Lecture 8: Earthquakes and Tsunamis: Science and Society Flashcards
How many earthquakes a year?
There are over a million earthquakes each year
What is an earthquake?
Earthquakes are dislocation of land (e.g. plates slide past each other) with energy spreading from focus (hypocentre) along fault plane.
Where do earthquakes occur?
Three large zones:
1) “Ring of fire”
2) Alpide
3) Mid-Atlantic Ridge.
Circum-Pacific seismic belt or “Ring of Fire”
About 90% of the world’s earthquakes and 81% of the world’s largest earthquakes occur along the Ring of Fire (USGS)
Alpide
From Java to Sumatra through the Himalayas, the Mediterranean, and out into the Atlantic
What is an earthquake caused by?
An earthquake is caused by a sudden slip on a fault.
Epicentre directly above focus on surface. Release seismic waves
What is a fault?
A fault is a fracture or zone of fractures between two blocks of rock.
Amplitude
Amplitude is measured in millimeters (mm) from the center of the seismic record to the maximum on the trace.
Large amplitude:
The earthquake epicenter is very close. The earthquake is far away but very large
What is wavelength?
Wavelength is the distance between successive points of equal amplitude
Body Waves
Primary waves
Primary waves (P-waves): compressional waves, travel fast and arrive at seismograph stations first.
Body Waves
Secondary waves
Secondary waves (S-waves): shear waves, S-waves arrive at seismograph sta>ons acer the faster-moving P-waves.
How to measure earthquake size?
“size” of earthquakes: amount of energy released by the event. Measurements: of the amount of energy releases:
1) Intensity: indirect, subjective descriptions, such as how strongly people reacted and the type and extent of building damage.
2) Magnitude: is a quantative measure of earthquake size and is based on the response of seismic instruments. More reliable parameter to use in this effort.
Tsunamis: Seismic sea waves that are caused by a major
displacement of the ocean:
1) earthquakes: the reverse fault is the source of most tsunamis. 15% of all damaging tsunamis were triggered by strike-slip earthquake.
2) volcanic erup>ons
3) submarine landslides
4) meteor impacts
NOAA has taken 3 approaches:
1) Computer models to show worse-case scenarios
2) Education
3) Sophisticated detection system
There are three basic types of faults:
1) Normal: two blocks are pulling away from one another
2) Reverse: two blocks of earth are moving toward each other (tsunamis!)
3) Strike-slip: two block move horizontally opposite to each other
