The Focus and Epicenter of an Earthquake

• The point within Earth where rock under stress breaks is called the focus

• The point directly above the focus on the surface is the epicenter

WHAT ARE SEISMIC WAVES?

Vibrations that travel through Earth carrying the energy released during an earthquake

Energy starts at the focus and moves outward

Two types: Body waves

P and S Surface waves

BODY WAVES: P AND S WAVES Body waves

P or primary/pressure waves

fastest waves travel through

solids, liquids, or gases

Waves compress and expand like an accordian

S or secondary/shear waves

slower than P waves

travel through solids only

Move up and down as well as side to side

Undisturbed Material

P Waves

S Waves

SURFACE WAVES:

Surface Waves Travel just below or along the ground’s surface Slower than body waves Surface waves are the most damaging and are

especially damaging to buildings.

R waves

L waves

R waves L waves

Seismographs record earthquake events

HOW IS AN EARTHQUAKE’S EPICENTER LOCATED? Seismic wave behavior

P waves arrive first, then S waves, then surface waves Average speeds for all these waves is known After an earthquake, the difference in arrival times at a

seismograph station can be used to calculate the distance from the seismograph to the epicenter.

WHERE DO EARTHQUAKES OCCUR AND HOW OFTEN?

~80% of all earthquakes occur in the circum-Pacific belt~15% occur in the Mediterranean-Asiatic belt~remaining 5% occur in the interiors of plates and on

spreading ridge centers more than 150,000 quakes strong enough to be felt are

recorded each year

HOW DO YOU LOCATE AN EPICENTER?

Surface

focus

epicenter

A BWhich seismicStation will Receive the Wave first? A or B

The Focus and Epicenter of an Earthquake•The point within Earth where rock under stress breaks is called the Focus •The point directly above the focus on the surface is the Epicenter.

HOW DO YOU LOCATE AN EPICENTER?

You must have three different seismographs at three differentLocations - then you can pinpoint the different times each pickedup the earthquake wave. A scientist will do this in a lab.

Surface

focus

epicenter

A B

HOW IS AN EARTHQUAKE’S EPICENTER LOCATED?

Three seismograph stations are needed to locate the epicenter of an earthquake

A circle where the radius equals the distance to the epicenter is drawn

The intersection of the circles locates the epicenter

HOW IS AN EARTHQUAKE’S EPICENTER LOCATED?

Time-distance graph shows the average travel times for P- and S-waves. The farther away a seismograph is from the focus of an earthquake, the longer the interval between the arrivals of the P- and S- waves

HOW ARE THE SIZE AND STRENGTH OF AN EARTHQUAKE MEASURED?

Magnitude: the measurement of earthquake strength based on seismic waves and movement along faults

THREE WAYS OF MEASURING EARTHQUAKES:

Mercalli Scale measures the intensity of an earthquake

Richter Scale is a rating of the size of seismic waves

Moment Magnitude Scale estimates the total energy released by an earthquake

MERCALLI SCALE

RICHTER SCALE

THE ECONOMICS AND SOCIETAL IMPACTS OF EQS

Damage in Oakland, CA, 1989• Building collapse• Fire• Tsunami• Ground failure

WHAT ARE THE DESTRUCTIVE EFFECTS OF EARTHQUAKES?

Ground Shaking The earthquake’s amplitude, how long it lasts,

and damage increases in poorly consolidated rocks

EARTHQUAKE HAZARDS Local soil conditions: loose soil shakes more

violently than the surrounding rock (house built on solid rock will shake less than a house build on sandy soil)

Liquefaction: earthquakes violent shaking suddenly turns loose soil into liquid mud

Aftershocks: buildings weakened by an earthquake may collapse during aftershocks

Tsunamis: earthquakes on the ocean floor cause large waves that can grow to the height of a six story building

CAN EARTHQUAKES BE PREDICTED?

Forecasting when and where earthquakes will occur and their strength is difficult.

One way seismologist forecast earthquakes in a place is to observe their past strength and frequency.

EARTHQUAKE HAZARD Earthquake hazard

is a measurement of how likely an area is to have damaging earthquakes in the future. An area’s earthquake-hazard level is determined by past and present seismic activity.

FORECASTING EARTHQUAKES A seismic gap is an area along a fault where

relatively few earthquakes have occurred recently but where strong earthquakes are known to have occurred in the past. Ex: California

The gap hypothesis states that sections of active faults that have had relatively few earthquakes are likely to be the sites of strong earthquakes in the future.