# 6.8: Earthquake Prediction

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Scientists are a long way from being able to predict earthquakes. A good prediction must be accurate as to where an earthquake will occur, when it will occur, and at what magnitude it will be so that people can evacuate. An unnecessary evacuation is expensive and causes people not to believe authorities the next time an evacuation is ordered.

Where an earthquake will occur is the easiest feature to predict. Scientists know that earthquakes take place at plate boundaries and tend to happen where they’ve occurred before. Earthquake-prone communities should always be prepared for an earthquake. These communities can implement building codes to make structures earthquake safe.

When an earthquake will occur is much more difficult to predict. Since stress on a fault builds up at the same rate over time, earthquakes should occur at regular intervals. But so far scientists cannot predict when quakes will occur even to within a few years. Signs sometimes come before a large earthquake. Small quakes, called foreshocks, sometimes occur a few seconds to a few weeks before a major quake. However, many earthquakes do not have foreshocks and small earthquakes are not necessarily followed by a large earthquake. Often, the rocks around a fault will dilate as microfractures form.Ground tilting, caused by the buildup of stress in the rocks, may precede a large earthquake, but not always. Water levels in wells fluctuate as water moves into or out of fractures before an earthquake. This is also an uncertain predictor of large earthquakes. The relative arrival times of P-waves and S-waves also decreases just before an earthquake occurs.

Folklore tells of animals behaving erratically just before an earthquake. Mostly these anecdotes are told after the earthquake. If indeed animals sense danger from earthquakes or tsunami, scientists do not know what it is they could be sensing, but they would like to find out.

The geology of California underlies the state’s wealth of natural resources as well as its natural hazards. This video explores the enormous diversity of California’s geology:

A YouTube element has been excluded from this version of the text. You can view it online here: http://pb.libretexts.org/pg/?p=116

## Damage from Earthquakes

Earthquakes are natural disasters that cause enormous amounts of damage, second only to hurricanes. Earthquake-safe construction techniques, securing heavy objects, and preparing an emergency kit are among the precautions people can take to minimize damage.Earthquakes kill people and cause property damage. However, the ground shaking almost never kills people, and the ground does not swallow someone up. The damage depends somewhat on the earthquake size but mostly on the quality of structures. Structures falling on people injure and kill them. More damage is done and more people are killed by the fires that follow an earthquake than the earthquake itself.

There are a variety of ways earthquakes become deadly. Probably the greatest influence is population density. The magnitude 9.2 Great Alaska Earthquake, near Anchorage, of 1964 resulted in only 131 deaths. At the time few people lived in the area. Oddly enough, size doesn’t matter. Only about 2,000 people died in the 1960 Great Chilean earthquake, the largest earthquake ever recorded. The Indian Ocean earthquake of 2004 was one of the largest ever, but most of the 230,000 fatalities were caused by the tsunami, not the earthquake itself.Ground type is an important factor in earthquake damage. Solid bedrock vibrates less than soft sediments so there is less damage on bedrock. Sediments that are saturated with water under go liquefaction and become like quicksand. This will have dangerous implications for Salt Lake City and the Wasatch Front in Utah. Much of the Wasatch Front is loose soil, left over from the remnants of Lake Bonneville. Along the middle of the two mountain ranges, between the Wasatch Mountains and the Oquirrh Mountains, is the Jordan River which flows from Utah Lake northward to the Great Salt Lake. Near the river and surrounding floodplain, the water table is near the surface. When the ground shakes, the water near the surface shifts upward and over-saturates the soil making it extremely unstable. Earthquake effects on buildings are seen in this animation.

## Earthquake-Safe Structures

Construction is a large factor in what happens during an earthquake. For example, many more people died in the 1988 Armenia earthquake where people live in mud houses than in the 1989 earthquake in Loma Prieta. Most buildings in California’s earthquake country are designed to be earthquake safe.The key to earthquake safety are the structures we live in. It is often said that earthquakes don’t kill people, it’s the buildings that kill people. But the reason why not all buildings are not built to withstand earthquakes is cost. More sturdy structures are much more expensive to build, so communities must weigh how great the hazard is, what different building strategies cost, and make an informed decision. But this is the crucial factor in earthquake safety.

Let’s compare Salt Lake City, UT and Port-au-Prince, Haiti. When Salt Lake City has its expected 7.0 magnitude earthquake, worst case scenario is that 3,000–4,000 will die. But on January 12, 2010, Haiti experienced a 7.0 magnitude earthquake that killed nearly 300,000 people. The difference—building codes.

Skyscrapers and other large structures built on soft ground must be anchored to bedrock, even if it lies hundreds of meters below the ground surface. Larger buildings must sway, but not so much that they touch nearby buildings. Counterweights and diagonal steel beams are used to hold down sway. Large buildings can also be placed on rollers so that they move with the ground. Earthquake prone areas should have building codes that require the use of correct building materials. Houses should also be built with wood and steel rather than brick and stone because they need to be able to bend and sway. New buildings should be built on layers of steel and rubber to absorb the shock of the waves. Connections, such as where the walls meet the foundation, must be made strong to withstand the shaking. In a multi-story building, the first story must be well supported.

To make older buildings more earthquake safe, retrofitting with steel or wood can reinforce a building’s structure and its connections. Elevated freeways and bridges can also be retrofitted so that they do not collapse.Fires often cause more damage than the earthquake. Fires start because seismic waves rupture gas and electrical lines, and breaks in water mains make it difficult to fight the fires. Builders zigzag pipes so that they bend and flex when the ground shakes. In San Francisco, water and gas pipelines are separated by valves so that areas can be isolated if one segment breaks.

## Human-Induced Earthquakes

Can humans create earthquakes? Maybe not intentionally, but the answer is yes—and here’s why. If a water reservoir is built on top of an active fault line, the water may actually lubricate the fault and weaken the stress built up within it. This may either create a series of small earthquakes or potentially create a large earthquake. Also the shear weight of the reservoir’s water can weaken the bedrock causing it to fracture. Then the obvious concern is if the dam fails. Earthquakes can also be generated if humans inject other fluids into a fault such as sewage or chemical waste. Finally, nuclear explosions can trigger earthquakes. In fact, one way to determine if a nation has tested a nuclear bomb is by monitoring the earthquakes and energy released by the explosion.