Lecture Notes: Earthquakes - PDF

EARTHQUAKES José Guadalupe Posada, 1894 Definitions: Important Terms EARTHQUAKE: A sudden and violent shaking of the ground as a result of movements within the Earth’s crust or volcanic activity. QUAKE: To shake or tremble. SEISMIC: Relating to EARTHQUAKES or other vibrations of th...

EARTHQUAKES José Guadalupe Posada, 1894 Definitions: Important Terms EARTHQUAKE: A sudden and violent shaking of the ground as a result of movements within the Earth’s crust or volcanic activity. QUAKE: To shake or tremble. SEISMIC: Relating to EARTHQUAKES or other vibrations of the Earth and its crust. SEISMOLOGY: The study of EARTHQUAKES. SEISMOLOGIST: Scientist who studies EARTHQUAKES. We have all heard of the devastation EARTHQUAKES can cause. Let us gain a better understanding of what causes them. Earthquakes: Frequency Each year, there are: ✓ over 500,000 detectable EARTHQUAKES ✓ 100,000 are felt by humans ✓ 100 cause damage and/or death. e.g., Since the start of October 2023, multiple EARTHQUAKES have hit western Afghanistan. Village destroyed by October 2023 earthquakes in Afghanistan Thus far approximately 1500 people have died and 2000 have been injured. Earthquakes: Causes To understand what causes most EARTHQUAKES, we must revisit TECTONIC PLATES. As previously indicated, Earth’s entire CRUST and UPPER MANTLE (i.e., the LITHOSPHERE) can be subdivided into large rocky plates known as TECTONIC PLATES. The LITHOSPHERE is divided into 7 MAJOR TECTONIC PLATES and 8 MINOR TECTONIC PLATES. Earthquakes: Causes LITHOSPHERE and therefore TECTONIC PLATES it is made of, sits on the ASTHENOSPHERE. ASTHENOSPHERE: partially molten layer of MANTLE. ASTHENOSPHERE moves due to heat transfer and rise and fall of partially molten rocks within it. When ASTHENOSPHERE below moves, TECTONIC PLATES are dragged along and move too! 5 Earthquakes: Causes Due to ASTHENOSPHERE movement, TECTONIC PLATE edges may slide against each other. FRICTION where plates meet may slow and/or prevent their motion, resulting in a build up of pressure. FRICTION: ✓ Resistance a surface or object encounters when moving over another. ✓ A force that opposes motion (sliding, rolling) of one object over another. FRICTION where plates meet may slow and/or Pressure build up can last for long periods of time – until plates overcome prevent their motion, FRICTION and move, releasing built-up pressure as SEISMIC WAVES. resulting in a build up of pressure. When this occurs, sections of Earth’s crust can literally break apart and be displaced, resulting in an EARTHQUAKE. Such EARTHQUAKES are known as TECTONIC EARTHQUAKES. 6 Earthquakes: Causes A majority (i.e., 81%) of large TECTONIC EARTHQUAKES occur along the “circum-Pacific seismic belt”/ along the rim of the Pacific Ocean. “Circum-Pacific seismic belt” is also home to some of the world’s most powerful volcanoes and is known as the RING OF FIRE. Compare it to location of CONVERGENT oceanic and continental TECTONIC PLATE boundaries around the Pacific. We see a trend! 7 Earthquakes: Causes Overall, most EARTHQUAKES can be linked to TECTONIC PLATE boundaries. Other examples of important EARTHQUAKE MID- zones: ATLANTIC RIDGE Alpide belt (home to the recent Afghanistan earthquakes). Mid-Atlantic ridge (where DIVERGENT PLATE boundaries are encountered). 8 Thinking Question! Epicenter versus Hypocenter Many of you have heard the term EPICENTER. What is an EPICENTER and how is the EPICENTER different from the HYPOCENTER? 9 Thinking Question! Epicenter versus Hypocenter Many of you have heard the term EPICENTER. What is an EPICENTER and how is the EPICENTER different from the HYPOCENTER? HYPOCENTER: point in Earth’s interior where EARTHQUAKE originates. EPICENTER: point ON Earth’s surface directly above the HYPOCENTER point ON Earth’s surface where SEISMIC WAVES are first felt/where waves first arrive. 10 Thinking Question! Epicenter versus Hypocenter When EARTHQUAKE occurs, SEISMIC WAVES are generated at HYPOCENTER and then travel through Earth’s interior, ultimately reaching surface. Word SEISMIC WAVES was utilized a few times now. What exactly are these SEISMIC WAVES? 11 Seismic Waves: Types SEISMIC WAVES are vibrations that travel through (body of) Earth or along its surface when an EARTHQUAKE occurs. SEISMIC WAVES Some move/travel through Earth (BODY WAVES). Some move/travel along Earth’s surface (SURFACE WAVES). If travel through If travel along Earth: surface: There are 2 types of BODY WAVES that travel through Earth: BODY WAVES SURFACE WAVES p-WAVES (short for PRIMARY WAVES): Named this way as they are first waves detected by SEISMIC recording stations (that monitor earthquake activity) following an earthquake. s-WAVES (short for SECONDARY WAVES) Arrive at SEISMIC recording stations after the p-WAVES. 12 Seismic Waves: Types BODY WAVES also have an impact on Earth’s surface. There are 2 types of BODY WAVES that travel through body of the Earth: p-WAVES (short for PRIMARY WAVES): ✓ cause surface of Earth to EXPAND and CONTRACT in direction of wave travel (PARALLEL to wave travel). ✓ can travel through SOLIDS, LIQUIDS and GASES. s-WAVES (short for SECONDARY WAVES): ✓ cause surface of Earth to RISE and FALL. ✓ surface moves PERPENDICULAR to direction of wave travel. ✓ can travel through SOLIDS. 13 Seismic Waves: Types There are 2 types of BODY WAVES that travel through body of the Earth: p-WAVES (short for PRIMARY WAVES): ✓ can travel through SOLIDS, LIQUIDS and GASES. ✓ can travel through CRUST, MANTLE, OUTER CORE (which is liquid) and INNER CORE (which is solid). s-WAVES (short for SECONDARY WAVES): ✓ can travel through SOLIDS. ✓ can travel through CRUST and MANTLE. ✓ cannot travel through OUTER CORE (which is liquid) and therefore never reach INNER CORE (which is solid). 14 Seismic Waves: Types There are 2 types of BODY WAVES that travel through body of the Earth: p-WAVES (short for PRIMARY WAVES): ✓ can travel through SOLIDS, LIQUIDS and GASES. ✓ can travel through CRUST, MANTLE, OUTER CORE (which is liquid) and INNER CORE (which is solid). s-WAVES (short for SECONDARY WAVES): ✓ can travel through SOLIDS. ✓ can travel through CRUST and MANTLE. ✓ cannot travel through OUTER CORE (which is liquid) and therefore never reach INNER CORE (which is solid). 15 Seismic Waves: Types SURFACE WAVES follow the faster BODY WAVES (i.e., p- WAVES and s-WAVES). ✓ much SLOWER than SURFACE WAVES. ✓ SURFACE WAVES ONLY travel along Earth’s SURFACE. ✓ SURFACE WAVES ONLY travel through SOLIDS. ✓ SURFACE WAVES more destructive than BODY WAVES. There are 2 types of SURFACE WAVES (that travel along Earth’s SURFACE): ✓ LOVE WAVES (the faster of the two) ✓ RAYLEIGH WAVES 16 Seismic Waves: Types LOVE WAVES: ✓ travel HORIZONTALLY ✓ cause Earth’s surface to move from SIDE to SIDE PERPENDICULAR to direction of WAVE motion. RAYLEIGH WAVES: ✓ travel HORIZONTALLY ✓ cause Earth’s surface to shake in an Elliptical pattern (i.e., ROLLING PATTERN). ✓ cause Earth’s surface to move like ocean waves. ✓ spread out most (of all waves) and last for longest period of time. 17 SEISMIC WAVES: Earth’s Internal Structure Fun video explaining how SEISMIC WAVES (p-WAVES and s-WAVES) helped researchers gain a better understanding of the internal structure of the Earth! https://www.youtube.com/watch?v=h4jvZ_zHKYY 18 Next Lecture: Studying SEISMIC WAVES. Earthquake magnitude. Other Earthquake sources. Notable Earthquakes. 19