EARTHQUAKES & EARTH’S INTERIOR Measuring Earthquakes CHAPTER 8.2

Section 8.2

EARTHQUAKE WAVES SEISMOLOGY The study of earthquake waves The study of earthquake waves Many cultures have attempted to determine the source of earthquakes Many cultures have attempted to determine the source of earthquakes Chinese as far back as 2000 years ago

MEASURING EARTHQUAKE WAVES SEISMOMETER / SEISMOGRAPH - Instrument that records earthquake waves (motions)SEISMOMETER / SEISMOGRAPH - Instrument that records earthquake waves (motions) Instrument attached to bedrock Instrument attached to bedrock Pen suspended above Pen suspended above

MEASURING EARTHQUAKE WAVES SEISMOGRAM - A printed record of ground motionSEISMOGRAM - A printed record of ground motion are traces of amplified, electronically recorded ground motion made by seismographs. (seismos = shake, gramma = what is written) shows all three types of seismic waves - surface waves, P waves, and S waves.shows all three types of seismic waves - surface waves, P waves, and S waves.

Seismic Waves Waves travel through the earth at different speeds depending on the material through which they travel

TYPES OF EARTHQUAKE WAVES SURFACE WAVES SURFACE WAVES seismic waves that travel along Earth’s outer layer seismic waves that travel along Earth’s outer layer One type of surface wave moves the ground from side to side Can damage the foundations of buildings Another type travels along Earth’s surface much like rolling ocean waves Arrow shows the movement of rock as the wave passes

TYPES OF EARTHQUAKE WAVES BODY WAVES can travel through the interior of the Earth can travel through the interior of the Earth P-Waves P-Waves (primary) (primary) S-Waves S-Waves (secondary) (secondary)

P-waves (Primary) Sometimes called Longitudinal or Compression waves Push - pull motion shakes ground back and forth in the same direction and opposite direction as the direction the wave is moving shakes ground back and forth in the same direction and opposite direction as the direction the wave is moving Can Travel through Solids, Liquids, & Gasses P-waves have the greatest velocity (compress) (expand)

S-waves (Secondary) Also called Shear waves Exhibits a Shaking motion shakes ground back and forth right angles, perpendicular, to the direction the wave is moving shakes ground back and forth right angles, perpendicular, to the direction the wave is moving Only travels through Solids Has a slower velocity than P-waves

LOCATING EARTHQUAKES Travel-Time Graph Earthquake Distance: Find time difference between: 1 st p-wave and 1 st S-wave Find time difference between: 1 st p-wave and 1 st S-wave P-wave arrival S-wave arrival

LOCATING EARTHQUAKES Travel-Time Graph Earthquake Distance: Find the time span between the P & S wave curves on the travel-time graph Find the time span between the P & S wave curves on the travel-time graph

LOCATING EARTHQUAKES Earthquake Direction & location:  found using Triangulation  three or more seismographs are used to find the exact location of an earthquake epicenter

LOCATING EARTHQUAKES Triangulation  Three Radial arcs or circles drawn around each station based on distance  intersection is the epicenter

EARTHQUAKE ZONES 95% of all major earthquakes occur in three narrow zones Circum-Pacific Belt - “Ring of Fire” Mediterranean Belt Oceanic-Ridge System Ocean Ridge Systems Closely match up with plate boundaries

MEASURING EARTHQUAKES

Historically: two different types of measurements have been used to describe the size of an earthquake two different types of measurements have been used to describe the size of an earthquake Intensity Intensity measures the strength of shaking produced by the earthquake at a certain location Magnitude Magnitude measures the energy released at the source of the earthquake

RICHTER MAGNITUDE SCALE Measures Magnitude of Earthquake Relative to maximum Amplitude of largest seismic wave Relative to maximum Amplitude of largest seismic wave Largest Earthquakes are above 8.9 Magnitudes < than 2.0 are usually not felt Measures both: Measures both: Amplitude: Amplitude: A Logarithmic,10-fold increase Energy release: Energy release: A ~32-fold increase The problem with this scale: The problem with this scale: Does not adequately estimate the size of very large earthquakes Amplitude

MOMENT MAGNITUDE Moment Magnitude measures total energy released by earthquake Derived from amount of displacement that occurs along the fault zone Derived from amount of displacement that occurs along the fault zone More widely used More widely used more precise than the Richter scale more precise than the Richter scale Measures very large earthquakes Measures very large earthquakes

MOMENT MAGNITUDE & RICHTER SCALE Comparison between the Richter and Moment Magnitude Scales EarthquakeRichter ScaleMoment Magnitude New Madrid, MO, San Francisco, CA Prince William, AK Northridge, CA, COMPARISON

Approximate Magnitude Approximate TNT for Seismic Energy Yield (Release) Example 1480gSmall construction site blast 215kg 3480kg 415 metric tons> Massive Ordnance Air Blast bomb 5480 metric tons 615 kilotons 7480 kilotonsHaiti Earthquake megatonsSan Francisco EQ megatonsTohoku, Japan EQ & Tsunami gigatonsNever recorded teratonsAsteroid impact –Dinosaur Extinction 65 Mya x10 43 tonsStarquake on neutron star

MERCALLI INTENSITY SCALE Measure of Earthquake Intensity Assesses damage at a specific location based on several factors Assesses damage at a specific location based on several factors Earthquake Strength Distance from Epicenter of earthquake Nature of Surface Material – (Geology of the area) Building Materials & Design – (Architectural design)

MEASURING EARTHQUAKES Mercalli Intensity vs. Richter Magnitude

EARTHQUAKE MAGNITUDES

SOME NOTABLE EARTHQUAKES