Earthquake Waves

Seismic Waves Seismic waves - Waves generated by an earthquake Focus – point in earth where EQ takes place

Types of EQ Waves P wave – primary - push/pull - compression -back and forth motion - fastest wave - travel through solid, liquid, gas

S waves – secondary - shear waves - waves travel at rt angles to wave - second fastest - only travel through solids

Surface waves – cause most of the damage of EQ. -rolling waves like the ocean waves -slowest - produced when seismic waves reach the surface

Epicenter – location where EQ occurs on the earth’s surface - directly above the focus pt.

Measuring seismic waves Seismograph – instrument to measure seismic waves and arrival times of each

http://www.wwnorton.com/college/geo/egeo/flash/8_3.swf

Seismogram seismogram                                                                                                         

Seismograph stations Each type wave reaches the station at a different time 3 or more stations needed to pin point the epicenter of an EQ Intersection of the circles is epicenter

EQ P and S travel time graph Used to find distances from epicenters

Ancient Chinese Seismograph Zhng Heng's seismograph was an urn-like instrument with a central pendulum. An earth tremor would cause the pendulum to loose balance and activate a set of levers inside. Then, one of the eight dragons placed in eight directions outside the urn would release a bronze ball held in its mouth. The ball would fall into the mouth of a toad and give off a sound in the meantime, letting people know when and in which direction an earthquake had occurred.

How waves travel through earth

Shadow Zone

Four seismograph stations receive data from the same earthquake Four seismograph stations receive data from the same earthquake. The table below shows the differences in travel times for the P- and S-waves recorded at each station. Which station is the closest to the epicenter of the earthquake? Station Travel-Time Difference A 4 min 32 sec B 3 min 52 sec C 3 min 10 sec D 4 min 17 sec A B C D

Seismograph station D is 7,700 kilometers from the epicenter Seismograph station D is 7,700 kilometers from the epicenter. If the P-wave arrived at this station at 2:15 p.m., at approximately what time did the earthquake occur? 1:56 p.m. 2:00 p.m. 2:04 p.m. 2:08 p.m. Seismograph station B recorded the arrival of P-waves at 2:10 p.m. and the arrival of S-waves at 2:15 p.m. Approximately how far is station B from the earthquake epicenter? 1400 km 2400 km 3400 km 4400 km

Difference in P & S arrival times

Locating epicenter