Earthquakes What is an Earthquake EarthquakeOccur when rocks

Earthquakes

What is an Earthquake? • Earthquake--Occur when rocks under stress shift along a fault (energy released) • Fault—break in rock where one block moves relative to another

Let’s Build a Fault Model!

Damage from Northridge Earthquake in Southern California--1994

Anatomy of an Earthquake • Focus--1 st motion of earthquake in Earth, along fault • Epicenter—point on Earth’s surface directly above focus • 90% of continental earthquakes have shallow focus—most damaging

Parts of an Earthquake

Elastic Rebound — sudden return of deformed rock to undeformed shape Elastic rebound causes earthquakes

Fence offset due to Elastic Rebound

Seismic Waves • When rocks on a fault move, they release energy as seismic waves • Seismic waves travel outward from the focus in all directions

Body wave—seismic wave that travels thru a medium –two types: P-wave & S-wave

Seismic Waves • P (primary) waves are the fastest & travel thru solids, liquids, & gases

Seismic Waves • S (secondary) waves are 2 nd fastest & travel thru solids only. Although slower they can cause the most damage

Seismic Waves Surface wave— a seismic wave that travels along surface of medium. These are S & P waves that reach the surface & can cause greatest damage

Water Wave and Slinky Activities!

WAYS TO MEASURE AN EARTHQUAKE • There are two major scales to measure an earthquake: – Mercalli Scale: measures earthquake intensity (by amount of damage caused) – Richter Scale: measures earthquake magnitude (strength)

Studying Earthquakes • Seismology— study of earthquakes & seismic waves • Seismograph— records vibrations in ground • Seismogram— tracing of EQ motion, recorded by seismograph

Bill Nye on the Richter Scale http: //www. youtube. com/watch? v=1 qbg 7 orb 1 lc Bill Nye on Waves Earthquake recording station/seismograph clip http: //www. youtube. com/watch? v=dx 4 Oq. T 0 PYn. U&lr=1

SEISMOGRAM Order of recording: P-waves, S- waves, then surface waves • Shows S-P lag time

P-Wave S-Wave P-Wave • To find S-P lag time, subtract where the P wave starts from where the S wave starts (S-P = Lag Time)

What is S-P lag time for El Paso, TX? • 28 sec – 0 sec = 28 sec lag time What is S-P lag time for San Francisco, CA? • 120 sec – 0 sec = 120 sec lag time What is S-P lag time for Seattle, WA? • 165 sec – 0 sec = 165 sec lag time Which city is closest to the epicenter? • The city with the shortest lag time is closest to the epicenter, so it’s El Paso, TX

Locating Earthquakes Through Triangulation • S-P lag times are analyzed to find the epicenter of Earthquakes using 3 stations • Why 3 stations? Epicenter • Because all we know is distance, so it takes 3 circles to narrow it down to one point

Let’s stop here and practice finding S P lag time

Fault Zones • Fault zone —region w/ lots of faults that typically form at plate boundaries • Fault zones and earthquakes can occur away from plate boundaries as well

Fault Zones—New Madrid • New Madrid fault line—New Madrid, MO— 120 miles southward on I-55 • 1811/1812—most widely felt series of E. Q. in U. S. history (2000+) • Five were 8. 0+ • Ancient fault zone deep in crust of Mississippi River region

Tsunamis • Tsunami—giant ocean wave that forms after volcanic eruption, submarine earthquake or landslide • May form when ocean floor rises due to earthquake Tsunami animation http: //embc. gov. bc. ca/em/tsunamis/cause s_2. htm

Japan’s Tsunami Before and After…(Google Earth)