Today’s Schedule: HAZARD UPDATE! Review Lecture 3 XXXX Earthquakes (cont.) Stress and Strain (energy transfer) Elastic (bounces back) vs. Plastic (stays deformed) Elastic Rebound Theory Today’s Material Earthquakes (cont.) Stress and Strain (energy transfer) Elastic (bounces back) vs. Plastic (stays deformed) Elastic Rebound Theory Class Review Preview Next Class Today’s Schedule: HAZARD UPDATE! Review Lecture 3 XXXX Earthquakes (cont.) Stress and Strain (energy transfer) Elastic (bounces back) vs. Plastic (stays deformed) Elastic Rebound Theory Today’s Material Earthquakes (cont.) Stress and Strain (energy transfer) Elastic (bounces back) vs. Plastic (stays deformed) Elastic Rebound Theory Class Review Preview Next Class

314 years ago (January 26, 1700), an estimated M9 earthquake unzipped the Cascadia Subduction Zone fault from northern California, USA to southern British Columbia, Canada, much like recent events in 2011 in Japan and 2010 in Chile. The newly released “Cascadia Subduction Zone Earthquakes: A Magnitude 9.0 Earthquake Scenario” examines how the Pacific Northwest may fare after the next great ‘megathrust’ earthquake and tsunami. s/cascadia_subduction_scenario_2013.pdf Turbidite Event History—Methods and Implications for Holocene Paleoseismicity of the Cascadia Subduction Zone

Earthquake Animation shows the buildup of stress along the margin of two stuck plates that are trying to slide past one another. The rock is deformed as it builds up strain in the plates; stress increases along the contact.

What is stress? Stress is defined as a force (F) acting on some area (A) Stress and Strain (Energy Transfer): What is stress and what is strain? How do these relate to earthquakes? Additional resources:

Stress: Tension vs. Compression vs. Shear Tensional stress is the stress that tends to pull something apart. It is the stress component perpendicular to a given surface, such as a fault plane, that results from forces applied perpendicular to the surface or from remote forces transmitted through the surrounding rock. Compressional stress is stress that squeezes something. It is the stress component perpendicular to a given surface, such as a fault plane, that results from forces applied perpendicular to the surface or from remote forces transmitted through the surrounding rock. Shear stress is the stress component parallel to a given surface, such as a fault plane, that results from forces applied parallel to the surface or from remote forces transmitted through the surrounding rock. Tensional stress is the stress that tends to pull something apart. It is the stress component perpendicular to a given surface, such as a fault plane, that results from forces applied perpendicular to the surface or from remote forces transmitted through the surrounding rock. Compressional stress is stress that squeezes something. It is the stress component perpendicular to a given surface, such as a fault plane, that results from forces applied perpendicular to the surface or from remote forces transmitted through the surrounding rock. Shear stress is the stress component parallel to a given surface, such as a fault plane, that results from forces applied parallel to the surface or from remote forces transmitted through the surrounding rock. Image courtesy of Michael Kimberly, North Carolina State Univ.

What is strain? Strain is the relative change in shape or size of an object due to externally-applied forces (e.g. stress). Hooke’s Law: Stress is directly proportional to strain. >>>>>>>>

What does “elastic” mean? Elastic refers to internal strain in a material. This internal strain is observed as a distortion of the material. Elastic deformation returns to it's original shape after a strain is applied. What does “plastic” mean? Plastic refers to internal strain in a material. This internal strain is observed as a distortion of the material. Plastic deformation stays in the deformed shape after a strain is applied.

Earthquake Machine On the graph, the yellow line shows the movement of the hand over time, thus a steady line. The blue line shows the movement of the block during slip on "earthquakes" thus the jumps in distance over time.

Activity 1

What is the Elastic Rebound Theory? The elastic rebound theory is an explanation for how energy is spread during earthquakes. As rocks on opposite sides of a fault are subjected to force and shift, they accumulate energy and slowly deform until their internal strength is exceeded. At that time, a sudden movement occurs along the fault, releasing the accumulated energy, and the rocks snap back to their original undeformed shape. The rocks accumulate energy and slowly deform until their internal strength is exceeded. The rocks accumulate energy and slowly deform until their internal strength is exceeded. At that time, a sudden movement occurs along the fault, releasing the accumulated energy, and the rocks snap back to their original undeformed shape. At that time, a sudden movement occurs along the fault, releasing the accumulated energy, and the rocks snap back to their original undeformed shape. The rocks accumulate energy and slowly deform until their internal strength is exceeded. The rocks accumulate energy and slowly deform until their internal strength is exceeded. At that time, a sudden movement occurs along the fault, releasing the accumulated energy, and the rocks snap back to their original undeformed shape. At that time, a sudden movement occurs along the fault, releasing the accumulated energy, and the rocks snap back to their original undeformed shape.

Reid's Elastic Rebound Theory From an examination of the displacement of the ground surface which accompanied the 1906 earthquake, Henry Fielding Reid, Professor of Geology at Johns Hopkins University, concluded that the earthquake must have involved an "elastic rebound" of previously stored elastic stress. If a stretched rubber band is broken or cut, elastic energy stored in the rubber band during the stretching will suddenly be released. Similarly, the crust of the earth can gradually store elastic stress that is released suddenly during an earthquake. This gradual accumulation and release of stress and strain is now referred to as the "elastic rebound theory" of earthquakes. Most earthquakes are the result of the sudden elastic rebound of previously stored energy.

The following diagram illustrates the process. Start at the bottom. 1.A straight fence is built across the San Andreas fault. 2.As the Pacific plate moves northwest, it gradually distorts the fence. Just before an earthquake, the fence has an "S" shape. 3.When the earthquake occurs the distortion is released and the two parts of the fence are again straight; but now there is an offset. This diagram greatly exaggerates the distortion. Actually, the distortion is spread over many miles and can only be seen with precise instrumentation (e.g. GPS).

Review Lecture 4 Earthquakes: What are earthquakes? What causes them? What is the results of an earthquake? Earthquakes: What are earthquakes? What causes them? What is the results of an earthquake? Stress and Strain (Energy Transfer): What is stress and what is strain? How do these relate to earthquakes? What are the 3 different kinds of strain? Stress and Strain (Energy Transfer): What is stress and what is strain? How do these relate to earthquakes? What are the 3 different kinds of strain? Elastic Rebound Theory: What is this and how does this relate to earthquakes? Elastic Rebound Theory: What is this and how does this relate to earthquakes?

One Minute Paper, that lasts three minutes. I want to read about what you do understand. What is the most exciting thing you learned today? I want to read about what you do not understand. What may have been confusing? What is one question that you have about today’s lecture? We will cover these issues during our review of today’s class at the beginning of our next class. One Minute Paper, that lasts three minutes. I want to read about what you do understand. What is the most exciting thing you learned today? I want to read about what you do not understand. What may have been confusing? What is one question that you have about today’s lecture? We will cover these issues during our review of today’s class at the beginning of our next class.

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