Causes of earthquakes An earthquake is defined as the shaking of the earth’s crust Can be large or small Earthquakes occur when energy is suddenly released Volcanic eruptions Most occur by rocks breaking or moving along a fault Most occur near plate boundaries San Andreas Fault (Pacific and NA plate meet)

Earthquake waves Energy from an earthquake travels through rock in waves P-waves (primary waves) vibrate rock back and forth S-waves (secondary waves) vibrate rocks up and down or side to side Both are inside the earth When P and S waves reach the earth, they become L-waves L-waves (long waves, surface waves) cause the earth to bend and twist Most destructive Measured with the use of a seismograph Pen does not move, paper does, measured in hertz (vibrations)

Locating the epicenter Point inside the earth where the earthquake starts is called the focus Point on the earth’s surface directly above the focus is the epicenter To locate the epicenter, scientists compare seismograph readings from at least three locations Point where the three circles meet is the earthquake’s epicenter

Earthquake strength and its effect The strength of an earthquake is measured on the Richter Scale Measured from 1 to 9 (one weakest and 9 highest) Strongest earthquake ever recorded was 8.9 Earthquakes on the ocean floor can trigger tsunamis Large sea waves

Predicting earthquakes Scientists hope to save lives by learning to predict where and when an earthquake will occur Watch for several signs Sudden drop in the level of well water Bulges in the earth’s surface near a fault Seismic activity with a sudden change in P-waves or an increase in frequency of P or S waves