Journal #60 What are earthquakes? Why do earthquakes occur?

Earthquakes Earthquakes are one of the most destructive natural disasters. Earthquakes- defined as movements of the ground that are caused by a sudden release of energy when rocks along a fault move. Earthquakes usually occur when rocks under stress suddenly shift along a fault. A fault is a break in a body of rock along which one block slides relative to another.

Why Do Earthquakes happen? The rocks along both sides of a fault are commonly pressed together tightly. Friction prevents them from moving past each other. In this immobile state, a fault is said to be locked. Parts of fault remain locked until the stress becomes so great they suddenly move past each other. This slippage causes trembling and vibrations of an earthquake.

Elastic Rebound (pg 295) Earthquakes are a result of elastic rebound. Elastic rebound is the sudden return of elastically deformed rock to its undeformed shape. The rocks on each side of a fault are moving slowly. If the fault is locked, stress in the rocks increases. When the rocks are stress passed a point, they fracture at their weakest point and rebound (spring back) to their original shape.

Anatomy of an Earthquake Focus- where the first motion of an earthquake occurs The point on the Earth’s surface directly above the focus is called the epicenter

Anatomy of an Earthquake Earthquakes that usually cause the most damage usually have a shallow focus. Earthquakes that have a deep focus, most of their energy dissipates before the vibrations reach the surface.

Seismic Waves As rocks along a fault slip into new positions, the rocks release energy in the form of vibrations called Seismic Waves. These waves travel outward in all directions from the focus. Similar to when you drop a stone into a pool of still water. Eathquakes produce 2 main types of waves: Body waves- travel through the body of a medium Surface waves- travel along the surface of a body rather than through the middle.

Body Waves P Waves- (Primary waves or compression waves) Are the fastest seismic waves The first waves of an earthquake to be detected Cause particles of rock to move in a back and forth motion that is parallel to the direction in which the waves are traveling. Move through solids, liquids and gases. The more rigid the material is, the faster the P waves travel through it.

Body Waves S Waves- (Secondary or Shear waves) Second-fastest waves and arrive at detection sites after P waves Cause particles of rocks to move in a side-to-side direction that is perpendicular to the direction in which the waves are traveling S waves can only travel through solids

Surface Waves Surface waves form from motion along a shallow fault when P and S waves reach the surface Although surface waves are the slowest moving waves, they may cause the greatest damage during an earthquake. Love waves- move rock side to side and perpendicular Rayleigh waves- cause the ground to move in an elliptical, rolling motion.

Seismic Waves and Earth’s Interior The composition of the material through which P waves and S waves travel affects the speed and direction of the waves. Example: P waves travel fastest through materials that are very rigid and are not easily compressed. By studying the speed and direction of seismic waves, scientists can learn more about the makeup and structure of the Earth’s Interior.

Shadow Zones Shadow Zones- are locations on Earth’s surface where no body waves from an earthquake can be detected. They exist because the materials that make up Earth’s interior are not uniform in rigidity. When seismic waves travel through materials of differing rigidities, the speed of the waves change direction as they pass through different materials.

Earthquakes and Plate Tectonics Earthquakes are the result of stresses in Earth’s lithosphere. Most earthquakes occur in 3 main tectonic environments These are generally located at or near tectonic plates boundaries, where stress on the rock is greatest.

Convergent-Oceanic Environments At convergent plate boundaries, plates move toward one another and collide. The plate that is denser subducts. As the plates move, the overriding plate scrapes across the tope of the subducting plate, and earthquakes occur. This can occur between continental and oceanic plates

Divergent Oceanic Environments At the divergent plate boundaries that make up the mid ocean ridges, plates are moving away from each other. This spreading motion causes earthquakes along the ocean ridges.

Continental Environments Earthquakes occur at locations where 2 continental plates converge, diverge or move horizontally. The rock surrounding the boundary experiences stress. The stress may cause mountains to form and also causes frequent earthquakes.

Fault Zones Fault zones form at plate boundaries because of the intense stress that results when the plates separate, collide, subduct or slide past each other. Ex: North Anatolian fault zone When enough stress is built up, movement occurs along one or more of the individual faults in the fault zone and sometimes causes major earthquakes.

Earthquakes away from plate boundaries Not all earthquakes result from movement along plate boundaries Some faults were formed millions of years ago within continental crust Ex: New madrid, Missouri in 1811

Your Assignment: Vocabulary Squares Earthquake Elastic rebound Focus Epicenter Body wave Surface wave P wave S wave Shadow zone Fault zone * you do not have to give examples of the ones in bold