Chapter 11. Earthquakes PDF

Summary

This document is a chapter on earthquakes, providing an introduction to the topic and covering key concepts such as seismology, locating earthquake sources, earthquake belts, depths, intensity, magnitude, predicting earthquakes, and types of seismic waves. It also details the causes of earthquakes, including elastic rebound theory, the difference between fault creep and stick-slip, and earthquake damage.

Full Transcript

Chapter 11. Earthquakes 1 from Lite Geology Chapter 11 Earthquakes What is an earthquake Seismology Locating earthquake source Earthquake – belts – depths – intensity and magnitude – destruction...

Chapter 11. Earthquakes 1 from Lite Geology Chapter 11 Earthquakes What is an earthquake Seismology Locating earthquake source Earthquake – belts – depths – intensity and magnitude – destruction Predicting earthquakes 3 Earthquake Vibration of Earth produced by the rapid release of energy Earthquake triggers – slippage along fault (primary cause) – volcanic eruption – nuclear blast focus - source at depth from which energy radiates as seismic waves in all directions – site of initial rupture epicenter - surface location above source (focus) 4 Earthquake waves are like waves radiating in water from the focus of the earthquake 5 Seismic waves are the elastic energy that radiates in all directions from the focus 6 Elastic Rebound Causes Earthquakes 1. Rock initially bent (rather than fractured) by tectonic forces. – Bending stores elastic energy in the rock 2. Frictional “lock” (resistance) holding the rock together is overcome. – slippage occurs at the weakest point (the focus) 3. Elastic rebound - strain is released and rock snaps back to flat shape – causes earthquake 4. Slippage occurs along the fault. – Cycle begins again until next earthquake 7 8 Fault creep versus stick-slip faults fault creep – gradual fault movement releases elastic energy so little is stored – regular slippage = small earthquakes – Faults with this situation are not a danger. Stick-slip fault – fault locks storing elastic energy for hundreds of years and its release produces large earthquake – Faults with this situation are a great danger. 9 Surface Expression of Faults vertical offset: produces a fault scarp – cliff horizontal offset – with no vertical offset displaces 1964 Alaska Earthquake structures along the fault. no offset: very deep faults will have no surface expression 1906 San Francisco Earthquake 10 Earthquake Duration Earthquake - last seconds to minutes Foreshocks - days to years before a major earthquake small earthquakes occur – monitor to predict earthquakes - poor results Aftershocks - for days after main earthquake smaller earthquakes occur 11 Rate of Movement Fault movement is listed as movement pre year, even if the fault only moves once every 100 years. For example, the San Andreas fault of San Francisco moves about 3 cm per year, but that is calculated from actual offset that occurs of 300cm every 100 years. 12 Seismology The study of earthquake and seismic waves 13 Seismograph (or seismometer) Seismograph measures seismic waves, by recording the movement of Earth in relation to a stationary mass on a rotating drum or magnetic tape. – Seismographs work on the principal of inertia - objects at rest tend to stay at rest unless acted on by an outside force Seismogram is the record provided by a seismograph 14 Seismograph types More than one type of seismograph is needed, because movements are both horizontal and vertical. Horizontal-motion seismograph – mass suspended from support attached to ground. The support vibrates the hanging mass remains stationary Vertical-motion seismograph – mass suspended on spring 16 Types of Seismic Waves surface waves - long (L) waves - travel in the rock just below the ground surface – Two types: up-and-down and side-to-side motions (*these are the most destructive to buildings) body waves – travel through Earth’s interior – Primary (P) waves – Secondary (S) waves 17 How do body waves (P and S) give us information on the interior of the earth? P wave - push-pull waves compress and expand the rock - like toy slinky – Can travel through solid and liquid S wave - shake the material – like shaking a rope – It changes the shape of the material – shears it so it cannot travel through liquid in the interior of the earth Used to identify the liquid outer core. 18 How does the relative velocity of P and S waves give us information on the location of earthquakes? The different types of seismic waves travel at different speeds, that results in a lag time that increases with distance from the focus. Relative speeds: P waves > S waves > surface (L) waves 20 Locating the epicenter from the lag time. Seismic-wave travel-time graph relates lag time to distance from the epicenter – Obtain the distance of the seismograph from the epicenter by matching the lag time between the P and S waves on the seismic-wave travel-time graph Then use triangulation - – use distance data from 3 different seismograph locations – draw circle with radius of epicenter distance – epicenter where the 3 circles intersect 22 Use the lag time from three different seismograph location anywhere in the world to obtain their distance from the earthquake source. 23 Triangulation to find earthquake source 24 Where Do Most Earthquakes Occur? Convergent plate boundaries: About 95% of energy released from earthquakes originates in two narrow zones at convergent plate boundaries. Location of the largest most destructive earthquakes. – Circum-Pacific belt and the Alpine-Himalayan belt Divergent plate boundaries have frequent, shallow weak earthquakes Transform faults tend to generate large earthquakes Measuring the size of earthquakes Mercalli scale an intensity scale – measures the degree of earthquake shaking at a given location based on the amount of damage – old scale measures amount of damage Richter scale a magnitude scale – estimate of the amount of energy released at the source of the earthquake 29 Modified Mercalli (Intensity) Scale – outdated based only on amount of damage. cartoon I Not felt II Felt only by persons at rest III–IV Felt by persons indoors only V–VI Felt by all; some damage to plaster, chimneys VII People run outdoors, damage to poorly built structures VIII Well-built structures slightly damaged; poorly built structures suffer major damage IX Buildings shifted off foundations X Some well-built structures destroyed XI Few masonry structures remain standing; bridges destroyed XII Damage total; waves seen on ground; objects thrown into air 30 Richter (Magnitude) Scale – Determined from the Energy amplitude of the largest seismic wave. – Logarithmic scale (not a linear scale) Each unit on the scale means a 10-fold difference in wave amplitude and a 32- fold difference in energy released 31 Earthquake Damage Building collapse Tsunami Landslides Fire Flood 32 Earthquake Damage – Building Collapse Factors that determine if the building will collapse Intensity and duration of the vibrations: increase intensity and duration, increases destruction of buildings Construction practices – steel is better than masonry Nature of the material the structure is built on: – bedrock is better than unconsolidated (loose) sediments – Dry sediment is better than water-saturated sediment – liquefaction - water-saturated sediments turns into a mobile fluid, loose cohesiveness and liquefies, then the buildings sink 33 Building Collapse: Steel better than masonry Masonry (brick/stone) structure Steel structure 1964 Alaska earthquake 34 Building Collapse - Liquefaction Liquefaction: Shaking of water saturated sediments causes them to loose their cohesion and become liquid. Then buildings sink into these liquified sediments 35 Earthquake Damage - Liquefaction Freeway collapsed in area where it was built on loose water- saturated mud. Freeway did not collapse where built on compacted sand and gravel or bedrock. Loma Prieta Quake, 1989, Cypress Freeway bridge collapse 38 Earthquake Damage – Tsunami Tsunami is a seismic sea wave – Most are generated by displacement of ocean water by fault movement at subduction zones or undersea landslide 39 Other destruction Landslides – unconsolidated sediments experience liquefaction sudden loss of strength of water-saturated material – bedrock more stable Fire - gas and electric lines severed – 1906 San Francisco quake fire caused more damage than vibrations Floods – dams break, rivers change course 42 Turnagain Heights slide, 1964 Alaska earthquake 43 Earthquake prediction Short-range (hours-days) - not possible Long-range (10’s of years) – based on repetitive, cyclic, nature of quakes – look for patterns where faults are “locked” areas with fault creep have a lower potential seismic gaps - no activity have high potential – used to set building codes not useful for evacuation 45 Long-Range Earthquake Forecasting: Paleoseismology Paleoseismology is the study of the timing, location, and size of prehistoric earthquakes By digging a trench across a fault zone, scientists look for evidence of ancient faulting (mud volcanoes and offset sedimentary strata) Long-Range Earthquake Forecasting: Seismic Gaps Seismic gaps are tectonically quiet zones along a fault where strain is currently building up The stored strain will be released in a future earthquake Strain can be estimated using known rate of plate movement

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