Earthquakes (Chapter 8) Earth Science Earthquakes (Chapter 8)

Earthquakes General features Vibration of Earth produced by the rapid release of energy. Focus – the place within Earth where earthquake starts. Epicenter - Point on the surface, directly above the focus Caused by slippage along a break in the lithosphere, called a fault. Fault: Fracture in earth where movement has occurred.

Earthquake focus and epicenter

Earthquakes Types of Faults (draw and label each fault type on right) Normal Fault: Fault in which the rock above the fault plane has moved down relative to the rock below. Reverse Fault: Fault in which the material above the fault plane moves up relative to the material below.

Earthquakes Types of Faults (draw faults on right) 3. Thrust Fault: Reverse fault with a dip less than 45 degrees. 4. Strike-slip fault: A fault along which the movement is horizontal and parallel to the trend of the fault.

The Cause of Earthquakes Deformation of Rocks Forces within Earth slowly deform the rock that makes up Earth’s crust causing the rock to change shape, or bend on both sides of a fault. When the rocks bend they store energy, much like a rubber band when stretched.

The Cause of Earthquakes Elastic Rebound sequence of events… Stored energy builds as the rock is deformed. Suddenly the rocks slip, or break at their weakest point (the focus) and the stored energy is released all at once. The deformed rock ‘springs back’ to its original shape. This is called elastic rebound.

Elastic Rebound

Earthquakes General features Earthquakes are often preceded by foreshocks and followed by aftershocks.

Earthquakes Earthquake waves Study of earthquake waves is called seismology Earthquake recording instrument (seismograph) Records movement of Earth Record of Earth’s movement is called a seismogram

Seismograph

Earthquakes Earthquake waves Types of earthquake waves P - waves Push–pull (compressional) motion Travel through solids, and liquids Greatest velocity of all earthquake waves

P - waves

Earthquakes Earthquake waves Types of earthquake waves S - waves Up-down motion Travel through solids, but NOT liquids. S - waves travel slower than P-waves

S - waves

Earthquakes Earthquake waves Types of earthquake waves Surface waves (occurs when an S-wave reaches the surface) Travel slower than P-waves and S-waves Move up and down as well as side to side (similar to an ocean wave) Most destructive seismic wave

A seismogram records wave amplitude vs. time

Earthquakes Earthquake intensity and magnitude Intensity Magnitude A measure of the degree of earthquake shaking at a given locale based on the amount of damage Most often measured by the Modified Mercalli Intensity Scale Magnitude Concept introduced by Charles Richter in 1935

Earthquakes Earthquake intensity and magnitude Magnitude (M) Often measured using the Richter Scale Based on the amplitude of the largest seismic wave A tenfold increase in wave height equals an increase of 1 on the Richter scale. Does not estimate adequately the size of very large earthquakes. Scientists no longer use it routinely.

Earthquakes Earthquake intensity and magnitude Magnitude Moment magnitude scale Measures very large earthquakes Derived from the amount of displacement that occurs along a fault zone More precise than the Richter Scale

Magnitude 5 or greater earthquakes over 10 years

Earthquakes Locating an earthquake Located using the difference in the arrival times between P and S wave recordings, which are related to distance (Travel-time graph)

Travel-time graph How long does it take a P wave and and S wave to travel 1000 km? P-wave = S-wave = Which seismic wave is faster?

Earthquakes Locating an earthquake Epicenter At least three station recordings are needed to locate an epicenter Circle equal to the epicenter distance is drawn around each station Point where three circles intersect is the epicenter

The epicenter is located using three or more seismic stations

Earthquakes Causes of Earthquake Damage Earthquake-related hazards Seismic shaking Liquefaction Landslides Mudflows Tsunamis

Earthquakes Earthquake destruction Destruction results from … Seismic shaking (the ground vibrating) May jolt and twist structures Unreinforced brick or concrete structures may collapse Liquefaction of the ground Saturated soil material turns fluid Underground objects may float to surface

Damage caused by the 1964 earthquake in Alaska

Damage from the 1964 Anchorage, Alaska, earthquake

Earthquakes Earthquake destruction Destruction resulting from… Landslides and Mudflows Movement of loose rock and soil down slope If the water content of the soil is high an earthquake can start a mudflow

Landslide

Mudflow

Earthquakes Earthquake destruction Destruction resulting from… Tsunamis Ocean wave formed when the ocean floor suddenly shifts. Move fast (100+ km/hr) across the open ocean A Tsunami slows down and increases in size as it approaches shore

Formation of a tsunami

Tsunami travel times to Honolulu

Earthquakes Assessing Earthquake risk Earthquakes are more frequent along the boundaries of Earth’s Tectonic plates. No reliable method of predicting an Earthquake occurrence in the near future.