Chapter 12: Earthquakes

Section 1: How and Where Earthquakes Happen Earthquakes are defined as movements of ground that are caused by a sudden release of energy when rocks along a fault move.

Why Earthquakes Happen Rocks along a fault are pressed tightly together (the fault is “locked) Eventually the stress will be too great and the rocks slide past each other.

Elastic Rebound Earthquakes are a result of elastic rebound The sudden return of a deformed rock shape to its undeformed original shape.

Anatomy of an Earthquake Focus is the location within the Earth along a fault where the first motion of an earthquake occurs. Epicenter is the point on the Earth’s surface directly above the focus. Earthquakes that cause the most damage have foci less than 70 km from the Earth’s surface.

Seismic Waves Energy is released as seismic waves that move outward from the foci. Earthquakes produce two types of waves. Body waves – waves that travel through the body of a medium. Surface waves- waves that travel along the surface of a body through the middle.

Earthquakes and Plate Tectonics Convergent Ocean Environments Plates move towards each other and collide The denser plate sinks underneath the other one Divergent Ocean Environments The divergent boundaries are pulling away from each other which causes an earthquake Continental Earthquakes As the continental plates interact by colliding, dividing and scraping past each other earthquakes occur.

Fault Zones Fault zones occur at some plate boundaries where there are lots of closely spaced faults

Earthquakes Away from Plate Boundaries Sometimes there are ancient fault zones buried under sedimentary rock in the middle of current tectonic plates. These can activate and cause earthquakes.

12.2 Studying Earthquakes Seismology is the study of earthquakes and seismic waves

Recording Earthquakes Seismograph detects and records vibrations in the ground Three sensory devices measure vertical motion of the ground, east-west motion and north-south motion.

Locating an Earthquake To determine the distance of the epicenter, scientists analyze the arrival time of the P and the S waves. The longer the time between the two the farther away the earthquake occurred.

Earthquake Movement Scientists that study earthquakes are interested in the amount of energy released by an earthquake. Magnitude – is the measure of the strength of the earthquake Richter scale – measures the ground motion from an earthquake to figure out the earthquake’s strength

12.3 Earthquakes and Society Movement of the ground during an earthquake seldom causes deaths or injuries. Injuries occur because of buildings collapsing or objects flying Other dangers include landslides, fires, explosions caused by electric and gas lines, as well as floodwaters from collapsing dams.

Tsunamis Is a giant ocean wave caused when the epicenter is on the ocean floor They form when there is a sudden drop or rise in the ocean floor.

Earthquake Warnings and Forecasts Scientists study past earthquakes to predict where future earthquakes will occur.

Seismic Gaps An area along a fault where relatively few earthquakes have occurred recently but where strong earthquakes occurred in the past.

Foreshocks Some earthquakes are proceeded by little earthquakes. In one case in 1975 geologists recorded foreshocks in a city in China. They evacuated the city and the next day there was a massive earthquake that caused mass destruction.

Changes in Rocks Scientists use a variety of sensors to detect slight tilting of the ground and to identify the strain and cracks in rocks to hope to predict future earthquakes.