Seismicity & Earthquakes
Questions 1) Where would you expect the higher earthquake insurance rates: Florida or Oregon? 2) Briefly explain. 3) Did you use inductive or deductive reasoning ? 4) explain
What is an Earthquake? Vibration of the Earth produced by the rapid release of energy. Energy release due to plate tectonics and failure (fault zones) of the Earth’s crust Energy radiates as waves in all directions from the focus (source) Epicenter – surface expression of the focus
Elastic Rebound Theory Tectonic forces slowly deform the rock As the rock bends, it stores energy When the rock’s resistance to the stress is overcome, the rock slips or breaks Slippage occurs at the weakest point (focus), rock “snaps” back to original position Energy released produces the vibrations we know as an earthquake
Seismogram
Seismic Waves P-waves and S-waves are body waves, they travel through the Earth’s interior. Surface waves travel along the Earth’s outer layer.
P-waves – Primary Waves Compressional wave – push and pull rock in direction of movement Can travel through all materials (solids, liquids, and gases) Fastest moving wave Smallest wave amplitude (lowest energy)
S-waves – Secondary Waves Shear wave - Waves shake material at right angles to their direction of movement TRAVEL ONLY THROUGH SOLIDS Intermediate speed – 2nd to arrive as seismic station
Surface waves Move up and down, similar to ocean waves Can also move side-to-side. Very damaging waves Travel the slowest – last to arrive at seismic station Greatest wave amplitude (highest energy)
Locating Earthquakes Need three seismic stations. Use the time difference (tH) in the arrival of the p-waves and the s-waves. Distance (d) extrapolated from a time-distance graph. A circle with a radius of d is drawn around the seismic station. Earthquake occurred somewhere on the circle. The intersection from three stations shows the epicenter location.
Locations of Earthquakes By studying 1000s of earthquakes, geologists learned most occur along plate boundaries
Earthquake Intensity and Magnitude Mercalli intensity scale Assesses the damage from an earthquake at a specific location Based upon human observation Can vary from location to location
Earthquake Intensity and Magnitude Richter Scale Describes the earthquakes magnitude (energy released) Measures amplitude of largest wave, adjusted for travel time Is the same from location to location A 1 digit increase in Richter scale is a 30 fold increase in energy released
Is the Earth Entirely Solid? What piece of the Earth is not solid? How do we know this? Seismic waves Different materials transmit waves at different speeds, i.e. p-waves change speed as they move from the crust to the mantle and from the mantle to outer core
Wave Movement Through Homogeneous Earth
Movement through material of different density
Velocity and Amplitude Are a Function of the Material
Velocity Profile Through Earth
P-wave Shadow Zone Liquid causes p-waves to bend (refract) Refraction of waves creates a zone where no p-waves are recorded (shadow zone) 103 to 143 degrees from focus
S-wave Shadow S-waves cannot pass through liquid Waves refract at surface of outer core, creating a zone where no s-waves are recorded (shadow zone) 103 to 180 degrees from focus
Why did the 1895 EQ have a wider radius of influence?
Earthquake Prediction
Time Interval Analysis Statistics is used to generate a “best-fit line” which can be used to predict the time of the next earthquake Not reliable This example has not occurred
Seismic Gap Look at the location of earthquakes Areas (Gaps) where no earthquake has happened is an area of accumulating strain Gaps represent locations of future earthquakes
Seismic Gap Examples California Alaska
Environmental Impacts Tsunami Giant tidal waves created by earthquakes Move at speeds between 500 and 950 km/hr (300 – 600 miles/hr) Waves reach height of over 30 meters (100ft)
Landslides and ground subsidence Vibrations cause unstable material to slide down slopes Vibrations cause stable material to turn into fluid – liquefaction