CH 8 Earthquakes

What is an Earthquake? - more than 30,000 earthquakes are measured each year, but only ~75 are major - usually occur along a break in the Earth’s crust called a Fault.

Focus vs. Epicenter Focus = point where the earthquake originates underground - energy radiates outward in all directions from the focus

Focus vs. Epicenter Epicenter = point on Earth’s surface that is directly above the focus. - point of most damage on surface

Elastic Rebound Hypothesis = energy is built up slowly underground and rock bends - eventually the energy gets too great and the rocks break at the focus - after energy is released, rocks snap back to original shape but at a new location

How do we study Earthquakes? Seismographs = instrument to measure energy of earthquake (ground shaking) 2. Seismogram = written record of ground movement  Energy from Earthquakes travel through Earth as Seismic Waves

2 Types of Seismic Waves 1. Surface Waves = travel only along Earth’s surface - can move up & down or side to side - most destructive wave to property - slowest speed

2 Types of Seismic Waves 2. Body Waves = travel through Earth’s interior P - Waves = primary waves - push/pull motion - fastest waves S - Waves = secondary waves - side to side motion - can’t travel through liquids

 Seismographs show all 3 specific waves for every earthquake

How strong are earthquakes? Scientists have used 2 types of measurements to describe the size of an earthquake……

How strong are earthquakes? Intensity = amount of shaking at a location based on damage Not really accurate Based on personal estimates of damage

2. Magnitude = measures size of seismic waves (amount of energy released) on seismograms »Based on 2 scales

Richter Scale = based on largest seismic wave recorded on seismogram - exponential scale - outdated

EX: magnitude 5 is 10X stronger than magnitude 4 EX: magnitude 5 is 100X stronger than magnitude 3 Problems measuring larger quakes

more accurate than Richter Moment Magnitude Scale measures ground movement - measures the energy more accurate than Richter more complicated Not used outside of science

Dangers of Earthquakes 1. Property damage = depends on strength & length (time) of earthquake - ground material = loose soil will shift during quake - building material = rigid buildings (steel) don’t bend, so they break during a quake - wood structures bend and sway

2. Tsunamis = large wave created by an underwater earthquake - very fast 500-1000 km/hr - in open ocean tsunamis appear small but near land, can become +100 m tall - tsunami warning system (US) monitors Pacific Ocean earthquakes

3. Landslides = shaking causes soil to slide downhill 4. Fire = gas & electrical lines can be cut during a quake causing a fire - water lines are also broken so fires can’t be put out

 Most of our knowledge of the Earth’s interior comes from the study of earthquakes As scientists studied seismic waves, they saw they do not travel straight through = curve, WHY???

- Because Earth’s interior is not the same throughout, it has layers with different properties

4 Layers of the Earth CRUST- thin rocky outer layer of the Earth - Oceanic Crust = ~7 km thick, dense, made of basalt, relatively young - Continental Crust = ~40 km thick, made of granite & other light, very old

2. Mantle = solid rocky layer that extends down to 2900 km deep, weakened because of increased temperature & pressure 3. Outer Core = made of liquid Fe & Ni, rotates to create Earth’s magnetic field, 2600 km thick

4. Inner Core = made of solid Ni & Fe , 1220 km thick

2 Special Zones Lithosphere = crust and outer mantle, rigid shell, forms the continental plates

Asthenosphere = below lithosphere, about 150 km thick, very weak, partially melted, allows lithosphere above to move slowly

VERY IMPORTANT FOR PLATE TECTONICS   VERY IMPORTANT FOR PLATE TECTONICS