Earthquakes I-880, Oakland, CA (October 1989)

Magnitude 5+ earthquakes

Earthquake Basics The elastic rebound theory is an explanation for how energy is spread during earthquakes.

Normal fault

Earthquake Basics  Elastic Rebound Theory: –Due to plate motion, rocks on each side of a fault are moving slowly. If the fault is “locked”, stress in the rock increases. Once they are stressed past a certain point, they fracture at their weakest point and spring back to their original shape (rebound) –Aftershocks—this release of energy often puts stress on other rocks and cause them to break and rebound too

Earthquake focus and epicenter

Types of Earthquake Waves  P (primary) waves –Compressional (push-pull) –Fastest waves (first to arrive at seismograph) –Move through all materials

Evidence of P-wave motion

Types of Earthquake Waves  S (secondary) waves –Move perpendicular to P wave –Second fastest waves (next to arrive at seismograph) –Move ONLY through solids

Types of Earthquake Waves  Surface waves –Rolling motion –Slowest waves (arrive last at seismograph) –Cause most damage

Seismometer

A seismogram records wave amplitude vs. time

Locating an Epicenter Locating an Epicenter 1.View Seismograms 2.Measure P and S wave arrivals 3.(cont’d on next slides)

PS

Locating an Epicenter 1.View Seismograms 2.Measure P and S wave arrivals 3.Measure S-wave “lag” time ( = S - P) 4.Use Travel-Time graph to correlate distance.

14.7 Locating an Epicenter 1.View Seismograms 2.Measure P and S wave arrivals 3.Measure S-wave “lag” time ( = S - P) 4.Use Travel-Time graph to correlate distance. 5.Triangulate Distances

PRACTICE TIME!!

Seismogram

Shadow Zone  P-waves reflect between  P-waves refract between  S-waves only reflect  Proves outer core is LIQUID!

Measuring Strength  Earthquake Magnitude Charles Richter designed first scaleCharles Richter designed first scale Based on height of the largest seismic waveBased on height of the largest seismic wave Every 1 increase in magnitude = 10x increase in strengthEvery 1 increase in magnitude = 10x increase in strength

Measuring Intensity  Earthquake Intensity A measure of earthquake shaking based on the amount of damageA measure of earthquake shaking based on the amount of damage Most often measured by the Modified Mercalli Intensity Scale (I-XII)Most often measured by the Modified Mercalli Intensity Scale (I-XII) Human basedHuman based Why needed?Why needed? Example: Central Michigan Quake 9/2/94Example: Central Michigan Quake 9/2/94

Earthquake Destruction  Earthquake can trigger Volcanic EruptionsVolcanic Eruptions Tsunami (seismic sea waves)Tsunami (seismic sea waves) Mass MovementsMass Movements Liquefaction of the groundLiquefaction of the ground Saturated material turns fluidSaturated material turns fluid Underground objects may float to surfaceUnderground objects may float to surface FiresFires

Earthquake Damage: Liquifaction

Tsunami Formation

Tsunami travel time (hours; simulation) NOAA

Dec Sumatra Tsunami

Current Seismic Events  Earthquakes  Tsunami

References Used   ml ml ml  29/web_pages/los_gatos.html 29/web_pages/los_gatos.html 29/web_pages/los_gatos.html   