Earthquakes Nelson’s Class 2008

Earthquakes An earthquake is the shaking and vibrating of the earth caused by large and sudden releases of energy that accompany movement, or displacement, of the outermost layer of the earth, called the crust. Energy can be released by stress in the Earth’s crust.

Large earthquakes can occur at depths of 650 to 700 km Prentice Hall defines earthquakes as the shaking and trembling that results from the movement of rock beneath Earth’s surface. Most earthquakes are too small to notice, but a large earthquake can cause changes in the Earth’s surface.

What are some forces that are found in Earth’s Crust? When Earth’s plates move, they create a great force that squeeze or pull the rock found in the crust. These forces are called stress. Stress is a force that acts on rock to change its shape or volume. Volume is the amount of space that the rock takes up. Stress is a force and it adds energy to the rock.

Types of stress: There are three types of stress: Tension Compression Shearing These forces cause rocks to become brittle and snap.

Tension Tension is a push or pull on the crust. It stretches the crust (stretches the rock). The crust becomes thin in the center when this happens. It occurs when two plates are moving apart. Plates can push apart or get pushed apart by melted rock from the core.

Compression Compression squeezes rock until it folds or breaks. One plate pushing against another can compress rock. When plates collide, the edges sometimes crumple up into mountain ranges. One plate can also slide under the other. Plates can get stuck and energy builds until it is released in an earthquake.

Shearing Shearing occurs when stress pushes a mass of rock in two opposite directions. Shearing can cause rock to break and slip apart or change its shape. During shearing two plates are trying to slide by each other. They may rub against each other, or stick and grind and cause energy to build up time and time again.

Faults There are three main kinds of faults: normal, reverse, and strike-slip faults. A fault is a fracture, or break, in Earth’s lithosphere. Most faults are located along tectonic plate boundaries. Faults are classified by how they move.

Normal Fault In a normal fault, the hanging wall slips down relative to the footwall. Stress that pulls rock apart causes a normal fault. Earthquakes along normal faults are common near boundaries where tectonic plates are moving apart, such as in the Great Rift Valley of Africa.

Reverse Fault Along a reverse fault, the block of rock moves up. Stress that presses rocks together causes reverse faults. These faults can occur near collision zone boundaries between plates.

Strike-Slip Fault Blocks of rock move sideways on either side of the fault plane. Stresses that push blocks of rock horizontally cause earthquakes along strike-slip faults. These faults occur where plates scrape past each other.

Over time: Movement of rocks along normal and reverse faults can push up mountains and form deep valleys. As rocks move along strike-slip faults, rocks that were once in continuous layers can become separated by hundreds of kilometers.

Earthquake Vocabulary: Focus: area beneath Earth’s surface where rock that is under stress breaks, triggering an earthquake. Epicenter: the point on the surface directly above the focus. Seismic waves: vibrations that travel through Earth carrying the energy released during an earthquake.

Main Categories of Seismic Waves An earthquake sends out two types of waves from its focus: S-waves, and P-waves. When S-waves and P-waves reach Earth’s surface at the epicenter, surface waves develop.

P-waves Primary Waves or P-waves are the fastest waves. They are the first to reach any location after an earthquake. They travel on an average of 5 kilometers per second (3 mi/s). They can travel through solids, liquids and gases. As they pass through materials they pull and push materials apart. Buildings experience this push and pull as primary waves pass through the ground where they are built.

S-Waves S-waves are the second waves to arrive at any location after an earthquake. They start at the same time as the P-waves but travel at about half the speed as primary waves. As they travel through a material, the materials particles move up and down, or from side to side. S-waves rock small buildings from side to side during an earthquake. S-waves can travel through rock but they can not travel through liquids or gases.

Surface waves Surface waves are seismic waves that move along the Earth’s surface, not through its interior. They make the ground roll iup and down or shake from side to side. Surface waves cause the largest ground movement and the most damage. Surface waves travel more slowly than P-waves or S-waves.

How are seismic waves measured? Seismic waves are measured with a seismograph. Seismograph: an instrument that constantly records ground movements. Separate seismographs are used to measure up- and- down movements and side- to -side movements. The recording produced by a seismograph is called a seismogram.

Locating an Earthquake In order to locate the epicenter of an earthquake, scientists must have seismograms from at least three seismic locations. Scientist find the difference between the arrival times of the P-waves and S-waves of the three stations. The time difference is used to determine the distance of the epicenter from each station. The greater the difference in time, the farther away the epicenter. A circle is drawn around each station, with a radius corresponding to the epicenter’s distance from the station. The point where the three circles meet is the epicenter.

Seismograms Scientist can use seismograms to help them locate the focus of an earthquake. The seismogram records the time when the first primary waves arrives. This wave travels by a direct path.

Magnitude Charles Richter and Beno Gutenberg invented the Richter Scale during the 1930’s. The Richter scale measures an earthquake according to how fast the ground moves at a seismic station. Today most scientists prefer the moment magnitude scale, which is more accurate for larger earthquakes. The moment magnitude scale is based on the total amounts of energy released by an earthquake. Both scales have top values of 10.

Damage from earthquake The Mercalli Scale was developed to rate an earthquake according to the amount of damage in a given location. The Mercalli Scale uses Roman numerals to rank earthquakes by how much damage they caused. Uses a rating of 1 to 12.

Safety The best way to protect yourself is to drop, cover and hold. Stay away from large objects.

Damages caused by earthquakes: Damage caused by earthquakes includes shaking, liquefaction, aftershocks, and tsunamis.

Summary Study your notes. Read sections assigned in the text.