HOW AND WHERE EARTHQUAKES HAPPEN

Why Earthquakes Happen? Rocks along faults pressed tightly together. Stress occurs but friction prevents them from moving – fault is said to be locked. Rocks under stress suddenly shift along a fault Fault – break in body of rock where one block slides relative to another When rocks along a fault move, a sudden release of energy occurs causing movements on the ground...

….This is an Earthquake

FOCUS Area along a fault where the slippage first occurs Focus is beneath the earth’s surface The epicenter is directly above The depth of the focus varies – 90% of continental earthquakes have shallow focus Shallow foci – within 70km from surface Deep foci – subduction zones farther from plate boundaries than shallow quakes

FOCUS DEPTH - 1975-95

FOCUS DEPTH OF EARTHQUAKES IN WASHINGTON 1977-1996

EPICENTER A point on the earth’s surface directly above the focus

Seismic Waves travel outward in all directions from focus What happens when rocks in motion release energy in the form of vibrations? Seismic Waves travel outward in all directions from focus

Seismic Waves Body waves – travel through body of a medium Two types of waves produced from earthquakes : Body waves – travel through body of a medium Surface waves – travel along surface of body rather than through middle

Surface Waves Form from motion along shallow fault or by energy transfer from P and S waves reaching earth’s surface. Slowest moving seismic waves Cause greatest damage at surface Two types : Love and Rayleigh

Love and Rayleigh Waves Love waves – rock moves side-to-side and perpendicular to wave direction Rayleigh waves – move ground in an elliptical rolling motion

Earthquakes and Plate Tectonics Most earthquakes are located at or near tectonic plate boundaries

Convergent Oceanic Environments Plates move toward one another and collide Subduction occurs as denser plate sinks under overriding plate - earthquakes occur Oceanic-oceanic or oceanic-continental

Divergent Oceanic Environments Plates moving away from each other and mid-ocean ridge Spreading motion causes earthquakes

Continental Environments Two continental plates converge, diverge, or move horizontally to one another Rocks build up stress Mountains form and earthquakes occur

MAJOR EARTHQUAKE ZONES Pacific Ring of Fire Mid-ocean ridges Eurasian-Melanesian Mountain Belt

MAJOR EARTHQUAKES AREAS

Fault Zones Intense stress along plate boundaries create fault zones Plates separate, collide, subduct or slide past one another Movement from stress buildup causes major earthquakes

PUGET SOUND FAULT ZONES

Ancient Chinese Method of Studying Earthquakes

Magnitude The measure of the strength of an earthquake, determined by measuring the amount of ground motion.

Richter Scale Used for most of the 20th century. Each number represents a factor of 30 (a 5 earthquake has 30 times the energy of a 4). We now prefer to use the……

Moment Magnitude Based on the size of the area of fault movement, the average distance that the fault blocks move, and the rigidity of the rocks in the fault zone. More accurate for large earthquakes.

Intensity Based on an earthquake’s effects. The Mercalli scale (p. 304) uses Roman numerals I-XII. Depends on magnitude, distance from the epicenter, local geology, earthquake duration, and human infrastructure. Charleston, SC - 1886

I & II Not felt except by a very few. Felt by a few persons on the upper floors of buildings.

III & IV Felt indoors. Hanging objects swing.

V & VI Felt outdoors. Felt by all, many scared and run outdoors.

VII & VIII Difficult to stand. Damage slight in specially designed structures.

IX & X Damage considerable in specially designed structures. Ground cracked, rails bent.

XI & XII Bridges destroyed, broad fissures in ground. Total destruction.

Earthquakes and Society Effects on Humans, Property & Infrastructure How to PREPARE for an Earthquake

MAJOR TSUNAMIS

A rapid drop or rise in the ocean floor Caused by an earthquake whose epicenter is on or below the ocean floor that causing: A rapid drop or rise in the ocean floor OR An underwater landslide that displaces a large volume of water

Much Safer To Be Out In The Open Most injuries result from the collapse of buildings, falling objects, and flying glass.

Other Dangers Include Landslides Fires Explosions (broken electric and gas lines) Floods (from collapsing dams)

Ground Type Is Important Loose soil and rock can vibrate like Jell-O.

KOBE SEISMOGRAPH Top - Station on solid ground Right - Station on water saturated, soft ground

LIQUEFACTION Kobe

Quake Duration A long duration moderate earthquake can cause more damage than a short one at a higher magnitude.

FIRES Kobe

Destruction to Buildings and Property

STRUCTURAL DAMAGE Kobe earthquake

CHILE – 1960 Largest Recorded 9.5

ALASKA- 1964 9.2

BEFORE - Be prepared DURING - Stay calm AFTER - Stay cautious EARTHQUAKE SAFETY People who live near active faults should be ready to follow a few simple earthquake safety rules to help prevent death, injury, and property damage BEFORE - Be prepared DURING - Stay calm AFTER - Stay cautious

Before an Earthquake Be prepared. Keep an adequate supply of food, water, batteries, flashlights and a radio. Prepare an earthquake plan and discuss it with your family. Learn how to turn off the gas, water, and electricity in your home.

During an Earthquake Protect yourself by moving to a safe position. Best way is the “Triangle of Life” (next slide--) Indoors – get under a table or if not possible in a inside door frame away from heavy furniture Outdoors – get away from tall buildings, tunnels, power lines or bridges out in the open

Triangle of Life during a Quake Newest research shows your first best position to get into during a quake is next to and below the level of a heavy object – table or furniture Cover your head in bracing position (like shown on a plane) Any falling debris will hit table or furniture and arc over you and you will be protected

Bracing Under a Table Especially at School

Earthquake Warnings and Forecasts Scientists study past earthquakes to predict where future earthquakes will occur. There is currently no way to predict exactly when an earthquake will occur.

Seismic Gaps – Areas along a fault where stress has built up due to a decrease in seismic activity recently but where strong earthquakes occurred in the past

Don’t be Scared, Be Prepared! 3 Days, 3 Ways Earthquake Preparedness Starts NOW! 3days3ways.org Mr. K will show you how starting Tuesday!