earthquakes geology 101, fall 2012. stress and strain stress is the stored energy built up due to...
TRANSCRIPT
![Page 1: Earthquakes Geology 101, Fall 2012. Stress and strain Stress is the stored energy built up due to plate motion; strain is the volume change on a rock](https://reader035.vdocuments.net/reader035/viewer/2022062803/56649f285503460f94c4047f/html5/thumbnails/1.jpg)
Earthquakes
Geology 101, Fall 2012
![Page 2: Earthquakes Geology 101, Fall 2012. Stress and strain Stress is the stored energy built up due to plate motion; strain is the volume change on a rock](https://reader035.vdocuments.net/reader035/viewer/2022062803/56649f285503460f94c4047f/html5/thumbnails/2.jpg)
Stress and strain
• Stress is the stored energy built up due to plate motion; strain is the volume change on a rock due to stress
![Page 3: Earthquakes Geology 101, Fall 2012. Stress and strain Stress is the stored energy built up due to plate motion; strain is the volume change on a rock](https://reader035.vdocuments.net/reader035/viewer/2022062803/56649f285503460f94c4047f/html5/thumbnails/3.jpg)
Faults
• At fracture point, the rock breaks; the resulting offset between the two parts of the rock is a fault. The energy released is an earthquake
![Page 4: Earthquakes Geology 101, Fall 2012. Stress and strain Stress is the stored energy built up due to plate motion; strain is the volume change on a rock](https://reader035.vdocuments.net/reader035/viewer/2022062803/56649f285503460f94c4047f/html5/thumbnails/4.jpg)
Types of faults
• The different types of plate boundaries lead to different types of faults
![Page 5: Earthquakes Geology 101, Fall 2012. Stress and strain Stress is the stored energy built up due to plate motion; strain is the volume change on a rock](https://reader035.vdocuments.net/reader035/viewer/2022062803/56649f285503460f94c4047f/html5/thumbnails/5.jpg)
Basic earthquake terminology
![Page 6: Earthquakes Geology 101, Fall 2012. Stress and strain Stress is the stored energy built up due to plate motion; strain is the volume change on a rock](https://reader035.vdocuments.net/reader035/viewer/2022062803/56649f285503460f94c4047f/html5/thumbnails/6.jpg)
Earthquake energy
• Radiates away as body waves
![Page 7: Earthquakes Geology 101, Fall 2012. Stress and strain Stress is the stored energy built up due to plate motion; strain is the volume change on a rock](https://reader035.vdocuments.net/reader035/viewer/2022062803/56649f285503460f94c4047f/html5/thumbnails/7.jpg)
Earthquake energy
• When body waves reach the surface, they transfer into surface waves
![Page 8: Earthquakes Geology 101, Fall 2012. Stress and strain Stress is the stored energy built up due to plate motion; strain is the volume change on a rock](https://reader035.vdocuments.net/reader035/viewer/2022062803/56649f285503460f94c4047f/html5/thumbnails/8.jpg)
Seismograph
![Page 9: Earthquakes Geology 101, Fall 2012. Stress and strain Stress is the stored energy built up due to plate motion; strain is the volume change on a rock](https://reader035.vdocuments.net/reader035/viewer/2022062803/56649f285503460f94c4047f/html5/thumbnails/9.jpg)
Seismogram is a record of all waves
![Page 10: Earthquakes Geology 101, Fall 2012. Stress and strain Stress is the stored energy built up due to plate motion; strain is the volume change on a rock](https://reader035.vdocuments.net/reader035/viewer/2022062803/56649f285503460f94c4047f/html5/thumbnails/10.jpg)
Measures of earthquakes
• Richter scale (Charles Richter and Beno Gutenberg, Caltech, 1935)
• Based on movement of the seismograph stylus
• Logarithmic scale, meaning that each 1 point increase in magnitude means a 10-fold increase in movement of the stylus
• Not accurate for big earthquakes, or distant ones
![Page 11: Earthquakes Geology 101, Fall 2012. Stress and strain Stress is the stored energy built up due to plate motion; strain is the volume change on a rock](https://reader035.vdocuments.net/reader035/viewer/2022062803/56649f285503460f94c4047f/html5/thumbnails/11.jpg)
Measures of earthquakes• Moment magnitude scale is
commonly used today to report earthquake “magnitudes”.
• It is still logarithmic, but is now based on a formula that looks at other parts of the seismogram than just the largest movement.
• The formula calculates the energy released by the quake, which is more relevant to engineers and planners than the Richter scale magnitude.
![Page 12: Earthquakes Geology 101, Fall 2012. Stress and strain Stress is the stored energy built up due to plate motion; strain is the volume change on a rock](https://reader035.vdocuments.net/reader035/viewer/2022062803/56649f285503460f94c4047f/html5/thumbnails/12.jpg)
Basic math of logarithmic scale
The point is, for each increase of 1 point on the moment magnitude scale, the energy released by the earthquake goes up by a factor of 30 or so.
![Page 13: Earthquakes Geology 101, Fall 2012. Stress and strain Stress is the stored energy built up due to plate motion; strain is the volume change on a rock](https://reader035.vdocuments.net/reader035/viewer/2022062803/56649f285503460f94c4047f/html5/thumbnails/13.jpg)
Measures of earthquakes
• Modified Mercalli Intensity (MMI), developed by Guiseppe Mercalli, 1884 and 1906
• Measures effects on humans and their structures
• Often a better indicator of potential earthquake hazard than the moment magnitude scale
![Page 14: Earthquakes Geology 101, Fall 2012. Stress and strain Stress is the stored energy built up due to plate motion; strain is the volume change on a rock](https://reader035.vdocuments.net/reader035/viewer/2022062803/56649f285503460f94c4047f/html5/thumbnails/14.jpg)
Nisqually (2001) quake intensity
![Page 15: Earthquakes Geology 101, Fall 2012. Stress and strain Stress is the stored energy built up due to plate motion; strain is the volume change on a rock](https://reader035.vdocuments.net/reader035/viewer/2022062803/56649f285503460f94c4047f/html5/thumbnails/15.jpg)
Aftershocks occur as stress transfers to unrelieved parts of fault or rock
![Page 16: Earthquakes Geology 101, Fall 2012. Stress and strain Stress is the stored energy built up due to plate motion; strain is the volume change on a rock](https://reader035.vdocuments.net/reader035/viewer/2022062803/56649f285503460f94c4047f/html5/thumbnails/16.jpg)
Earthquake prediction – which one is true?
![Page 17: Earthquakes Geology 101, Fall 2012. Stress and strain Stress is the stored energy built up due to plate motion; strain is the volume change on a rock](https://reader035.vdocuments.net/reader035/viewer/2022062803/56649f285503460f94c4047f/html5/thumbnails/17.jpg)
Interlude about resource extraction
![Page 18: Earthquakes Geology 101, Fall 2012. Stress and strain Stress is the stored energy built up due to plate motion; strain is the volume change on a rock](https://reader035.vdocuments.net/reader035/viewer/2022062803/56649f285503460f94c4047f/html5/thumbnails/18.jpg)
When organic material dies...
• If oxygen can be excluded, the decay process produces a material called kerogen from various fats and other biological compounds.
• They are rich in carbon (C), hydrogen (H) and oxygen (O), which will burn well, but also contain nitrogen (N), sulfur (S) and phosphorus (P) which produce noxious gases (like SO2 and NOx) when burned.
![Page 19: Earthquakes Geology 101, Fall 2012. Stress and strain Stress is the stored energy built up due to plate motion; strain is the volume change on a rock](https://reader035.vdocuments.net/reader035/viewer/2022062803/56649f285503460f94c4047f/html5/thumbnails/19.jpg)
Petroleum needs three rocks
• Source rock, which contains the organic material in the first place (and allows the organics to “cook”)
• Reservoir rock to which the petroleum migrates and is stored.
• Cap rock to prevent the petroleum from rising further.
![Page 20: Earthquakes Geology 101, Fall 2012. Stress and strain Stress is the stored energy built up due to plate motion; strain is the volume change on a rock](https://reader035.vdocuments.net/reader035/viewer/2022062803/56649f285503460f94c4047f/html5/thumbnails/20.jpg)
Oil and natural gas can be trapped
Since oil is less dense than water, it will “float” on ground-water. This allows trapping by an imperme-able layer.
![Page 21: Earthquakes Geology 101, Fall 2012. Stress and strain Stress is the stored energy built up due to plate motion; strain is the volume change on a rock](https://reader035.vdocuments.net/reader035/viewer/2022062803/56649f285503460f94c4047f/html5/thumbnails/21.jpg)
Using seismic studies profitably
Using vibrator trucks to shake the ground, and listening with an array of geophones, petroleum engineers can determine oil-rich layers.
![Page 22: Earthquakes Geology 101, Fall 2012. Stress and strain Stress is the stored energy built up due to plate motion; strain is the volume change on a rock](https://reader035.vdocuments.net/reader035/viewer/2022062803/56649f285503460f94c4047f/html5/thumbnails/22.jpg)
Interlude about wave behavior
• When a wave passes through a boundary, it can either reflect off the boundary or pass through it in a slightly different direction.
• First shown for light, the same physics works for seismic waves.
![Page 23: Earthquakes Geology 101, Fall 2012. Stress and strain Stress is the stored energy built up due to plate motion; strain is the volume change on a rock](https://reader035.vdocuments.net/reader035/viewer/2022062803/56649f285503460f94c4047f/html5/thumbnails/23.jpg)
Probing the Earth’s interior
![Page 24: Earthquakes Geology 101, Fall 2012. Stress and strain Stress is the stored energy built up due to plate motion; strain is the volume change on a rock](https://reader035.vdocuments.net/reader035/viewer/2022062803/56649f285503460f94c4047f/html5/thumbnails/24.jpg)
Changes in wave speed indicate deep Earth structures
Specifically, the Mohorovicic (Moho) discontinuity, where seismic speeds suddenly increase as they get deeper, shows the change from crust materials to mantle within the lithosphere.
Note that the mantle does get more solid toward the core (speeds increase), but that the outer core is liquid.
![Page 25: Earthquakes Geology 101, Fall 2012. Stress and strain Stress is the stored energy built up due to plate motion; strain is the volume change on a rock](https://reader035.vdocuments.net/reader035/viewer/2022062803/56649f285503460f94c4047f/html5/thumbnails/25.jpg)
S-wave properties give us the size of the outer core
![Page 26: Earthquakes Geology 101, Fall 2012. Stress and strain Stress is the stored energy built up due to plate motion; strain is the volume change on a rock](https://reader035.vdocuments.net/reader035/viewer/2022062803/56649f285503460f94c4047f/html5/thumbnails/26.jpg)
P-wave properties give us the size of the inner core