Determining time and distance • Focus – the point deep within Earth’s crust where the break occurs and seismic waves are released • Epicenter – The point on the surface directly above the focus • Seismograph machines are located around the world at seismic reporting stations. • Seismogram is the paper or digital record of Earth motions
Locating Earthquakes • Distance of a reporting station from the epicenter is calculated by the difference in arrival times between the p-waves and S-waves. • It takes data from three reporting stations to locate the epicenter of an earthquake. • One locates a circle; two locates two points on the circle; the third identifies which of the two points is the epicenter
Richter Scale • Most commonly used scale to measure magnitude or strength of earthquakes. • Measures ground motion -seismic waves and fault movement • Accurate measure for small or nearby earthquakes – can be adjusted for distant or stronger quakes • Each number increase on scale indicates 10x increase in ground motion • 2 can’t be felt; 5 causes damage at epicenter; 7 causes great widespread damage.