Seismicity, Earthquakes & Earth’s Structure

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

Earthquake Waves - Seismicity Seismic waves – elastic energy released at the focus following the rupture of rock Seismology- study of earthquake (seismic) waves Seismograph – instrument that records earthquake waves Seismogram – record of the seismic waves

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 – 2 nd 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 (t H ) 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.

Questions 1) Where would you expect the higher earthquake insurance rates: Georgia or California? 2) Briefly explain.

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

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 P-wave Shadow Zone

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

Interior Structure Chemical Composition

Crust 5 to 75 km thick Solid Silicon (Si) and Oxygen (O) Continental –5 to 75 km thick –Old (billions of years in age) –Deformed –Density 2.7 g/cm 3 - Granite Oceanic –5 to 8 km thick –Relatively young (less than 200 million years) –Undeformed –Density 3.0 g/cm 3 - Basalt

Mantle Approximately 2900 km thick 82% of Earth’s Volume 68% of Earth’s Mass –Density increases with depth through the mantle –3.2 g/cm 3 in upper portion –5 g/cm 3 in lower portion Silicon (Si) and Oxygen (O) with some iron (Fe) and magnesium (Mg)

Core Total diameter – 7000 km 16% of Earth’s Volume 32% of Earth’s Mass – Density 10.8 g/cm 3 Iron (Fe) Rich with some Nickel (Ni)

Interior Components Physical Properties

Lithosphere Solid, strong, and rigid outer layer Includes all of crust and the upper mantle 10 to 300 km thick

Asthenosphere High Temperature Moderate Pressure Soft-plastic zone (Rock can flow) Upper Mantle 100 to 500 km thick

Mesosphere High Temperature Zone High Pressure Zone Rock is stronger and more rigid than in the overlying Asthenosphere Remainder of Mantle

A Two Component Core Inner Core –  1200 km thick –Solid Outer Core –  2300 km thick –Liquid – Molten –Chemically may be different – may have nickel

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 Alaska California

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