Earthquakes Pt Reyes Station 1906
Earthquakes Ground shaking caused by the sudden release of accumulated strain by an abrupt shift of rock along a fracture in the earth.
You Live in Earthquake Country Be Prepared Don t expect someone to be there to help you! Have an emergency kit and a plan. Emergency kit: water and food (+3 days) blankets, coat 1 st aid kit, you are set if you have camping gear (shelter, water filter, light, stove, etc.) Plan: Know what the dangers are (gas lines, tsunami risk, fire risk, etc..), and how to react. Know where to go and where you will meet.
+10,000 Fatalities Since 1970 2010 2004 2005 2008 Japan 2011 ~25,000 fatalities (almost all due to the associated tsunami). 2003 2001
Turnagain Arm subsidence, Alaska 1964 Earthquake.
Sumatra, 2004 +250,000 dead (mostly from associated tsunami).
Turkey, 1999, +17,000 dead
Sichuan, China 2008 +80,000 deaths
San Francisco 1906 and 1989 (inset)
Earthquakes most frequently occur near plate boundaries---why?
1 2 Discuss each location. 6 8 10 5 4 9 3
Intra-plate Earthquakes New Madrid Seismic Zone Ancient failed rift (aulacogen)
1. Stress (pressure) is continually applied to rocks or preexisting faults. This stress most likely comes from plate tectonics and is at a plate boundary. 2. Stress builds where strong rocks or locked faults withstand it. (Friction is the internal force that locks a fault, making its two sides stick together.) 3. Rocks and faults deform elastically (strain) in response to the building stress. 4. Stress finally builds higher than rock strength or fault s friction and rock breaks or fault slips. 5. Stress is released in the form of energy waves that move outward in all directions. 6. The rocks or faults elastically rebound and strain energy is released. 7. After the rocks or faults have settled back into place, the stress begins building again.
Types of Fault Movement Behavior Fault Creep: Slow, gradual and gentle displacement along the fault. Segments of the San Andreas Fault show this behavior. Locked Faults: Faulted blocks are locked, thus stress builds and rocks strain (elastically deform). When the rocks break or friction between the blocks is exceeded, the blocks suddenly slip releasing the strain through elastic rebound (like a rubber band). This is an earthquake!
Simon Street, Hayward
Body Waves (P and S waves): Body waves travel through the Earth. Body waves are faster than surface waves and are recorded globally. Surface Waves (Love and Rayleigh waves): Surface waves are analogous to water waves and travel along the Earth's surface. Surface waves are low frequency, long duration, and can have large amplitudes, thus they can be the most destructive type of seismic wave.
Surface Waves
P-Waves Primary or Pressure Wave P-waves are compressional waves that travel through all materials including fluids such as gases and liquids. P-waves are the fastest waves (5 to 8 km/s) in the Earth s crust, so they are felt first. The speed increases with increased density and stiffness thus P-waves travel through granite in the crust at about 5 km/s and the deep mantle at about 13 km/s.
S-Waves Secondary or Shear Wave S-waves are shear waves so they do not travel through fluids. S-waves are slower (2-5 km/second in the Earth s Crust) than P-waves, so they arrive after the P-waves. The velocity of S-waves ranges from 2 3 km/s in light sediments and 4 5 km/s in the Earth's crust up to 7 km/s in the deep mantle.
Seismic Wave Motion
Turn to Neighbor: Draw cross-section through Earth s surface, where a fault comes through. Explain fault, epicenter, and focus.
Seismographs
How are earthquake locations pinpointed around the world?
The time lag between the arrival of the P waves and the S waves increases with distance from the epicenter. 10 km 30 km 70 km 110 km 140 km 280 km Arrival of P waves S waves 410 km
A single seismograph can determine the distance to an earthquake epicenter. Three or more seismographs are needed to locate an earthquake epicenter. 1 2 3
Magnitude There are different ways for measuring the magnitude of an earthquake. The Richter Scale: A quantitative way of measuring an earthquake. Other quantitative ways have been developed (the Moment Magnitude Scale being in favor in the U.S.) and they are calibrated to the popular Richter Scale. The Modified Mercalli Scale: A qualitative way of measuring an earthquake.
Richter Scale AMPLITUDE (of the waves) increases 10X for each increase of 1 in magnitude ENERGY increases 30X for each increase of 1 in magnitude
MAGNITUDE 6 = 10X AMPLITUDE OF 5 =30X ENERGY RELEASE OF 5 MAGNITUDE 6 = 100X AMPLITUDE OF 4 =900X ENERGY RELEASE OF 4
MAGNITUDE 8 = 30X ENERGY RELEASE OF 7 MAGNITUDE 8 = 900X ENERGY RELEASE OF 6 MAGNITUDE 8 = 27,000X ENERGY RELEASE OF 5 MAGNITUDE 8 = 810,000X ENERGY RELEASE OF 4 MAGNITUDE 8 = 24,300,000X ENERGY RELEASE OF 3
Magnitude The Richter Scale and Moment Magnitude Richter magnitude scale is based on amplitude of shaking as measured on a seismometer. It is limited in it s effectiveness to magnitudes of 6.8. The Richter scale becomes unreliable on large earthquakes. The Richter scale is older and now mostly replaced by the moment magnitude scale. Moment magnitude scale is based on crust character, rupture length and amount of offset. The moment magnitude scale was developed in the 1970s to succeed the Richter magnitude scale. The moment magnitude scale is better suited for studying large earthquakes. Even though the formulae are different, the new scale (moment magnitude scale) retains the familiar continuum of magnitude values defined by the older one. The moment magnitude scale is calibrated to give similar values for medium sized Earthquakes.
MOMENT MAGNITUDE SCALE IS BASED ON SEISMIC MOMENT SEISMIC MOMENT (LENGTH OF FAULT RUPTURE) X (DEPTH TO FOCUS) X (TOTAL AMOUNT OF SLIP ALONG RUPTURE) X (STRENGTH OF ROCK)
Modified Mercalli Scale Qualitative Description of the event.
NE Japan Tomography 0 Sea of Japan Moho Honshu Pacific Ocean 100 Zone of Melt Generation -4% 0% 5% Relative P-wave velocity
Seismic Gaps: fault segments with likely accumulated strain.