what is an earthquake?
the shaking and trembling that results from the movement of rock beneath earth's surface
what is the focus?
the area beneath earth's surface where rock that is under stress breaks, triggering an earthquake
what is the epicenter?
the point on the surface directly above the focus
what is the motion of seismic waves?
they carry energy from an earthquake away from the focus, through earth's interior, and across the surface
what are the 3 main categories of seismic waves?
P waves, S waves, surface waves
what are P waves?
primary waves are the first ones to arrive; they compress and expand the ground like an accordion; they travel through solids and liquids
what are S waves?
secondary waves come after P waves; these waves shake the ground from side to side and up and down; S waves only move through solids
what are surface waves?
when P and S waves reach the surface they become surface waves; surface waves are the slowest waves but they are very dangerous; they make the ground roll like ocean waves
what are 3 commonly used methods of measuring earthquakes?
mercalli scale, richter scale, moment magnitude scale
what is the mercalli scale?
rate earthquakes according to the level of damage at a given place
what is the richter scale?
rating of an earthquake's magnitude based on the size of the earthquake's seismic waves; works best for small and nearby earthquakes
what is magnitude?
a number based on the earthquake's size
what is a seismograph?
an instrument that records and measures seismic waves
what is a moment magnitude scale?
a rating system that estimates the total energy released by an earthquake
how do geologists locate the epicenter?
by using seismic waves; they measure the difference between the arrival times of the P and S waves. the farther away an earthquake is, the greater the time between the arrival of the waves