movement.
The relative size of earthquakes is measured by the Richter Scale, which measures the energy an earthquake releases. Each whole number increase in value on the Richter scale indicates a ten-fold increase in the energy released and a thirty–fold increase in ground motion. An earthquake measuring 8 on the Richter scale is ten times more powerful, therefore, than an earthquake with a Richter magnitude of 7. Another scale, called the Mercalli Scale uses observations of damage (such as fallen chimneys) or people's assessments of effects (such as mild or severe ground shaking) to describe the intensity of a quake. The Richter Scale is open-ended, while the Mercalli scale ranges from 1–12.
Catastrophic earthquakes happened just as often in past human history as they do today. Earthquakes shattered stone-walled cities in the ancient world, sometimes hastening the ends of civilizations. Earthquakes destroyed Knossos, Chattusas, and Mycenae, ancient cities in Europe located in tectonically active mountain ranges. Scribes have documented earthquakes in the chronicles of ancient countries. An earthquake is recorded in the Bible in the Book of Zachariah, and the Apostle Paul wrote that he escaped from jail when the building fell apart around him during an earthquake.
Many faults are located in California because two large plates are sliding past each other there. Of the 15 largest recorded earthquakes ever to hit the continental United States, eight have occurred in California, according to the United States Geological Survey (USGS). The San Francisco earthquake of 1906 is perhaps the most famous. It struck on April 4, 1906, killing an estimated 3,000 people, injuring thousands, and causing $524 million in property loss. Many of the casualties and much of the damage resulted from the ensuing fires. This earthquake registered a 7.7 magnitude on the Richter Scale and 11 on the Mercalli Scale. Four other devastating earthquakes have occurred in California in the twentieth century: 1933 in Long Beach, 1971 in the San Fernando Valley, 1989 in the San Francisco Bay area, and 1994 in Los Angeles.
The Long Beach earthquake struck on March 10, 1933, killing 120, injuring hundreds, and causing more than $50 million in property damage. It led to the passage of the state's Field Act, which established strict building code standards designed to make structures better able to withstand strong earthquakes.
Centered about 30 mi (48 km) north of downtown Los Angeles, the San Fernando earthquake killed 65, injured more than 2,000, and caused an estimated $505 million in property damage. The quake hit on February 9, 1971, and registered 6.5 on the Richter Scale and 11 on the Mercalli Scale. Most of the deaths occurred when the Veterans Administration Hospital in San Fernando collapsed.
The Loma Prieta earthquake occurred on October 18, 1989, in the Santa Cruz Mountains about 62 mi (100 km) south of San Francisco. It killed 63, injured 3,757, and caused an estimated $6 billion in property damage, mostly in San Francisco, Oakland, and Santa Cruz. The earthquake was a 6.9 on the Richter Scale and 9 on the Mercalli Scale.
The Northridge earthquake that struck Los Angeles on January 17, 1994, killed 72, injured 11,800, and caused an estimated $40 billion in damage. It registered 6.7 on the Richter Scale and 9 on the Mercalli Scale. It was centered about 30 mi (48 km) northwest of downtown Los Angeles.
In the past 100 years, Alaska has had many more severe earthquakes than California. However, they have occurred in mostly sparsely populated areas, so deaths, injuries, and damage have been light. Of the 15 strongest earthquakes ever recorded in the 50 states, 10 have been in Alaska, with the strongest registering a 9.2 (the second strongest ever recorded in the world) on the Richter Scale and 12 on the Mercalli Scale. It struck the Anchorage area on March 28, 1964, killing 125 (most from a tsunami or tidal wave caused by the earthquake), injuring hundreds, and causing $311 million in property damage.
The strongest earthquake ever recorded in the world registered 9.5 on the Richter scale and 12 on the Mercalli Scale. It occurred on May 22, 1960, and was centered off the coast of Chile. It killed 2,000, injured 3,000, and caused $675 million in property damage. A resulting tsunami caused death, injuries, and significant property damage in Hawaii, Japan, and the West Coast of the United States.
Every major earthquake raises the question of whether scientists will ever be able to predict exactly when and where one will strike. Today, scientists can
only make broad predictions. For example, scientists believe there is at least a 50% chance that a devastating earthquake will strike somewhere along the San Andreas fault within the next 100 years. A more precise prediction is not yet possible. However, scientists in the United States and Japan are working on ways they might be able to make predictions more specific.
Ultra sensitive instruments placed across faults at the surface can measure the slow, almost imperceptible movement of fault blocks. This measurement records the great amount of potential energy stored at the fault boundary. In some areas, small earthquakes called foreshocks that precede a larger earthquake may help seismologists predict the larger earthquake. In other areas where seismologists believe earthquakes should be occurring but are not, this discrepancy between what is expected and what is observed may be used to predict an inevitable large-scale earthquake.
Other instruments measure additional fault-zone phenomena that seem to be related to earthquakes. The rate at which radon gas issues from rocks near faults has been observed to change before an earthquake. The properties of the rocks themselves (such as their ability to conduct electricity) have been observed to change as the tectonic force exerted on them slowly alters the rocks of the fault zone between earthquakes. Unusual animal behavior has been reported before many earthquakes, and research into this phenomenon is a legitimate area of scientific inquiry, even though no definite answers have been found.
Techniques of studying earthquakes from space are also being explored. Scientists have found that ground displacements cause waves in the air that travel into the ionosphere and disturb electron densities. By using the network of satellites and ground stations that are part of the Global Positioning System (GPS), and data about the ionosphere that is already being collected, scientists may better understand the energy released from earthquakes. This may help scientists to predict them.
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