1989 San Francisco Earthquake • https://www.youtube.com/watch?v=uQLSkNUDTv8 • What are some of the dangers of an earthquake? • What was the magnitude of the 1989 San Francisco earthquake? • What was the total cost($) of damage? • How far north did the Santa Cruz mountains move?
Earthquakes • An earthquake is a shaking of the Earth, caused by the release of energy in the Earth’s crust. • Caused by the abrupt release of strain that has accumulated over a long time. • When the accumulated energy grows strong enough, the plates break free and shift.
Earthquakes • Typically occur along plate boundaries. • They can also occur away from active plate boundaries. • Scientists and engineers are challenging the assumption that earthquakes must present an uncontrollable and unpredictable hazard to life and property.
Earthquakes • Most EQs are part of a cluster or sequence of EQs. • An “aftershock” is a smaller EQ that occurs after a previous EQ (mainshock). • “Hypocenter” or focus is where the EQ originates (underground) • “Epicenter” is the point on the Earth’s surface that is directly above the hypocenter or focus.
History of Earthquakes • Used to think they were caused by air rushing out of caverns deep in the Earth’s interior. • Earliest described EQ in 1,177BC in China. • EQ mentioned in Europe as early as 580BC • Earliest known EQ in the Americas were in Mexico around 1300 AD.
New Madrid, Missouri 1811-1812 • Most widely felt earthquake in recorded history of North America. • Series of EQs between 1811-1812 • Magnitude 8 the morning of Dec. 16, 1811 • Another great EQ on Jan. 23, 1812 • The strongest EQ on Feb 7, 1812 • Nearly continuous aftershocks between the great EQs and continued for months afterwards
New Madrid, Missouri 1811-1812 • These EQs were felt by people as far away as Boston and Denver. • Very little damage and death because the strongest shock was in a very sparsely populated region.
New Madrid, Missouri 1811-1812 • http://www.sciencechannel.com/tv-shows/what-are-the-odds/videos/what-are-the-odds-earthquake.htm
San Francisco 1906 • One of the most destructive in the US history. • EQ and fire that followed killed nearly 700 people and left the city in ruins.
Where Earthquakes Occur • EQs typically occur at plate boundaries also known as faults. • A fault is a break (or rupture) in the Earth’s crust, along which movement has taken place. • Normal Fault • Reverse Fault • Strike-Slip Fault
Normal Fault • Caused by tension (pulled apart) • Occur primarily at divergent boundaries
Reverse Fault • Caused by compression • Occur primarily at convergent boundaries
Strike-Slip Fault • Caused by shear • Occur primarily at transform boundaries
Earthquakes within plates • Less than 10 percent of all EQs occur within the plate interiors. • Over time (millions of years) weak boundary regions become part of the interiors of plates • Eventually these weak spots can give out and cause an EQ • The New Madrid EQs were within the North American plate.
Seismic Waves • EQs are any vibration from the release of energy into the Earth’s crust. • Most destructive EQs are caused by parts of Earth’s crust snapping into a new position. • Either way the energy is transferred through a seismic wave.
Seismic Waves • Like all waves, seismic waves only transfer energy, not mass. • These waves travel outward from the source of the EQ in all directions. • The speed of the wave depends of the material it is traveling through. • Seismic waves travel faster in denser materials
Seismic Waves • Like seismic waves, sound waves are also just vibrations. • If the seismic wave shakes the air at the right frequency we can hear it • https://www.youtube.com/watch?v=qNf9nzvnd1k • These vibrations cause the entire planet to quiver or ring like a bell
Waves at Boundaries • When a particle hits a boundary it bounces back or is reflected. • Because waves do not have mass, like a particle, they act differently when encountering a boundary
Waves at Boundaries • When a wave hits a boundary some of its energy is reflected or bounces back towards the source, while the rest of its initial energy is transmitted into the new boundary. • Initial = Reflected + Transmitted • Waves also change speed when entering new mediums
Types of Seismic Waves • There are three types of seismic waves: • Primary (P) waves • Secondary (S) waves • Surface waves
Primary Waves • Compression waves • Fastest of the seismic waves, about 25,000mph (but changes in each medium) • Travel through solids and liquids • After an EQ this will be the first shockwave you feel • P waves travel through the Earth
Primary Waves • Refraction causes the P waves to bend as they travel through Earth and its layers • This creates P Wave “Shadow Zone” between 103 and 142 degrees
Secondary Waves • Shear waves • Slower than p-waves, about 15,000mph • They arrive after the p-waves • Do not travel through liquids because liquids do not support shear stresses • S waves travel through the Earth
Secondary Waves • Refraction causes the S waves to bend as they travel through Earth and its layers • This creates SWave “Shadow Zone” for any angle greater than 103 degrees
P vs S wave Demos • Telephone wire demo of p waves and s waves • https://www.youtube.com/watch?v=gjRGIpP-Qfw • I need five sturdy volunteers • P wave in a solid • S wave in a solid • P wave in a liquid • S wave in a liquid
P wave and S wave Questions • What are p waves in a gas called? • Why aren’t s waves transmitted through liquids or gases? • Which type of wave only transfers energy?
Surface Waves • Complex circular motion • Two types: • Rayleigh (R) • Love (L) • These travel along the surface of the Earth • Not through it • Slowest and most destructive of the seismic waves. • They are destructive because of their long deration and large amplitude.
Rayleigh Waves • Surface waves that roll across the surface like ripples in a pond. • Predicted by John William Strutt, Lord Rayleigh • About 90% of the speed of S waves (13,500mph)
Rayleigh Waves • Large EQs may generate Rayleigh waves that travel around the Earth several times before dissipating.
Rayleigh Waves • They’re generated by the interaction of P and S waves at the surface of Earth • They travel slower and arrive at a certain area after the P and S waves
Love Waves • Surface waves that only move shake horizontally. • Travel slightly faster than Rayleigh waves • Named after A.E.H. Love, a British mathematician who created a mathematical model of the waves in 1911. • Like Rayleigh waves, they will arrive after the P and S waves.
Seismograph/Seismometer • This is an instrument used to detect and record earthquakes. • Generally it consists of a heavy suspended mass that remains stationary as the table below it shakes. • Either an electric current is generated to show the amplitude of a wave on a screen or a pen marks the amplitudes of a piece of paper.