Transcript Slide 1
• Chapter 10 Objectives 1. Discuss the elastic rebound theory. 2. Explain why earthquakes generally occur at plate boundaries. 3. Compare the 3 types of seismic waves. 4. Discuss the method scientists use to pinpoint an Earthquake (EQ). 5. Discuss the method most commonly used to measure the magnitude of EQs. Earthquakes & Plate Tectonics • Earthquakes (EQs): vibrations of the Earth’s crust – Occur when rocks under stress suddenly shift • Elastic Rebound Theory: the theory geologists use to explain many EQs – Rocks on each side of fault are moving – If fault is locked, stress increases – At certain point rocks fracture & rebound • Focus: the area along a fault where slippage 1st occurs. – Depths vary • Epicenter: the point on the Earth’s surface directly above the focus. • Aftershocks: series of smaller tremors. – Caused by released energy (seismic waves) • Increases stress in other rocks along the fault • Focus depths vary – Shallow focus - w/in 70 km of surface • Tend to cause the most damage • Focus depths vary – Intermediate focus - 70 to 300 km • Focus depths vary – Deep focus (occur in subduction zone) • 300 to 650 km • Focus depths vary – About 90% of Continental EQs are shallow focus 3 Major EQ Zones • Links btwn EQs & Plate Tectonics – Plate movements = stress buildup • 1. Pacific Ring of Fire: ring of tectonic/volcanic activity surrounding the pacific. • 2. Mid-ocean ridges: spreading motion = stress • 3. Eurasian-Melanesian mountain belt: Eurasian plate colliding w/African & Indian plates = stress • Fault Zones: are groups of interconnected faults. – Form at plate boundaries b/c of intense stress • e.g. San Andreas fault • Not all EQs occur at plate boundaries! – e.g. New Madrid, Missouri, 1812 • Ancient fault? (600 million yrs ago) 3 major Types of Seismic Waves • P waves: primary wave. – Move fastest – 1st to be recorded – Travels through solids & liquids • More rigid matl. = faster 3 major Types of Seismic Waves • S waves: secondary waves. – 2nd waves to be recorded – Only travel through solids 3 major Types of Seismic Waves • L waves: surface waves, also called long waves. – Converted from P & S waves when they reach the surface – Slowest-moving waves • Last to be recorded – Cause surface to rise & fall Recording EQs • Seismograph: instrument used to detect and record seismic waves. – 3 separate devices • 1 records vert. motion • 2 record horz. motion – One N to S – One E to W • Seismogram: seismograph’s record of waves; has wavy lines recorded on paper. Locating an EQ • Scientists compare time diff. btwn P & S waves. – Plot on graph to get distance to epicenter Locating an EQ • Need 3 plots to locate epicenter – Point where all 3 circles meet is location of epicenter Blue primary waves followed by red secondary waves move outward in concentric circles from the epicenter of an earthquake off British Columbia and Washington State. EQ Measurement • Richter Scale: used to express magnitude of an EQ. – Measure of energy released EQ Measurement • Richter Scale: used to express magnitude of an EQ. – Measure of energy released – Major EQ = 7 or above • Causes widespread damage EQ Measurement • Richter Scale: used to express magnitude of an EQ. – Measure of energy released – Moderate EQ = 6 to 7 EQ Measurement • Richter Scale: used to express magnitude of an EQ. – Measure of energy released – Minor EQ = 2.5 to 6 EQ Measurement • Richter Scale: used to express magnitude of an EQ. – Measure of energy released – Microquakes = < 2.5 • Not felt by people EQ Measurement • Mercalli Scale: expresses the intensity of an EQ. • Intensity: the amount of damage the EQ causes. – Roman numerals from I to XII Seismic Monitor EARTHQUAKE DAMAGE Movement of the ground itself seldom causes many deaths or injuries. Most injuries result from the collapse of the buildings, flying glass and falling objects. EARTHQUAKE DAMAGE EQs and under water landslides can cause huge ocean waves called tsunamis. Tsunamis can cause massive death and damages.