EARTHQUAKES

Earthquakes & Plate Tectonics

• It is a vibration in the crust due to the release of stress

• Causes a shift in the rocks along a fault or plate boundary

• Explained by the Elastic Rebound Theory“Locked” rocks build up stress,

fracture, & slip back to original position• Slipping causes release of energy =

seismic waves

• Focus is the location (within the earth) along the fault where slippage occurs

• Epicenter (what you hear on the news) is the point, on the surface, directly above the focus– Where most damage occurs

• Approximately 90 % of all earthquake foci are at depths of less than 100 km (62 miles)

3 Major Earthquake Zones

• Pacific Ring of Fire• Mid-oceanic ridges• Eurasian-Melanesian mountain belt

EQ occur along plate boundaries

Recording Earthquakes

• Seismographs detect motion of the crust

• Creates series of wavy lines called seismogram

3 Types of Seismic Waves• BODY= a. Primary waves-

move together & apart like accordion (fastest)

b. Secondary waves- move at right angles to direction of wave travel like a snake

• SURFACE= Love & Rayleigh waves- move up & down like ocean waves (slowest) w/ side-to-side component

These cause most damage Earthquakes seismic waves animation

Locating an Earthquake

• To find epicenter of an earthquake, analyze difference between arrival of P & S waves

• P waves travel 1.7 X faster than S waves

If S waves arrive shortly after P waves = close If S waves arrive long after P waves = far away

Locating an Earthquake

• Need seismographic data from 3 different locations

• Distances to epicenter plotted on map & their intersection determines epicenter

Earthquake Measurement• Magnitude expressed by Richter ScaleMeasures energy released by EQ as

measured by the amplitude (height) of the seismic wave increase by 1 = 31.7 times more energy

Ex. EQ = 7 had 31.7 X more energy released than an EQ = 6

• Largest EQ = 9.5, Chile, 1960

Earthquake Measurement

• Mercalli scale expresses intensity or amount of damage done

• Indicated by Roman numerals I to XII

• Ex. II = low intensity described as “Felt only by a few persons at rest, especially on upper floors of buildings”

Earthquake Damage

• Determined by type of ground that building is located on

Loose soil & rock = more damage

• Most injuries from collapsing buildings & flying debris

• May cause landslides, fires, & explosions

EQ occur along plate borders, little “tears” in the crust create faults

One of the world’s

most active = San Andreas Fault System (made of 100s of faults) in California, it separates the N Am. & Pacific plates

800 mi long

10 mi deep

Seismic Activity in the U.S.

Red/orange = areas of high seismic activity

Yellow = medium seismic activity

Blue/white= low seismic activity

Tsunami

• Created when an the epicenter of the earthquake is on the ocean floor

• Displaces (and pushes) a huge volume of water

• Creates massive “tidal waves” of moving water

As a tsunami approaches the shore

Wavelength decreased & wave height increase

December 2004 Tsunami Disaster

Earthquake Detection

• Scientists try to detect changes in the crust by:

instruments used to detect movementimmobile area = seismic gaps(fault is locked)– Future earthquake

locations

Evidence of Pending Earthquake

Slight tilting of ground just prior to EQIncrease in strain in rocks

• may inject water in cracks to reduce strain

Change in electrical & magnetic propertiesIncreased natural gas seepageDecrease in speed of P waves from

distant earthquakes

• 10 things you didin't know about earthquakes