# EARTHQUAKES.

## Presentation on theme: "EARTHQUAKES."— Presentation transcript:

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 Scale Measures 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 movement immobile area = seismic gaps (fault is locked) Future earthquake locations

Evidence of Pending Earthquake
Slight tilting of ground just prior to EQ Increase in strain in rocks may inject water in cracks to reduce strain Change in electrical & magnetic properties Increased natural gas seepage Decrease in speed of P waves from distant earthquakes

10 things you didin't know about earthquakes