Earthquakes and Seismology Dr Stephen Crabtree November 1
Earthquakes and Seismology Dr. Stephen Crabtree November 1, 2019
Earthquakes & Seismology • Stress vs. Strain • Causes and Evidence • Seismic Waves • Defining Locations • Magnitude vs. Intensity • Earthquake Prediction
Fundamental Terms – Stress: Force applied over a specified area – Strain: Deformation caused by stress
What Causes Earthquakes? • • Build-up and Release of Stress and Strain Release the pressure all at once Focus – 3 D location of earthquake origin Epicenter – Location on surface above focus
Build-Up and Release • Most faults are not perfectly smooth planes • Stress and strain are concentrated at lock-points • Release of strain at these points releases energy – Energy goes out in 360°
Earthquakes ALWAYS going on • There are (probably) earthquakes going on in Minnesota right now • Most are so small or weak they’re never felt • Most just interpreted as regular “background noise” • US Geological Survey only bothers documenting those with magnitudes 2. 0 -3. 0 or above
Current USGS Earthquake Map http: //earthquake. usgs. gov/ earthquakes/map/
Evidence of Substantial Plate Motion over Time Point Reyes Nat’l Seashore Granodiorite outcrops at Pt. Reyes Nat’l Seashore Identical to outcrops from Sierra Nevadas (hundreds of miles away
Evidence of Abrupt Movement • Movement as jumps, not just slow, gradual creep • Leaves behind fault surfaces, called “scarps” • Sites of abrupt upward/downward movement
Evidence of Abrupt Movement • May also show abrupt horizontal movement • Example: 1906 San Francisco, 20 -feet of slip
Tracing Earthquake Origins • Remember: Two types of seismic waves travelling through the Earth – P-waves • Primary Waves/Compression Waves – S-waves • Secondary Waves/Shear Waves
P-waves vs S-waves • P-waves travel faster, arrive at stations first • S-waves move slower, but give bigger signal P-Waves • Compression waves • Displacement parallel to wave path S-Waves • Shear Waves • Displacement 90° to wave path
Wave Speeds Velocity (km/s) 0 5 10 15 0 • Generally increases with density and rigidity of material passed through Vp Lower Mantle (Mesosphere) 2, 000 Vs 3, 000 4, 000 Outer Core • cannot travel through liquids (i. e. , the outer core) 1, 000 5, 000 6, 000 Inner Core S-waves: • 2. 5 – 7. 5 km/s thru mantle Aestheno. Depth (km) P-waves: • 5. 5 – 13. 5 km/s thru mantle Lithosphere
Wave Paths through the Earth P or S Only P P or S
How a Seismograph Works • Secure seismograph to bedrock • Very large/heavy weight remains stationary while the rest of the machine moves
P-waves vs S-waves • P-waves travel faster, arrive at stations first • S-waves move slower, but give bigger signal 0 10 Minutes after Earthquake Starts 20
Defining Earthquake Locations
Defining Earthquake Locations
Defining Earthquake Locations Use distances from three seismic stations to define actual location of earthquake epicenter
A THIRD type of Wave Surface Waves • • • Similar to S-waves Much slower-moving Far more destructive Far larger amplitudes Don’t travel “through” the Earth, just along the surface
Surface Waves • Far larger signal than P-waves or S-waves • Slower moving, more destructive
Earthquake Magnitude • Calculation based on maximum surface wave amplitude and distance from epicenter • Define the total amount of energy released
Japan Earthquake Seismogram L I S I S I L I
Earthquake Magnitude: Richter Scale • Specific measure of total energy released • NOT a linear scale – logarithmic factor of 30 – 2. 0 is 30 times more energy than a 1. 0 – 4. 0 is 900 times more energy than a 2. 0 – 7. 0 is 27, 000 times more energy than a 4. 0 • Each earthquake has only ONE defined magnitude – an objective measure
Earthquake Magnitude: Richter Scale Measurement Energy released, relative to 1. 0 1 2. 0 30 3. 0 900 4. 0 27, 000 5. 0 810, 000 6. 0 24, 300, 000 7. 0 729, 000 8. 0 21, 870, 000 9. 0 656, 100, 000 10. 0 19, 683, 000, 000
Earthquake Intensity: Mercalli Scale • Definition of the amount of shaking felt and degree of destruction witnessed • Much more subjective than Richter Scale • Can have different Mercalli Scale measurements depending on different factors: – Distance from epicenter – Type of material built on – Standards of construction – Etc…
Earthquake Intensity: Mercalli Scale Measurement Degree Felt by Individuals Degree of Damage – example I Not felt by people II Felt by a few at rest III Felt indoors by several Vibration like passing light trucks IV Felt indoors by many, outdoors by few Vibration like passing heavy trucks Windows, dishes and doors rattle V Felt by nearly all Some dishes and windows broken VI Felt by all people Slight damage, furniture moved VII Difficult to stay standing Mod. damage to well-built buildings VIII Many people frightened Partial collapse of buildings IX General Panic! Hanging objects may swing Great damage to most buildings X Most buildings destroyed XI Few buildings remain standing XII Damage total
Surface waves cause destruction Why are they so destructive?
Amplification of Wave Effects • Liquefaction – Water in rock/dirt/soil reduces ground to a flowing liquid – no stability – Can create “mud volcanoes” from pressure • Tsunamis – ocean waves triggered by seismic disturbances – Sudden uplift/downdrop of area of oceanic crust • Damage to Infrastructure – Snapping gas and water lines, cause further damage
Example: Liquefaction • Water in rock/ dirt/soil reduces ground to a flowing liquid
Tsunami Damage – Sumatra, 2004
Tsunami Damage – Sumatra, 2004
Tsunami Damage – Japan, 2011 • 2011 Japan Earthquake
Tsunami Damage – Japan, 2011
Example: Infrastructure Damage More damage from 1906 San Francisco Earthquake due to snapped gas and water mains than from just building collapse
Shaking causes plenty of damage, too Example: Destroyed overpass during rush-hour in 1989 59 people killed when bridges collapsed
How to Predict Earthquakes • Short-range vs. Long-range prediction • Relies on accurate knowledge of past earthquakes – Where were they, precisely – When were they – how long ago – How strong were they (magnitude) • Need to know average rate of slip/strain building up on fault
Most fault systems are complicated • Pressure is built and released on different branches of faults are at different rates and times
Most fault systems are complicated • Plates keep moving! • Use known dates and earthquake magnitudes to define when strain will need to be released again
Earthquake Hazards in North America
Earthquakes, M > 2. 5
Summary • Stress vs. Strain • Causes and Evidence • Seismic Waves • Defining Locations • Magnitude vs. Intensity • Earthquake Prediction
- Slides: 42