The Focus and Epicenter ofan Earthquake
•The point withinEarth where rockunder stress breaksis called the focus
•The point directlyabove the focus onthe surface is theepicenter
Vibrations that travel throughEarth carrying the energyreleased during an earthquake
Energy starts at the focus andmoves outward
Two types:
Body waves
P and S
Surface waves
![MCj02345410000[1]](data/images/img4.png)
Body waves
P or primary/pressurewaves
fastest waves
travel throughsolids, liquids, orgases
Waves compressand expand like anaccordian
S or secondary/shearwaves
slower than Pwaves
travel throughsolids only
Move up and downas well as side toside
Undisturbed Material
P Waves
S Waves
Surface Waves
Travel just below or along the ground’s surface
Slower than body waves
Surface waves are the most damaging and areespecially damaging to buildings.
R waves
L waves
R waves L waves
Seismographsrecord earthquakeevents
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 aseismograph station can be used to calculate the distancefrom the seismograph to the epicenter.
~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 onspreading ridge centers
more than 150,000 quakes strong enough to be felt arerecorded each year
Surface
focus
epicenter
A
B
Which seismic
Station 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 theFocus
•The point directly above the focus on the surface is the Epicenter.
You must have three different seismographs at three different
Locations - then you can pinpoint the different times each picked
up the earthquake wave. A scientist will do this in a lab.
Surface
focus
epicenter
A
B
Three seismographstations are neededto locate theepicenter of anearthquake
A circle where theradius equals thedistance to theepicenter is drawn
The intersection ofthe circles locatesthe epicenter
Time-distance graphshows the averagetravel times for P-and S-waves. Thefarther away aseismograph isfrom the focus ofan earthquake, thelonger the intervalbetween thearrivals of the P-and S- waves
Magnitude: themeasurement ofearthquakestrength based onseismic waves andmovement alongfaults
Mercalli Scale measures the intensity of anearthquake
Richter Scale is a rating of the size of seismicwaves
Moment Magnitude Scale estimates the totalenergy released by an earthquake
Damage in Oakland, CA, 1989
•Building collapse
•Fire
•Tsunami
•Ground failure
Ground Shaking
The earthquake’s amplitude, how long it lasts, anddamage increases in poorly consolidated rocks
Local soil conditions: loose soil shakes more violentlythan the surrounding rock (house built on solid rockwill shake less than a house build on sandy soil)
Liquefaction: earthquakes violent shaking suddenlyturns loose soil into liquid mud
Aftershocks: buildings weakened by an earthquakemay collapse during aftershocks
Tsunamis: earthquakes on the ocean floor cause largewaves that can grow to the height of a six storybuilding
Forecasting when and where earthquakeswill occur and their strength is difficult.
One way seismologist forecast earthquakesin a place is to observe their past strengthand frequency.
Earthquake hazard isa measurement ofhow likely an area isto have damagingearthquakes in thefuture. An area’searthquake-hazardlevel is determinedby past and presentseismic activity.
A seismic gap is an area along a fault whererelatively few earthquakes have occurredrecently but where strong earthquakes areknown to have occurred in the past. Ex:CaliforniaThe gap hypothesis states that sections of activefaults that have had relatively few earthquakesare likely to be the sites of strong earthquakes inthe future.