Notes for lecture 4 - Earthquakes L4 What is an Earthquake? Causes, Elastic rebound theory, Volcanism, PDF

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Earthquakes L4
What is an Earthquake? Causes, Elastic rebound theory, Volcanism, Landslides, Detecting and Measuring Earthquakes
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Earthquakes Lecture 4 Summary • An earthquake is a shaking of the ground caused by seismic waves, emanating from the focus along a fault that moves suddenly. When the fault moves, the strain built up over the years of slow deformation by tectonic forces is released in a few moments as seismic waves. • Elastic rebound theory explains why earthquakes occur. Over a period of time the application of stress causes rock to slowly deform (bend or stretch) elastically until it breaks, and the rock snaps back as the fault moves. This is analogous to stretching a rubber band until it breaks and snaps back to sting your hand. • There are three major types of seismic waves. Two types of waves travel through the Earth’s interior: P (primary/compressional waves), which move through all forms of matter and move the fastest, and S (secondary/shear waves) waves, which move through solids only and move at half the speed of P waves. Surface waves need a free surface like the Earth’s surface to ripple. They move more slowly than the interior waves but cause most of the destruction associated with earthquakes . • Earthquake magnitude is a measure of the size of the earthquake. The Richter magnitude is determined from the amplitude of the ground motion. The movement magnitude is closely related to the amount of energy radiated by the earthquake. The moment magnitude scale depends on the product of the slip of the fault when it broke, the area of the fault break, and the rigidity or stiffness of the rock. The Modified Mercalli Intensity Scale provides a way of estimating the intensity of an earthquakes shaking directly from an event’s destructive effects. • Most earthquakes occur along crust plate boundaries but not all. Earthquakes at divergent plate boundaries are usually shallow, of lower magnitude, and a consequence of tensional stress. Convergent plate boundaries produce shallow and deep earthquakes of low to high magnitudes and are commonly due to compressive stress. Transform

faults produce shallow to moderately deep earthquakes of low to high magnitude and are usually in response to shear stress. • The destructiveness of an earthquake does not depend on the magnitude alone. In addition to ground motion, the duration of the earthquake, avalanches, fires, liquefaction, tsunamis, proximity to population centers, and the construction design of buildings all help to determine the destructiveness of an earthquake. • The damage caused by earthquakes can be mitigated by: regulating the construction design of buildings in earthquake zones; bolting houses to their foundation; in your home, securing appliances connected to gas lines and tall furniture to walls; keeping heavy items at low levels; and having a community plan for dealing with emergencies generated by earthquakes. Geologists generate seismicrisk maps to aid public authorities with their evaluations. • Scientists can characterize the degree of risk in a region, but they cannot consistently predict earthquakes. Recurrence intervals—the average time between large earthquakes—are used for long-term forecasting of earthquakes.

Learning Objectives Knowledge • Know the factors that define an earthquake (ground shaking caused by seismic waves that emanate from a fault that moves suddenly). • Know the three types of seismic waves and their basic characteristics. • Understand what is meant by earthquake magnitude and intensity and the scales used to measure them. • Know that most earthquakes are associated with active tectonic plate boundaries. • Know that earthquake activity at each type of tectonic plate boundary has distinctive characteristics. • Know what governs the type of faulting that occurs in an earthquake. Skills/Applications/Attitudes • Evaluate the geologic circumstances that contribute to the destructiveness of earthquakes? • Appreciate the importance of mitigating damage by earthquakes and understand the steps that should be taken by threatened communities. • Given first motion data at different locations, determine the direction of movement and type of fault associated with earthquake activity. General Educational Skills

• Given the arrival time of S and P waves at an observation point compute the distance from the observation point to the epicenter of an earthquake.

Content What is an Earthquake? Causes Elastic rebound theory—release of strain energy built up in the Earth’s crust beyond the elastic limit Volcanism Explosions Landslides Detecting and Measuring Earthquakes Seismometer Seismograph Seismic Waves Body waves P waves—compressional S waves—shear Surface waves Location Focus Epicenter Measuring the Size of an Earthquake Magnitude Richter Scale (ground motion) Moment magnitude (energy released) Intensity Mercalli Scale (damage) Earthquakes and Patterns of Faulting Three main types of fault movements First motions Global Pattern of Earthquake Activity Divergent boundaries Transform-fault Convergent boundaries Intraplate Destructiveness of Earthquakes Ground motion Tsunamis Landslides Earth cracks and fissures Fires Building materials and methods

Protection programs Earthquake Risk Assessment and Prediction What Should You Do Before and During an Earthquake?...