EARTHQUAKES

 

Forces inside the earth’s mantle and near its surface push, stress, and deform rocks. At some point the stress can cause the rocks to suddenly shift or break and produce a transform fault, or fracture in the earth’s crust. When a fault forms, or when there is abrupt movement on an existing fault, energy that has accumulated over time is released in the form of vibrations, called seismic waves, which move in all directions through the surrounding rock. This internal geological process is called an earthquake. 

Most earthquakes occur at the boundaries of tectonic plates, when colliding plates create tremendous pressures in the earth’s crust or when plates slide past one another at transform faults. The place where an earthquake begins, often far below the earth’s surface is called the focus. The earthquake’s epicenter is located on the earth’s surface directly above the focus. The energy of the earth’s tremendous internal stress is released in the form of seismic (shock) waves, which move upward and outward from the earthquake’s focus like ripples in a pool of water. 

Scientists measure the severity of an earthquake by the magnitude of its seismic waves. The magnitude is a measure of ground motion (shaking) caused by the earthquake, as indicated by the amplitude, or size of the seismic waves when they reach a recording instrument, called a seismograph.

Scientists use the Richter scale, on which each unit has an amplitude 10 times greater than the next smaller unit. Thus, a magnitude 5.0 earthquake would result in 10 times more ground shaking than a magnitude 4.0 earthquake and the amount of ground movement from a magnitude 7.0 quake is 100 times greater than that of a magnitude 5.0 quake. Seismologists rate earthquakes as insignificant (less than 4.0 on the Richter scale), minor (4.0–4.9), damaging (5.0–5.9), destructive (6.0–6.9), major (7.0–7.9), and great (over 8.0). The largest recorded earthquake occurred in Chile on May 22, 1960 and measured 9.5 on the Richter scale. Each year, scientists record the magnitude of more than 1 million earthquakes, most of which are too small to feel.

Effects: 

The primary effects of earthquakes include shaking and sometimes a permanent vertical or horizontal displacement of the ground. These effects may have serious consequences for people and for buildings, bridges, freeway overpasses, dams, and pipelines. A major earthquake is a very large rock-and-roll geological event. 

Steps to stay safe: 

• One way to reduce the loss of life and property damage from earthquakes is to examine historical records and make geologic measurements to locate active fault zones. 
• We can then map high-risk areas and establish building codes that regulate the placement and design of buildings in such areas. 
• Then people can evaluate the risk and factor it into their decisions about where to live. 
• Also, engineers know how to make homes, large buildings, bridges, and freeways more earthquake resistant.