Earthquakes Chapter 8

Forces Within Earth Stress & Strain Most earthquakes are the result of movement of Earth’s crust produced by plate tectonics Tectonic plates tend to move gradually Over time, stress builds up Stress: the total force acting on crustal rocks per unit area Movement occurs when stress overcomes strength Strain: the deformation of material in response to stress

Types of Stress Compression Tension Shear - Stress towards each other - Stress in opposite directions (object becomes long and skinny) Shear - Top and bottom move in different directions - Causes a material to twist

Earthquakes Shaking of the ground caused by the sudden release of energy stored in the rocks beneath Earth’s surface Rocks can be twisted, squeezed and stretched. They fracture when stress and strain reach a critical point.

Why do earthquakes occur? Elastic rebound theory: Sudden release of progressively stored strain in rocks causes movement along faults Stage 1: Stress acts on a rock Stage 2: Stress causes strain in rock Stage 3: Rock breaks suddenly, releasing energy, and rock moves along fault *Similar to the process of pulling a rubber band and it snapping

Faults Fault: any fracture or system of fractures along which Earth moves A. Dip-Slip Faults Movement up or down parallel to the fault plane Hanging wall and footwall B. Strike-slip faults Movement is horizontal and parallel to the fault surface

A. Dip-slip Fault Hanging Wall: sits above footwall Footwall: below the fault line

A. Dip-Slip Faults 1. Normal Fault 2. Reverse Fault Hanging wall moves down Tensional Stress 2. Reverse Fault Hanging wall moves up Compressional stress

B. Strike-Slip Faults 1. Right Lateral 2. Left Lateral * Use movement of feature to determine left vs. right lateral

Describing Earthquakes Location Focus- center of movement of the earthquakes (occurs underground) Epicenter- point on Earth’s surface directly above the focus (can be a city)

Describing Earthquakes Seismic Waves: waves of energy A. Body waves- travel through Earth’s interior Move through the rocks in the subsurface B. Surface waves- travel along Earth’s surface

A. Body Waves 1. P waves: compressional waves Motion parallel to direction P stands for primary – first to arrive (because they travel the fastest) Speed: 4-7 km/sec Travel through solids or liquids

A. Body Waves 2. S waves: shearing waves Motion transverse to direction S stands for secondary – arrive after P waves Speed: 2-5 km/sec Travel through solids only

B. Surface Waves Surface waves: travel on the surface Cause LOTS of damage Love waves: side to side 2. Rayleigh waves: up and down (similar to the way waves move on water)

Measuring Earthquakes Seismometer: recording device that produces a record of earthquake motion The first seismic wave detected by a seismometer is a P wave Seismogram: output of the seismometer Can calculate strength and location of earthquake Now digitized

Seismogram

Locating Earthquakes Each station calculates a distance radius to earthquake 3 stations are required for an unique epicenter solution Epicenter Location: where all three distances overlap

Locating Earthquakes Depth of focus beneath Earth’s surface can also be determined Shallow focus: 0-70 km Intermediate focus: 70-350 km Deep focus: 350-670 km

Earthquake Distribution Along Plate Boundaries Divergent: Shallow, low magnitude Convergent (Subduction Zones): Shallow to Deep focus, low to high magnitude Transform: Shallow, along faults, low - medium magnitude Most earthquakes are shallow focus (<70km) The deepest earthquakes are associated with subduction convergent plate boundaries (Caused from friction on the plate as its subducted)

Describing “size” of Earthquake Mercalli Index: Earthquake intensity measured by damage (close to epicenter, the intensity is higher) Values I to XII Varies with distance from quake and type of geologic material underlying buildings

Mercalli Index

Describing “size” of an Earthquake Richter Scale: Measure of earthquake magnitude = energy releases How much energy came away from the site of the earthquake Height of “wiggles” on seismogram Values 1 to 10: Logarithmic scale Logarithmic scale- when you move up one value, the energy increase by 10x

Earthquake Prediction Earthquakes cannot be predicted to an exact time or place unless the earthquake has already occurred They MAY be predicted in a time frame and general area Based on change in rock properties Small tremors Water pressure Surface elevation or tilt

Earthquake Prediction Based on patterns of earthquake in space and time Seismic gaps: a segment of an active fault known to produce significant earthquakes that has not slipped in an unusually long time Calculate average time between earthquake and estimate probability of another occurring Historical record analysis

Effects of Earthquakes Ground motion Displacement of land surface Breaks windows, cracks walls Topples buildings Most injuries or fatalities are from falling debris Building design Building location

Effects of Earthquakes Fire – broken gas and water mains Landslides – triggered by ground shaking Liquefaction – water saturated soil turns from solid to liquid when shaken

Tsunami Tsunami: A wave of ocean water generated by an earthquake Seismic sea waves Caused by sudden upward or downward movement of the sea floor Can also be caused by underwater landslides or volcanic eruptions Generally produced by magnitude 8+ earthquake Wavelength: 160 km Speed: up to 700 km/hr (430 mi/hr) Can cross the entire Pacific Ocean in less than a day Waves: 45-90 ft high

In the deep ocean, tsunami waves may appear only a foot or so high As they approach shoreline and enter shallower water, they slow down and begin to grow in energy and height A tsunami’s trough, the low point beneath the wave’s crest, often reaches shore first. When it does, it produces a vacuum effect that sucks coastal water seaward and exposes harbor and sea floors A warning sign of a tsunami

26 December 2004 Earthquake in the Indian Ocean off the coast of Sumatra, Indonesia Magnitude 9.3 Waves up to 30 meters (100 ft) high Over 230,000 causalities One of the deadliest natural disasters in recorded history Earthquake was caused when the Indian Plate was subducted by the Burma Plate