Lecture 31 EARTHQUAKES About Earthquakes Earthquakes can be

Lecture 31 EARTHQUAKES

About Earthquakes • Earthquakes can be understood in terms of the basic mechanisms of deformation. • Most earthquakes occur at plate boundaries (convergent, divergent, and sliding). • Earthquakes cannot yet be reliably predicted or mitigated.

Basics 1. Earthquakes generate waves that travel through the earth 2. Earthquakes occur when the stress on a fault overcomes its strength and leads to slip along the fault plane 3. Faults are classified by the kinds of movement that occur along them 4. Earthquakes don’t kill people, buildings kill people 5. Difference between Magnitude and Intensity 6. Predicting earthquakes is not yet possible but we can make hazard maps

TYPES OF EARTHQUAKES 1) Tectonic Earthquake 2) Volcanic Earthquake 3) Collapse Earthquake 4) Explosion Earthquake CLASSIFICATION DISTANCE 1) Teleseismic Earthquake > 1000 km 2) Regional Earthquake > 500 km 3) Local Earthquake < 500 km

Largest earthquake in the world Chile : 1960 May 22 19: 11: 14 UTC Magnitude 9. 5 • More than 2, 000 killed, 3, 000 injured, 2, 000 homeless, and $550 M damage in southern Chile • Tsunami caused 61 deaths • $75 million damage in Hawaii; • 138 deaths and $50 million damage in Japan; • 32 dead and missing in the Philippines; • $500, 000 damage to the west coast of the US

Earthquake Terminology • Earthquake = slip on a fault plane resulting in the rapid release of energy • Seismic waves = Energy moving outward from the focus of an earthquake • Focus= location of initial slip on the fault; where the earthquake origins • Epicenter= spot on Earth’s surface directly above the focus

Earthquake Basics • • Earthquakes occur along a fault or fault system Magnitude = a measure of how much energy is released in all directions • Bigger faults make bigger earthquakes • Bigger earthquakes last a longer time

Why do earthquakes occur? • • epicenter focus Rupture occurs when the stress of the rocks overcome the strength and the rocks Energy is released in waves that radiate outward from the fault

Sources ・ What is the Earthquake Source? Fault slip Elastic Rebound Measurement - Magnitudes (1/quake, related to energy released, Richter or Moment scale) - Intensity (many for each earthquake, measure of effects Mercalli scale)

1. What Is an Earthquake? ● Global forces at work ● stress ● strain ● strength

1. What Is an Earthquake? ● Earthquakes occur where rocks being stressed suddenly break along a new or pre-existing fault. ● Seismic waves are ground vibrations caused by rocks slipping along opposite sides of a fault.

1. What Is an Earthquake? ● Why earthquakes occur ● elastic rebound theory ● fault rupture ● epicenter ● focus

1. What Is an Earthquake? Example of Elastic Rebound

1. What Is an Earthquake? Fault Rupture

Elastic Rebound 2. Earthquakes occur when rocks slip along faults

Seismic cycle

Elastic Fault Deformation Can divide fault zone based on how fault slips – Seismogenic Crust exhibits stick slip – Aseismic Crust exhibits stable sliding • Crustal earthquakes involve slip of seismogenic crust and possibly transitional zone Seismogenic Crust: Stick Slip Transitional Zone Aseismic Crust: Stable Sliding or plastic (flow) deformation

Interseismic: Strain accumulation • Between earthquakes: Slip – Shallow fault is locked – Deeper fault is creeping at long-term slip rate – Stress builds up: elastic strain energy stored in crust ? – Stress on fault reduced • Cycle repeats forever Depth • During earthquake, shallow fault slips Locking Depth • Between earthquakes – Fault does not slip from surface to locking depth – Fault slips continuously beneath locking depth – May be finite transition zone between locked and slipping parts • During earthquake – Shallow fault slips

Interseismic: Strain accumulation • Between earthquakes: interseismic Slip coseismic – Shallow fault is locked – Deeper fault is creeping at long-term slip rate – Stress builds up: elastic strain energy stored in crust – Stress on fault reduced • Cycle repeats forever Depth • During earthquake, shallow fault slips Locking Depth • Between earthquakes – Fault does not slip from surface to locking depth – Fault slips continuously beneath locking depth – May be finite transition zone between locked and slipping parts • During earthquake – Shallow fault slips

Elastic Rebound Interseismic + Coseismic = Long-term Motion Interseismic No Strain Interseismic strain Coseismic strain

2. How Do We Study Earthquakes? ● Seismographs are machines that record the seismic waves generated by earthquakes. ● vertical ground movements ● horizontal ground movements

Measuring earthquakes • Seismometers: instruments that detect seismic waves • Seismographs Record intensity, height and amplitude of seismic waves Chang Heng ‘Seismometer’ AD 132

How do Seismometers work? 1. Earthquakes generate waves that

2. How Do We Study Earthquakes? ● Seismic wave types ● P waves (primary waves) ● S waves (secondary waves) ● Surface waves

Rayleigh Love

Seismic Waves: properties • Velocity: function of the physical properties of the rock the wave is traveling through – Velocity increases with rock density – Velocity changes when passing from one material to another (increases/decreases) – Liquids: S-waves do not get transmitted through liquid; P-waves slow down • Why is this important? –If we know the velocity of the wave, we can infer the type of rock it traveled through- that’s how we map the interior of the Earth