How do scientists know the structure of the Earth? Rock samples (direct evidence) Seismic waves (indirect evidence) –Vibrations that travel through Earth carrying energy released during an earthquake. –The speed and paths of waves reveal the structure of the planet. –We will learn more about these later. Rock samples (direct evidence) Seismic waves (indirect evidence) –Vibrations that travel through Earth carrying energy released during an earthquake. –The speed and paths of waves reveal the structure of the planet. –We will learn more about these later.

Earthquakes Occur along faults Caused by stressed rock during plate movement. This energy travels as seismic waves. Occur along faults Caused by stressed rock during plate movement. This energy travels as seismic waves.

Earthquake Zones Along tectonic plate boundaries where a large number of faults are located Example: San Andreas Fault Zone in California Along tectonic plate boundaries where a large number of faults are located Example: San Andreas Fault Zone in California

Seismic Waves P waves: or primary waves –Fastest, so they travel ahead of other waves and are the first to be detected. –Can travel through all media solid, liquid, AND gas! P waves: or primary waves –Fastest, so they travel ahead of other waves and are the first to be detected. –Can travel through all media solid, liquid, AND gas!

Seismic Waves S Waves: or secondary waves –Second-fastest waves –Only travel in solids (think S = solid) S Waves: or secondary waves –Second-fastest waves –Only travel in solids (think S = solid)

Seismic Waves Surface waves –Move along the Earth’s surface (in the upper few kilometers) of the earth’s crust. –Move the ground much like ocean waves. –Travel the slowest. –The most destructive. Surface waves –Move along the Earth’s surface (in the upper few kilometers) of the earth’s crust. –Move the ground much like ocean waves. –Travel the slowest. –The most destructive.

S and P waves give an idea of the inside the Earth. –P waves can travel in solid, liquid, and gas Crust - solid - YES! Mantle - solid / liquid - YES! Core (outer) - liquid - YES! Core (inner) - solid - YES! –S waves can only travel in solids Crust - solid - YES! Mantle - solid / liquid - YES! Core (outer) - liquid - NO! Must be liquid! Core (inner) - solid - YES! –Solid due to intense pressure S and P waves give an idea of the inside the Earth. –P waves can travel in solid, liquid, and gas Crust - solid - YES! Mantle - solid / liquid - YES! Core (outer) - liquid - YES! Core (inner) - solid - YES! –S waves can only travel in solids Crust - solid - YES! Mantle - solid / liquid - YES! Core (outer) - liquid - NO! Must be liquid! Core (inner) - solid - YES! –Solid due to intense pressure

Earthquake Measurement Seismologist - a person who studies earthquakes Seismographs - instruments that record seismic waves Seismogram - the “picture” that is created by the seismograph Seismologist - a person who studies earthquakes Seismographs - instruments that record seismic waves Seismogram - the “picture” that is created by the seismograph

Determining Time and Location Seismologists look at the seismogram to note the difference in arrival times of P waves and S waves Seismographs also help find the epicenter and focus Seismologists look at the seismogram to note the difference in arrival times of P waves and S waves Seismographs also help find the epicenter and focus

Measuring Strength and Intensity Charles Richter created the Richter scale in the 1930’s. Magnitude - a measure of the strength / energy of an earthquake –Each unit on the Richter scale represents motion that is 10 times larger than the previous unit. A magnitude of 5 is ten times stronger than a 4. A magnitude of 6 is 100 times stronger than a 4. Charles Richter created the Richter scale in the 1930’s. Magnitude - a measure of the strength / energy of an earthquake –Each unit on the Richter scale represents motion that is 10 times larger than the previous unit. A magnitude of 5 is ten times stronger than a 4. A magnitude of 6 is 100 times stronger than a 4.

Modified Mercalli Intensity Scale Intensity - a measure of the degree it is felt by people and the amount of damage caused Uses Roman numerals I to XII I = not felt, XII = total damage of an area The number changes depending on where you are (highest number is close to the epicenter). Intensity - a measure of the degree it is felt by people and the amount of damage caused Uses Roman numerals I to XII I = not felt, XII = total damage of an area The number changes depending on where you are (highest number is close to the epicenter).

Earthquake Forecasting The Gap hypothesis –Sections of active faults that have had few earthquakes are likely to be sites of strong earthquakes in the future. –These sites are called seismic gaps The Gap hypothesis –Sections of active faults that have had few earthquakes are likely to be sites of strong earthquakes in the future. –These sites are called seismic gaps

Where Volcanoes Form Over plate boundaries –Ex. Ring of Fire surrounding the Pacific Ocean (contains 75% of active volcanoes on land) –80% (on land) where plates collide –15% (on land) where plates separate Over plate boundaries –Ex. Ring of Fire surrounding the Pacific Ocean (contains 75% of active volcanoes on land) –80% (on land) where plates collide –15% (on land) where plates separate