Earthquakes Chapter 19

Stress and strain Earthquakes occur when rocks fracture Fractures occur when stress exceeds strain Stress: forces per unit area acting on a material Three types COMPRESSION TENSION SHEAR Strain: deformation of materials in response to stress

Rock behavior ELASTIC BEHAVIOR: return to original shape after stress DUCTILE BEHAVIOR: rocks are deformed or change shape Ductile deformation is permanent Material stays deformed even if stress is reduced to zero Material can break or fail depending on its strength

Earthquakes http://www.youtube.com/watch?v=VSgB1IWr6O4 Produced by the release of elastic energy stored in rock that has been subjected to great forces When the strength of the rock is exceeded by these forces, it breaks. Causes vibrations of an earthquake http://www.youtube.com/watch?v=VSgB1IWr6O4

faults FAULT: resulting fracture or system of fractures where movement occurs Types of Faults 1. Reverse Fault Horizontal compression 2. Normal Fault Horizontal tension 3. Strike-slip Fault Horizontal shear

Earthquake waves The vibrations of the ground during an earthquake are called SEISMIC WAVES. THREE TYPES 1. Primary waves: pull rocks in same direction 2. Secondary waves: rocks move at right angles 3. Surface waves: rocks move in two directions, up and down or side to side

Earthquake waves Surface waves: travel along Earth’s surface Body waves: travel through Earth’s interior P-waves, S-waves Body waves spread out from the point of failure of rocks.

Earthquake waves Focus: Where earthquake originates Epicenter: point on Earth’s surface directly above the focus

vibrations Vibrations sent out by earthquakes shake the entire globe. Seismometers: detect and record these vibrations in the form of a seismogram

How does a seismometer work? Take a piece of tape and attach a piece of paper to the desk. One student hold the pencil to the paper while another slowly pulls the table. What do you see? Now, the third student should begin to shake the table opposite the direction it is being pulled. Now what do you notice?

Seismometer questions 1. Explain how a seismometer works. 2. What is a seismogram? 3. Describe how each type of wave (P-waves, S-waves, and surface waves) causes rock particles to move. [You can use your book to help answer these questions]

Earthquake magnitude and intensity Magnitude: amount of energy released Measured on the Richter scale Based on the size of the largest seismic wave Each point is 10x larger than previous How much larger is a magnitude-8 earthquake than a magnitude-7? What about magnitude-6? Moment magnitude scale: takes into account the size of the fault rupture, amount of movement along the fault, rock’s stiffness

Earthquake intensity Intensity: the amount of damage done to structures involved Measured on the modified Mercalli scale Rates the types of damage and other effects of an earthquake as noted by observers Uses roman numerals I-XII I: not felt except under unusual conditions XII: Damage is total. Objects are thrown upward into the air.

Structural failure Pancaking: supporting walls of the ground floor fail and cause the upper floors to fall and collapse Building height: intermediate buildings (5- 15 stories) collapse more readily than shorter or higher buildings due to the frequency of the Earthquake waves. http://www.youtube.com/watch?v=4Y-62Ti5_6s&safety_mode=true&persist_safety_mode=1&safe=active

Tall or Small? Which is Safer? It depends on resonance!! Small buildings are more affected, or shaken, by high- frequency waves (short and frequent). For example, a small boat sailing in the ocean will not be greatly affected by a low-frequency swell where the waves are far apart. On the other hand several small waves in quick succession can overturn, or capsize, the boat. Large structures are more affected by low-frequency, or slow shaking. Ex: An ocean liner will experience little disturbance by short waves in quick succession. However, a low-frequency swell will significantly affect the ship.

Earthquake Hazards Land and soil failure Can trigger massive landslides Subsurface material in fluid-saturated sand may liquefy and form quicksand Soft soils can actually amplify the waves Why do you think this is?

Fault scarps Great vertical offset where the fault intersects the ground surface

tsunami A large ocean wave generated by vertical motions of the seafloor during an earthquake http://www.youtube.com/watch?v=QIy_m_BCFEA

Predicting an earthquake We use two factors to estimate the probability of an earthquake 1. Earthquake History Seismic gaps: sections of active faults that haven’t experienced significant earthquakes for a long period of time Lasers 2. Strain Accumulation The rate at which strain builds up in rocks Strain accumulated in a particular location of a fault Strain released during last earthquake Amount of time passed since last earthquake